College of Technological Studies

Department of Power & Refrigeration Technology

Course Contents

Course Designation: Air Conditioning Control systems

Course No. : 0463 272

Credit Hrs.: 3 Lecture Hrs.: 2 Lab Hrs.: 2

Lecturer: Eng. Raad Alsaleh

Course Topics

i. Background

ii. Compressor Control System

Reciprocating Compressors Control

Capacity, safety and operation controls.

Centrifugal Compressor Control

Capacity, Surge, and safety control.

iii. Air handling systems control

Outside Air Control Minimum Outside Air, Economy Cycle Outside Air, Enthalpy Control, Static

pressure Control, Air Stratification

Heating Coil Control

Preheat of outside or mixed air, Normal heating, Reheat

cooling Coil Control Direct Expansion (DX) coils, Chilled water coils

Humidity Control humidifying, Dehumidifying

iv. Applications

Single Zone, Multi zone and dual-duct, 3 deck Multi zone, Variable Air Volume

Textbooks

1- Control Systems for Heating, Ventilating and Air Conditioning

by Roger W. Haines

2- Handbook of Air Conditioning System Design,

by Carrier

Grading Policy

1. Attendance - 10

2. Homework - 10

3. Exam 1 - 15

4. Exam 2 - 15

5. Lab. - 20

6. Final - 30

I. Background

A. The purpose of the control system:

1. Provide automatic operation.

2. Maintain the controlled conditions closer than could be achieved by manual

operation.

3. Provide maximum efficiency and economy of operation.

4. Ensure safe operation.

B. Control Fundamentals:

1. The desired temperature is ________________________________.

2. _____________________________ the actual quantity which is being controlled.

Examples: _________________, ______________________, __________________________.

3. The _________________________ measures the controlled variable and convey

the value to the controller.

Examples: Electric _______________________

Pneumatic ______________________

Mechanical _____________________

4. The _________________________ compares the actual quantity (Controlled

variable) with the desired quantity (set point).

Examples: _______________________

______________________

_____________________

5. The _________________________ reacts to signals received from the controller

to vary a flow

Examples: _______________________

______________________

______________________

6. The ________________________ is the medium manipulated by the controlled

device.

Examples: _______________________

______________________

______________________

7. The mathematical relationships of control systems are usually represented

by

________________________________. A ______________________ is the symbol which

is used to indicate a summing operation, and a _______________________ is the

symbol for multiplication.

C. Components of Control Systems:

Control systems are consisting of a loop that contain:

1. _______________________

2. _______________________

3. ________________________

4. ________________________

The communication signals between control system components are:

1. _______________________

2. _______________________

3. ________________________

D. Control Action:

1. Two-Position action:

It is the simplest of automatic regulation. There is no intermediate position

between the two extremes of full ON and full OFF.

2. Timed Two-Position action:

A heater element is added to offset operation time lag.

3. Floating action:

The control device will be turned off at any position if it reaches upper limit

or lower limit.

4. Proportional action:

Continuous action either toward OFF position or ON position, and it is of three

types:

a. Proportional.

b. Proportional Plus Integral.

c. Proportional plus derivate

Figure 3.1 Reciprocating compressor, (a) Suction stroke, (b) Discharge stroke

II. Compressor Control System:

A. General:

The purpose of the compressor in the vapor compression

cycle is to accept the low-pressure dry gas from the

evaporator and raise its pressure to that of the condenser.

Compressors may be of two types:

1. _______________________________ the general form is the piston type,

or Reciprocating compressors, being adaptable in size, number of

cylinders, speed and method of drive.

2. _____________________________ they impart energy to the gas by

velocity or centrifugal force and then convert this force to pressure

energy. The most common type is the Centrifugal compressors.

B. Reciprocating Compressors Controls:

1. Capacity Control:

A refrigeration system will be designed to have a maximum duty to

balance a calculated maximum load, and for much of its life may work

at some lower load. Such variations require capacity reduction devices.

Capacity reduction means mainly reducing the __________________________________

compressed by the compressor according to a set heat load.

Methods of capacity control are:

a) ON - OFF Control:

This is generally used with residential air conditioners, where starting

and stopping the compressor may be done directly by room

thermostat. On a rise in room temperature the thermostat opens a

Solenoid valve. The low-pressure switch closes and starts the

compressor. When the Solenoid valve closes, the refrigerant is pumped

out of the evaporator, opens the low-pressure switch, stopping the

compressor.

This type of capacity control is recommended only when the load on

the system is moderately constant.

b) Multispeed Compressor

Since the capacity of a compressor is proportional with its speed, multispeed

motor may be used to regulate its capacity (Two-speed motor). This motor

may be controlled by two stage thermostat and two speed magnetic starter.

The added cost is the disadvantage of this control method.

c) Multiple Compressors:

There are some applications where failure of refrigerant in equipment could

result in financial loss beyond the equipment repair expense. In such cases it

advisable to break the refrigeration load into multiple stages, each has its own

compressor.

d) Loading and Unloading Cylinders:

Except in very small sizes, reciprocating compressors have multistage capacity

control. This is generally achieved by loading and unloading cylinders under

control of a suction pressure controller, by raising the suction valve off its

seat.

When the load increase: (Unloading stage)

Suction pressure is high

Bellow shrink and Bleed port arm go up closing bleed port

Pressure in the piston room increase

Piston pin moves down

Suction valve operates No load

When load decrease or in starting: (Loading stage)

Suction pressure decrease

Bellow expands moving bleed port arm down opening bleed port

Pressure in piston room decrease

Piston pin moves up

Suction valve (on load)

e) Hot Gas Bypass Control:

Hot gas bypass may also be used for capacity control, for system that operates at

or below minimum temperature of compressor unloading.

A constant pressure expansion valve is used to maintain the evaporator pressure

and temperature at constant level, regardless of load.

2. Safety Controls

Any gas control system must include such safety controls as high or low

temperature limits and high or low pressure limits

a) Oil safety switch

All compressors except the smallest have mechanical lubrication and will

fail if the oil pressure falls because of a pump fault or oil shortage A

safety cut-out is required which will stop the compressor. This takes the

form of a differential pressure switch with starting time delay.

A pressure gauge on the pump discharge will indicate the total pressure

at that point Any detection element for true oil pump pressure must

sense both suction and pump outlet pressures and transduce the

difference Oil safety cut-out have pipe connections to both sides of the

oil pump and two internal bellows to measure the difference.

Since there will be no oil pressure at the moment of starting, a time delay

must be fitted to allow the oil pressure to build up This timer may be

thermal or mechanical or electric.

Oil safety cut-out control indicates an unsafe condition and such controls are

made with hand reset switch, and normally operate an alarm to warn of the

malfunction.

b) Refrigerant Pressure Controls

Pressure controls are used for safety protection with compressor and

condenser type systems.

High-pressure control actuates the system if high-side pressure

exceeds the preset level When the switch in the pressure control

opens, the compressor is stopped.

This type of safety control is required to:

Prevent the compressor motor from overloading.

Prevent the system from rupture.

Low-pressure control is used to stop the compressor motor

whenever the low-side pressure falls below a preset level Too low

suction pressure mast often occurs when there is a refrigerant leak in

the system. Damage to the system might result if the unit to operate

under this condition, drawing moisture and air.

High and low pressure switches are similar in appearance. The pressure range at

which the switch bellows mechanism operates determines whether it is a high or

a low pressure control. All pressure switches regardless of type have a common

element which is the ______________________________.

PRESSURE OPERATED SWITCH

3. Operation Control

a) Water Tower Controls

A reverse-acting fan pressure control in a water-cooled air conditioning

system. A water tower is often used to supply cooled water to the

condenser. For the system to operate efficiently, the head(high-side)

pressure of the compressor must maintained within certain limits. If the

head pressure becomes too low, the evaporator does not function properly.

Low head pressure could be the result of too cool water being supplied to

the condenser. To solve this problem, a reverse-acting fan pressure control

connected to the high side pressure line of the compressor is used. When

pressure is below a set limit, the fan motor will stop.

b) Pump-Down circuit control:

A low-pressure switch is used in conjunction with a thermostat and a

solenoid valve to form the pump-down circuit. In this method of control,

the thermostat does not stop the compressor but de-energizes the liquid

line solenoid valve to stop the supply of refrigerant to the evaporator. The

compressor continues to run and pumps down the evaporator until stopped

by the low-pressure switch. When the thermostat again calls for cooling, it

opens the solenoid valve to let the liquid refrigerant enters the evaporator

and the compressor restart again by the low-pressure switch. This method is

used to ensure that the evaporator is kept clear of liquid refrigerant when

the plant, is off.

C. Centrifugal compressors Control:

Centrifugal compressors are built for heavy duty continuous operation.

Centrifugal compressors are of two types:

a. Open - have a shaft which projects outside the compressors housing

OPEN CENTRIFUGAL MACHINE

b. Hermetic - have the driver built into the unit, completely

isolating the refrigerant space from the atmosphere

Hermetic Centrifugal Machine

1. Capacity Control

a. Hermetic Centrifugal

Temperature control is obtained by means of variable inlet guide

vanes at the suction inlet of the compressor.

This control reduces capacity by varying the angle at which the suction gas

is directed into the eye of the impeller. At low flow the change of inlet

gas direction has little effect on capacity and the control operates

primarily as a suction damper. The minimum partial load capacity of the

machine is based upon the amount of gas leakage thru the fully closed

capacity regulating vanes.

When the temperature changes, the thermostat signals the control to

reposition the capacity regulating vanes which changes the capacity of the

system, to maintain the desired temperature. When the vanes reach the closed

position and the leaving temperature continues to decrease to a predetermined

minimum, the low temperature cutout switch stops the compressor.

b. Open Centrifugal

Capacity control on an open centrifugal compressor may be obtained with

a suction damper, variable inlet guide vanes or variable speed drive.

Suction damper is controlled by a thermostat to reduce the capacity

of the compressor by throttling the gas suction inlet.

Variable inlet guide vanes control is identical to that discussed

under Hermetic centrifugal.

Variable speed drives may be controlled manually when the change

in loading is gradual or when a suction damper is used for automatic

control. Automatic speed control is used with steam, gas turbine or

gas engine drives. Automatic speed control provides very

economical operation, and require less input than other methods of

control.

COMPARATIVE PERFORMANCE, CENTRIFUGAL COMPRESSOR CAPACITY CONTROL

1. Safety Control

a. Surge Control

Surge is a characteristic of centrifugal compressors which occurs at

reduced capacities. This condition is a result of the breakdown in flow

which occurs in the impeller. When this happen, the impeller can no longer

maintain the condenser pressure, and there occurs a momentary reversal of

flow and the compressor is unable to force the gas into the condenser.

The following are some conditions which may cause compressor "surge":

Air in System - increases condenser pressure.

Low Refrigerant Charge - lowers cooler pressure.

Dirty Condenser Tubes - increases condenser pressure.

Low Condenser Water Flow - may increase condenser pressure.

High Entering Condensing Water Temperature – increase condenser

pressure

Faulty Float Valve Operation - decreases cooler pressure or increases

condenser pressure.

Division Plate By-pass - increases condenser pressure & decreases

cooler pressure.

Notice all tend to increase lift by either raising condenser pressure or

lowering cooler pressure.

Surge is solved by lowering the condenser pressure This allow the impeller

to function normally again, and gas flow returns to its normal direction. The

operation is stable until the condenser pressure builds up and surge occurs

again. Surging can be detected primarily by the change in sound level of

the machine.

Surge in a centrifugal machine does not occur at partial loads if the

head or lift decreases sufficiently with the load.

“Lift is defined as the difference between chiller

suction and discharge pressures.”

The figure below shows a typical lift versus load diagram at different positions

of the inlet guide vanes:

Line (B) when the loading is such that the total lift of the compressor

reduces sufficiently as the loading also reduces.

Line (A) when the lift remains almost constant or decreases slightly, it

can be seen that line (B) does not enter surge until load is under

minimum. Line (A) enters the surge region above minimum load.

IIFT-IOAD DIAGRAM, HERMETIC CENTRIFUGAL

To control surge occurring at partial load, a hot bypass valve between the

condenser and the evaporator is used to load the compressor artificially The

valve may be either manual or automatic, and controlled in sequence with the

automatic suction damper

or speed of the compressor

or position of the inlet guide vanes

So that the valve starts to open just before there is an indication of surge

HOT GAS BYPASS VALVE

b. Condenser high pressure cutout switch

It stops the compressor when the condenser pressure becomes

too high due to a condenser water stoppage, excessive

condenser scaling or air in the system.

c. Evaporator low pressure cutout switch

It stops the compressor when the evaporator pressure

becomes too low due to a chilled water stoppage, excessive

cooler scaling, or insufficient refrigerant charge.

d. Low oil pressure cutout switch

stops the compressor when the oil pressure drops below the

required minimum.

e. Low chilled water temperature cutout switch

stops the compressor when leaving chilled water temperature

drops below the minimum allowable temperature

f. Chilled water flow switch

stops the compressor when chilled water ceases to flow, and

prevents a start-up of the compressor motor until chilled

water flow is established.

TYPICAL SAFETY CONTROL SYSTEM. HERMETIC MACHINE

TYPICAL SAFETY CONTROL SYSTEM, OPEN MOTOR DRIVEN MACHINE

III. Air Handling System Control:

A. Outside Air Control:

Before deciding on how to control the amount of outside air it is

necessary to determine how much is required by the HVAC system

and why.

For example,

Laboratories require 100% exhaust and makeup; so chemical

labs require a negative pressure to prevent exfiltration.

Clean rooms require that a positive internal pressure be

maintained to prevent infiltration from surrounding areas.

When there are no special requirements, the minimum amount of

outside air required is that needed to meet the code requirements for

ventilation rates.

Once the criteria have been determined, one of the following methods of

control can be used

1. Minimum Outside Air

The simplest method of outdoor air control is to open a minimum

outside air damper whenever the supply fan is running.

2. Economy Cycle Outside Air

There many times when it is necessary to operate the cooling coil even

when outdoor air temperatures are near or below the freezing mark, with

outside and return dampers controlled by air temperature.

a. When the outside air at design winter temperature

Outside air dampers are in minimum open position

Return air dampers are in maximum open position

b. When outside air temperature increases

The mixed air thermostat (T1) gradually opens the outside air

damper to maintain constant mixed air temperature.

Return dampers modulate correspondingly

c. When outside air temperature become moderate

100* outside air will be provided and used for

cooling.

d. As outside air temperature continues to increase

Outdoor air high-limit thermostat(T2) cut the system back to

minimum outside air, decreasing cooling load

An interlock from the supply fan to close the outside air damper

when the fan is off to keep out dirt and unwanted things from

entering the system.

3. Enthalpy Control

The outside air economy cycle control based on dry bulb temperatures is

not always the most economical. That is, in very humid climates the

total heat (Enthalpy) of the outside air may be greater than of the return

air even though the dry bulb temperature is lower

Since the cooling coil must remove the total heat from the air to

maintain the desired condition, it is more economical in this case to hold

outside air to a minimum.

4. Static pressure Control

For those spaces requiring a constant positive or negative pressure with

respect to their surroundings, the outside and return air dampers will be

controlled by static pressure controllers.

The static pressure controller senses the difference in pressure between the

controlled space and a reference location outdoors, and adjusts the dampers

to maintain that pressure differential.

5. Air Stratification

Stratification of return air and outside air streams in mixing plenums can be a

serious problem. In a worst condition case, two air streams will not mix and

remain separate for a long distance through filters and coils. If the outside air

temperature is below freezing, the separate air stream can cause localized

freezing in heating and cooling coils. Or if a good low temperature safety

control is provided, the HVAC system will cycle OFF and ON.

The mixing plenum and its dampers should therefore be designed to promote

good mixing of the air streams.

Some of these methods are:

a. Parallel Blade Dampers - to have the air streams meet head on

b. Opposed Blade Dampers - the air streams enter opposite sides of

the mixing plenum.

c. Dynamic mixing - propeller fan is added, set at right angles of the

air streams, to provide mechanical mixing

B. Heating Coil Control:

Heating may be done as:

Preheat of outside or mixed air

Normal heating

Reheat for humidity control

1. Preheat

Preheating is used with large percentages of outside air to prevent freezing

of downstream heating and cooling coils and to provide a usable mixed air

temperature.

a. The simplest control Is a two-position valve in the steam or hot water

supply, with an outdoor thermostat which opens the valve whenever

the outdoor temperature is below 35-40o F.

b. Face and bypass dampers are added at the coil and controlled by

means of a downstream thermostat (T2) to provide a usable mixture

temperature. The difficulty here is stratification of the two air streams.

c. In many cases a sufficient distance for mixing is not available. The best

solution in this case is to use hot water with recirculating pump. Now

there can be full flow through the coil at all times with the temperature

of the water varied to suit the requirements. No air is bypassed, so

there are no mixing problems. Very accurate control of air

temperature is possible. Notice the opposed flow arrangement, with

the hot water supply entering the air leaving side of the coil. When

dealing with freezing air certain precautions are necessary. For hot

water, it has been shown experimentally that water velocities of 2.5- 3

ft./sec (0.9 m/s) in the coil tubes are sufficient to prevent freezing at

outdoor temperatures down to -30oF.

2. Normal Heating

Normal heating refers to the coil in a single-zone, multi zone or dual-duct air

system which handles all or a major portion of the system air at entering

temperature of (45o to 50o F) or higher.

3. Reheat

is used mainly with humidity control or individual zone control. In either

case, control of steam or hot water supply valves is usually done by room

thermostat.

C. cooling Coil Control:

There are two types of cooling coils:

Direct Expansion (DX) coils.

Chilled water coils.

1. Direct Expansion coils:

DX coils must by their nature use two-position control with its wide operating

differential. This system is often used in small units.

a. The room thermostat opens the solenoid valve, allowing refrigerant

liquid to flow through the expansion valve to the coil. The expansion

valve modulates to maintain a minimum refrigerant suction

temperature. A low limit discharge thermostat (T2) keeps the supply air

temperature from becoming too cold.

b. Controllability can be improved by providing face and bypass

dampers, but this may lead to lack of humidity control and coil icing

at high bypass rates.

c. A different approach adds a variable back pressure valve in the

refrigerant suction line, controlled by room thermostat. As the room

thermostat decreases, the valve is throttled, increasing the suction

temperature at the coil and decreasing the coil capacity.

d. Hot gas bypass may also be used for capacity control. There are

limitations on the percentage of total refrigeration flow which may be

bypassed, and on pressure drops in the piping system.

e. Two-Stage direct expansion will often provide adequate capacity

control. The stages should be made by rows of the coil rather than by

sectioning the coil.

In sectioning the active section may ice up forcing most of the

air flow through the inactive section and reducing the coil

capacity.

In multi-row coil the first stage should be first row in the

direction of air flow, and second stage the rest of the rows A

two stage thermostat is used

2. Chilled Water Coils:

Chilled water coils are controlled by

Three-way valve (Two-position or Modulating).

Recirculating pump.

Recirculating pump is used in two cases:

For extremely accurate temperature control.

To avoid coil freezing.

Parallel and Counter flow:

Consider the cooling coil with air flowing through it, decreasing in

temperature from 80oto 55oF, and with water flowing parallel or

counter to the air, and increasing in temperature from 42oto 50oF.

The heat transfer from air to water in the coil is a function of

Tube wall

Air and water films inside the tube wall.

Total internal tube surface area.

MED (Mean Equivalent Temperature Difference).

where:

CTD = greatest temperature difference between water and air.

LTD = least temperature difference between water and air.

MED = mean equivalent temperature difference.

ln = natural log (log to base).

If we calculate the MED for each of the two flow

arrangements, we get:

1. Parallel flow:

GTD = 80 - 42 = 38

LTD = 55 - 52 = 3

𝑴𝑬𝑫 = 𝟑𝟖−𝟑

𝐥𝐧𝟑𝟖

𝟑

= 13.3

2. Counter flow:

GTD = 80 - 52 = 28

LTD = 55 - 42 = 13

𝑴𝑬𝑫 = 𝟐𝟖−𝟏𝟑

𝐥𝐧𝟐𝟖

𝟏𝟑

= 18.7

This is an increase of nearly 1/3 in the heat transfer capacity. This

should allow water temperature of 47oF in and 57oF out, and still get

the design condition (55oF leaving air), which is impossible with

parallel flow.

The higher water temperature, increases chiller efficiency and

capacity.

D. Humidity Control:

Sometimes it may be necessary to raise or lower the humidity of

the supply air in order to maintain selected humidity conditions

in the air conditioned space.

1. Air Washer Humidifier:

Often used for its sensible cooling capability, it is then known as

an evaporative loader. The cooling is accomplished by using the

sensible heat of the air to evaporate water. Thus the air passing

through the washer changes conditions along a constant wet

bulb line, with the final state being dependent on the initial

state and the saturation efficiency of the washer. The is no

control of humidity.

2. Steam Humidifier:

Steam humidifiers are often used because of their simplicity. A

piping manifold with small orifices is provided in the air duct or

plenum. The steam supply valve is controlled by space or duct

humidistat. A duct high-limit humidistat must be provided to

avoid condensation in the duct.

3. Chemical Dehumidifier:

Chemical dehumidifiers use a chemical adsorbent. One form of

dehumidifiers uses a wheel containing silica gel, which revolves

first through the conditioned air stream, absorbing moisture,

and then through a regenerative air stream of heated outside

air, which dries the gel. In the process, a great deal of heat is

transferred to the conditioned air, and a cooling is necessary.

Space humidistat control the heating coil in the regenerative

(drying) air stream, and room thermostat control the space

temperature.

4. Dehumidifying by Refrigeration:

Low temperature cooling coil is used to reduce humidity to low

values. Special DX coils with wide fin spacing must be used.

A provision must be made for defrosting by:

a. Hot gas

b. Electric heater

c. Warm air

This approach has some limitations:

a. Inefficient at low humidifies.

b. Intermittent shutdown for defrosting.

c. Reheat is necessary.

Since space humidity is largely a function of coil temperature, fairly

good control can be achieved through a humidistat.

A room thermostat controls the cooling coil when the humidistat is

satisfied.

V. Applications:

A. Single Zone:

A space thermostat controls heating and cooling directly.

If used, a humidifier is controlled by the humidistat.

B. multi Zone and dual duct:

The mixing dampers controlled by the zone space

thermostat for each zone. If used, humidifiers are usually

controlled by a return air humidistat.

C. Three-deck multi Zone system:

The zone dampers operate with sequenced damper motor

either with:

1. Mix hot supply air with bypass air when the cold deck

damper is closed.

2. Mix cold supply air with bypass air when the hot deck

damper is closed.

D. Variable air volume system:

Motorized dampers in each zone supply duct. A related

zone space thermostat controls each damper by a (flow

sensor controller), that is reset by the thermostat. If used,

humidifiers are controlled by a return air humidistat.