Air Conditioning Controls NR15 - Wikispaceshvaceducation.wikispaces.com/file/view/ACControls.pdf · Resources and references Recommended textbooks Automatic Control Principles. Honeywell,

Module Resource Manual

Air Conditioning Controls

NR15

First published in April 2009 by Manufacturing and Engineering Educational Services

NSW TAFE Commission

PO Box 218 Bankstown NSW 2200

This work is copyright. Any inquiries about the use of this material should be directed to the publisher.

©

New South Wales Technical and Further Education Commission

AIR CONDITIONING CONTROLS

NR15

FEEDBACK We value your opinion and welcome suggestions on how we could improve this resource manual.

CONTENTS

INTRODUCTION 1

1. CONTROL SYSTEM FUNDAMENTALS AND DIAGRAMS 3

PRACTICAL EXERCISE 1 - CONTROL CIRCUIT DIAGRAMS 24

PRACTICAL EXERCISE 2 - CONTROL CIRCUIT DIAGRAMS 27

REVIEW QUESTIONS 30

2. TYPES OF CONTROL SYSTEMS – GENERAL OVERVIEW AND FLUID FLOW

CONTROL 35

PRACTICAL EXERCISE 1 – CONTROL SYSTEMS 50

PRACTICAL EXERCISE 2 – FLUID FLOW CONTROL 53

REVIEW QUESTIONS 56

3. ENERGY MANAGEMENT AND BUILDING MANAGEMENT PRINCIPLES 61

4. ELECTRIC CONTROL SYSTEMS 65

PRACTICAL EXERCISE 1 – ELECTRIC CONTROL SYSTEM 76

PRACTICAL EXERCISE 2 – ELECTRIC CONTROL SYSTEM COMMISSIONING 78

PRACTICAL EXERCISE 3 – ELECTRIC CONTROL SYSTEM FAULT FINDING 80

REVIEW QUESTIONS 82

5. ELECTRONIC CONTROL SYSTEMS 87

PRACTICAL EXERCISE 1 – ELECTRONIC CONTROL SYSTEM 98

PRACTICAL EXERCISE 2 – ELECTRONIC CONTROL SYSTEM

COMMISSIONING 100

PRACTICAL EXERCISE 3 – ELECTRONIC CONTROL SYSTEM FAULT

FINDING 103

REVIEW QUESTIONS 105

6. PNEUMATIC CONTROL SYSTEMS 109

PRACTICAL EXERCISE 1 - PNEUMATIC CONTROL SYSTEM 122

PRACTICAL EXERCISE 2 - PNEUMATIC CONTROL COMMISSIONING 124

PRACTICAL EXERCISE 3 - PNEUMATIC CONTROL FAULT FINDING 127


7. PROGRAMMABLE LOGIC CONTROLLERS AND DIRECT DIGITAL

CONTROLS 135

PRACTICAL EXERCISE 142


SAMPLE TESTS 149

SAMPLE THEORY TEST 1 150

PRACTICAL TEST 1 162


SAMPLE PRACTICAL TEST 2 179

ANSWERS 185






Resources and references

Recommended textbooks Automatic Control Principles. Honeywell, USA. Boyle, G. Australian Refrigeration and Air Conditioning. 3rd Edition Volume 1 and 2. ISBN 1 86442 037 5 TAFE Publication of Western Australia, Perth, WA

Additional Reference The following texts and videos may be of further assistance for this module.

Coffin, M.J. Direct Digital Control for Building HVAC Systems ISBN 0 442 23797 9 Van Nostrand Reinhold, New York, USA. Electrical and Electronic Drawing Practice for Students - SAA/SNZ HB3-1996. ISBN 0 7337 0246 5 Standards Australia, Homebush, New South Wales. Engineering Manual of Automatic Control (SI Ed). Library of Congress Catalog Card Number: 94-073455 Honeywell, USA, 1995. General Information and General Index (AS 1102.101-1996) to Analogue Elements (AS 1102.113-1996) Standards Australia, Homebush, New South Wales. Heating Ventilation Air Conditioning Seminar Booklet produced by Honeywell Langley, B.C. Control Systems for Air Conditioning and Refrigeration Prentice-Hall ISBN 0 13 171679 4 01 New Jersey, USA

Suggested Videos Air Conditioning 84-065 Regency College of TAFE, SA

Acknowledgments TAFE NSW acknowledges and thanks all companies and individuals who generously supplied diagrams, pictures and information. The following companies provided information: Atlas Cop Co Belimo Australia Pty Ltd Celsius Magazine Daikin Australia Email Major Appliances Honeywell Limited Johnson Controls Kirby Refrigeration- Erie Controls Landis and Staefa Division – of Siemens Building Technology SMC Pneumatics (Australia) Pty. Ltd Toshiba International Corporation Pty. Ltd

NR15 Air Conditioning Controls Module Resource Manual April 2009

1

Introduction This resource manual contains learning exercises, review questions and sample assessment instruments. It is designed to assist students achieve the outcomes and purpose described in the national module descriptor NR15 and is an example of the depth and breadth of learning expected.

The topics listed in the content are arranged in the preferred learning sequence. It is recognised that this is not the only sequence in which the material could be learnt. Assessment arrangements and sample assessment instruments are based on the sequence of topics listed above. A teacher may decide that for a particular student or group of students it is more effective to present the topics in a different sequence. In this case the students must be informed in writing of the resulting changes in the assessment events before starting the module.

Learning plan The following topic weighting will help you plan and allocate the effort needed to achieve the purpose and outcomes of the module.

1. Control System Fundamentals and Diagrams 2.5%

2. Types of Control Systems – General Overview and Fluid Flow 12.5%

3. Energy Management Principles 12.5%

4. Electric Control Systems 12.5%

5. Electronic Control Systems 18.75%

6. Pneumatic Control Systems 18.75%

7. PLC and DDC Control Systems 12.5% Imagine a world where humans manually operated air conditioning systems. Whenever the temperature and / or humidity of a room moves outside pre-agreed parameters, someone would have to start either the refrigeration or heating equipment. Once the conditions move back into the agreed parameters, the equipment would then have to be manually turned off. This scenario is most impracticable and fortunately we know this is not necessary because of the use of automatic controls. This module is designed as an introduction to the various control systems used in the air conditioning industry. The systems that will be covered include electromechanical, pneumatic, electronic and microprocessor systems. If at the end of this module you feel you wish to learn more about control systems at a greater depth than that provided by this module, you should consider doing further modules in the controls area.


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Topics and Exercises

Topic Exercise Practical exercise

1. Control System Fundamentals and Diagrams

Drawing - Control circuit diagrams from a circuit diagram

Drawing - Control circuit diagrams

2. Types of Control Systems – General Overview and Fluid Flow Control

Drawing - Observe and draw an existing fluid flow control system

Control systems observation

3. Energy Management and Building Management Principles

A 600 word assignment on the principles behind Energy Management and Building Management

4. Electric Control Systems

Explain the operation of various electric controls.

Electric control system commissioning

Electric control system fault finding

5. Electronic Control Systems

Explain the operation of various electronic controls.

Electronic control system commissioning

Electronic control system fault finding

6. Pneumatic Control Systems

Explain the operation of various pneumatic controls.

Pneumatic control commissioning

Pneumatic control fault finding

7. Programmable Logic Controllers and Direct Digital Controls

Observe the operation of a DDC air conditioning control system and / or a PLC air conditioning control system.


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1. Control System Fundamentals and Diagrams Purpose

This topic is divided into two sections. In the first section you will learn about the principles, concepts, terminology and applications of air conditioning control systems. The second section of this topic will allow you to read, design and explain the sequence of operation of simple air conditioning circuit and control circuit diagrams.

Objectives At the end of this topic you should be able to:

list and explain the principles of air conditioning control

define various terms used in air conditioning control

describe the operation of various simple control diagrams

list various applications employing air conditioning control

explain the sequence of operation of a simple air conditioning circuit diagram

draw a circuit diagram for a simple air conditioning system

explain the sequence of operation of a simple air conditioning control system diagram

draw a control system diagram for a simple air conditioning system.

Content - Terminology and Definitions - Symbols - Control System Fundamentals - Principles - Method of Control - Closed Loop - Open Loop - Factors that Affect Loop Stability - Automatic Control Elements - Six Basic Functions of Automatic Control - Symbols and Diagrams - Diagrams

- Air Conditioning Diagrams - Pneumatic and Logic Control Diagrams - Electrical Diagrams


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- Block Diagrams - Circuit Diagrams (Schematic Diagram) - Wiring Diagrams

- Revision of Control Circuits - Control Symbols - Air Conditioning Circuit Diagrams

References

ARAC Volume 2, pages 29.3; Volume 1, page 12.35; pages 12.43 – 12.44 and Glossary of Terms

Terms and definitions

ARAC Volume 2, pages 29.1 – 29.3

Loops, control elements and functions of automatic control

ARAC Volume 2, pages 28.16 – 28.18; pages 29.7 – 29.13; pages 29.38 – 39

Symbols and electrical diagrams

NR08 Appliance Motors and Circuits and NR12 System Control – module books

Discusses circuit components


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Terminology and definitions Many of the Terminology and Definitions found below have been used with permission of Honeywell Air Conditioning. Actuator - ARAC Volume 2, page 29.4 Algorithm - A calculation method that produces a control output by operating on an error signal or a time series of error signals. Analog - Continuously variable (e.g. a faucet controlling water from off to full flow). Authority - The influence that a second sensor has on a controller to produce an output. Automatic control / system - A system that reacts to a change or imbalance in the variable it controls by adjusting other variables to restore the system or a time series or error signals. Binary - A numbering system used by computers to i) count and ii) perform tasks. Calibrate - Adjustment of control devices to ensure accurate operation. Compensation control - A process of automatically adjusting the control point of a given controller to compensate for changes in second measured variable (e.g. outdoor air temperature). For example, the hot deck control point is normally reset as the outdoor air temperature decreases. Also called “reset control." Control agent - The medium in which the manipulated variable exists. In a steam heating system, the control agent is the steam and the manipulated variable is the flow of the steam. Control point - ARAC Volume 2, page 29.3 Control system - Made of all the equipment in which the controlled variable exists but does not include the automatic control equipment. Controlled medium - The medium in which the manipulated variable exists. In a space temperature control system, the controlled variable is the space temperature and the controlled medium is the air within the space. Controlled value / desired value - ARAC Volume 2, page 29.4 Controlled variable - ARAC Volume 2, page 29.3


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Controller - A device that sense change in the controlled variable (or receives input from a remote sensor) and derives the proper correction output. Corrective action - Control action that results in a change of the manipulated variable. Initiated when the controlled variable deviates from setpoint. Cycle - One complete execution of a repeatable process. In basic heating operation, a cycle comprises one on period and one off period in a two-position control system. Cycling - ARAC Volume 2, page 29.3 Cycling rate - The number of cycles completed per time unit, typically cycles per hour for a heating or cooling system. The inverse of the length of the period of the cycle. Dead band - A range of the controlled variable in which no corrective action is taken by the controlled system and no energy is used. See also “Zero Energy Band." Desired value / controlled value - ARAC Volume 2, page 29.4 Deviation - the difference between the set point and the value of the controlled variable an any moment. Also called “Offset." Differential gap / differential - ARAC Volume 2, page 29.4 ARAC Glossary page 7 Digital - A series of on and off pulses arranged to convey information. Morse code is an early example. Processors (computers) operate using digital language. Digital control - A control loop in which a microprocessor-based controller directly controls equipment based on sensor inputs and setpoint parameters. The programmed control sequence determines the output to the parameter. Direct Acting (DA) - A positive operation of the actuator caused by a positive signal from a controller or visa versa. Direct Digital Control (DDC) - ARAC Volume 2, page 12.36 Also see Digital and Digital control. Droop - A sustained deviation between the control point and the setpoint in a two-position control system caused by a change in the heating or cooling load. Electric control - A control circuit that operates on line or low voltage and uses a mechanical means, such as temperature-sensitive bimetal or bellows,


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to perform control functions, such as actuating a switch or positioning a potentiometer. The controller signal usually operates or positions an electric actuator or may switch an electric load directly or through a relay. Electronic control - A control circuit that operates on low voltage and uses solid-state components to amplify input signals and perform control functions, such as operating a relay or providing an output signal to position an actuator. The controller usually furnishes fixed control routines based on the logic of the solid-state components. Final control element - A device such as a valve or damper that acts to change the value of the manipulated variable. Positioned by an actuator. Hardware - Physical components of a computer, (not including the software). Interface - A device that a computer uses to communicate with another computer. Input / Output (I/O) - Input is where information passes into a controller and output is where information leaves the controller. Lag - ARAC Volume 2, page 29.4, ARAC Glossary page 12 Load - ARAC Glossary page 12 Logic - The process of arriving at a decision based upon information that has been provided. Manipulated variable - The quantity or condition regulated by the automatic control system to cause the desired change in the controlled variable. Measured variable - A variable that is measured and may be controlled (e.g. discharge is measured and controlled, outdoor is only measured). Microprocessor - ARAC Glossary page 13 Microprocessor-based Control - ARAC Volume 2, page 12.35 Modulating - An action that adjusts by minute increments and decrements. Offset - ARAC Volume 2, page 29.4 On / Off control - A sustained deviation between the control point and setpoint of proportional control system under stable operating conditions. Pneumatic control - A control circuit that operates on air pressure and uses a mechanical means, such as a temperature-sensitive bimetal or bellows, to perform control functions, such as actuating a nozzle and flapper or a


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switching relay. The controller output usually operates or positions a pneumatic actuator, although relays and switches are often in the circuit. Program Logic Control (PLC) - This is a computer based device that replaces the “hard wired” control circuit and allows connections to be done by way of a program. Potentiometer / POT - ARAC Glossary page 15 Primary element - The proportion of the controller that senses the controlled medium. For example, a thermostat bi-metal is a primary element. Process - A general term that describes a change in a measurable variable (e.g. the mixing of return and outdoor air streams in a mixed-air control loop and heat transfer between cold water and hot air in a cooling coil) Usually considered separately from the sensing element, control element, and controller. Proportional band - In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through its full operating range. Expressed in percent of primary sensor span. Commonly used equivalents are “throttling range” and “modulating range." Proportional control - A control algorithm or method in which the final control element moves to a position proportional to the deviation of the value of the controlled variable from the setpoint. Proportional - Integral (PI) control - A control algorithm that combines the proportional (proportional response) and integral (reset response) control algorithms. Reset response tends to correct the offset resulting from proportional control. Also called “proportional-plus-reset’ or ‘two-mode” control. Proportional-Integral-Derivative (PID) control - A control algorithm that enhances the PI control algorithm by adding a component that is proportional to the rate of change (derivative) of the deviation of the controlled variable. Compensates for system dynamics and allows faster control response. Also called “three-mode” or “rate-reset’ control. Relay - ARAC Volume 2, page 29.4 & ARAC Glossary page 16 Reverse acting - To reverse the signal from a controller and used that signal to open instead of close an actuator or visa versa. Sensing device / element - ARAC Glossary page 17 Set point - ARAC Volume 2, page 29.3


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Signal - The message that is sent between components. Changes in voltages or air pressures are primarily used in air conditioning control systems Software - ARAC Volume 2, pages 12.43 - 44 Programs for computers. Throttling range - In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through its full operating range. Expressed in values of the controlled variable (e.g. degrees C, percent relative humidity, kPa). Also called “proportional band." In a proportional room thermostat, the temperature change required to drive the manipulated variable from full off to full on. Time constant - The time required for a dynamic component, such as a sensor, or a control system to reach 63.2 percent of the total response to an instantaneous (or “step”) change to its input. Typically used to judge the responsiveness of the component or system. Two-position control - See On / Off control. Variable - Something that can be changed or adapted (e.g. pressure is variable) Zero energy band - An energy conservation technique that allow temperatures to float between selected settings, thereby preventing the consumption of heating or cooling energy while the temperature is in this range. Zone / zoning - ARAC Glossary page 22


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Symbol Description Symbol Description

Direct Current

Double junction of conductors

This shall be of used if required by layout considerations

Alternating Current

Double junction of conductors

This shall be used if required by layout considerations

Indicates suitability for use on either direct or alternating supply.

Normally Closed Contacts

Positive Polarity

Normally Open Contacts

(Can be used as a switch)

Negative Polarity

i.

ii.

Contactor main contacts - load bearing.

i. Main make contact

ii. Main break contact

Battery of accumulator or primary cell.

NOTE: The longer line represents the positive pole, the short line represents the negative pole.

Change-over break before make contact.

Earth (General symbol)

Two-Way contact with center-off position

i.

ii.

i. Connection of conductors

ii. Terminal

(NOTE: The terminal may be filled in.)

Triple Pole Switch


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Conductor or group of conductors. (A line for a particular path may be emphasised by increasing its thickness.)

Circuit – breaker

Conductor crossing (no connection)

Temperature-sensitive make contact

i.

ii.

i. Manually operated control.

ii. Manually operated switch - Normally open (General symbol)

Mechanical interlock between two devices.

i.

ii.

i. Operated by push button

ii. Normally closed push button switch -non-latching.

i.

ii.

i. Automatic return (reset)

ii. Non-automatic return (reset)

i.

ii.

i. Emergency switch (mushroom-head safety feature)

ii. Normally closed emergency stop button.

i.

ii.

i. Resistor (General symbol)

ii. Temperature - dependent resistor with negative resistance - temperature coefficient (thermistor)

Break contact, delay when the device containing the contact is being activated.

Heating element

Break contact, delayed when the device containing the contact is being de-activated.

Single acting pneumatic or hydraulic control

Make contact; delay when the device containing the contact is being activated.

Double acting or hydraulic control

Make contact; delay when the device containing the contact is being de-activated.

Operated by stored mechanical energy

t°

t°


12


Fuse

Operated by electromechanical effect

Fuse switch

Information showing the form of energy stored may be added, e.g. t° Temperature p Pressure r.h. Relative humidity Flow Fluid level Thermal Sail

i.

ii.

Connecting link

i. Closed

ii. Open

Time Delay Relay Coil - (delay off)

Capacitor (General symbol)

Time Delay Relay Coil

(Delay on)

Inductor coil winding

Solenoid valve

Transformer with ferromagnetic core

General Symbol for a motor

Single phase automatic transformer.

(If used as an autotransformer, tapping % can be written on the diagram.)

Three-phase, squirrel cage induction motor.

Simplified form of a transformer with two windings.

Three phase, squirrel cage induction motor, star / delta connected.

i.

ii.

i. Terminal strip

ii. Terminal marks may be added

t°

M

M 3

1 2 M 3


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General symbol for a clock

Meters

A = Amp meter

V = Volt meter

Clock with switch

Watt-hour meter

Signal Lamp

Contactor Coil

K1 = component being supplied and number

4 = Number of contacts

Incandescent Lamp

(a)

(b)

(a) Three pin male plug

(b) Three pin female socket

General symbol for a discharge lamp, e.g. fluorescent lamp

i ii. Contact Identification

Identification numbers are always to be opposite the moving contact

Manual Reset Overload contact

(Contactor)

Manual Reset Thermostat

Automatic Reset Overload contact

(Contactor)

Multi-stage Thermostat

(Makes on a rise in temperature.)

A

K1

4

K1-1

K2-2

Wh

t°

t°


14


Pressure Control

(Breaks on rise in pressure.)

Multi-stage Thermostat Two Stage Cooling (Make on a rise in temperature.)

Two Stage Heating (Makes on a fall in temperature)

Pressure Control (Makes on rise in pressure.)

Pressure Differential Switch

(Contacts make on the correct air pressure differential.)

Manual Reset Pressure Control

(Break on rise in pressure.)

Manual Reset Oil Pressure Failure Switch

Note the doted lines around the control shows that all the components are in the same body.

Dual Pressure Control

Note the doted lines around the controls to show that they’re in the same body.

HP: Break in rise

LP: Makes in rise

Domestic Defrost Timer - Time initiated, temperature terminated.

(Make one contact, break the other.)

Thermostat

(Breaks on a rise in temperature.)

Domestic Defrost Timer -

Time initiated, temperature terminated.

(Make one contact, break one contact

Thermostat

(Make on a rise in temperature.)

Single Phase Motor Compressor Starter

HP

t°

LP

t°

t°

HP

t°

LP

OP

t°

HP LP

LP

HP

OR

R L R

S


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Control system fundamentals

Principles

Method of control (ARAC Volume 2, page 29.1) Air conditioning systems are designed to perform a number of processes in order to maintain constant conditions within a controlled space. Most systems are able to cool and / or heat but there are other processes they can carry out, depending on the equipment supplied. ARAC Volume 2, pages 23.38 - 40 describes the following:

Cooling

Cooling and Heating

Cooling and Dehumidification

Heating

Heating and Humidification

Evaporative Cooling (Used in warmer and drier climates) No matter the process the air conditioning system is trying to control, they all operate by using one or a combination of the two control methods described below, those being Closed Loop and Open Loop Control. (Other control loops include cascade loops and interactive loops.)

Closed loop control (ARAC Volume 2, page 29.1) Closed loop control systems provide feed back from the sensor so conditions will be maintained within preset parameters. This is the most common control loop used.

If conditions move outside preset parameters, the sensor sends feedback to the controller for a corrective action

Actuator

Sensor

Controller

Heating

or Cooling

Coil

Air Flow Air Flow

Closed loop control system


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Open loop control (ARAC Volume 2, page 29.2) Open Loop systems receive no feed back from the space being conditioned and will operate regardless of the effect of the conditions being prevailed on the space. Open loops are not used by themselves but are used in connection with other control loops. A typical example of where this loop is used is on perimeter zone air conditioning using induction units.

Open loop control system

Factors that affect loop stability

There are four main factors that affect how accurately the control loop will maintain the space conditions, they are:

1. The Speed of Operation of the Control Equipment

Excess cycling will occur if the control system can cause a change faster than it can sense the change.

2. The Speed of the Controlled Equipment and Thermal Inertia

The equipment that the controlled system controls must be considered as part of the control loop. Compressor delay on start up, thermal inertia of heat exchangers, transport of the air through the ducts, etc., all slow the corrective action and hence the loop stability.

3. Air Change Rate.

If there is too high an air change rate, there can be problems with ‘swamping’ of the conditions within the space being controlled. The control equipment will respond according to the swamping which in turn cycles the equipment excessively.

4. Sensor Location

The location of the sensor is also important to loop stability. It must be located so as to avoid being:

affected by residual cooling and or heating from coils

Controller

Sensor

Wall

Air Flow

Heating

or Cooling

Coil

Conditions of space being controlled externally, [no feedback].


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placed in stagnant, non consistent areas, e.g. bends and outlets of ductwork.

Automatic control elements (ARAC Volume 2, page 29.2)

In all control systems there are three elements necessary to control conditions automatically, they are:

1. A sensing device: used to sense a change in the ‘Controlled Variable’ whether that controlled variable be temperature, humidity, pressure, etc.

2. A controller: which responds to the sensing device to initiate some form of corrective action.

3. A controlled device: used to carry out the actual corrective action.

The three elements can be seen in the controlled loop diagrams previous. The control device in this instance is an actuator on a water valve.

Six basic functions of automatic control (ARAC Volume 2, page 29.3) The three elements perform the following six functions (see ARAC for greater detail of each step):

2. Controller Amplifies the Sensor Signal

3. Transportation of Amplified Signal to Control Device

4. Corrective

Action

5. Sensor Senses Corrective Action and Signals Control Device

6. Control Device Ends Corrective Action

1. Sensor Senses Change to

Controlled Variable


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Symbols and diagrams

Symbols Symbols are used to represent components in a variety of diagram types, i.e. mechanical, pneumatic, electrical, electronic, etc. Without the use of symbols the components would have to be drawn as seen. It would depend on the ability of the diagram's author as to how recognisable each component would become and hence how easy the diagram would be to read. For this reason there must be some standard for all to use to ensure consistency. A variety of symbols are found in ARAC, they are: Symbolic Representation - Air Conditioning Drawings (ARAC Volume 2,

pages 28.16-18) Pneumatic and Logic Controls - which can be seen in ARAC Volume 2,

pages 29.38-39. Electrical Symbols - (ARAC Volume 2, pages 29.10 - 13) which will be

looked at and discussed in another section of this module. It should be noted that many of the symbols are found across all types of diagrams

Diagrams

Air conditioning diagrams (ARAC Volume 2, pages 28.16-18) Air conditioning diagrams are used to represent components involved in the transportation of air throughout a ducted system. The types of equipment that may be represented are the ductwork, fans, dampers, vents, and even control equipment. Filter Fan Humidifier Air Cooler Air Heater

Basic Air Supply with Humidification

Pneumatic and logic control diagrams (ARAC Volume 2, pages 29.38-39)

The following diagram shows a typical simple pneumatic control diagram used in the Air Conditioning Industry. Pneumatic control circuit diagrams will be discussed in a later section.

Mixed Air Supply Air


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Electrical diagrams (ARAC Volume 2, pages 29.7-9) Approximately 75% of fault finding in the air conditioning industry involves the need for electrical diagnosis. If a mechanic is unable to read the various types of electrical diagrams available, the task of fault finding is made that much harder than is necessary. There are a number of electrical diagram types commonly used in the air conditioning industry. Three of the typical diagrams are: 1. Block Diagram: Block diagrams tend to be used to aid in the understanding of a circuit operation. It identifies exactly what the circuit does without giving any information on the circuit itself.

Block Diagram of a Primary Resistance Starter

Contactor Coil A

Contactor Coil B

Timer

Contactor A and Overload

Resistors (for reduced voltage starting)

Contactor B

Motor

On / Off / Safety Controls

Supply

ASV1

+

Basic Pneumatic Circuit with Heating and Cooling Coil

Restrictor

CHWV HWV

T

MA SA

Thermostat

Return Air

Supply Air

Outside Air


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2. Circuit Diagrams (Schematic Diagram) Circuit Diagrams are ideal for fault finding as it shows a circuit operation in a logical sequence. Energy flows is from top to bottom and / or left to right. Circuit diagrams can be either of a vertical or horizontal layout.

Contactor A O/Load Resistors

Horizontal Layout of a Circuit Diagram Showing a Primary Resistance Starter

A4

A4

Start Stop

L1L2L3N

T

T1B3

Motor


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3. Wiring Diagrams Wiring diagrams are used to allow unskilled workers to complete wiring of equipment. Often it is drawn on a panel layout drawing and shows point to point connection of cables. A numbering system is often used on more complicated diagrams.

L1 L2 L3 N

1 10 4 5 6

11 7 8 9 7 8 9 Motor

Revision of control circuits

The modules Motors (NR08) and System Controls (NR12) are designed as introductions to the different types of motors and control circuits found in the HVAC industry. Having completed these two subjects you should by now be aware that:

Motors are found in the power side of a wiring diagram along with other load bearing components like heaters.

Wiring Diagram of a Primary Resistance Starter

4 5 6 14

Contactor B 7 8 9 N

1 2 3 1 12

Contactor A

10 N

Overload

4 5 6 11 12

1 12

Timer 14 N

Start Button

Start / Stop Station

Stop Button

Resistors


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Components like thermostats, head pressure controls, low pressure controls, etc are found in the control side of the diagram.

You should also be able to design basic electrical diagrams (with both power and control circuits) and in addition be able to read a basic wiring diagram.

If you are not confident with working with control diagrams, ARAC 29.14 - 29.24 shows how control systems are build by using various components through to a completed wiring diagram. It is from the understanding of the basic control system that an understanding of more complicated Air Conditioning Control systems can be gained and hence the ability to trouble shoot these systems in event of a fault.

Control symbols (ARAC Volume 2, pages 29.10 - 29.13, SA/SNZ HB3-1996 and AS 1102-1996)

In the Air Conditioning industry, it appears that every controls diagram differs from one to the next in the symbols they used. Australian Standards HB3-1996 Electrical and Electronic Drawing Practice for Students and AS 1102-1996, shows electrical symbols that should be used to standardise diagrams within both the Electrical and Air Conditioning industry. ARAC pages 29.10 - 29.13 shows a number of typical symbols used in the industry but it should be noted that they were written from HB3-1986 (an out dated standard). An updated set of symbols has been supplied with this module to conform with the new HB3-1996 standard and AS 1102-1996 standard.


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Air conditioning circuit diagrams

Air conditioning circuit diagrams range in complexity from the simple domestic air conditioning unit with mechanical switches through to the more complicated industrial air conditioning systems using electronic control systems.

Electrical Diagram of a Room

Air Conditioning Unit

Kelvinator Room Air Conditioning Unit (Reproduced with the permission of

Email Major Appliances)

SCS-klimo Controls (Staefa Controls)

Diagram of an Electronic Connection Circuit of a Landis and Staefa Controller.

(Reproduced with permission of Landis and Staefa Division of Siemens Building Technology)


24

Practical exercise 1 - Control circuit diagrams

Task Describe the sequence of operation of a simple air conditioning circuit

diagram.

Draw a circuit diagram for a simple air conditioning system.

Procedure 1. Briefly write the sequence of operation for the wiring diagram of a

Kelvinator Heat Pump air conditioning unit (found below and in ARAC Volume 2, page 29.9).

2. Convert the diagram into a circuit diagram so that it can be used for fault

finding.

Notes for Diagrams Be sure to use the symbols supplied in this module. When you draw a line on the circuit diagram, cross out the original line on

the wiring diagram as you go. This avoids repeating wires that you have already done on the circuit diagram or missing any wires.

Draw sketches elsewhere so only your finished drawing appears in the

spaces provided.

Wiring Diagram from ARAC page 29.9.

(Reproduced with the permission of Email Major Appliances)


25

Sequence of operation for the wiring diagram above

Circuit diagram for the Kelvinator Heat Pump air conditioning unit

Neutral Active


26

Check your work with your teacher to be sure that you have identified the correct sequence of operation and correctly converted the diagram. Note below anything you wish to further investigate. NB: If you have any questions don’t hesitate to ASK! The teacher is there to help you.


27

Practical exercise 2 - Control circuit diagrams

Task Draw a circuit diagram for a simple air conditioning system.

Describe the sequence of operation of a simple air conditioning circuit

diagram.

Procedure 1. Draw a control system diagram for a simple air conditioning system

using the components listed below. 2. Briefly explain the sequence of operation for the air conditioning system

diagram

Notes for Diagrams

Be sure to use the symbols supplied in this module.

Draw sketches elsewhere so only your finished drawing appears in the spaces provided.

Components

Three phase compressor fitted with sump heater

Three phase evaporator fan motor

Condensing unit containing three (3) single phase condenser fans.

Three phase electric heater bank protected by an over temperature manual reset thermostat.

One heat / one cool thermostat

Safety controls for compressor

Fuses and / or circuit breakers

Control switches


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Sequence of operation for the wiring diagram above

L1 L2 L3 Neutral


29

Check your work with your teacher to be sure that you have identified the correct sequence of operation and correctly converted the diagram. Note below anything you wish to further investigate. NB: If you have any questions don’t hesitate to ASK! The teacher is there to help you.


30

Review questions These questions will help you revise what you have learnt in this topic. 1. What are the three essential components of a control loop?

2. Describe the six major functions of these components.

3. What is the controlled variable in the control loop?

4. What is the set point of a controller?

5. What is the control point of a control loop?

Review questions


31

6. What is the differential gap?

7. What is the offset of a control loop?

8. Define lag in a control loop.

9. What is meant by the term cycling in a control loop?

10. What does an actuator do?

11. What is the difference between a closed loop control system and an open

loop control system?

Review questions


32

12. Define the term control point.

13. What are the three types of diagrams discussed in section 1?

14. What is a typical application for each of the diagram types: Air Conditioning Diagrams

Pneumatic and Logic Control Diagrams

Block Diagrams

Circuit Diagram

Wiring Diagrams

Control Circuit Diagrams

Review questions


33

15. Write down any control system components that you would expect to find in a control diagram for a basic system to control conditioned air.

16. Identify the control system components that you would expect to find in

a control diagram for a system to condition water.

17. Draw the symbol for the following components:

A thermostat that makes on rise.

A thermostat that makes on fall.

A two stage heating and two stage cooling thermostat

A motor operated valve

A sail switch


34

Notes


35

2. Types of Control Systems – General Overview and Fluid Flow Control

Purpose In this topic you will learn about the different types of control circuits, sensing elements control actions, types of drives and fluid control.


list and explain the principles of air conditioning control

explain the operation of various fluid flow control devices and systems.

Content - Control Systems - Control Signal Types - Energy Transmission Types

- Electric Control Systems - Electronic Control Systems - Microprocessor - Pneumatic

- The Three Elements of a Control System - Sensing Elements

- Pressure Sensing - Temperature Sensing - Humidity Sensing - Fluid Flow Sensors - Orifice Plate or Vortex Nozzles

- Control Action - On / Off or Two Position Controller - Anticipators - Multiposition or Multistage control - Step Control - Floating Control - Proportional Control (PI & PID)

- Operation Type - Control Valves – Liquid and Air Flow Control - Other Forms of Drive


36

References ARAC Volume 1, pages 12.5 – 12.7

Discusses On / Off control, humidity sensing ARAC Volume 2, pages 29.5 – 29.7

Discusses anticipators, floating control and proportional control ARAC Volume 1, pages 11.36 – 11.38; Volume 2, pages 29.23; 29.32 -

34 Discusses actuator / drive types

Honeywell’s Heating, Ventilation Air Conditioning Training Manual

Discusses principles of automatic control Automatic Control Principles

Discusses principles of automatic control


37

Control systems

Automatic control systems are used to maintain a controlled variable to a desired condition. They are classified by the type of control signal it uses to perform its function, either analog or digital and the type of energy transmission. There are four energy transmission types commonly used in the air conditioning industry, they are electric, electronic, microprocessor and pneumatic control systems. Many control systems use a combination of the above categories.

Control signal types

Analog and digital control (Excerpt from the Heating Ventilation Air Conditioning Training Manual, page 25) Used with permission of Honeywell Ltd.

Traditionally, analog devices have performed HVAC control. A typical analog HVAC controller is the pneumatic type that receives and acts upon data continuously.

The digital controller receives electronic signals from sensors, converts the electronic signals to digital pulses (values), and performs the mathematical operations on these values. The controller reconverts the output signal to operate an actuator. The controller samples digital data at set time intervals, rather than reading it continually.

The diagram below compares analog and digital control signals.

Comparison of Analog and Digital Control Signals (Honeywell’s Heating Ventilation and Air Conditioning Training Manual page 2-5)

(Reproduced with permission of Honeywell Ltd)


38

Energy transmission types

Electric control systems Electric control is the most basic control system of the three in its operation. It tends to lend itself to mechanical modes of operation and uses higher voltage in its control. They are generally slower to respond and are bulkier in size.

Electronic control systems Electronic control systems use sensitive equipment for faster more accurate control. They have greater control capability when compared to other control types and they usually take up less space. Lower voltages are used which makes them safer to work with. Unfortunately, electronic controls tend to be more expensive to purchase, though they are becoming more reasonable as time goes on.

Microprocessor Microprocessor-based controllers use digital control for a wide variety of control sequences.

Pneumatic Similar to electric but use compressed air as its energy source.

The three elements of a control system

Each control system, no matter the type requires three elements to control conditions automatically, they are the sensing device, the controller and the control device. This section of the module looks at:

1. the sensing elements used in the sensing device

2. the control action of the controller

3. the types of actuator / drive used by control devices.

Sensing elements

As mentioned previously, electric and pneumatic controls are similar in operation and so use similar sensing equipment, called primary elements, to attain automatic control. Electronic sensing is done by using low mass primary elements that respond quickly to changes in the controlled condition. Sensors are used to sense:

Pressure

Temperature

Humidity


39

Fluid flow

Other sensor types are available but will not be discussed in this module.

Pressure sensing

The following primary elements are used to sense pressure:

Electromechanical (ARAC Volume, pages 12.5 – 6 & Automatic Control Principles page 7)

Diaphragm

Bellows

Inverted bells immersed in oil

Electronic (Excerpt from Honeywell Engineering Manual of Automatic Control page 127)

An electronic pressure sensors is usually a transmitter which converts pressure into a variable such as voltage, current or resistance that can be used by an electronic controller.

Strain Gauge As pressure is exerted onto the strain gauge the length of the fine wire / thin film metal stretches. The wires’ resistance changes according to the amount of stretching that it undergoes. Capacitance Types Pressure Sensor

Fixed Plate

Flexible Plate

Pressure being exerted from system

The capacitance type pressure sensor has two plates in its assembly, a fixed plate and a flexible plate. As the pressure on the flexible plate varies it moves closer to the fixed plate and changes the capacitance.

Fine (Serpentine) wire / thin film metal

Amplifier Connection

Strain gauge shown flexing

Pressure being exerted from system

Flexible base


40

Differential Pressure Sensor

Temperature sensing (ARAC Volume 1, pages 12.4 – 5 & Automatic Control Principles pages 7 –9)

The following primary elements are used to sense temperature:

Electromechanical

Bimetal strips

Rod-and-Tube Element

Sealed Bellows

Sealed bellows attached to a remote capsule or bulb.

Electronic

Thermocouples

Resistance Temperature Detectors – (RTD)

Integrated Circuit Temperature Transducer (ICTT)

ICTTs are one of the latest progresses in sensor technology being that it is a silicon chip soft soldered into a printed circuit board. They operate on the principle that with a change in temperature the output, either a voltage in millvolts or a current in milliamps, will vary accordingly.

Thermistors

Thermistors can have either a positive or negative temperature co-efficient.

Positive Temperature Co-efficient (PTC): with a rise in temperature there will be a rise in the resistance.

Negative Temperature Co-efficient (NTC): with a rise in temperature there will be a reduction in the resistance.

Force is exerted onto a strain gauge in two directions. As with the strain gauge mentioned above, the resistance varies depending on the pressure exerted. This type of sensor can measure small differential pressure changes even with high static pressure.

Strain Gauge

Movement due to change in pressure


41

Humidity sensing (ARAC Volume 1, pages 12.6 –12.7 & Automatic Control Principles, page 10 – 11)

The following primary elements are used to sense humidity:

Electromechanical

Nylon Ribbon or hair, either human or horse

Wood

Any material that responds to humidity like leather, horn and silk.

Electronic

Hygroscopic (Gold – Foil Grid)

The Capacitive Sensor (Excerpt from Honeywell Engineering Manual of Automatic Control page 126)

The capacitive sensor is a capacitor that has lithium chloride as a dielectric. As the resistance of the lithium chloride varies with a change in humidity so does the capacitance between the plates. With a change in capacitance there will be either more or less current flow from the plates.

Ohms

Temperature 0 +

+

Ohms

Temperature 0 +

+

Positive Temperature Co-efficient (PTC)

Negative Temperature Co-efficient (NTC)

Moisture Sensitive Polymer

Gold foil or other type of electrode plates

Leads to controller or sensing circuit


42

Fluid flow sensors Fluid flow control sensors are used to i. show fluid flow, ii. measure flow or iii. measure temperature. Many of the sensors discussed will include both the sensor and switchgear in the one body. Fluid control sensors include: Air pressure sensor – including pressure differential switch These switches are used to sense a positive pressure in the duct to indicate that the fan is running. The pressure differential switch can be used to indicate a positive pressure, a negative pressure as well as differential pressures and are used on applications according to the application.

(Reproduced with permission of Kirby Refrigeration - Erie Controls)

Air flow sensor – sail switch

Sail switches are directional components that rely on airflow to make the switch. Air movement lifts the sail that in turn makes a microswitch. Sail switches are used for interlocking purposes; i.e. to hold off conditioning until the fan is running.

Air velocity sensor – using microelectronic circuitry.

A heated resistance element on a microchip is used as the primary sensing element. By comparing the resistance of the heated element to the resistance of an unheated element, the air velocity can be indicated.

Liquid flow sensor – paddle switch

Paddle switches are used to detect water flow and are used for interlocking purposes, i.e. as an indication that water is flowing before starting a chiller.

Differential Pressure Switch Air Pressure Switch

Sail Switch

Ductwork

Airflow

Sail

Sail movement Sail Switch shown mounted in ductwork.


43

Differential Water Pressure Switch

Water movement makes a paddle move in the direction of the water flow, see the diagram below. The paddle is connected to a microswitch that makes with movement. It should be noted that this sensor / switch is a directional component and must be installed accordingly.

Differential water pressure sensor These switches are designed both as a safety cut out and is used to detect water flow. A typical application would be to mount the switch across a water vessel (either a water cooled condenser or chilled water evaporator) to confirm there is water flow.

(Reproduced with permission of Kirby Refrigeration)

Orifice plate or vortex nozzles

Flow meters measure the rate of fluid flow. Principle types of flow meters use orifice plates or vortex nozzles that generate pressure drops proportional to the square of the fluid velocity.

Microswitch

Liquid Flow

Pivot

Paddle fitted perpendicular to flow

On off signal to controller Paddle Switch shown

fitted to into pipe work

No flow position (Reproduced with permission of Kirby Refrigeration)


44

Control action Controllers are the link between a sensor and the equipment used to change the controlled variable. There are two basic control types: On / Off and Proportional though there are several variants of each. The following diagrams are used to represent the behavior of a controller to show the output versus the input relationship.

On/Off or two position controller (ARAC Volume 2, page 29.5 & Automatic Control Principles, page 2)

As the name of this control type suggests, the control will either turn a component on or off. On / Off control is the most common control type used in the air conditioning industry. There are many examples of where this form of control is used; on contactors (used for heating, cooling, motors, etc), solenoid coils, reversing valves, etc. Different controls can and often do combine different functions, like thermostats controlling both heating contactors for heating and compressors for cooling. On

The above diagrams represent compressors cycling on and off on cooling

Diagrams used to represent cycling in electronic controls

On

State Off

Set point

Holes used for the release of air bubbles

Concentric Orifice Plate (other plate types are available)

Orifice Plate shown connected to a differential pressure water sensor

To Controller Differential Pressure Water Sensor

Orifice Plate

On Off


45

Anticipators (ARAC Volume 2, pages 29.5-6)

Heaters are used in conjunction with the bimetal strip (primarily element) to turn on and off the controlled device quicker. Anticipators are used for more accurate control by minimising overshoot and can be used on both heating and cooling.

Without Heat Anticipator With Heat Anticipator

Multiposition or multistage control (Automatic Control Principles, page 3)

Multiple stages are used to attain smoother operation than for example just one stage of heating or one stage of cooling.

Step control Step Control uses proportional input to obtain proportional output using equipment with On / Off control. This type of control is typically used on larger air conditioning systems. Step controllers can be mechanical in that they use cams to drive open and close microswitches or they can be electronic. 3 2 1

Setpoint 20 21 22 23 24 Space Temperature °C

Step control showing Three Heat Three Cool Operation

S T A G E S

On

On Off

On Off

Off

Off

On

On

Differential Off

Throttling Range

Off On

On

Off

Overshoot

Undershoot


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Maximum

Minimum

Output

Set Point

Heating Cooling

Proportional Control Showing Both Heating and Cooling

Full On

Full Off

Increasing Temperature

Floating Control (ARAC Volume 2, page 29.7 & Automatic Control Principles, page 3)

Floating control uses fast responding sensors and slow moving actuators. The sensor adjusts the controlled device until the controlled medium moves back into the dead band area. The following graph demonstrates how floating control is constantly moving the damper to match the load.

Proportional Control (ARAC Volume 2, page 7 & Automatic Control Principles, pages 4 & 16-17)

This type of control proportions the equipment to match the load. As the load increases the controller opens an actuator at a rate that matches load increase and visa versa. Proportional control tends not to bring the control point to the set point and so is not used in the industry.

A Graph Showing Floating Control (Used with permission of Honeywell)


47

Fig. 1 Proportional

Fig. 2 Proportional-Integral

Control

Fig. 3 Proportional-Integral-

Derivative Control

The proportional band can be calculated by the formula: % Proportional Band = Throttling Range x 100 Span of Sensor Example: A sensor has a range of 0 to 30°C, it would a range (called sensor span) of 30K. If the final control element has a throttling range of 3K with the set point at 22°C, the proportional band would be: % Proportional Band = Throttling Range x 100 Span of Sensor = 3K x 100 30K = 20% Other types of proportional control used include Proportional – Integral (PI) and Proportional – Integral – Derivative (PID). A brief description and a graphical comparison of each are given below. PI and PID controls will be discussed at greater depth in latter control modules.

Proportional – Integral (PI) Control Proportional – Integral acts as the proportional control described above but with an automatic reset function. Integral shifts the proportional band to bring back the controlled medium to the set point but over a period of time, see Fig 2. This form of proportional control is the most common type used.

Proportional – Integral – Derivative (PID) Control The derivative function of the control opposes any change in temperature and is proportional to the rate of change. PID is quicker to bring the control point back to set point, see Fig 3. Though this control is tighter in the parameters it maintains, it is more expensive and hence generally used in applications where accuracy is needed.

A Graphical Comparison of the Three Forms of Proportional Control (Honeywell’s HAVC Training Manual p.2-16)


48

Actuators/drives Many of the actuators listed below will be discussed at greater depth in other sections of this workbook. An actuator is a device that converts electric or pneumatic energy into a rotary or linear action. Electronic systems do not have the inherent power to drive valves or dampers. Electronics are used for the logic part of the operation that in turn operates electric relays to do the physical work of actuating the valves or dampers. An actuator creates a change in the controlled variable by operating a variety of final control devices such as valves and dampers.

Operation type Motorised Actuators including pneumatic actuators and motors: (ARAC

Volume 2, page 29.23, ARAC Volume 2, pages 29.32 – 34 and Automatic Control Principles, page 28)

Electronic Actuators

Control valves - liquid and air flow control (Automatic Control Principles pages 17-18 and 35-42)

A control valve is any device that can be opened, closed, started or stopped so as to regulate the flow of fluid being controlled. Actuators are fitted to control valves to allow for automatic control. Typical flow control valves used in air conditioning are classified by: Construction

Single seated valves

Double seated valves

Three-way mixing valves

Three-way diverting valves

Method of controlling flow

Sliding plug valve

Rotary plug valve

Butterfly valve

Type of actuator

Solenoid valve

Three Way Mixing and Diverting Valves



49

Diaphragm valve

Motorised valve

Type of dampers

Flap type

Splitter damper

Pinch damper

Louver damper

Parallel blade damper

Opposed blade damper

Other forms of drive

Electric

- Contacts for example on a relay (ARAC Volume 2, pages 12.19 –20)

Electronic

- Triac - (ARAC Volume 1, page 11.38)

- Transistor (ARAC Volume 1, pages 11.36 - 37)

Thermal Expansion Fluid

A fluid that is used in an actuator that when heated will expand moving a shaft that in turn can be used to drive open or closed dampers or valves. See electronic control for further details.

Solenoid Coil Fitted to a Single Seated Valve



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Practical exercise 1 – Control systems

Task

Observe the operation of various simple air conditioning control devices.

Explain the operation of various air conditioning control devices.

Procedure 1. Observe the operation of simple air conditioning control devices, one

system using On / Off control, one using step control and one system using proportional control. (Do not observe Honeywell/ Johnson electric modulating motors at this stage, they will be investigated in electric control).

2. Briefly explain the operation of the simple air conditioning control

devices shown.

Remember: Work safely at all times!

Systems may start automatically so make sure you keep fingers, tools, hair, clothing etc away from rotating machinery.

You cannot see electricity so consider all systems to be live until proven otherwise.

Make sure you are supervised when working on live circuits.

If you are working with live circuits to diagnose electrical problems, used extreme caution so as to avoid damage to testing equipment, electrical shock or even electrocution.

.


51

The operation of a On / Off control

The operation of a step control

The operation of proportional control


52

Check your work with your teacher to be sure that you have carried out the work required by this exercise. Note below anything you wish to further investigate. NB: If you have any questions don’t hesitate to ASK! The teacher is there to help you.


53

Practical exercise 2 – Fluid flow control

Task

Observe the operation of various fluid flow control devices and systems.

Explain the operation of various fluid flow control devices and systems.

Draw a control system circuit diagram.

Procedure 1. Observe the operation of a simple fluid flow control system, either a

water distribution system / or an air distribution system. 2. Briefly explain the operation of the water distribution system and / or an

air distribution system. 3. Draw a control system circuit diagram of the fluid flow control circuit.






Cooling towers may be contaminated, use safety equipment. The operation of a water distribution system and / or an air distribution system


54

Control system circuit diagram


55



56

Review questions These questions will help you revise what you have learnt in this topic. 1. What are the four major types of control systems available in the air

conditioning industry?

2. Briefly describe the difference between a digital and analog signal.

3. What is the function of a sensor?

4. How does a bimetal strip thermostat work?

5. How does a mercury tilt switch work?

6. What is the primary function of a Fluid Flow Sensor within an air

conditioning control system?

Review questions


57

7. Where would a sail switch be fitted into a control circuit to keep conditioning (heating and cooling) off until airflow was established?

8. Why would a paddle switch be fitted into a chilled water circuit?

9. What is the “controlled differential”?

10. What is the “throttling range” of a proportional controller?

11. What are the operating differences between “on-off” and “floating”

control types?

12. What are the operating differences between “floating” and

“proportional” control types?

Review questions


58

13. What is the term for the device, which incorporates an actuator that drives a shaft that has many cams, mounted on it to operate micro-switches for a number of output devices?

14. What is the purpose of a relay in a control system?

15. What are the major design and operational difference between opposed

blade dampers and parallel blade dampers?

16. What effect would a parallel blade air damper and an opposed blade

damper have on the airflow at the half-open position?

Review questions


59

17. In the following sketch are the valves mixing or diverting types? 18. Sketch in the correct location of a mixing valve on the cooling coil

below. Show all piping and indicate water flow directions.

_

Review questions


60

19. Sketch in the space below a face and bypass damper control arrangement and describe its operation.

Operation


61

3. Energy Management and Building Management Principles

Purpose In this topic you will learn about the principles of both energy and building management and the various systems used in air conditioning.

Objectives At the end of this topic you should be able to: Explain the principles of both energy and building management and the

various systems used in air conditioning.


62

Energy management and building management principles The buildings that are currently being constructed in the major cities use a great deal of glass that makes them aesthetically pleasing. For all their beauty though, the use of so much sealed glass makes air conditioning an absolute must throughout. The cost of air conditioning these buildings makes up for a large part of the energy expense for running the building. The following graph shows a comparison of energy usage in the commercial sector of the USA:

The graph shows that HVAC makes up the largest percentage of the consumption of energy, consumption that has to be paid for, consumption that can be reduced. Energy management has become a major concern for building owners in that large savings can be made once different energy saving methods have been installed. You are required to investigate the various methods that can be used to attain those energy savings, the assignment for this section can be found on following page.

Other 14%

Cooking 4%

HVAC 39%

Lighting 26%

Office Equipment 3%

Water Heating 7%

Refrigeration 7%

US Commercial Sector Primary Energy Usage - 1993 (Celsius Magazine Vol. 26, No. 5, May 1998, p.10.) Reproduced with permission of Celsius Magazine


63

Energy management and building management assignment You are required to do an assignment of approximately 600 words to investigate the principles behind Energy Management and Building Management. Time will be provided during class hours to do this assignment. It will be due in hour 24 though this us up to the teacher in charge of the module. The assignment will be worth 15% of the total mark for this module. The following topics are to be addressed: What is:

Energy Management

Building Management and what type of systems are available?

Economizer System

Night Purge

Thermal Storage Other information to be investigated is:

Running Costs What are the potential savings (if any)?

Capacity Control What type of capacity control is / can be used in Building Management and Energy Management?

Many of the topics listed above will discuss by your teacher throughout this module. Suggested area for your research:

1. Text Books

ARAC

Honeywell Engineering Manual of Automatic Control.

2. Industry Magazines

Celsius Magazines

AIRAH Journals

3. The Internet

Hints on using the Internet.

Use just the words you are looking for, i.e. night purge in the search box. The Internet will look for the words night and purge and will show you a list of matches to your inquiry. You then can go through the findings as you wish.


64

To narrow the findings, many of the search engines allows you to link the words and so receiving findings closer to your needs. Try Night and Purge, “Night Purge” etc. for example.

Use questions to provide you with the answers you require, like: What is night purge? etc.

Use a variety of search engines. The author of this module has found the search engine ‘Infoseek’ to be a good for technical information. Other search engines include Excite, Yahoo, Alta Vista and Lycos, etc.

4. Manufacturers of BMS systems, like: Honeywell, Landis and Staefa, etc

5. Ring building owners / building managers who are currently using Energy Management / Building Management systems and speak to them about their systems. Your college may supply you with a list of building owners / managers that you may be able to approach.

Note: References should be provided on the last page of your assignment.

It should be noted that questions will be asked on this topic in the theory tests.


65

4. Electric Control Systems Purpose

In this topic you will learn about electric control systems, the advantages and disadvantages, major components, how to safely and correctly commission and fault find electric control systems.


list and explain the principles of electric control systems

determine the settings for various control devices

safely and correctly start up, adjust and commission a simple electric air conditioning control system

identify electric system faults and their cause

repair electric control system faults.

Content

- What is Electric Control

- Advantages of Electric Control

- Disadvantages of Electric Control

- What is in an Electric Controller

- Electric Control Components

- Integral Sensor / Controllers

- Electric Controller Types

- Step Controllers

- Electric Actuators

- Modulating Motors

- Application of Electric Control

- Electric Control System Commissioning

- Equipment Required When Testing and / or Commissioning Electric Controls

- Controlled Variable Calibration Testing

- Two Stage Temperature Control Adjustments

- Setpoint Adjustment


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- Differential Adjustment

- Cutout Adjustment - Other Tests that Can be Done to Electric Control Systems

- Insulation Resistance Test

- Resistance Tests

- Line Current Tests

- Volt Meter Tests

- Trouble Shooting of Electric Systems

- The Use of the Four Senses

References Automatic Control Principles



67

Electrical control systems

What is electric control? (Automatic Control Principles, page 5) Electric controls have been around from the advent of air conditioning. It is the most basic of control systems, using heavy robust components to provide On / Off control. Variants of proportional control are possible either by using modulating motors or step controllers (the use of On / Off control to achieve a stepped output similar to proportional). Typical voltages used with electric control range anywhere from 12 volts through to 415 volts. All connection between control components is hard wired.

Advantages of electric control (Automatic Control Principles, page 5)

See Automatic Control Principles

Other advantages include:

Control components tend to be more robust in their construction.

They tend to be an integral sensor / controller

The sequence of control tends to be simple.

Disadvantages of electric control (Automatic Control Principles, page 5)


Other disadvantages include:

Greater care must be taken when servicing electric controls because of the higher voltages used

Minor changes can require major wiring alterations to achieve the desired result.

A high degree of accuracy is often difficult to achieve.

Electric controls are typically large therefore requiring more room for mounting.

The modulating actuators can be complex.


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What is in an electric controller? For the most basic electric controller, the sensing element and switching components are located within the one body. The switching arrangement is directly coupled to the sensing element. As mentioned previously, hard wiring is used to interlink other controls or actuators within the control system.

Electric control components

Integral sensor / controllers As the name Integral sensor / controller implies, both the sensor and the controller are contained within the one body. This is typical of most electric control equipment. See below for example of typical sensor / controllers.

(Reproduced with permission of Johnson Controls)

Terminals for Switching

Bulb Type Sensing Element

Setpoint Knob

Picture showing both the switching and sensing element all in the one

Multistage Thermostat (Reproduced with permission of of Johnson

Controls)

MERCURY BULB

BI-METAL STRIP

Mercury Bulb, Bi-Metal Thermostat with Anticipation

Potentiometric Thermostat fitted with a Sensing Bulb

BULB SENSING ELEMENT


69


Electric controller types

Step controllers Electric step controllers are being replaced with modern electronic step controllers but they are still in the field and an awareness of them is necessary in the event that you will need to service one. On / Off sensor / controllers are typically used to control step controllers. The sensors used are generally located remote to (away from) the step controller. Whenever the controller makes contact, a motor is powered. The motor drives a set of adjustable cams (see diagram below) that can make and or break a series of microswitches. These microswitches can be used to sequence on and off a range of air conditioning equipment like liquid line solenoid coils compressors, heaters etc.

SENSING ELEMENT Made of a continuous band

of cellulose acetate butyrate.

Humidistat

Low Range Pressure Control

Microswitches Common (1) Switch Position (2) Switch Position (3)

A 1 2

A B

1 2

B

0° 90°

Cams

Motor Terminal

Wiring from motor terminals to motor

Bi-directional synchronous motor

Position Dial Shown as a percentage (%) of travel

Diagram Showing Step Controller and Microswitch Operation

Drive between motor and cam


70

Electric actuators (Automatic Control Principles, pages 27-42) The types of electric actuators used with electric control include:

Magnetic coils

Relays

Contactors

Motors – ranging from the synchronous motor shown in the step controller above to the reversible motors seen on pages 28 - 29 of Automatic Control Principles.

Synchronous

Permanently Split Capacitor

Shaded Pole

Spring Return

Modulating motor – Described as electronic motors in Automatic Control Principles (pages 32-35).

Modulating motor (Automatic Control Principles, pages18-20)

The motors shown are the precursor to the modern day electronic actuator. These motors use a potentiometric controller, a second potentiometer and a balancing relay (part of the motor) as its basis of operation.

Light Duty Motor Actuator (Reproduced with permission of Johnson

Controls)

Relay (Reproduced with permission of

Johnson Controls)

Three Stages of Heating Three Stages of Cooling

°t Reverse

Forward

Motor Terminal Block

Step Controller Wiring Diagram Showing One of Many Combinations of Output


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Application of electric control Electric control: was originally and still is to varying degrees was across the spectrum of

the air conditioning industry. is used in domestic room air conditioning units through to large

commercial and industrial applications. can and often is used with other control systems.

Electric control system commissioning (Video: Air Conditioning Commissioning – 88-042 - SA TAFE)

Equipment required when testing and / or commissioning electric controls

Before the commencement of any commissioning exercise, the technician should become familiar with the operating parameters for the plant being commissioned. The manufacturer of the equipment, the consultant or the design engineer looking after the job will provide the technician with the relevant data to be able to commission the job correctly. Once the commissioning data is attained, the correct testing equipment is required to properly carry out the job. A list of the typical equipment that you will need when working on electric based control systems is listed below.

Controlled Variable Measuring Devices – ie Sling Psychrometers, Digitemps, etc.

Insulation tester (Megga)

Multimeter

A resistance meter

Modulating Motor incorporating a balancing relay



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Volt meter

Amp meter (tong tester)

Controlled variable calibration testing Controlled variable calibration testing is done using test instruments like Digitemps for temperature, Sling Psychrometers for both temperature and humidity, pressure gauges for pressure, etc. Many of the air conditioning integrated sensor / controllers have preset differentials and the only adjustment is the set point. Each adjustment recommended below would depend on the type and brand of the control you are working on. To test a control for calibration:

1. Measure the controlled variable condition – ie pressure, temperature or relative humidity using the appropriate measuring instrument.

2. Adjust the set point of the control until it corresponds to the control variable condition. If the control is not asking for a corrective action, (sitting within the dead band) test for differentials. If the control is not sitting within the dead band, an adjustment will be required.

3. Test the differentials of the control and adjust if possible accordingly.

If in doubt of any adjustments call the manufacturer of the control and get them to send out the relevant information on their product.

The information below shows typical adjustments that can be made on a variety of controls:

Two stage temperature control adjustments

(Pictures reproduced with permission of Johnson Controls)


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Setpoint adjustment To adjust the setpoint turn the adjustment shaft in the desired direction required.

Differential adjustment Rotating the adjusting cam counter clockwise as shown in the diagram will increase the differential between the stages.

Cut out adjustment The cut out is adjusted by sliding the stop to the front of the thermostat. Other controls have adjustments located on the front of the control like that of the Low Pressure control shown earlier in this section.

Other tests that can be done to electric control systems

Insulation resistance test NEVER TEST ELECTRONIC COMPONENTS WITH AN INSULATION TESTER AS ELECTRONICS WILL NOT TAKE HIGH VOLTAGES. Testing should be done between all live conductors and earth (without power on). According to the Electrical Wiring Rules – AS 3000:

You should always test the electrical component at approximately twice the rated voltage, ie. 240 component – use the 500V setting of the meter.


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An electrical component is bad between zero (0) and one (1) and good between one (1) megohm and infinity. Between one (1) and two (2) megohm you would be very wary of using it.

Resistance tests

Test circuit wiring, switches, safety controls, etc for continuity.

Check coils on relays, motors and heater elements for correct resistance.

Line current tests

Check motors and equipment for operating currents compared to rated line currents.

Check all currents in each phase are approximately the same.

Voltmeter tests

Check for correct voltage across:

Supply

Motors

Coils

Heater etc.

The voltmeter can be a very useful tool to detect open circuit by simply turning the circuit on and checking where the supply stops.

Trouble shooting of electric systems

Regardless of the type of control system used, whenever a technician is required to trouble shoot faults on an air conditioning system they must do four things.

1. Recognise that a fault or faults has occurred on the plant, this may occur during general servicing or when a customer makes a complaint.

2. With the recognition that there is a fault, the next thing to do is to locate the fault(s) and the reason for the fault.

It should be remembered that the most valuable asset in fault finding is knowledge and common sense.

Fault finding requires a knowledge of both circuit operation and equipment operation.

When checking for faults use logic by starting at the beginning of the circuit and work through.


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3. Once the fault and its cause have been identified, the technician must carry out all necessary repairs to the equipment.

4. With all repairs complete, the plant can be restarted and a final check done to ensure its correct operation.

The use of the four senses

Four senses should be used when fault finding and can be used as follows:

Look - Check for broken wires, blown fuses, broken switches or burnt out coils, etc.

Smell - Burnt varnish (i.e. burnt out coils) is a distinctive smell that should be acquired.

Listen - Listen for buzzing or chattering relays or contactors, noisy or groaning motors.

Touch - Feeling for excess component temperatures. Care should be taken when touching components for overheating as many components in an electrical system operate in excess of 100°C.

When touching electric component, care should also be taken to avoid electric shock.


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Practical exercise 1 – Electric control system

Task Explain the operation of various electric controls.

Procedure You will observe the operation of various electric control systems including an example of on/off control, proportional controller (e.g. a Johnson or Honeywell modulating motor), etc. From your observations you are required to describe the operation of various electric controls that your teacher will identify.






Operation of electric controls as identified by your teacher Name of Component: On / Off Control Operation:


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Name of Component: Johnson or Honeywell Modulating Motor Operation:

Name of Component: Operation:




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Practical exercise 2 – Electric control system commissioning

Task

Wire a basic air conditioning electric control system

Determine the settings for various electric control devices.

Safely and correctly start up, adjust and commission a simple electric control system.

Procedure

From the commissioning sheet provided, you will:

1. Wire a basic electric air conditioning control system as per the wiring diagram provided (using the equipment available in your college.) Your teacher will provide you with any alterations that are necessary.

2. Test the integrated sensor / controller for accuracy and make any adjustments that are required.

3. Commission all actuators.








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Commissioning electronic controller data

Control circuit diagram

Systems operating parameters

The conditions of the space are to be maintained at 22.5°C

The compressor is to cycle on and off between 23°C and 24°C.

The heater bank is to cycle on and off between 21°C and 22°C.

A humidistat is required to maintain conditions in the space between 55 to 60% RH.

The heater safety (usually non-adjustable fitted with a manual reset), but for this exercise a manually adjustable thermostat is to cut out the heater if the temperature reaches 60°C.

The sail switch will make whenever the fan is operating.

°t

°t

RH

On / Off Switch

Sail Switch

K1

3

K2

3

K3

5

Humidistat

One Heat / One Cool Thermostat

Heater Safety

Safety Control Circuit

Evaporator Fan

Compressor Contactor Coil

Heater Contactor Coil


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Practical exercise 3 – Electric control system fault finding

Task

Identify an electric control system fault and it’s cause.

Repair the fault.

Procedure

1. One or a number of faults will be placed on an electric fluid control system by your teacher and you will be required to identify the fault/s as you see it / them.

2. Your teacher may require you to repair the fault/s and then test the system for correct operation or you may simply be required to describe the repairs required in the space provided below.






Fault Identification Type of Fault: Possible Cause for Fault:


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Repairs Required:

Type of Fault:

Possible Cause for Fault:

Repairs Required:



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Review questions These questions will help you revise what you have learnt in this topic. 1. What control action does the electric control system lend itself to best?

2. Is it possible to attain true proportional control from electric control? Explain your answer.

3. Name three advantages of electric control.

4. Name three disadvantages of electric control.

5. Step Controllers are used on large air conditioning applications.

Describe how a Step Controller would operate in conjunction with an integrated temperature sensing / floating controller.

Review questions


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6. Step controllers are fitted with microswitches. What are microswitches and what is their function?

7. What is the simplest type of control system actuation?

8. Permanently split capacitor motors can be used as actuators. Describe

how the motor’s direction can be changed.

9. What is the function of limit switches on modulating motors?

10. Briefly describe the operation of the balancing relay in the modulating

motor.

11. What is an integrated sensor / controller?

Review questions


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12. How would an integrated humidity sensor / controller be tested for accuracy when it was being commissioning?

13. What is the insulation resistance tester used to test?

14. Describe how the insulation resistance tester is used (ie, the settings and

expected outcomes)

15. What are the four things technicians must do every time they fault find

on an air conditioning system?

16. When fault finding, why was it suggested that the senses be used?

17. Which senses were suggested and what faults could they be used to

identify?

Review questions


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18. In the following drawings of simple electric control circuits, identify the various components as indicated by the numbers.

1.

2.

3.

4.

5.

6.

7.

8.

Heater Stage 1

Heater Stage 2 °t

°t High-Limit Manual Reset ?

Three Phase Power

From Supply Fan

Two Stage Electric Heating

1 2

3 4

5

6 7

8


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Notes


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5. Electronic Control Systems

Purpose In this topic you will learn about electronic control systems, the advantages and disadvantages, major components, how to safely and correctly commission and fault find electronic control systems.

Objectives At the end of this section you should be able to: list and explain the principles of electronic control systems determine the settings for various control devices safely and correctly start up, adjust and commission a simple electronic

air conditioning control system identify electronic system faults and their cause repair electronic control system faults.

Content

- What is Electronic Control

- Advantages of Electronic Control

- Disadvantages of Electronic Control

- What is in the Electronic Controller

- The Bridge

- The Amplifier

- The Output Circuit

- Electronic Controller Types

- Magnetic Actuators

- Motorised Actuators

- Thermal Actuators

- Electro-hydraulic Actuators

- Applications of Electronic Control Systems


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- Electronic Control System Commissioning

- The Sensor

- The Controller

- Final Control Devices

- Specialised Service Tools Required for Electronic Control Work

References Automatic Control Principles



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Electronic control system

What is electronic control?

Electronic control systems use electronic components like resistors, diodes, transistors, silicon controlled rectifiers, diacs, triacs, etc, to produce more accurate control of environmental conditions. (Electronic components will not be discussed at any great length in this module). They use extra low input signals (millivolts etc) and increases the signal to a useable values at the output of the controller (ie 0 –10 V).

Sensing elements in the input circuit are generally of the resistive type (see section 2 for electronic sensor types). The outputs are fundamentally proportional but On / Off control is also available.

Advantages of electronic control (Automatic Control Principle, page 5)


Disadvantages of electronic control (Automatic Control Principles, page 6)



Initial purchase cost of electronic control equipment more expensive than electric control equipment.

The actuators and controllers are more complex.

What is in the electronic controller? (Automatic Control Principles, page 20, Control Systems for Air Conditioning and Refrigeration, pages 113-115)

There are three basic parts of a simple single-element electronic controller, they are:

The bridge The bridge incorporates the sensing part of the controller. It works on the principle of the Wheatstone Bridge described in the Automatic Control

The Bridge

The Amplifier

The Output Circuit


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Principles (page 18 - 20) but with the addition of an electronic sensing element and a variable resistor for set point adjustments.

The amplifier As the signal from the bridge circuit of the controller does not have enough power to operate an actuator connected to the output, the signal must be amplified. The amplifier must increase the millivolt signal from the bridge to an output voltage of between 0 – 10 volts. The controller generally has two amplifiers fitted, one for direct acting signals and one for reverse acting signals.

The output circuit The output circuit is where the actuators are connected on the controller to provide the correct sequence of operation. The diagram below shows a typical one heat / one cool proportional control that could be connected to the output circuit.

t°

Resistor Resistor

Variable Resistor Required to adjust set point

Electronic Sensing Element

The Bridge Circuit

Setpoint Temperature Increase

Reverse Acting Direct Acting

10V 0V 10V

Outputs on Electronic Controller

For a balance to occur in the bridge, both the sensing element and the set point variable resistor must be in balance. When this occurs the output will be at zero volts.


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With a drop in temperature below the set point temperature, the reverse acting output signal will increase proportionally to 10 volts and the direct acting output will be at zero. At set point both outputs will be at zero, this point is considered to be the null position. With an increase in temperature above the set point temperature, the direct acting output signal will increase proportionally to its maximum output and the reverse acting output will be at zero.

Electronic controller types Electronic controllers are available in a wide variety of types and styles depending on the manufacturer. Typical controls that Landis and Staefa produce include: On / Off Control

- One, two and three outputs Proportional Control

- One, two and three outputs Proportional Integral (PI) and Proportional Integral Derivative (PID) Step Controllers Two and Three in One Controller

- Both temperature and humidity is contained in the one controller body

Energy Recovery Controller Transducer Modules

SCS-klimo On / Off

Controller

SCS-klimo Proportional

Controller

(Reproduced with permission of Landis and Staefa Controls)


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Other control types are available like high and low selectors, etc. Electronic actuators (Control Systems for Air Conditioning and

Refrigeration, pages 117 – 119)

Electronic Actuators are available as proportional, two-position, and two-position with spring return configurations. There are primarily four main types of electronic actuators, they are:

Magnetic

Motorised

Thermal

Electro Hydraulic

Magnetic actuators

There are two types of magnetic actuator:

On / Off - Solenoid type actuator

Variable - The core of the actuator changes its position against a counter spring with each change in current. This allows for small movements to be transferred to the valve being controlled.

Example of a Magnetic Actuator Steam Humidifier Valve



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Motorised actuators The motorised actuator makes use of synchronous motors that convert rotary motion into linier motion.

Thermal Actuators Thermal actuators are made up of a heating element and a solid expansion medium. The heater heats up and cools down according to the variation in the output voltage from the electronic controller. With an increase in temperature, the solid expansion medium will expand causing a force to be exerted creating a stroke movement. If the voltage is reduced from the controller, the heater cools, the expansion medium contracts again causing the opposite effect in stroke movement.

SCS-klimo Thermal Actuators 3 Way Valve


Shaft and Sring

Heater

Expansion Medium ie Wax

Seal

Thermal Actuator

SCS-klimo Motorised Control Valve Belimo Motorised Damper Actuator Actuator

(Reproduced with permission of Landis and Staefa Controls) (Reproduced with permission of Belimo

Aust Pty Ltd)


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Electro-hydraulic actuators Electro hydraulic actuators use an oil pump and a pump valve (similar to a solenoid) to create valve or damper movement. If the voltage from the output of the electronic controller increases the pump valve closes isolating the actuators bottom chamber from the top. The pump fills the bottom chamber, extending the actuator shaft. If the voltage from the controller decreases, the servo valve opens relieving pressure back into the top chamber. The actuator shaft retracts.

Applications of electronic control systems

Electronic controls are used in all forms of air conditioning systems from domestic through to large industrial applications.

Electronic control system commissioning

You will be required to commission an electronic control set up applicable to your college. The following control components will be commissioned: a sensor, a controller and at least one final control device. The exercise assumes that the wiring of the control circuit has been done to an acceptable Australian standard and there are no faults. WARNING NEVER TEST / COMMISSION ELECTRONIC CONTROLS WITH AN INSULATION RESISTANCE METER (MEGGA). YOU WILL DESTROY THE CONTROLS THAT ARE CONNECTED AT THE TIME!

The sensor

Electronic sensors are used to measure a variety of mediums, for example, temperature, humidity, pressure, pressure difference, etc. Where there is a

Actuator Shaft and Spring

Piston

Diaphragm

Pump Valve Hydraulic Pump

Electro-hydraulic Actuator

Oil Reservoir

Pressure Chamber

Hydraulic Fluid Flow


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change in the medium condition, the sensor senses the change and reacts by varying its output to the controller. Sensor outputs vary between manufacturers, and the output types used are listed below:

resistance (PTC or NTC), voltage (mV) amperage (mA).

The manufacturer of the sensor will produce a table or a chart that compares the change in medium to the output for the sensor in question. If you find that the sensor being tested does not compare to the manufacturer’s specifications, either adjust the sensor (if possible) or replace it.

The controller

The controller takes the signal from the sensor and amplifies it into some form of energy that can be used to turn on and off components, drive actuators open and closed etc. Again as with sensors, each controller will vary in the type of adjustments available. Many of the domestic controllers only have simple adjustments available to both the service person as well as the customer. They tend to have a combination of the following alterations available:

temperature fan speed zoning

Your teacher will show you typical domestic electronic controls with simple adjustments. Other controllers, typically commercial and up are more complicated in that they have a wide variety of alterations available to the service person and minimal alterations available to the customer. The controls described can include the following alterations:

Set point Differential Dead band Proportional band, etc

Your teacher will show a controller or controllers that contains some / all of the above adjustments. The adjustments are typically done using a ‘pots’ or button adjustment.


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Final control devices

Final control devices are components that receive the output signal from the controller and make the corrective action occur. Final control devices can include:

Motors Actuators Contactors Relays, etc.

Again each final control device adjustment will vary depending on the manufacturer. Adjustments can include:

Direction Reverse acting Direct acting Length of stroke Degree of rotation, etc.

Specialised service tools required for electronic controls work Electronic control manufactures produce a variety of tools specific to their control system for ease of commissioning and service work. The diagram below shows a tool used to allow for easy access of operating conditions / outputs of a Landis and Gyr electronic controllers. See other manufacturers for tools specific to their control systems.

Landis and Gyr (now Landis and Staefa Controls) Electronic Controller Access Tool



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Fault finding electronic control systems Many of the fault finding techniques and meters used in electronic fault finding are the same as those discussed in electric control fault finding. The one major difference is that you should NEVER use an Insulation Resistance Tester (Mega) to test electronic equipment! Again a good working knowledge of electronic control systems is a benefit when fault finding. If in doubt, do not hesitate to call the electronic manufacturer for technical data. They are generally open to offering product specific advice.


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Practical exercise 1 – Electronic control system

Task Explain the operation of various electronic controls.

Procedure You will observe the operation of a complete electronic control system and from your observations briefly explain the operation of various electronic controls that your teacher will identify.






Operation of electronic controls identified by your teacher Name of Component: Operation:


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Practical exercise 2 – Electronic control system commissioning

Task Wire a basic air conditioning electronic control system Determine the settings for various electronic control devices. Safely and correctly start up, adjust and commission a simple electronic

control system.

Procedure

From the wiring diagram and commissioning sheet provided by your teacher, you will:

1. Wire a basic electronic air conditioning control system such as Micro Air, Eberle, Carel, Innotceh etc.

2. Identify the operating temperatures, setpoint, dead bands / zones, proportional bands and differentials that the electronic controller/s and actuators will operate at to satisfy the diagram requirements.

3. Commission the electronic controllers within the settings provided.

4. Commission all actuators attached to the electronic controllers.







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Step 1 The operating temperatures, setpoint, dead bands / zones, proportional bands and differentials identified on the commissioning sheet. Controller 1 (Type) _______________________________________

Setpoint ____________________

Proportional Band ____________________

Differential ____________________

Dead Zone ____________________

Dead Zone ____________________

Forward / Reverse Acting ____________________

Forward / Reverse Acting ____________________ Actuator Operating Range (in volts)

Full open: ______________ Volts

Full closed: ______________ Volts Step 2 Commission the controllers and actuators according to the figures identified above. Do not forget to set the switches to ensure correct operation. Your teacher MUST check your wiring before power is supplied to the controllers!


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Step 3 Confirm that the electronic controllers are operating as per the commissioning sheet. Is the controller operating as per the commissioning sheet?

Yes / No Explain your answer

Is the actuator operating as per the commissioning sheet? Yes / No Explain your answer



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Practical exercise 3 – Electronic control system fault finding

Task

Identify an electronic control system fault and it’s cause.

Repair the fault.

Procedure

1. One or a number of faults will be placed on the electronic system by your teacher and you will be required to identify the fault/s as you see it / them.







Fault Identification

Type of Fault: Possible Cause for Fault:


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Repairs Required:

Type of Fault: _________________________________________________ Possible Cause for Fault:

Repairs Required:



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Review questions These questions will help you revise what you have learnt in this topic. 1. What is the major difference between the electronic control system and

the electric control system?

2. What are the two types of control action provided by electronic controllers?

3. What are the three major components in a simple electronic control

system?

4. Briefly describe the operation of a Wheatstone Bridge.

5. Name the three advantages of electronic control

Review questions


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6. Name three disadvantages of electronic control

7. Name the three basic parts of a simple single element controller

8. Name the four types of actuator used in the AC industry

9. In your own words briefly describe the operation of a thermal actuator

10. Briefly describe the operation of a electrohydraulic actuator

Review questions


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11. What meter should never be used when servicing electronic control systems? Explain your answer.

12. Draw a graphical representation using the following system description

on the graph provided below.

System description

The room temperature in the controlled space is to be maintained at 19°C. It is not to deviate further away than + 1K or – 0.5 K. There is to be no dead zone.

13. Using the information provided in the description and the graph from

question 10, complete the following table.

Controller Type: Set Point Proportional Band 1 Dead Zone 1 Proportional Band 2 Dead Zone 2

17 18 19 20 21

Full Open / On Full Closed / Off

Review questions


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14. Using the information provided in the graph shown below, complete the following table.

Controller Type:

Set Point

Differential 1

Dead Band 1

Differential 2

Dead Band 2

19 20 21 22 23 24 25


1 2


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6. Pneumatic Control Systems

Purpose In this topic you will learn about pneumatic control systems, the advantages and disadvantages, operating pressures, major components, how to safely and correctly commission and fault find pneumatic systems.

Objectives At the end of this topic you should be able to: list and explain the principles of pneumatic control systems determine the settings for various control devices safely and correctly start up, adjust and commission a simple pneumatic

air conditioning control system identify pneumatic control system faults and their cause repair pneumatic control system faults.

Content

- What is Pneumatic Control

- Advantages of Pneumatic Control

- Disadvantages of Pneumatic Control

- What is in a Pneumatic System

- Air Compressors

- Operating Conditions of a Pneumatic Circuit

- Keeping the Air Supply at the Correct Operating Pressure, Clean and Dry

- Pressure Reducing Device

- Main Line Filter

- Refrigerated Air Dryer

- Pneumatic Transmission

- Controllers

- Other Pneumatic Controls


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- Humidity and Air Pressure Sensors

- Pneumatic Actuators or Motors

- Applications of Pneumatic Control Systems

- Pneumatic Control System Commissioning

- Specialised Service Tools Required for Pneumatic Controls Work

- Component Commissioning

- Air Regulator Commissioning

- Pneumatic Thermostat Commissioning

- Testing the Operating Range of a Water Valve

- Setting the Stroke of a Damper

- Fault Finding Pneumatic Control Systems


Discusses pneumatic control


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Pneumatic controls (ARAC Volume 2, pages 29.27 – 29.39)

What is pneumatic control? Pneumatic control is the use of clean dry air as a power source in place of voltages and currents in the operation of control equipment within the air conditioning plant. In place of a mains power supply, an air compressor is used to supply the systems energy needs. Instead of copper conductors, soft drawn copper tubing with soldered fittings or nylon-reinforced plastic or polyethylene tubing is used to transport the air around the control system. Apart from these significant differences much of the controls equipment stays the same (other than the fact that they operate by air.) Sensors are still required to sense the conditions; controllers to operate according to the sensor signal and pneumatic controlled devices to operate to control the load. Electricity and / or electronics can be coupled into the pneumatic system. This is done so i. equipment like compressors, heaters, etc can be initiated and ii. to get the benefit of accuracy that electronics give. Control equipment like Pneumatic – Electric (PE) relays (switching devices) and Electronic – Pneumatic transducers (sensing devices) are used to link the different systems together.

Advantages of pneumatic control (ARAC Volume 2, pages 29.27-29.28)

Other advantages include:

Can be used in hazardous environments. If electric switching were to be used in explosive atmospheres like armament factories there could be the potential for explosion every time the contacts opened. (Pneumatic controls do not arc when they open and close.)

The actuators for valves and dampers are simple, powerful and reliable

Pneumatic control provides the simplest of modulating control

Disadvantages of pneumatic control


Some mechanics consider pneumatic control systems to be difficult to work with. Many pneumatic systems have been replaced with electronic systems due to the lack of knowledge about a systems operation.

At temperatures below zero, the air lines may provide trouble

Clean dry air is required to make the pneumatic control system working to it maximum potential.


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What is in a pneumatic system?

Air compressors Air compressors are generally single stage, air-cooled and of the reciprocating type. They range in size from 200 watts to 13 kW and the smaller sized compressors are mounted on the receiver tank. Pressure switches are used to start and stop the compressor and therefore maintain the operating pressures required by the system.

Operating Conditions of a Pneumatic Circuit (ARAC Volume 2, pages 29.28-29)

The above diagram helps to identify the names and the extent of the various air supply regions of a pneumatic control system. The list below is a description of the different supplies and the operating pressures that they operate within. System Air (SA): is the air that is held in the stored in the receiver tank

of the compressor and feeds to the pressure reducing

Diagram of a basic pneumatic control circuit.

SA MA

PE

Air Solenoid Valve

Thermostat

Pressure - Electric Relay

Actuator

Supply Air Main Air Branch Lines

T

Reciprocating Air Compressor

Air Receiver

Air Compressor (Reproduced with permission of Atlas Copco)


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device. ARAC suggests the operating pressure to be approximately 700 kPa.

Main Air (MA): is the supply to the components like controllers and

valves etc, after the system air has been reduced to between 100 and 120 kPa.

Branch (B): is the air that leaves a controller and that is varied

according to load. ARAC suggests that the operating pressure in the branch to be between 20 and 90 kPa. The Branch Line is sometimes called the Pilot line (P).

Keeping the air supply at the correct operating pressure, clean and dry

As was mentioned in the introductory paragraph, the air supply must be kept both clean and dry to ensure the reliable operation of the pneumatic control system. Equipment throughout the control system has many small orifices and hence the potential for blockages due to impure air. There are two minimum requirements necessary for the supply air, to be kept clean and dry. The components use to meet these requirements are shown below.

Pressure reducing device The air pressure being supplied to the main system must be reduced from the system air pressure of approximately 700 kPa to between 100 and 120 kPa. A pressure reducing device called an air pressure regulator is used, (see the diagram above) and can be fitted with a filtering device if required.

A Main Line Filter / Regulator (Reproduced with permission of SMC Pneumatics)

AX Valve

Condenser

Water Separator

Compressor

A Refrigerated Air Dryer (Reproduced with permission of Atlas

Copco)


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Main line filter The main line filter must be capable of eliminating small particles from the supply and will usually be able to filter oil to prevent oil contamination of the system. Most filters capable of absorbing oil will change colour, often becoming red. The extent of colour change indicates the need for filter replacement, (see manufacturer notes for further details.)

Refrigerated air dryer As well as the need for cleanliness, there is the need for dry air. Refrigerated air dryers are usually fitted and should be checked regularly for its proper operation. Some plants are equipped with chilled water air dryers. Another method of dehydration is to use chemical driers. Silica gel is generally used as the drying agent. Dehydration is required to prevent the:

Collection of moisture in the lower sections of piping.

Collection of moisture that will reduce airflow in the piping and in instruments causing orifice openings to block.

Corrosion of instruments rendering them inoperative. It should be noted that oil discharged into the main supply line would eventually penetrate the whole system causing similar problems to those mentioned under dehydration.

Pneumatic transmission The transmitter is a sensor that is used to measure temperature and relative humidity. It measures the controlled variable and converts it into a pressure signal of between 20 and 90 kPa. The sensitivity of a transmitter is equal to the change in output pressure divided by the change in the measured variable.

Controllers (ARAC 29.29-31) A pneumatic controller is a device that reduces mains air pressure into a signal that varies between 20 kPa and 90 kPa. They are either two position or proportional. Control types include:

The bleed-type controller – This system wastes a lot of air as it is continually exhausting air from the branch line.

The non bleed system – Only exhaust while reducing the branch line pressure.


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The pilot bleed system – A combination of the bleed-type and non-bleed systems.

High Capacity controller Controllers may be either direct acting or reverse acting. (See the Glossary in this module for further details.)

Other pneumatic controls (ARAC Volume 2, pages 29.31–32) As with all the other control systems, they can become quite complex in operation requiring a variety different control types to make them work effectively. The following list identifies a number of typical relay types that can be found in pneumatic control systems and shows a typical application for each. See the ARAC reference for details on these relays. (The following pictures have been reproduced with permission of Honeywell Australia.) Two position relays

Selector relays

Typical Application for a Snap Acting Relay

Load Analyser

Higher-of-Two-Pressures Relay

A Typical Application for the Load Analyser

Snap Acting Relay


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Proportional relays Reverse-acting relays

Transducers There are many more control types available and these can be found in the Honeywell Engineering Manual of Automatic Control. Other suppliers of pneumatic controls (like Johnson Controls) can be approached for information relating to their products.

Humidity and air pressure sensors (ARAC Volume 2, page 29.32) See the ARAC reference and earlier information in this module for further details.

Ratio Relay

A Typical Application for a Ratio Relay

Revering Relay

Typical Application for a Reversing Relay

Electronic Pressure Transducer Other transducer types are available like electromagnetic transducers or relay etc. (Reproduced with permission of Johnson Controls)


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Pneumatic actuators or motors (ARAC Volume 2, pages 29.32 – 34)

A pneumatic actuator is a controlled device that is the part of the system that does the work, i.e. opens dampers, opens valves etc. When air from the controller enters the space above the piston (see the diagram below) it causes the piston to move against the spring. The force exerted and the amount of movement depends on two factors:

1. The strength of the spring

2. The amount of pressure above the spring.

If greater forces are required than can be delivered by a piston type actuator, the diaphragm actuator is commonly used. (To calculate the force (Newton) being exerted, multiply the diaphragm area ( d2 ) by the pressure (Pascal).

4

Applications of pneumatic control systems (ARAC Volume 2, pages 29.34 – 36)

See the ARAC reference for details.

Pneumatic control system commissioning The following information will show you how to commission a selection of pneumatic equipment found typically in the simplest pneumatic system.

Cover

Piston

Diaphragm

Yoke

Retaining Nut

Cutaway of an Oval-Top Actuator (Reproduced with permission of Johnson Controls)

Air From Controller

Spring


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Specialised service tools required for pneumatic controls work (ARAC Volume 2, pages 29.36 – 37)

The tools used to fault find and commission is different than those used on other control systems. Pneumatic tools read air quantities, not voltages, amperage and resistance. Meters are still required though wherever transducers are used. Both Honeywell and Johnson Controls have pneumatics tools used to calibrate and test pneumatic controls. They are sold either as a service test kit or individually. The kit includes: A stab. See the description of its purpose in ARAC Volume 2, page

29.37. A hand pump (also known as a squeeze bulb) is used to test actuator

operation.

Stab Gauge Test Probe Assembly with hypodermic needle. (Reproduced with permission of Johnson Controls)

Pneumatic Tubing is used to connect onto actuators

Hand pump similar to the blood pressure tester that your local doctor would use.

Knurled knob used to relieve pressure slowly when testing actuators

Squeeze Bulb (Reproduced with permission of Johnson Controls)


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Component commissioning

Air regulator commissioning

1. Check the supply pressure before adjusting the main pressure.

2. By turning the adjusting knob clockwise (No.1) there will be an increase in the main air pressure and by turning the knob counter clockwise the pressure will be reduced. A pressure gauge (No.2) is generally fitted to the regulator to show the operating pressure.

3. Test the setting by bleeding air from the drain (No.3) to make sure the regulator will maintain the correct pressure feed that has just been set.

Pneumatic thermostat commissioning

1

Pressure Gauge No.2

Drain No.3

Adjustment Knob No.1

A Main Line Filter / Regulator (Reproduced with permission of SMC Pneumatics)

1 Throttling Range

Adjustment

2

Calibration Screw

3

(Reproduced with permission of Honeywell Controls)



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For both the following steps, the stab gauge must be installed in the thermostat and air must be supplied to the system. Step 1 Adjusting the throttling range

1. Move the temperature adjusting dial (No. 1) until the stab gauge pressure reads zero.

2. Move the temperature so the pressure on the stab gauge moves in a positive direction noting how many Kelvin it takes for the pressure to raise from the minimum operating pressure to the maximum operating pressure, see commissioning diagram for throttling range.

3. If the pressure range did not occur over the throttling range, adjust the throttling range – Honeywell, Sensitivity –Johnson Controls (No.2) until it does.

4. Retest and recalibrate if necessary. The throttling range is now set. Step 2 Calibrating the set point

1. Measure the control point of the area as close as possible to the thermostat. Do this three times to be sure of an accurate reading.

2. Set the setpoint dial (N0.1) until the needle lines up with the temperature you had just taken. Be sure not to submit the thermostat to any outside temperature influence, i.e. your breath or heat from your hand etc as you do this.

3. Adjust the set point calibration screw (N0.3) until the pressure on the stab gauge reads the set point pressure, see the commissioning diagram for details. Again this must be done without disturbing the sensor by breathing on it or touching it, etc.

4. Return the setpoint dial to its correct setting, usually 22° C but check the diagram.

5. Allow time for the thermostat to settle and regain control. Take further temperature readings to ensure the correct operation of the system, whilst remembering that offset will increase with deviation of the control point from the set point.

The thermostat is now calibrated to the requirements of the diagram.

Testing the operating range of a water valve

Note the operating range of the water valve, usually stamped on the head of the actuator. Compare this pressure range to that stated on the pneumatic diagram, they should be the same. Also check if the valve is normally open or normally closed again comparing this to the diagram requirements.

1. Disconnect the valve actuator from the system and connect the hand pump to it.


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2. Pump up the pressure on the valve to its minimum operating pressure while watching the valve’s spindle. The valve should not have moved.

3. Continue to pump while watching the valve as you do. If the valve is operating OK, it should be completely open at its rated maximum operating pressure.

4. Open the knurled knob on the pump to slowly release the pressure on the valve. Watch as the pressure on the valve is reduced back to its minimum.

5. The check is now complete, reconnect the valve to the system.

Setting the stroke of a damper (Honeywell pages 26-27)

Adjust the stroke of the outside air damper according to the requirements of the pneumatic diagram. See the Honeywell reference for further details.

Fault finding pneumatic control systems

Learning how to use the tools described in the commissioning section of this topic and the ability to read a pneumatic diagram will allow you to cope with most faults you may come across.

Remember when fault finding:

Fault finding of pneumatic control systems requires as with other control system types a basic understanding of the equipment and the operating ranges that they must operate over. Without this understanding it will be difficult to identify the fault. If unsure, attain documentation on the equipment contained in the installation. System diagrams showing operating conditions are usually kept near hand or if they are not available approach the manufacturers of the equipment for data sheets on individual pneumatic components.

Electronic control system operating voltages were designed around the pneumatic operating pressures. If you are used to working with electronic systems, pneumatic systems should not be too much of a worry for you.


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Practical exercise 1 - Pneumatic control system

Task Explain the operation of various pneumatic controls.

Procedure You will observe the operation of a complete pneumatic control system and from your observations briefly explain the operation of various pneumatic controls that your teacher will identify.


Air compressors may start automatically so make sure you keep fingers, tools, hair, clothing etc away from rotating machinery.

High air pressures can be present throughout pneumatic systems.

Operation of pneumatic controls identified by your teacher Name of Component: __________________________________________ Operation:

Name of Component: __________________________________________ Operation:


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Practical exercise 2 - Pneumatic control commissioning

Task

Determine the settings for various pneumatic control devices.

Safely and correctly start up, adjust and commission a simple pneumatic control system.

Procedure

From the commissioning sheet provided, you will:

1. Identify the operating temperatures, setpoint and pressures that the thermostat, chilled water valve and outside air damper will operate at to satisfy the diagram requirements.

2. Commission an air regulator to maintain a constant pressure of 120 kPa.

3. Commission a thermostat to operate within the settings provided.

4. Confirm that the chilled water valve will open and close at the desired operating pressures by testing with the hand pump.

5. Stroke an outside air damper to operated within the parameters set out below.




The operating temperatures, setpoint and pressures that a thermostat, water valve and outside air damper.

Thermostat

Setpoint _________________________ °C

Setpoint Pressure: ________________________ kPa

Throttling Range: _________________________ K

Operating Pressure Range: ________________________ kPa


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Chilled Water Valve

Operating pressures on diagram: ________________________ kPa

On the valve made available to you: ________________________ kPa

Outside Air Damper

Fully Opened Pressure: ________________________ kPa

Fully Closed Pressure: ______________________ kPa Confirm that the chilled water valve will open and close at the desired operating pressures

Is the chilled water valve working within the range required by the:

Diagram: Yes / No

Pressures recorded on the head of the actuator on the valve: Yes / No

Stroke an outside air damper

Has the outside air damper been stroked as per the diagrams requirements?

Yes / No

If no, what is the problem and what can be done to rectify it?



126

Commissioning sheet for an outside air sequence with chilled water valve

T

MA

SA O/A

D/A R/A

SP 220C TR 20C

R/A

DM1

DM3 DM2

CWV 56–91 kPa 21-56 kPa ASV2

LEGEND SP = Setpoint TR = Throttling Range ASV = Air Solenoid Valve SA = System Air MA = Main Air R/A = Return Air O/A = Outside Air S/A = Supply Air D/A = Discharge Air

Further Notes ASV1 is energised through the supply fan contactor. ASV2 is energised through an outside air thermostat which is set to make at 180C and break at 220C.

21 22 23

Closed

CHWV

O/A

Open

Open 91 kPa 56 21

Setpoint

°C


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Practical exercise 3 - Pneumatic control fault finding

Task

Identify pneumatic control system fault/s and their cause.

Repair the fault/s.

Procedure

1. One or a number of faults will be placed on the pneumatic system by your teacher and you will be required to identify the fault/s as you see it / them.





Fault Identification Type of Fault: _________________________________________________ Possible Cause for Fault:

Repairs Required:


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Type of Fault: _________________________________________________ Possible Cause for Fault:

Repairs Required:



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Review questions These questions will help you revise what you have learnt in this topic. 1. What is pneumatic control?

2. What is the purpose of combining electronic / electric control with pneumatic control?

3. What is the purpose of fitting refrigeration units in the supply air of the

air compressor?

4. What are the pressure ranges mentioned in the pneumatic section and

what lines would you find them in?

Range Lines Pressure Range Used In

5. What type of primary element is used in pneumatic thermostats?

6. In your own words briefly describe the operation of a ‘bleed’ thermostat.

Review questions


130

7. In ARAC, pneumatic controllers are available in two styles, they are?

8. It is not effective to use a bleed thermostat on large capacity actuators.

Why is this the case and what is done to overcome this problem?

9. What is the difference between direct acting and reverse acting sensors?

10. What are the following relays used for:

Two Position Relay

Select relay

Reverse acting relay

11. What materials are used to sense humidity?

Review questions


131

12. Describe the method of operation of a pneumatic actuator.

13. A spring is also incorporated in the actuator. What is the purpose of the

spring?

14. What problem/s can occur if the air compressor breaks down on a system

without a ‘fail safe’ operation? What can be done to overcome the problem / s? (See Applications for Pneumatic Control Systems)

15. List the steps to service a pneumatic system on a three monthly

maintenance program.

16. What tools are required to commission pneumatic control equipment

like thermostats, valves and dampers and describe what they are used to do.

Review questions


132

17. To increase the mains pressure at the regulator, the regulator would need to be turned Clockwise / Counter Clockwise

18. From the diagram below, identify the:

Set Point: ___________ °C

Hot Water Valve Full Open Position Pressure: __________ kPa

Hot Water Valve Full Closed Position Pressure: __________ kPa

Chilled Water Valve Full Open Position Pressure: __________ kPa

Chilled Water Valve Full Closed Position Pressure: __________ kPa

Dead Band: between __________°C

19. In the diagram in question 18, if the thermostat reaches 24°C the pressure in the branch line from the thermostat would be 91 kPa. What is the problem with this scenario and what can be done to rectify the problem?

20 21 22 23 24 °C

91 kPa 56 21

CWVHWV

Review questions


133

20. Briefly describe in your own words how to calibrate the setpoint of a pneumatic thermostat.

21. Describe how to test the hot water actuator operation if the system had

no air being supplied to it and what tool/s would be required?


134

Notes


135

7. Programmable Logic Controllers and Direct Digital Controls

Purpose In this topic you will learn about Programmable Logic Controllers (PLC) and Direct Digital Controls (DDC).

Objectives At the end of this topic you should be able to: list and explain the principles of PLC and DDC control systems.

Content

- Microprocessors

- Programmable Logic Controllers (PLC)

- Advantages of PLC

- Disadvantages of PLC

- Application of PLC Systems

- Direct Digital Control (DDC)

- What is Direct Digital Control

- Sensing Function

- Decision Function

- Memory Function

- Action Function

- Advantages of DDC

- Disadvantages of DDC

- Applications of DDC Systems


Discusses programmable logic control


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Microprocessors

The microprocessor is a computer-based system that can be used to control air conditioning systems. The main benefit of microprocessor control is that it can control more accurately than other forms of control systems. The two main types of microprocessor control are:

Programmable Logic Controllers

Direct Digital Controls

The following block diagram shows the basis of how a microprocessor systems work.

Microprocessor Block Diagram

Programmable logic controllers (PLC)

What is a programmable logic controller (ARAC Volume 2, pages 29.25 – 27)

Programmable Logic Controllers (PLC) tend not to be as popular in use in the HVAC industry as Direct Digital Control (DDC) though it still can be used. A PLC is a computer-based system that converts analog signals from the sensor into digital values. It compares the sensor input against its programmed parameters and then produces a corrective action. PLCs simplify circuits by having many control circuit components within the microprocessor (the brain of the controller), components like:

Inputs

Outputs

Internal relays

Latching relays

Timers, etc

The following diagram shows the control panel with the programming keys and the program display window. Large programs can go over a number of pages (screens) and can be scrolled through at any time.

Input Information from Sensors

Analog to Digital Converter

Central Processor (CPU)

Digital to Analog Converter

Output Information to Control Devices

Microprocessor


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The following diagram shows how a typical cool only ladder diagram would appear as a PLC ladder diagram. Further note the key used to identify the inputs and the outputs.

Inputs On / Off Switch X1 Sail Switch X2 Thermostat X3 Thermal Overload X4 Outputs Evaporator Fan Y1 Compressor Y2 Condenser Fan Y3

Representation of a Toshiba PLC Programming Keypad (Reproduced with permission of Toshiba International Corp)

Display Window where ladder diagram is drawn.

Programming Panel showing input keys

Compressor

On /off Switch

t

Evaporator Fan Motor

Sail Switch Thermostat O/Load

Condenser Fan Motor

Typical Ladder Diagram of a Cool Only Air Conditioning System

Key to Ladder Diagram

X1

Y1 X2 X3 X4

Y1 Y2

Y1

Y2

Y3

PLC Ladder Diagram Note the differences between

the two diagrams


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Advantages of PLC

Less hard wiring is required with PLC controls.

Changes to the control circuit can be done with a program alteration where hard-wired systems (like electric and electronic systems) may need wiring alterations to effect the same outcome.

Less output components are required like timers, relays, etc as they are incorporated within the control body

Disadvantages of PLC

Programming can be difficult to grasp as unlike typical ladder diagrams, the ladder diagrams used in programming do not allow for crossing over of lines. Other peculiarities exist between the normal ladder diagram and programming ladder diagram.

Application of PLC Systems

PLC systems are generally used with the fire alarm system of a building and are maintained by the fire control company looking after the site.

If you wish to learn more about PLCs, you should consider doing further studies with the electrical trades’ section at your local TAFE.

Direct Digital Control (DDC)

References

Direct Digital Control for Building HVAC Systems

What is direct digital control?

PLCs are the early relatives of the current day DDC controller. Digital Control was introduced to HVAC applications during the 1970s as a result of the energy crisis of the time. A second factor that lead to move to DDC control is that it allowed a variety of functions to be performed from the one control. The following three screens show various parts of a building air conditioning plant controlled by a DDC system. If this were an electronic system there be would at least one controller looking after the chilled water circuit, one for cooling tower water circuit and one for each air handling unit.


139

As mentioned, DDC are computer based (microprocessor) machines that can be used to maintain set conditions of an air conditioning application. To maintain those conditions, the DDC system must perform four functions, they are:

Sense Function

Decision Function

Memory Function

Action Function

Sensing function

As with the other control systems, the sensor senses the control variable and transfers the information to the microprocessor for interpretation and response through the input of the controller. The input can be either On / Off (binary) or analog.

Sensing inputs can include temperature, humidity, pressure, etc. The typical signal input can include one or more of the following types:

A variable resistance signal

Variable milliamps signal i.e. 0-20 mA

Variable DC voltage signal i.e. 0-12 V

A Chilled Water Piping Schematic as Seen on a DDC Screen

Cooling Tower Schematic as Seen on a DDC Screen

Air Handling Unit as Seen on a DDC Screen (Reproduced with permission of Honeywell Controls)


140

Decision function

This is where the input information is processed. It compares the input to the information stored in the memory by making calculations on the deviation. Once the calculations are made a logical decision on the corrective action is taken.

Memory function

The memory of the processor is like our brain. It is where the DDC remembers what to do, how it does it, and even analyzes the result of every completed task. It does this with the use of a program.

The program provides both the information and instructions for the DDC system. If programmed correctly the system will perform the correct sequence of operations to provide optimal conditions of the medium being controlled.

Action function

Once the decision has been made for some form of action, the microprocessor carries out the corrective action by using action units of the DDC. The action functions are called the outputs of the system.

The two types of outputs include Binary (On / Off) and Analog (variable)

The action unit can be used to turn on and off supply fans, compressors, etc, communicate information to humans and / or other computers (alarms, etc).

Advantages of DDC

DDC equipment tends to be smaller and sometimes simpler.

DDC systems are very accurate and they do not loose their set point, i.e. they do not drift like other control systems.

They are energy efficient under all load conditions; this makes for long term cost savings.

DDC control is compatible with building management systems.

Adjustments can be done through the program. The building manager / owner / control company can even make alterations remotely to where the air conditioning system is located, i.e. in another building, in another state, etc.

Programs can incorporate adaptive control capability that allows the air conditioning system to adjust itself to changing conditions in the controlled environment.

Programs can be password protected to protect against tampering.

DDC can use pneumatic actuators.


141

Disadvantages of DDC

The initial cost of installing a DDC system.

- Equipment

- Computers, desktop and / or lap top computer.

- The cost of someone writing the system specific program, etc.

Programs can be password protected to protect against tampering but this may incur the added cost of employing a controls company to make a minor alteration.

Applications of DDC systems

As mentioned previous, DDC systems are an energy efficient control system and therefore are used as a result of their energy savings. DDC systems are frequently used for the control of larger applications like multistory buildings, hospitals, clubs etc.

If you wish to learn more about Direct Digital Controls you should consider further studies in the controls area.


142

Practical exercise

If there is no operational DDC or PLC system available for viewing in the section, it is recommended that an excursion to a DDC or PLC controlled building be arranged!

If you are on an excursion, remember that you are a guest in someone’s establishment. Be courteous to the people showing you around and don’t tamper with any equipment that you see.

Task

Observe the operation of a DDC air conditioning control system and / or a PLC air conditioning control system.

Procedure

Observe the operation of a DDC controlled building.

Identify the inputs, microprocessor, and the outputs of the DDC system

Draw and label one page of the program as seen on the monitor of the computer

Describe the operation of the drawn screen.



Types of inputs used

Type of microprocessor used

Manufacturer of the program: __________________________________

Is it Windows or DOS base: ___________________________________

What computer language was used to write the product:______________


143

If the building was another form of control system before being converted to computer based system: has it saved money due to energy conservation YES / NO

Explain your answer

what benefits over the previous system has been attained

has there been any disadvantages by going to a computer based system

Types of outputs used

Draw and label one page of the program as seen on the monitor of the computer.


144

Describe the operation of the drawn screen



145

Review questions These questions will help you revise what you have learnt in this topic. 1. What is the major difference between an electronic system and a

microprocessor based system?

2. What is the major difference between a programmable controller and a

conventional computer?

3. Why would a building owner install DDC control in place of either an

electric or electronic control system?

4. Direct Digital Controls perform four functions, they are:

Review questions


146

5. Briefly describe each function

6. What is the function of the program?

7. What functions can the terminal be used for?

8. What is the function of the central processing unit?

9. Write down the typical input signal values for a DDC control

Review questions


147

10. Name two devices used to provide input into the input interface.

11. Name two devices used to receive control system output.

12. What is the name of the device located between the input and the output

of a PLC control system?

13. Why is it easier to change the performance of the PLC controller than an

electronic controller?


148

Notes


149

Sample tests The following tests are included as an example of the expected coverage and depth. The assessment of this module is holistic in nature and to successfully pass the module, you must show evidence that you have achieved the module purpose, which incorporates the module outcomes.

The Sample Tests provide for an overall assessment of the module, as follows:

Test Topic Covered Timing Duration Theory test 1 1, 2, 3 and 4 18/36 60 minutes

Practical test 1 2 and 4 18/36 30 minutes

Theory test 2 1 to 7 36/36 60 minutes Practical test 2 2 to 6 36/36 60 minutes


150

Sample theory test 1 Time Allotted: 75 minutes Multiple Choice – Circle the best answer 1. The diagram below is an example of a:

(A) a resistive loop circuit

(B) a capacitive loop circuit

(C) a closed loop circuit

(D) a open loop circuit 2. A high temperature alarm sensor is part of:

(A) a resistive loop circuit

(B) a capacitive loop circuit

(C) a closed loop circuit

(D) a open loop circuit

Control Mechanism

Servo Mechanism

Switch Valve or Actuator

Perimeter Temperature

Temperature Sensor

Desired Value

Error Correction

Sample theory test 1


151

3. The following diagram is an example of a:

(A) reverse acting two position controller schedule

(B) direct acting two position controller schedule

(C) reverse acting proportional controller schedule

(D) direct acting proportional controller schedule 4. The following diagram is an example of a:

(A) reverse acting two position control

(B) direct acting two position control

(C) reverse acting proportional control

(D) direct acting proportional control

CONTROL POINT (°C)

THROTTLING RANGE

22 23 24

100% OPEN 50% OPEN CLOSED

ACTUATOR POSITION

22C



152

5. Flow control sensors include:

(A) air pressure switch, sail switch and paddle switch.

(B) differential water pressure sensor, orifice plate and sail switch

(C) air pressure switch, pressure differential switch and orifice plate

(D) sail switch, paddle switch and orifice plate.

6. The objective of Building Management is to:

(A) centralise the monitoring, operation and management of a building.

(B) decentralise the monitoring, operation and management of a building.

(C) allow building owners to manage the air conditioning system.

(D) allow the service technician to manage the air conditioning system.

7. Economiser Cycle is:

(A) the use of iced water that was produced during low tariff periods.

(B) the use of smaller capacity compressors during low load conditions

(C) the use of return air conditions to cool the building in preference to mechanical cooling

(D) the use of outdoor conditions to cool a building in preference to mechanical cooling.

8. Electric control is described as the most basic of control systems. Electric control primarily lends itself to which type of control?

(A) On / Off Control

(B) Proportional Control

(C) Step Control

(D) Analog Control



153

9. The use of four senses were recommended when fault finding. The four senses are:

(A) Sight, Smell. Taste and Touch

(B) Taste, Touch, Sound and Sight

(C) Sight, Smell, Sound and Touch

(D) Smell, Taste, Touch and Sound

10. In an automatic electric control system a Permanently Split Capacitor Motor’s direction is changed using:

(A) A thermostat with a double pole double throw switching arrangement

(B) A thermostat with a single pole double throw switching arrangement

(C) A Potentiometric controller

(D) A spring return



154

Short answer questions 1. Explain what is meant by the term “Automatic Control System”.

2. Name three factors that affect loop stability

3. List six functions that should be performed by an automatic control

system

4. What is the difference between digital control and analog control?



155

5. Describe the function of a sensor.

6. Briefly describe the difference between the output signal of a two-

position control and the output signal of floating control.

7. What type of control method does the graph (below) represent? 8. What control characteristics are indicated by the letters on the graph?

Overshoot

Undershoot

Setpoint

Differential

A

B

D

C



156

9. Describe the function of an actuator.

10. Fluid control sensors are used to do two things, they are:

11. Explain what is meant by the term ‘Energy Management’.

12. What is the purpose of Night Purge in relation to Energy Management?

13. What is thermal storage and what is its benefit?



157

14. Draw an integrated cool only sensor / controller labeling all parts and describe its operation.

15. With the aid of the wiring diagram for a package unit on the next page

answer the following questions. (a) During what part of the operating cycle is the crankcase heater

energised.

(b) What is the supply voltage for the control circuit?

(c) What is the function of the normally closed contact 52C [at terminal

point (7)]?



158

(d) What is the supply voltage to the control transformer, in Australia?

(e) What is item 62C, what is its function?

(f) What is the result if MF1 fails to energise due to a burnt out

contactor coil? List all results of this failure.



159



160

Wiring Diagrams In the space below draw a 240 volt circuit diagram in a ladder format for a simple air conditioning system containing the following components: Compressor Evaporator Fan Condenser Fan Electric Heater (including a heater safety thermostat) One Stage Heat / One Stage Cool Thermostat All Relevant Operating and Safety Controls No Lockout Relay Use pencil to sketch out your diagram before completing it in pen.

Active Neutral



161

Describe the operation of your diagram.

Fan only

Cooling

Heating

Sample practical test 1


162

Practical test 1 Time Allotted: 30 minutes Your teacher will set a series of fault finding and practical commissioning exercises on Electric and / or Fluid Flow control systems to reflect what you have learnt in class. The following examples of fault finding and commissioning exercises are typical of what your teacher could provide you. The following are examples of typical faults that could be fitted on electric and fluid flow control systems. Identify the fault and recommend the repairs that would be required to correct the fault.

Electrical control fault finding

A customer has called you out and complains that during the winter period, as conditions outside drop in temperature the space temperature drops also. Upon inspection of the outdoor coil it is found that the coil is iced up completely. From the diagram on next page of a split system, identify the fault (if any) and recommend any repairs required (if any).



163

Fault:

Repairs Required:

Indoor Unit

Fan Motor

Compressor

Reversing Valve Solenoid Coil

Defrost Timer

24 V Relay 415 Volt 50Hz

3 Phase Supply

Pressure Control

Fan Speed Selection

Compressor Contactor

Crankcase Heater

Outdoor Unit

°t

Vent Condition



164

Fluid control fault finding The air conditioning system in a shop will neither heat nor cool though ventilation (the fan) is operating. Upon inspection you find the air pressure switch to have 240 volts across it, all other safeties in the circuit are found to be OK What does this tell you?

What could have caused this problem?

Recommend the repairs (if required).



165

Electrical control commissioning For the electrical commissioning practical exercise you would be supplied with a simple air conditioning system ready for commissioning. The system would be fully charged and with no refrigeration problems fitted as well there would be no electrical faults fitted. Describe in point form the procedures that you would take to commission the air conditioning system found below (also found in ARAC p. 29.23, fig 29.23) noting all settings where possible.



166

Sample theory test 2 Time Allotted: 75 minutes Multiple Choice – Circle the best answer

1. Resistance type sensing elements are only used in:

(A) Electronic control systems.

(B) Electric control systems.

(C) Pneumatic control systems.

(D) Temperature control systems.

2. Changing the proportional band adjustment on an electronic controller is the same as the:

(A) Dead band adjustment

(B) Throttling range adjustment

(C) Authority adjustment.

(D) Set Point adjustment

3. Dead band on a cooling electronic controller is:

(A) The difference between cut in and cut out

(B) The difference between set point and end of the proportional band (max. end)

(C) The difference between set point and start of the proportional band (min. end)

(D) The difference between the start and the end of the proportional band

4. When controlled by an electronic control system, if a heating water valve is ‘hunting’, the most likely cause would be:

(A) A proportional band setting that is too narrow

(B) A proportional band that is too wide

(C) A dead band setting that is too wide

(D) A dead band setting that is too narrow



167

5. The simplest pneumatic control is:

(A) a non-bleed type controller

(B) a bleed type controller

(C) a pilot-bleed controller

(D) a sensor controller

6. The control (branch line) pressure for pneumatic control system actuators is:

(A) 20 Pa to 150 Pa

(B) 20kPa to 150 kPa

(C) 150 kPa to 400 kPa

(D) 150 Pa to 400 Pa

7. The main (line) pressure used in pneumatic control systems is:

(A) 20 Pa to 150 Pa

(B) 20kPa to 400 kPa

(C) 150 kPa to 400 kPa

(D) 150 Pa to 400 Pa

8. The following diagram is an example of a:

(A) reverse acting bleed type controller

(B) direct acting bleed type controller

(C) reverse acting proportional controller

(D) direct acting proportional controller



168

9. DDC stands for:

(A) Direct Digital Control

(B) Direct Digital Controller

(C) Direct Digit Control

(D) Direct Digit Controller

10. DDC systems is made up of three main parts, they are:

(A) the input, the controller and the output

(B) the input, the switching mechanism and the output

(C) the input, the microcomputer and the output

(D) the input, the microprocessor and the output



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Short answer questions 1. Describe the operation of a PTC (Positive Temperature Co-efficient of

resistance) sensor.

2. Briefly explain the operation of the Bridge in the electronic controller.

3. Briefly explain the operation of the Amplifier in the electronic

controller.

4. What is the output circuit?



170

5. What are three advantages and three disadvantages of electronic control? Advantages:

Disadvantages:

6. What is a Two in One Controller?



171

7. Name the Actuator seen in the diagram below and describe its operation. Actuator Name:

Operation:

8. Name the Actuator seen in the diagram below and describe its operation. Actuator Name:

Operation:

Shaft and Sring

Heater

Expansion Medium ie Wax

Seal

Actuator Shaft and Spring

Piston

Diaphragm

Pump Valve

Hydraulic Pump Oil Reservoir

Pressure Chamber

Hydraulic Fluid



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9. For electronic controls define the following terms: Set point:

Deadband:

Differential:

Control Point:

Offset:

10. How would you test a resistive sensor if you suspected it to be inaccurate?

11. Identify two advantages and two disadvantages of pneumatic systems

when compared to electrical systems. Advantages

Disadvantages



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12. From the following diagram below name the components indicated.

(Reproduced with permission of Honeywell Controls)

A_________________________________________________________ B_________________________________________________________ C_________________________________________________________ D_________________________________________________________ E_________________________________________________________ F_________________________________________________________ G_________________________________________________________ H_________________________________________________________ I _________________________________________________________ 13. Describe the operation of a bleed thermostat.

A B

E F

G



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14. Name the two primary types of pneumatic actuators:

15. What is the purpose of a refrigerated air dryer?

16. Briefly explain the function of the following pneumatic control

equipment: Two position relay

Selector switch

Electro-pneumatic transducer

17. What is the difference between a pneumatic direct and reverse acting

temperature sensor / controller?



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18. The following diagram shows a basic pneumatic control system. Name and explain the function of the four components numbered. Explain the sequence of operation of this circuit.

___________________________________________________________________________ ______________________________________________________________________________

1

2

3

4

Sequence of Operation:

1

2

4

3



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19. Why do pneumatic control systems require regular maintenance?

20. What should that maintenance include, list five points?



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21. Identify the type of control systems from the diagrams provided. (Reproduced with permission of Honeywell Controls)

22. Identify the blocks marked A, B and C of the following Microprocessor

diagram A: ___________________________________________________________ B: ___________________________________________________________ C: ___________________________________________________________

1. Type of System

2. Type of System

Input Information

from Sensors

A B C Output Information to

Control Devices

Microprocessor



178

23. Name an application for a PLC

24. What are the four functions that a DDC must perform to maintain a set

of conditions of an air conditioning application?

25. Briefly describe each function

26. Name an application of a DDC system



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Sample practical test 2 Time Allotted: 60 minutes Your teacher will set a series of fault finding and practical commissioning exercises on Electronic and Pneumatic control systems to reflect what you have learnt in class. The following examples of fault finding and commissioning exercises are typical of what your teacher could provide you. The following are examples of typical faults that could be fitted on electric and pneumatic control systems. Identify the fault and recommend the repairs that would be required to correct the fault.

Electronic control fault finding You have been called on a service call because the customer is complaining that the temperature out of the air conditioning vent is constantly going from hot to cold. Upon inspection, the following information is has been identified: The temperature of the space is found to 22.5°C. The reversing valve is found to keep turning on and then off while the

compressor continues to run. The following graph shows what you have found.

Identify the fault and recommend the repairs / alteration required. Fault: ______________________________________________________________

Set Point 20 21 22 23 24

On / Open Off / Closed

Y2 Y1



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Repairs / Alterations Required:

Pneumatic control fault finding Another of your fellow mechanics had completed a series of pneumatic line replacements the day because of water in the lines. The mechanic had also made minor alterations throughout the pneumatic system. You have been called out to the job the next day, as the whole building is too hot. Upon arrival you find all the clubs heating to be on. See the diagram below to identify the problem and recommend repairs.

SA MA

PE

Air Solenoid Valve

Thermostat

Pressure - Electric Relay

Actuator

Supply Air Main Air Branch Lines

T

700 kPa 70 kPa 20 –50 kPa



181

2 1

Fault:

Repairs Required:

Electronic control commissioning You are to commission the following electronic controllers to operate to the specification shown in the diagrams below. Your teacher would check each controller to make sure all settings are correct and the system they are connected with works accordingly. 1.

Setpoint 21 22 23 24 Temperature (°C)

ON Output OFF



182

Type of Controller: ____________________________________________

Setpoint: __________________________

Deadband 1: __________________________

Differential 1: __________________________

Deadband 1: __________________________

Differential 2: __________________________ 2. Type of Controller: _________________________________________

Setpoint: ____________________

Deadband 1: ____________________

Proportional band: ____________________

Setpoint 21 22 23 24

Temperature (°C)

Open Actuator Position Closed



183

Pneumatic control commissioning You are to commission a bleed thermostat to operate to the specification shown in the diagram below ensuring the thermostat’s set point is correctly calibrated. Your teacher would check the controller to make sure all settings are correct.

Setpoint: ____________________ °C

CWV

Full Open Position: ____________________ kPa

Full Open Position: ____________________ °C

Full Closed Position: ____________________ kPa

Full Closed Position: ____________________ °C

HWV

Full Open Position: ____________________ kPa

Full Open Position: ____________________ °C

Full Closed Position: ____________________ kPa

Full Closed Position: ____________________ °C

Throttling Range: ____________________ K

91 Pressure (kPa) 56

21

Setpoint 21 22 23 24

Temperature (°C)

CWV

HWV


184

Notes


185

Answers

Review questions

1. Control system fundamentals and diagrams 1. The three essential components of a control loop are: Sensing device Controller Control device

2. The six major functions of these components are: The sensor senses a change to the controlled variable. The controller amplifies the sensor signal. The amplified signal is transported to the control device. A corrective action takes place. The sensor senses the corrective action and signals the control device. The control device ends the corrective action.

3. The controlled variable in the control loop is the control medium, i.e.

water, air or many other material whose condition is being controlled. 4. The set point of a controller is the value at which the indicator is set to

on the control’s scale. 5. The control point of a control loop is the value of the condition actually

maintained in the space as recorded by the control. 6. The differential gap is the range through which the space condition must

travel from on to off. 7. The offset of a control loop is the difference between the set point and

the control point. 8. The lag in a control loop is the delay in the effect on the controlled space

if the corrective action. 9. Cycling in a control loop is the consistent repetition of change in the

control point. 10. An actuator is a motor, relay, solenoid etc., in which energy from the

controller is converted into rotary, linear or switching action to bring about change in the controlled space or condition.


186

11. The difference between a closed loop control system and an open loop control system is that closed loop systems receive feed back and controls accordingly. The open loop system on the other hand operates regardless of the impact on the space, i.e., has no feedback.

12. The control point is the value of the conditions actually maintained in

the space. 13. The three types of control diagrams are: Air conditioning diagrams Pneumatic and Logic Control diagrams Electrical diagrams

14. A typical application for each of the following diagrams would be: Air Conditioning Diagrams: used to show the transport of air

throughout a ducted system. Pneumatic and Logic Control Diagrams: Used to show circuit

operation of pneumatic and logic circuit. Block Diagrams: used to aid in the understanding of circuit operation Circuit Diagram: used for fault finding Wiring Diagrams: used to complete wiring equipment Control Circuit Diagrams: used to show circuit operation and can be

seen in most air conditioning units circuit boards from the simple domestic through to industrial applications.

15. The control components that you would expect to find in a control

diagram for a basic system to control conditioned air would be: Switches to turn on and off the system Thermostat to automatically turn on and off the compressor Contactors for starting and starting the compressor, fan motors, etc Protective controls like:

Overloads to protect against excess current High pressure, low pressure or dual pressure controls to protect

against high and low system pressures. Over temperature devices like thermistors to protect motors

against excessive operating temperatures. 16. Other control system components that you would expect to find in a

control diagram for a system to condition water would be: Safety thermostats: to protect chillers against icing up. Contactors for starting and stopping chilled water pumps. Water valves Flow switches to allow the system to know that the pump is

circulating water.


187

t°

17. The symbol for the following components are: A thermostat that makes on rise. A thermostat that makes on fall. A two stage heating and two stage cooling

thermostat A motor operated valve

A sail switch

M M or

t°

t°


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2. Types of control systems – general overview and fluid Flow Control

1. The four major types of control systems available in the air conditioning

industry are electric, electronic, microprocessor and pneumatic.

2. The difference between on a digital signal and an analog signal is that the analog signal is continuous where the digital signal controller samples digital data at set time intervals.

3. The function of a sensor is to detect and measure changes in the

controlled variable.

4. A bimetal strip thermostat works on the principle that with a temperature change the bimetal strip (fitted with contacts and supplied with electricity) will either springs open or close due to the different coefficients of the two metals in the bimetal strip.

5. A mercury tilt switch is fitted to a plate that can be rotated by a linkage

from a sensing element like a bimetal strip. When the switch tilts the mercury pool flows to the lower end and makes contact between the contacts. (See page 12 of Automatic Control Principles for further details.)

6. The primary function of a Fluid Flow Sensor within an air conditioning

control system is to show fluid flow or measure flow.

7. In order to keep conditioning off until airflow is established a sail switch would be fitted in series with the conditioning circuit (see below).

8. A paddle switch is fitted into a chilled water circuit to stop the compressor/s starting and icing up the evaporator causing major damage.

9. The “controlled differential” is the difference between the cut in and cut

out temperatures or pressures.

Condition Switch


Vent Switch

°

To compressor and condenser etc.

Sail Switch

Thermostat

M


189

10. The “throttling range” of a proportional controller is the control point range through which the controlled variable must pass to move the final control element through its full operating range.

11. The operating differences between “on-off” and “floating” control types

is that on-off has only two positions where floating control has a sensing element that is attached to a selector that is free to drift between contact points. When the selector makes, it drives an actuator motor in one direction until the sensed condition moves back into tolerable limits and then the contacts open.

12. The operating differences between “floating” and “proportional” control

types is that proportional is always on and there is constant feedback between the controlled device and the sensing element. Floating is like an On-Off version of proportional control.

13. The term used for the device, which incorporates an actuator that drives a

shaft that has many cams, mounted on it to operate micro-switches for a number of output devices is a step controller.

14. The purpose of a relay in a control system is to perform a function

beyond the capacity of the original controller.

15. The major design and operational difference between opposed blade dampers and parallel blade dampers is that: Parallel blade dampers: all the blades rotate in the same direction Opposed blade dampers: alternate blades rotate in opposite directions.

16. The effect on the airflow of a parallel blade air damper and an opposed

blade damper at the half-open position would be that: Parallel blade air damper: there would be approximately 80% of the

supply air moving through the damper. Opposed blade damper: there would be approximately 40% of the

supply air moving through the damper.

17. The valves shown in the sketch are mixing valves. Mixing valves have two lines in and one line out. See Automatic Control Principles (p.39) to see the difference between the two valve types.


190

18. The correct location for the mixing valve on the cooling coil is shown below with directional arrows to identifying water flow.

19. The following sketch shows a face and bypass damper control

arrangement. Operation As the temperature of the space rises the modulating motor opens the

face damper opens allowing air to pass across the cooling coil and then to the space. The bypass damper closes off at the same time as the face damper opens. As the temperature of the space drops the face damper closes back down again while the bypass damper opens allowing air to bypass the cooling coil.

-

Cooling Coil

Air Flow

Bypass Damper

Face Damper Damper Motor


191

4. Electric control systems 1. The control action that the electric control system best lends itself is the

On / Off control action. 2. Yes it is possible to attain true proportional control. Proportional control

is attained in electric control with the use of a potentiometer and a proportioning (modulating) motor.

3. The advantages of electric control include:

Available wherever there is electricity

Simple and easy to install

Amplification of signals is readily accomplished

Signals from sensing elements can be combined and or sequenced to perform several functions

Remote control capability

No time lags

Linear response

Wide ambient operating temperature range

Components and functions are easily integrated with other control types

Robust in their construction

Tend to be an integral sensor / controller

Sequence of operation tends to be simple. 4. The disadvantages of electric control include:

More complex than pneumatic control

Often needs more maintenance

Elaborate protection required in hazardous locations

Needs skilled service personnel

Higher voltages so greater care must be taken

Many control changes require wiring alterations to achieve the change

Accuracy can be difficult to achieve

Physical size of control components can be a problem

Modulating actuators can be complex


192

5. If the temperature varies above or below the set point, the thermostat makes, driving the step controller in one direction or the other. The cams in the step controller are turned making microswitches in their travel. The microswitches are used to energise or de-energise heating and cooling components according to the load requirement.

6. A microswitch is a snap acting switch whose contacts are made or broken

by the movement of a sensing element or cam mechanism. 7. The simplest type of control system actuation is On / Off. 8. The permanently split capacitor motors direction can be changed simply

with the use of a single pole double pole switch. 9. The function of limit switches on modulating motors is to stop the motor

from continually driving in one or the other direction. 10. The balancing relay in the modulating motor works on the principle that

if more current flows through one coil and less to the second coil, the magnetic fields created causes the relay to pivot.

11. An integrated sensor / controller has both the sensor and the contacts

contained within the one body. 12. To test for an integrated humidity sensor / controllers accuracy you must: Take the humidity of the area being controlled. Set the controller’s set point to the same humidity setting as the

condition that was just taken. Make adjustment if necessary. Check the differential and adjust if necessary.

13. The insulation resistance tester is used to test for earth leakage.

Forward Winding

Reverse Winding

Single Pole, Double Throw Switch

Coil with more current Coil with the least current


193

14. To use the insulation resistance tester for earth leakage:

Connect one of the meter leads to the electrics of the component being tested and the other lead to the metal frame.

Set the meter to that which is recommended in the wiring rules, (approximately double the voltage rating of the component being tested.

Take a reading. If the component reads from zero to 1 megohm the component is faulty. If the meter reads from 1 megohm to infinity the component is OK.

15. The four things technicians do every time they fault find are:

Recognise that a fault or faults has occurred on the plant.

Locate the fault(s) and the reason for the fault.

Carry out all necessary repairs to the equipment.

Restart the plant and do a final check 16. The use of the senses can be a quick way of identifying faults.

Verification with the appropriate testing equipment should always be done to confirm the fault.

17. The senses suggested and the types of faults that can be identified are

listed below:

Sight: Visually identifies breaks or damaged components.

Smell: The scent of burnt components and wiring

Hearing: Listen for unusual noises like chattering, buzzing or groaning coming from components.

Touch: Feel for excess temperature 18. The components indicated are:

1. Sail switch

2. Transformer

3. Three phase switch

4. Fuses

5. Heater

6. Contactor coil

7. Two stage heating thermostat

8. High-limit manual reset thermostat.


194

5. Electronic control systems 1. The major difference between the electronic control system and the

electric control system is in the symbols they use, components they use, voltages they use, etc.

2. The two types of control action provided by electronic controllers are On

/ Off and proportional control. 3. The three major components in a simple electronic control system are

the same as electric and pneumatic controllers, they are the sensor, the controller and the controlled device.

4. The Wheatstone Bridge operates on the principle that in the event of an

imbalance or resistance between two paths, a voltage difference will occur at points A and B (see below). The extent of imbalance can be calculated by using Ohms Law. This out of balance can be amplified and used to serve other functions in the control system.

5. Listed below are typical advantages of electronic control systems:

Simple , low voltage interconnections

Smaller equipment size

Sensing elements are faster to respond

Can be mounted in any position

Vibration and dirt resistant

Remote set point capability

Application flexibility

Superior calibration

Simple adjustment

6. The three disadvantages of electronic control are:

Some temperature sensing elements will continue with temperature change beyond their own control range.

The initial purchase cost can be expensive.

Actuators and controls tend to be complex.

A B V


195

7. The three basic parts of a single element controller and the function for each are listed below:

Bridge - The sensing circuit for the controller

Amplifier - Increases the signal into a useable signal

Output Circuit - Connection point for actuators, contactors, relays, etc.

8. The four major types of actuator used in the air conditioning industry is:

Magnetic

Motorised

Thermal

Electrohydraulic

9. Operation of a Thermal Actuator:

Thermal actuators are made up of a heating element and a solid expansion medium. The heater heats up and cools down according to the variation in the output voltage from the electronic controller. With an increase in temperature, the solid expansion medium will expand causing a force to be exerted creating a stroke movement. If the voltage is reduced from the controller, the heater cools, the expansion medium contracts again causing the opposite effect in stroke movement.

10. Operation of a Electrohydraulic Actuator

Electro hydraulic actuators use an oil pump and a pump valve (similar to a solenoid) to create valve or damper movement. If the voltage from the output of the electronic controller increases the pump valve closes isolating the actuators bottom chamber from the top. The pump fills the bottom chamber, extending the actuator shaft. If the voltage from the controller decreases, the servo valve opens relieving pressure back into the top chamber. The actuator shaft retracts.

11. An insulation resistance tester should never be used on electronic controls because the excess voltage that it produces will damage the electronic equipment.


196

12. Your graphical answer to question 13 should look the same as the one shown below.

13. The answers are both bolded and italic

Controller Type: Proportional Controller

Set Point 19°C

Proportional Band 1 0.5 K

Dead Zone 1 0 K

Proportional Band 2 1 K

Dead Zone 2 0 K

14. The answers are both bolded and italic

Controller Type: On / Off Controller

Set Point 22°C

Differential 1 2.5 K

Dead Band 1 - 0.5 K

Differential 2 1.5 K

Dead Band 2 0.5 K

17 18 19 20 21


1 2


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6. Pneumatic controls

1. Pneumatic control is the use of air as an energy source in place of electric or electronics.

2. The purpose of combining electronic / electric control with pneumatic control is to allow for:

Electronic- for the greater accuracy of electronic control.

Electric – for the initiation of electric components.

3. Refrigeration units are fitted to the supply air of the air compressor to remove any moisture before it enters the rest of the system where it may cause problems.

4. Range Lines Pressure Range Used In

700 kPa From the air compressor tank and the supply lines to the regulator

100 – 120 kPa From the regulator through the mains lines to controllers. This pressure can also be supplied to control devices.

20 – 90 kPa From controllers to control devices.

5. The type of primary element used in pneumatic thermostats is the bimetal

strip. 6. Bleed thermostats operate by bleeding various amounts of branch line

pressure to the atmosphere. The position of the ‘flapper’ with respect to the nozzle will determine how quickly the air is bled off and vary the branch line accordingly. The position of the flapper is determined by a temperature sensitive bimetal element. The branch line is used to control an actuator or become the input signal to a controller.

7. The two styles of controller described in ARAC are: Proportional

controllers and High capacity controllers. 8. A simple bleed thermostat’s capacity would be too small to operate large

capacity actuators at a reasonable speed. High capacity controllers or pilot operated actuators are used to overcome this problem.

9. The difference between direct acting and reverse acting thermostat is:

Direct Acting: With an increase in temperature there is an increase in the pressure output.

Reverse Acting: With an increase in temperature there is a decrease in the pressure output.


198

10. A two position relay is used to open one branch and close another.

A select relay is used to select a highest or lowest signal from a number of controllers and direct an actuator.

A Reverse-Acting Relay is used to reverse the signal from a controller so that an actuator may open instead or close or vice versa.

11. The following materials are used to sense humidity: human hair, wood or nylon, or electric-pressure transducers may be used.

12. The method of operation of a pneumatic actuator is that as air is

admitted into the actuator, it fills the space between the rubber sealing diaphragm on top of the piston. As the pressure rises, it overcomes the spring pressure (and the load) to force the piston and shaft down.

13. Springs are used in actuators to set the operating range that they will

operate over. 14. The problem/s that can occur in the event of an air compressor breaks

down on a system without a ‘fail safe’ operation is that without pneumatic pressure full cooling would be applied. To overcome this problem a reversing relay and a normally closed valve with the same operating range would need to be fitted.

15. The steps described to service a pneumatic system on a three monthly

maintenance program include:

Cleaning of all components

Checking all moving parts for operation (thermostats, actuators, etc).

Check to ensure that the control sequences are effective as originally commissioned.

Recalibrate where necessary.

Check all airlines, actuators, diaphragms, etc, for air leaks.

Other checks could include: check operation of automatic water bleeds, check strainers, filters, refrigeration equipment in supply air from air compressor, etc, clean / repair as necessary.

16. The tools that would be required to commission pneumatic control equipment include:

Stab Gauge: used to identify system operating pressures.

Hand pump: used to pressure test operating ranges of pneumatic of pneumatic actuators.

Small Flat Blade Screwdriver: used to calibrate the pneumatic actuators.


199

Large Flat Blade Screwdriver and or shifter: used to stroke dampers and valves.

Sling Psychrometer or Digitemp thermometer: used to calibrate the thermostat.

17. To increase the mains pressure at the regulator, the regulator must be

turned clockwise.

18. From the graph in question 4:

Set Point: 22 °C

Hot Water Valve Full Open Position Pressure: 91 kPa

Hot Water Valve Full Closed Position Pressure: 21 kPa

Chilled Water Valve Full Open Position Pressure: 91 kPa

Chilled Water Valve Full Closed Position Pressure: 21 kPa

Dead Band: between 21.5 and 22.5 °C

19. If the thermostat reaches 24°C the pressure in the branch line from the

thermostat would be 91 kPa. The problem with this scenario is that both the chilled water valve and the hot water valves would be open. To rectify this problem a reversing relay would have to be fitted to the hot water circuit.

20. To calibrate the setpoint of a pneumatic thermostat

Measure the control point of the area as close as possible to the thermostat. Do this three times to be sure of an accurate reading.

Set the setpoint dial until the needle lines up with the temperature scale. Be sure not to submit the thermostat to any outside temperature influence, i.e. your breath or heat from your hand etc as you do this.

Adjust the set point calibration screw until the pressure on the stab gauge reads the set point pressure, in this instance 56kPa. Again this must be done without disturbing the sensor by breathing on it or touching it, etc.

Return the setpoint dial to its correct setting in this case 22° C.

Allow time for the thermostat to settle and regain control. Take further temperature readings to ensure the correct operation of the system, whilst remembering that offset will increase with deviation of the control point from the set point.


200

21. To test the hot water actuator operation if the system had no air being supplied to it you would have to use a hand pump.

Disconnect the valve actuator from the system and connect the hand pump to it.

Pump up the pressure on the valve to its minimum operating pressure while watching the valve’s spindle. The valve should not have moved.

Continue to pump while watching the valve as you do. If the valve is operating OK, it should be completely open at its rated maximum operating pressure.

Open the knurled knob on the pump to slowly release the pressure on the valve. Watch as the pressure on the valve is reduced back to its minimum.

The check is now complete, reconnect the valve to the system.

7. Programmable logic controllers and direct digital controls

1. The major difference between an electronic system and a microprocessor based system is that the electronic system uses an analog signal throughout the controlling process where PLC use digital in the processor.

2. The major difference between a Programmable Logic Controller and a conventional computer is the language they use. PLCs use ladder diagrams where computers use computer language like BASIC and Fortran.

3. When compared to either electrical and electronic control systems, a DDC system will provide more accurate conditions and at the same time provide energy efficient cost savings to the building owner.

4. The four functions that a direct digital control performs are:

Sensing Function

Decision Function

Memory Function

Action Function


201

5. The Sensing Function is the sensing of control variable and the transferring of that information to the microprocessor for interpretation and response.

The Decision Function is the processing of the input information

The Memory Function is where the controller remembers what to do, how to do it, and the analysis of the result of every completed task.

The Action Function is the corrective action of the DDC system, the output of the system.

6. The program is a set of instructions that tells the microprocessor what to do.

7. There are two functions of the terminal: to allow for programming and trouble shooting the circuit.

8. The function of the central processing unit is to establish the electronic circuitry needed to carry out the instructions and to perform those instructions.

9. The typical input signals for a DDC control could include:

A variable resistance signal

Variable milliamps signal ie. 0-20 mA

Variable DC voltage signal ie. 0-12 V

10. Examples of devices used to provide input into the input interface are

start / stop switches

flow switches

pressure switches (ie. HP / LP controls)

time clocks

thermostats, etc.

11. Examples of devices used to receive control system output are: motors (if small enough), lights, solenoid coils, contactor coils, etc.

12. The name of the device located between the input and the output of a PLC control system is called a microprocessor.


202

13. To change the performance of the PLC controller it is simply a case of changing the program. With than an electronic controller, hard wiring may have to be changed to attain the same performance.


203

Sample theory test 1 Multiple choice questions 1. (C) A closed loop circuit 2. (C) A closed loop circuit 3. (D) Direct acting proportional controller schedule 4. (A) Reverse acting two position control 5. (A) Air pressure switch, sail switch and paddle switch. 6. (A) Centralise the monitoring, operation and management of a building. 7. (D) The use of outdoor conditions to cool a building in preference to

mechanical cooling. 8. (A) On/ Off Control 9. (C) Sight, Smell, Sound and Touch 10. (B) A thermostat with a single pole double throw switching arrangement

Short answer 1. An “Automatic Control System” is a system that will automatically

regulate a HVAC system output in response to varying indoor and outdoor conditions to maintain general comfort conditions.

2. The factors that affect loop stability include: Speed of operation of the control equipment Speed of the controlled equipment and Thermal Inertia Air change rate Sensor Location

3. The six functions which should be performed by an automatic control

system is The sensor senses a change to the controlled variable The controller amplifies the sensor signal The amplified signal is transported to the control device A corrective action takes place The sensor senses the corrective action and signals the control device The control device ends the corrective action


204

4. The difference between digital control and analog control is in the control signal it uses. Analog controller receives and acts upon data continuously. The digital controller samples digital data at set time intervals.

5. The function of a sensor is to detect and measure a disturbance in the

control variable. 6. The difference between the output signal of a two-position controller and

the output signal of a proportional controller is: The two-position controller signal is either On / Off or open / closed. The proportional controller signal is variable, it can range anywhere

between open and closed or on and off. 7. The type of control method represented in the graph is On / Off control. 8. The control characteristics indicated by the letters on the graph are: Overshoot A Undershoot D Stepoint B Differential C

9. The function of the actuator is to convert electric energy into a rotary action

10. Fluid control sensors are used to show fluid flow or measure flow 11. Energy Management is the optimisation of the operation, temperature

and process of an air conditioning system within a building. 12. Night Purge in relation to Energy Management is where outside night air

is used to cool a building space when outside conditions are favourable. 13. Thermal storage is the process of making use of lower tariff periods to

store energy, i.e. turning water into ice. The energy stored can then be reused at a more expensive tariff period, i.e. remelting the ice to chill water which is then fed to air handling units. This is to in place mechanical cooling.


205

14. The diagram of the integrated cool only sensor / controller could look like the following diagram, it is an example only:

Operation: As the temperature of the controlled variable warms, the bulb

pressure increases forcing the contacts to close. Cooling would be initiated by the making of the contacts. With a decrease in the controlled variable temperature, the bulb pressure decreases opening the contacts. The cooling cycle is complete.

15. The answers that relate to the wiring diagram are as follows: The crankcase heater is energised during the off cycle. The supply of the control circuit is 24 volts. The function of the normally closed contact 52C at terminal point 7

is to open and de-energise the crankcase heater. The supply voltage to the control transformer in Australia is 240

volts at 50 hertz. Item 62C is a recycle guard timer and its function is stop the

compressor short cycling. (Ensures the compressor will not stop and then try to restart too quickly.)

The results of MF1 failing to energise due to a burnt out contactor coil is that the compressor, evaporator fan and condenser fans will not operate. The system will not therefore cool.

Bulb

Bellows Contacts

Terminals for hard wiring

Mechanical connection between sensing element and contacts


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Wiring diagrams The following diagram is only meant to show one possible answer to the wiring diagram exercise. Operation Fan Only The fan will operate once the vent (fan) switch is turned on. Conditioning cannot come on until the vent switch is turned on even if the condition switch is turned on. Cooling The compressor will only start once the vent and condition switches are turned on, the thermostat is calling for cooling and all the safety controls are closed. As the condenser fan is in parallel with the compressor, it will start as well.

Compressor

Heating Element

VENT SWITCH

CONDITION SWITCH

Active Neutral

°t

LP HP

°t

M

M

M


Condenser Fan Motor

Manually Reset Over-temperature Thermostat

Thermostat


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Heating As with cooling, heating will only come on once the vent and condition switches are turned on and there is a call for heating. The over temperature thermostat will trip in the event of excess heat (i.e. due to no fan).


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Practical test 1

Electrical control fault finding

Fault:

The neutral of the reversing valve solenoid coil is connected straight to the neutral (at the defrost timer) so it will never change over and deactivate the coil.

Repairs Required:

The neutral of the reversing valve solenoid coil must be reconnected at the same terminal as the outdoor fan motor on the defrost timer.

Fluid control fault finding

What does this tell you?

By having a 240 volt potential difference across the air pressure switch there is no circuit through the switch, the switch has not made when air has started to flow.

What could have caused this problem?

There may be a number of reasons why the air pressure switch did not make. They could include the following answers:

The microswitch may have become open circuited (damaged points, etc.)

Line/s form the air pressure switch may have fallen out of the ductwork.

Blockages of air lines not allowing air pressure to be registered by the switch, etc.

Repairs Required:

The repairs could include, depending on the fault:

Replacement of the switch.

Refit the line/s back into the ductwork securing them so that they will not fall back out.

Unblock the air line/s if possible, etc.


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Electrical commissioning

Your answer to the electrical commissioning exercise could include the following:

Set the compressor overload to 7 amps (FLA), NO HIGHER.

Set the HP/LP control to the required pressures and test their operation. (See NR12 - System Control how to calculate cut out and cut in pressures).

Set the thermostat to 22°C and test for calibration, dead bands, differentials, etc.

Carry out an amperage test (using a tong tester) on all motors to ensure that they are operating within amperage limits. (See diagram for FLA ratings).

Check the system on both heating and cooling operation to make sure that the system operates correctly on both cycles.

Test both the de-ice thermostat and the de-ice heating element for correct operation, (the operating conditions are not described on the diagram so you may have to call the manufacturer for details).

Test the low ambient thermostat and low ambient heating element / circuit operation.

Further commissioning tests you may need to do: (see NE172 - Electrical Wiring and Equipment and Australian Standards 3000 - SAA Wiring Rules (current edition).

Check to make sure that there is no excess uninsulated conductors hanging out of terminals, etc.

Test to make sure all terminals are tight (no loose joints.)

Ensure correct fixing of conductors.

Test for earth leakage and resistance between conductors. (The electrician wiring the job should have carried out these tests before handing it over to you.)


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1. (A) Electronic control systems.

2. (B) Throttling range adjustment.

3. (C) The difference between set point and start of the proportional band (min. end).

4. (A) A proportional band setting that is too narrow.

5. (B) A bleed type controller.

6. (B) 20kPa to 150 kPa.

7. (C) 150 kPa to 400 kPa.

8. (A) Reverse acting bleed type controller.

9. (A) Direct Digital Control.

10. (D) The input, the microprocessor and the output.

Short answer 1. The PTC (Positive Temperature Co-efficient of resistance) sensor’s

resistance increases with a rise in temperature and visa versa. 2. The Bridge of an electronic controller operates on the principle of a

Wheatstone Bridge. It operates by using two sets of two resistors connected in parallel across a DC power source. In the event that the sensing element resistance changes due to temperature change, the bridge would no longer be in balance and a voltage difference will occur. The difference in voltage between the two paths can then be amplified.

3. The Amplifier of an electronic controller is to increase the signal from

the bridge circuit. 4. The output circuit is where the actuators are connected on the controller

to provide the correct sequence of operation.


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5. Listed below are typical advantages and disadvantages of electronic control systems that you could have written:

Advantages

Simple , low voltage interconnections

Smaller equipment size

Sensing elements are faster to respond

Can be mounted in any position

Vibration and dirt resistant

Remote set point capability

Application flexibility

Superior calibration

Simple adjustment

Disadvantages

Some temperature sensing elements will continue with temperature change beyond their own control range.

The initial purchase cost can be expensive.

Actuators and controls tend to be complex. 6. A Two in One Controller is a controller that can control two types of

variables, i.e. temperature and humidity. 7. The type of actuator shown is an Electro-hydraulic Actuator. Its operation: Electro hydraulic actuators use an oil pump and a pump valve (similar to

a solenoid) to create valve or damper movement. If the voltage from the output of the electronic controller increases the pump valve closes isolating the actuators bottom chamber from the top. The pump fills the bottom chamber, extending the actuator shaft. If the voltage from the controller decreases, the servo valve opens relieving pressure back into the top chamber. The actuator shaft retracts.

8. The actuator shown in question 7 is a Thermal Actuator Its Operation Thermal actuators are made up of a heating element and a solid

expansion medium. The heater heats up and cools down according to the variation in the output voltage from the electronic controller. With an increase in temperature, the solid expansion medium will expand causing a force to be exerted creating a stroke movement. If the voltage is reduced from the controller, the heater cools, the expansion medium contracts again causing the opposite effect in stroke movement.


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9. The Set Point: is the desired value that the controller is expected to maintain

The Dead Band: is the range through which an input signal will be varied without initiating a response from the controller.

The Differential: is the difference between on and off state.

The Control Point is the actual value of the controlled variable that the controller maintains.

The Offset is the sustained deviation between the control point and the setpoint of a control.

10. To test the resistive sensor, you would:

Disconnect the sensor from the controller and measure the resistance.

Measure the ambient air close to the sensor.

Compare the temperature and the resistance of the sensor against the relevant resistance curve for its accuracy.

11. Advantages of a pneumatic systems when compared to electrical systems:

Pneumatic systems can be safely used in hazardous environments.

Pneumatic actuators can be built to operate from the smallest to the largest valves and dampers.

The controller is naturally and inherently operates as proportional control but tow-position is easily provided.

It is easy to include a great variety of control sequences.

Disadvantages of a pneumatic systems when compared to electrical systems:

Expensive to purchase and more equipment to maintain (i.e. air compressors, driers, etc.)

Many mechanics consider pneumatic controls difficult to work with.

12. The components identified in the pneumatic diagram are:

(A) Intake filter

(B) Air compressor

(C) Pressure switch

(D) Motor

(E) Refrigerated air dryer


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(F) Pressure regulator

(G) Thermostat

(H) Actuator

(I) Valve 13. The bleed thermostat operates that with a change in temperature a

bimetal strip will either open or close a tiny nozzle, so varying the air pressure in the valve or damper actuator.

14. The two primary types of pneumatic actuator include:

Two position actuator

Proportional actuator 15. The purpose of the refrigerated air dryer is to remove all moisture before

it enters the system and causes blockages in the many small orifices throughout.

16. The function of each component listed is explained below:

Two position relay:

To open one branch line and close another

Selector switch:

To choose either the highest or lowest signal from a number of controllers and with this signal /s operate an actuator/s.

Electro-pneumatic transducer:

To change an electric signal into a pneumatic signal or vice versa. 17. The difference between a direct and a reverse acting sensor is that with

an increase in temperature the direct acting sensor will deliver an increase in output where the reverse acting sensor would deliver a decrease in output.

18. The name and the function of the four components numbered are:

1. Reverse acting thermostat: to send a signal to the hot water valve actuator and also to the reversing relay.

2. Chilled Water Valve: meters the flow of chilled water into the chilled water coil

3. Hot Water Valve: meters the flow of hot water into the hot water coil.

4. Reversing relay: changes the signal from the thermostat to operate the chilled water valve.


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The sequence of operation:

With a decrease in temperature, the reverse acting thermostat’s branch press increases and modulates the hot water valve open (between 60 and 90 kPa). The chilled water stays closed because the branch pressure has been reversed by the reversing relay.

As the temperature increases the branch pressure decreases modulating the hot water valve closes at 60 kPa. With a further increase in temperature the thermostat branch pressure continues to drop. The reversing relay will reverse this signal until the chilled water range is begins to operate.

Between 50 and 60 kPa branch pressure (from the thermostat), both valves will be closed

19. The recommended period for maintenance of a pneumatic control system

is three monthly. 20. The maintenance should include the following:

Clean all components

Check all moving parts for operation (thermostats, actuators, etc.)

Check the sequence of operation as installed

Recalibrate where necessary

Check for air leaks on all lines. 21. The two systems shown are

1. Programmable Logic Controller

2. Direct Digital Control 22. A: Analog to Digital Converter

B: Central Processor (CPU)

C: Digital to Analog Converter 23. An application for a PLC would be in the fire alarm system of a

building. 24. The four functions that of a DDC must perform to maintain a set of

conditions are:

Sense Function

Decision Function

Memory Function

Action Function


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25. Sense Function: As with the other control systems, the sensor senses the control variable and transfers the information to the microprocessor for interpretation and response through the input of the controller.

Decision Function: This is where the input information is processed. It

compares the input to the information stored in the memory by making calculations on the deviation. Once the calculations are made a logical decision on the corrective action is taken.

Memory Function: The memory of the processor is like our brain. It is

where the DDC remembers what to do, how it does it, and even analyzes the result of every completed task. It does this with the use of a program.

Action Function: Once the decision has been made for some form of

action, the microprocessor carries out the corrective action by using action units of the DDC. The action functions are called the outputs of the system.

26. An application of a DDC system might include larger applications like

multistory buildings, hospitals, clubs, etc.


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Practical test 2

Electronic control fault finding Fault: The controller has been set up without a dead band between heating and cooling. The unit will constantly go from heating to cooling and back again without ever turning off. Repairs / Alterations Required: The dead band between heating and cooling must be reset to allow for an off cycle.

Pneumatic control fault finding

Fault:

The mains air pressure is too low and thus reducing in turn the branch pressures.

Possible Repairs Required:

Check the air pressure regulator and dryer for incorrect adjustment. If incorrect adjust the pressure until the mains pressure is maintained between 100 and 120 kPa.

Check to make sure the air pressure regulator is not blocked or partially blocked. Repair the blockage.

If the air pressure regulator is operating OK and there is still reduced pressure in the mains, check the refrigerated air dryer for blockages, check manual and automatic purges etc. Repair as necessary.

Electronic control commissioning

1. Type of Controller: Two Position Controller

Setpoint: 22 °C

Deadband: 0 K

Differential 1: -1.5 K

Setpoint 2: 0.5 K

Deadband 2: -1.5 K


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2. TYPE OF CONTROLLER: PROPORTIONAL

Setpoint: 22 °C

Deadband: 0.5 K

Proportional band : 2K

Pneumatic Control Commissioning

Setpoint: 22.5 °C

CWV

Full Open Position: 91 kPa

Full Open Position: 24 °C

Full Closed Position: 56 kPa

Full Closed Position: 22.5 °C

HWV

Full Open Position: 21 kPa

Full Open Position 21 °C

Full Closed Position: 56 kPa

Full Closed Position: 22.5 °C

Throttling Range: 3 K

Documents

Air Conditioning Controls NR15 - Wikispaceshvaceducation.wikispaces.com/file/view/ACControls.pdf · Resources and references Recommended textbooks Automatic Control Principles. Honeywell,