MECHANICS HVAC

L | C | LOGISTICS

PLANT MANUFACTURING AND BUILDING FACILITIES EQUIPMENT

Engineering-Book

ENGINEERING FUNDAMENTALS AND HOW IT WORKS

September 2014

Expertise in Process Engineering Optimization Solutions & Industrial Engineering Projects Management 

Supply Chain Manufacturing & DC Facilities Logistics Operations Planning Management

Thermal comfort

Conditions/variables that contribute to making space comfortable for its occupants

Human comfort

● Dry-bulb temperature

● Humidity

● Air movement

● Fresh air

● Clean air

● Noise level

● Adequate lighting

● Proper furniture and work surfaces

● ……

Air-conditioning systems: providing thermal comfort

Mechanics HVAC

radiation

hotwater conduction

convection

cool air

warm air

Air-conditioning is a process of heat transfer

● Types of heat Sensible heat results in a change of

dry-bulb temperature. Latent heat is associated with the addition

or removal of moisture, or phase change, for example water converted to steam.

● Quantity & quality of heat Heat energy cannot be destroyed Heat always flows from a higher temperature

substance to a lower temperature substance

● How is heat being transferred? Convection, Conduction, Radiation Working fluid: water and air

100oCsensible heat

latent heat

30oC

100oC100oC

Mechanics HVAC

Air property and Psychometric process

● Parameters of the air dry-bulb / wet-bulb / dew point temperature, relative humidity, etc

● Process of air-conditioning

A

B

A B

dry-bulb temperature

hu

mid

ity ra

tio

26.7� C21.2� Ccomfort zone

Mechanics HVAC

Central HVAC system

● Chiller plant Chillers Cooling tower Pumps

● Water system Water distribution Pipes & accessories

● Air system Air handling units Air distribution Terminal…

● Instrument & controls

● ……

Mechanics HVAC

Type of water chillers

● Chemical absorption Heat source needed -- steam, hot

water, etc.

● Vapor-compression Electrical power needed to drive

the compressors

● Air-cooled or Water-cooled

Mechanics HVAC

Chiller plant schematics

● Condenser water

● Chiller water Primary–Secondary flow system Primary–only flow system

Mechanics HVAC

Chiller plant operation

● Sequencing Turn-on / turn-off to meet the demand

● Parameters to be monitored Temperature, flow rate, capacity, motor current, etc

● System timers: to prevent frequent on/off of equipment Confirm the demand before starting the next unit Staging timer to prevent more units from starting Minimum period required to run a unit prior to turning it off

Too fast response causes:• system unstable, waste energy• unnecessary wear & tear on mechanical components

Mechanics HVAC

Chiller in operation

The compressor compresses the refrigerant gas. Raising its temperature and pressure.

The hot gas dissipates its heat to condenser water, and condenses into liquid at high pressure.

The high-pressure refrigerant liquid flows through the expansion device (valve or orifice); its pressure and temperature are lowered down.

Chilled water flows through the evaporator and cooled down by the cold refrigerant; vaporized refrigerant gas is returned to the compressor again.

Chiller

Mechanics HVAC

Refrigeration cycle

R134a

R123Atmospheric pressure

Temperature

Pre

ssur

eCommon Refrigerants used in chillers R134a, R123, R132, R139

Chillers operate at different pressure with different refrigerant

Mechanics HVAC

Refrigeration cycle

Low-pressure/temp refrigerant absorb heat from chilled water

Compressor & Expansion device: refrigerant heated up/cooled down

High-pressure/temp refrigerant release heat to condenser water

Mechanics HVAC

Compressor

The main function of the compressor is to increase pressure/temperature of refrigerant.

The core components of a centrifugal compressor is the impeller. Rotation of the impeller creates a low pressure at the volute which draws in refrigerant vapor and accelerates it.

The pressure and temperature of refrigerant is increased by centrifugal acceleration.

Mechanics HVAC

Condenser

Ref

riger

ant

Ref

riger

ant

Ref

riger

ant

Ref

riger

ant

CWS/R in the tubes

CHWS/R in the tubes

A shell & tube heat exchanger. Refrigerant is cooled down by condenser water from cooling towers.

Mechanics HVAC

Evaporator

A shell & tube heat-exchanger. Chilled water is cooled down by refrigerant.

Mechanics HVAC

Expansion device

To maintain the pressure difference between Condenser (high pressure) and Evaporator (low pressure).

Pressure drop occurs when high- pressure refrigerant liquid flows through the expansion device. Pressure drop creates a small portion of liquid to flash, and reduces the remaining refrigerant to evaporator temperature.

Mechanics HVAC

Operation log

● Review daily operation log is necessary

● Important parameters

Evaporator and Condenserrefrigerant pressure

CHWS/R and CWS/Rtemperature

Oil pressure and level

Motor current, winding temperature, etc

• Vibration

• ……

Review operation parameters for early

alert of problems

Mechanics HVAC

Maintenance tasks

● Mechanical, electrical and controls Visual inspection, check for leakage Tighten electrical/controls terminals Safety interlocks, etc

● Follow recommendations from the manufactures Change of oil and oil filters Oil analysis Check shaft alignment Replace shaft seal Inspect purge system ……

● Good practices Maintain good quality of water treatment Cleaning of Condenser tubes ……

If applicable

Mechanics HVAC

Static

Pressure

Friction Flow

Velocity

pressure

DischargeSuction

Impeller

Basics

● Pressure, friction and flow

● Water pumps Increase pressure of the water and move it

from one point to another

Centrifugal pumps are commonly used in HVAC systems

● Centrifugal pumps Rotating impeller creates

centrifugal force and a pressure difference across the impeller.

Mechanics HVAC

Pumps, pipes and accessories

● Pump components Impeller, shaft & seal, bearing, casing, etc.

● Piping and accessories Typical installation

Valves, strainer, flexible connection, pressure gauges, thermometers, etc.

Globe valve

Check valve

Pump

Pressure gaugeThermometerFlexible connection Strainer

Gate valve

Thermometer

Pressure gauge

Mechanics HVAC

Single pump

performance curve

System curve

Parallel pump performance curve

Single pump operating point

Parallel pump operating point

Operation and maintenance

● Important parameters Suction & discharge pressure, flow, vibration, bearing temperature, motor KW,

etc

● NPSH (Net Positive Suction Head) and cavitations Suction pressure to be maintained.

● Parallel operation

Mechanics HVAC

Operation and maintenance

● Common breakdown Mechanical seal failure

Excessive vibrations

Pump rubbing or seizure

Inadequate performance (flow rate, head developed, power consumption)

Leaking casing

● Main areas of maintenance Lubrication

Seal replacement

Shaft alignmentLaser alignment

Mechanics HVAC

Basics

● Function of cooling towers When warm water flows through the tower, heat is absorbed by air and the remaining

water is cooled, through evaporation of a small percentage of the total water flow.

● Type of cooling towers Natural draft, mechanical draft Forced draft, induced draft Counter-flow, cross-flow

Release heat from HVAC system to surrounding air

Forced draft, counter flow

Induced draft, counter flow

Induced draft, cross flow

Mechanics HVAC

Components

● Fill

● Structure, frame, casing

● Fan, motor and drive

● Hot water distribution

● Cold water basin

● Make-up water

Mechanics HVAC

Operation and Maintenance

● Air side Motor, shaft, gearbox, fan

● Water side Fill

Hot & cold water basin/nozzle

Make-up water, drain, overflow

Water treatment

• Corrosion, scaling, bacteria

● Structural integrity Structure

casing

Follow O & M checklist

Mechanics HVAC

Fan basics

● Types of fans Centrifugal fan – Forward and Backward curved

Axial fan – positive discharge head

● Fan system components Impeller, motor, drive, ducts, damper, air-

conditioning equipment.

Axial fan

Air

Backward curve

Forward curve

Centrifugal fan

Air Air

AirAir

Mechanics HVAC

Maintenance of fans

● Common problems Belt drive wear, rupture, noisy, etc

Bearing wear, noisy, high temperature

Dirty fan blades

…

● Common maintenance tasks Regular inspection of all components

Bearing maintenance: lubrication, replacement

Belt tightening and replacement

Cleaning

Mechanics HVAC

Air distribution, ducts & accessories

● Equipment

● Ducts Rectangular, Round, flexible VAV box (variable air volume) Dampers

● Terminals Supply air diffuser Return air grills

Damper

Diffusers

Ducts

Mechanics HVAC

Configuration of AHUs

● Functions of AHUs Conditions the air and distributes it to various

spaces.

Fresh air and re-circulation air are often mixed and conditioned.

● Components Casing, Cooling/dehumidification coil, Heating

coil, Humidifier, Filters.

Controls – temperature/humidity, fan speed, interlock, etc.

Fan, motor, drive

Modular design

Typical arrangement

Mechanics HVAC

Operation and Maintenance

● Monitoring operation conditions Supply/return air temperature

Filter condition and pressure-drop

Supply/return water temperature

Strainer pressure-drop

Functioning of controls -- sensors, actuators, interlock, etc

Drain pipes operation

Moving parts – motor, fan, drive

● Maintenance tasks Water side

Air side

Motor/fan/drive

ControlsMaintain good performance of equipment:

1. Review operation log sheets2. Follow maintenance checklist

Mechanics HVAC

30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30

Basic functions of control systems

● To maintain environment conditions in the space: Temperature

Humidity

Air distribution

Indoor air quality

● Components of control systems Sensor

Controller

Actuator + damper/valve, etc

The controller compares the

temperature of the air leaving

coil to the setting, and adjusts

the valve to meet the setting.

Mechanics HVAC

Maintenance of control systems

● Common issues: Damage of electronic components/device

Loosening of damper linkage fastenings

Loosening of control wiring connections

● Planned maintenance: Re-calibration

Functioning test

Control valve not closed…when AHU stopped

Mechanics HVAC

The float valve maintains the constant level of the liquid in the flooded evaporator irrespective of the pressure and the temperature inside it

The float of the low side float valve is placed in the evaporator, which is at low pressure

The construction and the working of the low side refrigeration float valve are similar to the float valve used in the water tank used for maintaining the level of the liquid

In the water tank the float keeps floating inside on the water and its arm is connected to the water connection

When the level of the water drops down the arm of the float valve moves to open the water connection and allows the flow of the water to the tank

When the tank gets filled the float rises up and the arm closes the water connection

The float valve in the refrigeration plant also works in a similar manner

Mechanics HVAC

The valve assembly of the low side float valve comprises of the hollow ball, a float arm, needle valve and the seat

The valve seat and the needle forms the orifice opening of the valve that provides the throttling effect to the refrigerant passing through it and through which the regulated amount of the refrigerant can pass

The valve seat and the needle are located inside the chamber of the float valve which is connected to the evaporator

The hollow ball or float floats on the refrigerant inside the evaporator and moves up and down as per the level of the liquid

The hollow ball is connected to the needle and valve seat via the float arm

Thus as the ball moves up and down the float arm also moves that allows for the opening or the closing of the orifice.

Mechanics HVAC

When the level of the refrigerant drops inside the evaporator due to high refrigeration load the float moves down, this allows for the opening of the orifice of the valve increasing the flow of the refrigerant

When the sufficient amount of refrigerant enters the evaporator the level of the float rises due to which moves the float valve closes

The movement of the float and the opening of the float valve is in accordance to the refrigeration load on the evaporator

High Side Float ValveWhile in the low side float valve the float chamber is placed in the evaporator on low pressure side, in the high pressure side float valve and the float chamber is placed on the high pressure side between the condenser and the evaporator

In low side float valve the valve opens as the level of the refrigerant drops in the evaporator, but in high side float valve the valve opens when the level of the refrigerant increases in the chamber

Mechanics HVAC

The refrigerant condensed in the condenser moves to the chamber of the high pressure float valve

As the level of the refrigerant rises the float ball moves up and opens the float valve that allows for the passage of the refrigerant through needle valve

The level of the refrigerant would rise in float chamber when more refrigerant is coming from the condenser that means there is more load on the plant

Thus when there is higher load on the plant there is increase in the flow of the refrigerant through the float valve

The level of the refrigerant coming from the condenser reduces when there is less load on the plant

When the level of the refrigerant drops down the orifice of the needle valve closes, thus reducing the flow of the refrigerant through it

The high side float valves are usually used with the centrifugal refrigeration plants.

Mechanics HVAC

Capillary tube used as the throttling device in the domestic refrigerators, deep freezers, water coolers and air conditioners

When the refrigerant leaves the condenser and enters the capillary tube its pressure drops down suddenly due to very small diameter of the capillary

In capillary the fall in pressure of the refrigerant takes place not due to the orifice but due to the small opening of the capillary

The decrease in pressure of the refrigerant through the capillary depends on the diameter of the capillary and the length of the capillary

Smaller is the diameter and more is the length of the capillary more is the drop in pressure of the refrigerant as it passes through it

In the normal working conditions of the refrigeration plant there is drop in pressure of the refrigerant across the capillary but when the plant stops the refrigerant pressure across the two sides of the capillary equalize

Mechanics HVAC

Due to this reason when the compressor restarts there won’t be much load on it. Also, due to this reason one cannot over-charge the refrigeration system with the refrigerant and no receiver is used

The capillary tube is non-adjustable device that cannot control the flow of the refrigerant through it as one can do in the automatic throttling valve

Due to this the refrigerant flow through the capillary changes as the surrounding conditions changes. For instance as the condenser pressure increases due to high atmospheric pressure and the evaporator pressure reduces due to lesser refrigeration load the flow of the refrigerant through the capillary changes

Thus the capillary tube is designed for certain ambient conditions. However, if it is selected properly, it can work reasonably well over a wide range of conditions

The length of the capillary of particular diameter required for the refrigeration applications cannot be found by fixed formula rather it is calculated by the empirical calculations

Some approximate length required for certain application is found out and it is then corrected by the experiments

Mechanics HVAC

When the refrigerant leaves the condenser and enters the capillary tube its pressure drops down suddenly due to very small diameter of the capillary

In capillary the fall in pressure of the refrigerant takes place not due to the orifice but due to the small opening of the capillary

The decrease in pressure of the refrigerant through the capillary depends on the diameter of the capillary and the length of the capillary

Smaller is the diameter and more is the length of the capillary more is the drop in pressure of the refrigerant as it passes through it

The capillary tube limits the maximum amount of the refrigerant that can be charged in the refrigeration system due to which the receiver is not required in these systems

When the refrigeration plant stops the pressure across the capillary tube becomes same and also along the whole refrigeration cycle the pressure is constant

This means that when the plant is stopped the pressure at the suction and discharge side of the compressor are same

Mechanics HVAC

Thus when the compressor is restarted there is not much load on it since it does not have to overcome very high pressures

Due to this the compressor motor of smaller torque can be selected for driving the compressor, thus reducing the cost of the compressor

When the fresh refrigerant is charged into the refrigerator or the deep freezers, the capillary of the system should also be changed

This is because when the machine is stopped some oil particles may clog the capillary as the refrigerant leaks to the atmosphere

Mechanics HVAC

Accumulator is a small hollow cylindrical shape vessel made of copper

It is fitted between the evaporator and the compressor of the refrigeration system towards the suction side of the compressor

Sometimes the refrigerant leaving the evaporator carries liquid particles

These particles get separated in the accumulator

The liquid refrigerant collected in the accumulator slowly gets vaporized and is then sucked by the compressor

The accumulator also prevents the flooding of the liquid refrigerant to the compressor when the load on the evaporator drops down drastically

Mechanics HVAC

Simple ones are made from fiberglass or polyester, and allow for the passage of air but trap large particles of dust

These filters are often placed in front of fans because the large particles of dust can accumulate over time and eventually clog up the machinery. These basic filters serve no real protection, but they do protect the machinery from burning out

Some types of filter can be washed and reused, but the majority of them are meant to be thrown away after they become too dirty to use anymore. Then they are simply replaced with a new one

They do nothing against harmful moulds and bacteria. They are equally useless against airborne viruses

Mechanics HVAC

Another form of air filtration system is the needle ionizer and the plate ionizer, also known as Electrostatic Precipitators. These devices use electricity to filter out all of the smoke and dust out of the air

Plate ionizers use electricity to give the incoming air a static chargeThe air then is passed through a series of metal “plates” that contain the opposite charge of the air surrounding it

This causes all of those charged particles to be attracted to the plates, and in this way the air is effectively purified

Needle ionizers use high voltage electricity to create negative electronsThese electrons run up the length of a pointed spike, or needle, where they stream into the air and attract oxygen molecules

At this point, they become negative ions. Negative ions occur naturally in the air we breathe, and they are quite harmless. These negative ions attach themselves to airborne particles

When enough negative ions attach to a particle, it gets too heavy to float in the air and drops to the ground

Mechanics HVAC

An inverter in an air conditioner is used to control the speed of the compressor motor to drive variable refrigerant flow in an air conditioning system to regulate the conditioned-space temperature.

By contrast, traditional air conditioners regulate temperature by using a compressor that is periodically either working at maximum capacity or switched off entirely.

Inverter-equipped air conditioners have a variable-frequency drive that incorporates an adjustable electrical inverter to control the speed of the motor and thus the compressor and cooling output

The variable-frequency drive uses a rectifier to convert the incoming alternating current (AC) to direct current (DC) and then uses pulse-width modulation in an electrical inverter to produce AC of a desired frequency.

The variable frequency AC drives a brushless motor or an induction motor. As the speed of an induction motor is proportional to the frequency of the AC, the compressor can now run at different speeds

A microcontroller can then sample the current ambient air temperature and adjust the speed of the compressor appropriately. Eliminating stop-start cycles increases efficiency

Mechanics HVAC

Variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable-speed drive, AC drive, micro drive or inverter drive) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage. Where process conditions demand adjustment of flow from a pump or fan, varying the speed of the drive may save energy compared with other techniques for flow control.

Mechanics HVAC

An adjustable speed drive can often provide smoother operation compared to an alternative fixed speed mode of operation

For example, When fixed speed pumps are used, the pumps are set to start when the level of the liquid in the reaches some high point and stop when the level has been reduced to a low point

Cycling the pumps on and off results in frequent high surges of electric current to start the motors that results in electromagnetic and thermal stresses in the motors and power control equipment, the pumps and pipes are subjected to mechanical and hydraulic stresses

When adjustable speed drives are used, the pumps operate continuously at a speed that increases as the level increases

This matches the outflow to the average inflow and provides a much smoother operation of the process

Mechanics HVAC

The variable frequency drive controls the speed of compressor motor

The compressor is specifically designed to run at different motor speeds to modulate cooling output

Variable speed operation requires an appropriate compressor for full speed operation and a special compressor lubrication system

Proper oil management is a critical requirement to ensure compressor lifetime

Proper oil management provides proper lubrication for scroll set at low speed and prevents excess oil from being injected into the circuit when operating at full speed

Inverter compressor: uses an external variable frequency drive - to control the speed of compressor. The refrigerant flow rate is changed by the change in the speed of compressor.

The turndown ratio depends on the system configuration and manufacturer. It modulates from 15 or 25% up to 100% at full capacity with a single inverter from 12 to 100% with a hybrid tandem

Mechanics HVAC

Importance of the inverter drive: The compressor and drive need to be qualified to work together and for dedicated applications

The drive modulates the compressor speed and prevents it from operating out of the compressor operating limits

The inverter frequency drives need to use algorithms developed specifically for heating, ventilation and air conditioning (HVAC) or for refrigeration

They ensure that the system will run within the application constraints

The drive can also manage other devices such as oil injection valves or multiple compressors.

As the compressor rotational speed changes, the amount of refrigerant — and oil — flowing through the compressor increases or decreases

The drive ensures that the compressor and bearings are optimally lubricated at all compressor speeds

Mechanics HVAC

AC MotorThe AC electric motor used in a VFD system is a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred, and are the most economical motor choice

ControllerThe VFD controller is a solid state power electronics conversion system consisting of three distinct sub-systems: a rectifier bridge converter, a direct current (DC) link, and an inverter.

Voltage-source inverter (VSI) drives are by far the most common type of drives. Most drives are AC-AC drives in that they convert AC line input to AC inverter output.

The most basic rectifier converter for the VSI drive is configured as a three-phase, six-pulse, full-wave diode bridge. In a VSI drive, the DC link consists of a capacitor which smoothes out the converter's DC output ripple and provides a stiff input to the inverter. This filtered DC voltage is converted to quasi-sinusoidal AC voltage output using the inverter's active switching elements. VSI drives provide higher power factor and lower harmonic distortion than phase-controlled current-source inverter (CSI) and load-commutated inverter (LCI) drives (see 'Generic topologies' sub-section below).

The drive controller can also be configured as a phase converter having single-phase converter input and three-phase inverter output

Mechanics HVAC

VRF / VRV technology—basic principle:

Air conditioning removes heat from the space to be cooled by pushing refrigerant through a cycle

The cycle comprises four elements common to all HVAC systems, based on the fluid dynamics

When a refrigerant expands, it becomes cooler; When it is compressed, it becomes warmer; Changing phases from fluid to gas or back again adds to the cooling/warming effect

The system is composed of a compressor, a condensing unit, a metering device (or expansion valve), and an evaporator or heat sink.

Mechanics HVAC

Window units, pack all the elements of the cycle into one small devicethe hot side being on the outside the cool part facing the space to be cooled.

Split-system units split the hot side of the cycle (placed outside the building) from the cold side (placed inside the building)

Cool air is often transferred from the evaporator to many different rooms by an air-handling unit, which distributes the conditioned air through a series of ducts

Mechanics HVAC

Direct expansion (DX) system

The “hot” part of the cycle starts at the compressor, which compresses refrigerant vapor and turns it into a high-temperature gas

The refrigerant then goes through a condensing unit, a series of coils in which the gas loses heat and becomes liquid

The “cold” part of the cycle begins as the liquid refrigerant passes through the metering device, which causes a drop in pressure

The refrigerant then goes through the evaporator (another series of coils)

In the process of evaporating it absorbs heat from the surrounding area

Producing a cooling effect that is dissipated through fans

After completing the cycle, the refrigerant goes back to the compressor in its initial low-pressure, gaseous state

Mechanics HVAC

Industry standards set limits on the length of piping running between the condenser and the evaporator in DX systems.

When the needs of a particular project exceed such limits, chilled water systems are often used as an alternative

In chilled water systems water is cooled by a regular refrigeration system and then circulated through ducts to air handlers throughout the building

Because there is no limit to the permitted length of water pipes, these systems are often used to cool large buildings or entire campuses

Chilling is often cycled at night to take advantage of off-peak energy rates

Mechanics HVAC

DX systems configurations vary among the types of air-conditioning systems available, but always one condensing unit to one evaporator

once a condensing unit is connected to an evaporator inside the building,

providing cool air to several spaces requires either ductwork or additional condensing units and evaporators

Not so with VRF systems, in which one condensing unit can be connected to multiple evaporators, each individually controllable by its user

Similar to the more conventional ductless multi-split systems, which can also connect one outdoor section to several evaporators

Mechanics HVAC

Multi-split systems, like DX systems, turn on and off depending on whether the room to be cooled is too warm or not warm enough

VRF systems are different VRF systems constantly modulate the amount of refrigerant being sent to each evaporator

By operating at varying speeds, VRF units work only at the needed rate, which is how they consume less energy than on/off systems, even if they run more frequently

Less likely candidates to benefit from VRF technology are large open volumes, such as gyms, theaters, or sanctuaries

These building types often fail to maximize the potential of the system, which is ideal for areas with different zones

Mechanics HVAC

VRF systems offer an energy-efficient solution that provides considerable flexibility

But, as with any other HVAC system, their cost-effectiveness and usefulness need to be evaluated on a building-by-building basis

VRF systems are a good option for buildings with varying loads and different zone structures such as hotels, schools, and office buildings where individual users want to have control over the temperature in their areas.

VRF systems tend to have greater piping length allowances than DX systems and use copper piping with small diameters, which makes them suitable for buildings with low-ceiling spaces

Mechanics HVAC

VRF systems with one evaporator in every single room may be more costly initially, but the installation might require less ductwork

In a different arrangement, several spaces might share a nearby evaporator

The smaller footprint of VRF equipment can also reduce costs; the system eliminates the need to have mechanical rooms, so useable space is given back to the client

Concerning outside air ventilation, can turn into a hurdle, because VRF units may require a separate ventilation system

Especially in hot and humid climates or when dealing with high occupancy areas.

Major manufacturers do generally offer outside air processing solutions that can be tied into the same control systems used for VRF units

Mechanics HVAC

A fan coil unit (FCU) is a simple device consisting of a heating or cooling coil and fan.

Typically a fan coil unit is not connected to ductwork, and is used to control the temperature in the space where it is installed, or serve multiple spaces.

It is controlled either by a manual on/off switch or by thermostat.

Due to their simplicity, fan coil units are more economical to install than ducted or central heating systems with air handling units.

However, they can be noisy because the fan is within the same space.

A concealed fan coil unit will typically be installed within an accessible ceiling void or services zone. The return air grille and supply air diffuser, typically set flush into the ceiling, will be ducted to and from the fan coil unit and thus allows a great degree of flexibility for locating the grilles to suit the ceiling layout and/or the partition layout within a space.

It is quite common for the return air not to be ducted and to use the ceiling void as a return air plenum.

Mechanics HVAC

The centrifugal fan has a moving component (called an impeller) that consists of a central shaft about which a set of blades, or ribs, are positioned

Centrifugal fans blow air at right angles to the intake of the fan, and spin the air outwards to the outlet (by deflection and centrifugal force)

The impeller rotates, causing air to enter the fan near the shaft and move perpendicularly from the shaft to the opening in the scroll-shaped fan casing

A centrifugal fan produces more pressure for a given air volume

One phenomenon particular to the cross-flow fan is that, as the blades rotate, the local air incidence angle changes

The result is that in certain positions the blades act as compressors (pressure increase), while at other azimuthal locations the blades act as turbines (pressure decrease)

Mechanics HVAC

The cross-flow or tangential fan, is usually long in relation to the diameter, so the flow approximately remains two-dimensional away from the ends

The CFF uses an impeller with forward curved blades, placed in a housing consisting of a rear wall and vortex wall. Unlike radial machines, the main flow moves transversely across the impeller, passing the blading twice

The flow within a cross-flow fan may be broken up into three distinct regions: a vortex region near the fan discharge, called an eccentric vortex, the through-flow region, and a paddling region directly opposite

Both the vortex and paddling regions are dissipative, and as a result, only a portion of the impeller imparts usable work on the flow. The cross-flow fan, or transverse fan, is thus a two-stage partial admission machine.

The popularity of the cross-flow fan in the HVAC industry comes from its compactness, shape, quiet operation, and ability to provide high pressure coefficient.

Effectively a rectangular fan in terms of inlet and outlet geometry, the diameter readily scales to fit the available space, and the length is adjustable to meet flow rate requirements for the particular application

Mechanics HVAC

Valves measure and control flowThey operate either with linear force or with torque, which is rotaryLinear valve types (operating with force) include:

Gate valves, Knife Gate valves, Globe valves, Diaphragm Valves, Pinch valves, Slide Gate valves and Rising Stem Ball valves

Rotary valve types (operating with torque) include: Ball valves, Plug valves and Butterfly valves

Actuators provide the muscle that creates the force or torque to operate the valves, by opening, closing or modulating

Without the actuator, the valve is useless

Mechanics HVAC

What is an Air-Cooled Chiller?

Air-cooled chillers are refrigeration devices of a sort

They utilize a process of evaporation and condensation within a closed system to chill the surrounding air

Typically, such devices are used for large industrial purposes, as they are more energy efficient than traditional freon-powered refrigerators

It is a common misconception that air-cooled chillers do not use water.

What the name actually means is that no water is used to absorb waste heat from the chiller's closed system

Mechanics HVAC

Structure

An air-cooled chiller is a closed system

It starts with a device called an evaporator

It has a shell of tubes surrounding a central chamber

The tubes surround whatever item or material is meant to be cooled by the chiller

The central chamber of the evaporator then connects with a compressor

The compressor connects with a condenser, which then connects back to the evaporator

Mechanics HVAC

How Do Air-Cooled Chillers Work?

The process starts with the evaporator, which contains a liquid refrigerantThe refrigerant radiates out cold to the surrounding tubes filled with waterThe water is chilled and pumped through a circuit, absorbing heat from whatever items the chiller is meant to cool

When the water has finally reached a high enough temperature, it radiates the heat back at the refrigerant in the evaporator, causing it to turn into vaporThe vapor passes through a pipe into the compressor, which, compresses the vapor into a smaller space, putting it under high pressure and heat

This superheated vaporized refrigerant is then pumped through a condenserThe condenser is a series of air-cooled vanes, similar to those in a car's radiator The vapor gives off its heat into the air and then condenses back into a liquidThe liquid flows back into the evaporator to repeat the chilling process

Mechanics HVAC

OperationChillers send heat from air to water by circulating chilled water to air handlers, or devices used to condition and circulate air during the HVAC process

This water is sent back to the evaporator side of the chiller Heat then passes from the water to freon, a liquid refrigerant

Freon exits the evaporator in the form of a cold vaporThis vapor enters the compressor in the chiller and is pressurized, or compressed, converting the cold vapor to a heated vapor

As the heated vapor enters the chiller's condenser side, heat is transferred from the freon and is passed to a cooling tower, where it is removed through evaporation

Mechanics HVAC

Efficiency

•Chiller efficiency refers to the electricity amount needed to generate one cooling ton •Tons are measured in kilowatts per ton (kw/ton)•When commissioned, chillers are designated with a specific kw/ton efficiency rating

Applications

•Chillers are utilized in such applications as mechanical maintenance and water chemistry• Chillers are often used to determine mechanical lubrication levels, liquid refrigerant levels, and meter and gauge calibrations

Mechanics HVAC

.

Air Cooled Vs. Water Cooled Chillers | Centrifugal Chiller Compressors

Conventional thinking has been that water cooled chillers are more efficient than air cooled chillers

If we only look at compressor costs, this may be true

It is important to look at the total operating costs involved with the chillers, not just the compressor costs.

Cooling tower operating costs should be added. It includes the tower fan, water and sewer costs, chemical costs and pumping costs; process pumps and recirculation pumps

However, using state-of-the-art technology with centrifugal compressors and variable speed control, air-cooled chillers are often the better choice

Mechanics HVAC

The illustration compares the operating cost of an air cooled variable speed centrifugal chiller versus a water cooled screw chiller

This comparison is based upon a 140 ton load, $.07/kwh electrical costs, $5.00/1000 gallon water and sewer costs and 6,000 hrs/year operation.

Mechanics HVAC

A heat pump is a device that provides heat energy from a source of heat to a destination called a "heat sink"

While air conditioners and freezers are familiar examples of heat pumps, the term "heat pump" is more general and applies to many HVAC devices used for space heating or space cooling

When a heat pump is used for heating, it employs the same basic refrigeration-type cycle used by an air conditioner or a refrigerator, but in the opposite direction - releasing heat into the air-conditioned space rather than the surrounding environment

In this use, heat pumps generally draw heat from the cooler external air or from the ground

Operating principlesMechanical heat pumps exploit the physical properties of a volatile evaporating and condensing fluid known as a refrigerant. The heat pump compresses the refrigerant to make it hotter on the side to be warmed, and releases the pressure at the side where heat is absorbed.

A simple stylized diagram of a heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor

Mechanics HVAC

The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, now hot and highly pressurized vapor is cooled in a heat exchanger, called a condenser, until it condenses into a high pressure, moderate temperature liquid

The condensed refrigerant then passes through a pressure-lowering device also called a metering device. This may be an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine

The low pressure liquid refrigerant then enters another heat exchanger, the evaporator, in which the fluid absorbs heat and boils. The refrigerant then returns to the compressor and the cycle is repeated

Insulation is used to reduce the work and energy required to achieve a low enough temperature in the space to be cooled

Mechanics HVAC

It is essential that the refrigerant reaches a sufficiently high temperature, when compressed, to release heat through the "hot" heat exchanger (the condenser).

Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or else heat cannot flow from the ambient cold region into the fluid in the cold heat exchanger (the evaporator).

In particular, the pressure difference must be great enough for the fluid to condense at the hot side and still evaporate in the lower pressure region at the cold side.

The greater the temperature difference, the greater the required pressure difference, and consequently the more energy needed to compress the fluid.

Thus, as with all heat pumps, the coefficient of performance (amount of thermal energy moved per unit of input work required) decreases with increasing temperature difference.

Mechanics HVAC

Heat transport

Heat is typically transported through engineered heating or cooling systems by using a flowing gas or liquid

Air is sometimes used, but quickly becomes impractical under many circumstances because it requires large ducts to transfer relatively small amounts of heat

In systems using refrigerant, this working fluid can also be used to transport heat a considerable distance, though this can become impractical because of increased risk of expensive refrigerant leakage

When large amounts of heat are to be transported, water is typically used, often supplemented with antifreeze, corrosion inhibitors, and other additives

Mechanics HVAC

RefrigerantsR-12 (dichlorodifluoromethane) replacement refrigerant is the hydro fluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane)

Heat pumps using R-134a are not as efficient as those using R-12 and therefore, more energy is required to operate. Other substances such as liquid R-717 ammonia are widely used in large-scale systems, or occasionally the less corrosive but more flammable propane or butane, can also be used.

Since 2001, carbon dioxide, R-744, has been used, utilizing the transcritical cycle, although it requires much higher working pressures.

Mechanics HVAC

Mechanics HVAC

In residential and commercial applications, the hydro chlorofluorocarbon (HCFC) R-22 is still widely used, however, HFC R-410A does not deplete the ozone layer and is being used more frequently.

Hydrogen, helium, nitrogen, or plain air is used in the Stirling cycle, providing the maximum number of options in environmentally friendly gases.

More recent refrigerators use R600A which is isobutane, and does not deplete the ozone and is friendly to the environmentDimethyl ether (DME) is also gaining popularity as a refrigerant

One observation is that while current "best practice" heat pumps (ground source system, operating between 0 °C and 35 °C) have a typical COP around 4, no better than 5, the maximum achievable is 8.8 because of fundamental Carnot cycle limits

This means that in the coming decades, the energy efficiency of top-end heat pumps could at least double.

Cranking up efficiency requires the development of a better gas compressor, fitting HVAC machines with larger heat exchangers with slower gas flows, and solving internal lubrication problems resulting from slower gas flow.

Mechanics HVAC

Depending on the working fluid, the expansion stage can be important also.

Work done by the expanding fluid cools it and is available to replace some of the input power. (An evaporating liquid is cooled by free expansion through a small hole, but an ideal gas is not.)

SEER Seasonal energy efficiency ratio SEER = EER ÷ 0.9 SEER = BTU ÷ (W·h)coefficient of performance (COP) SEER = COP × 3.792 SEER = (BTU / h) ÷ WEnergy Efficiency Ratio (EER) EER = COP × 3.413

where "W" is the average electrical power in Watts, and (BTU/h) is the rated cooling power

For example, a 5000 BTU/h air-conditioning unit, with a SEER of 10, would consume 5000/10 = 500 Watts of power on average

The electrical energy consumed per year can be calculated as the average power multiplied by the annual operating time: 500 W × 1000 h = 500,000 W·h = 500 kWh

Assuming 1000 hours of operation during a typical cooling season (i.e., 8 hours per day for 125 days per year). 5000 BTU/h × 1000 h = 5,000,000 BTU

Then, for a SEER of 10, the annual electrical energy usage would be:5,000,000 BTU ÷ 10 = 500,000 W·h = 500 kWh

Mechanics HVAC

Mechanics HVAC

Regular AHU maintenance system should include the following:

•Coil Cleaning•Coil Treatment•Filtration Maintenance •Damper Maintenance•System Fan, Bearing & Belt Inspection / Maintenance•Air Intake Inspections•Cabinet & Supply Duct Inspection

While there are different methods for coil cleaning, high pressure power washing is the most effective way to handle condenser and evaporator coil cleaning

Damp, moist and humid areas present an ideal environment for mold growth.

This type of microbial growth on the HVAC coils poses a cleaning challenge due to the fact that this material is quite sticky, creating a bio-film that  seemingly ‘cements' particulate matter to the growing microorganism.

Mechanics HVAC

For proper AHU maintenance service when coil cleaning, it is essential that an effective cleaning solution is used and that it is allowed ample time to do its job on the coils before being rinsed off.

This includes not just the evaporator & condenser coils, but anywhere microbial growth may be growing on related metal surfaces

Use of antimicrobial treatments can be an effective way to curtail mold/bacterial growth by disrupting the mold spore reproductive cycle

One suggestion is that after a thorough coil cleaning the entire air handling unit should be treated

This will help ensure all metal surfaces stay free of mold/bacterial growth until the next scheduled cleaning.

Mechanics HVAC

Basic system maintenance is ensuring your AHU systems filter is replaced regularly, depending on the dirt/particulate load. This may mean every month for very heavy loads, or less frequently, perhaps once every six months

It's not just dust and dirt that filters capture. Anything airborne including small insects, organic fibers, etc. can be caught in filters. Keep in mind that these various particulates, especially if they are organic, can be a source for mold growth and mold spores, depending on the type of filter used

Also ensure your system's filter has the right capacity. The type of filter can impact the indoor air quality as well, with certain filter fabrics like cotton or other synthetics which help boost filtration and particulate matter capture.

This increases the overall filtration efficiency and ultimately your system's effectiveness

When dampers aren't cleaned and well lubricated, they get sticky & stuck. When this happens the HVAC unit can overload the cooling coil with too much hot outside air, or rob the unit of free cooling potential if the dampers are stuck in the closed position

Mechanics HVAC

Inspecting the AHU unit fan, bearings and belts for dirt and dust buildup, along with regular cleaning of the fan and blades will help keep your commercial AHU air duct cleaning system's efficiency and air flow operating at optimal levels

Conventional greased ball bearings should include checking for under or over-greasing, which can be damaging

proper belt alignment (helps avoid lateral wear)proper belt tension (avoiding rapid wear and torque loss due to belt slippage on the pulley wheels)

In addition, look for belts that are overly tight. This can lead to bearing and/or belt early failure due to the increased belt tension which puts an excessive load on the motor and fan shaft bearings

Inspecting the Cabinet & Supply Duct look for an air leaks in the cabinet and supply duct

Mold spores can be sucked into the commercial ventilation system. This can lead to further system contamination and indoor air quality issues

Mechanics HVAC

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another

The media may be separated by a solid wall to prevent mixing or they may be in direct contact

They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment

The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air

Mechanics HVAC

Countercurrent flow - almost full transfer. In countercurrent flow, the two flows move in opposite directions. Two tubes have a liquid flowing in opposite directions, transferring a property from one tube to the other

For example this could be transferring heat from a hot flow of liquid to a cold one

The counter-current exchange system can maintain a nearly constant gradient between the two flows over their entire length of contact

With a sufficiently long length and a sufficiently low flow rate this can result in almost all of the property transferred

So, for example, in the case of heat exchange, the exiting liquid will be almost as hot or cold as the original incoming liquid's heat / cold

Mechanics HVAC

For maximum heat transfer, the average specific heat capacity and the mass flow rate must be the same for each stream

If the two flows are not equal, for example if heat is being transferred from water to air or vice-versa, then, similar to concurrent exchange systems, a variation in the gradient is expected because of a buildup of the property not being transferred properly

Heat capacity, or thermal capacity, is a measurable physical quantity; it's the ratio of the heat added or subtracted to an object to the resulting temperature change. The SI unit of heat capacity is joule per Kelvin,

and the dimensional form is M1L2T−2Θ−1

Volumetric heat capacity (VHC), describes the ability of a given volume of a substance to store internal energy while undergoing a given temperature change, but without undergoing a phase transition

VHC is a 'per unit volume' measure of the relationship between thermal energy and temperature of a material, while the specific heat is a 'per unit mass' measure

If given a specific heat value of a substance, one can convert it to the VHC by multiplying the specific heat by the density of the substance

Mechanics HVAC

Heat transfer occurs at a higher rate across materials of high thermal conductivity than across materials of low thermal conductivity

Correspondingly materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation

Thermal conductivity of materials is temperature dependent. The reciprocal of thermal conductivity is called thermal resistivity

Ceramic coatings with low thermal conductivities are used on exhaust systems to prevent heat from reaching sensitive components

Air and other gases are generally good insulators, in the absence of convection. Many insulating materials function by having a large number of gas-filled pockets which prevent large-scale convection

An object's heat capacity (symbol C) is defined as the ratio of the amount of heat energy transferred to an object and the resulting increase in temperature of the object

Mechanics HVAC

Heat Capacity Ratio for various gases[1][2]

Temp. Gas γ

 

Temp. Gas γ

 

Temp. Gas γ

−181°C

H2

1.597 200°C

Dry Air

1.398 20°C NO 1.400

−76°C 1.453 400°C 1.393 20°C N2O 1.310

20°C 1.410 1000°C 1.365 −181°CN2

1.470

100°C 1.404 2000°C 1.088 15°C 1.404

400°C 1.387 0°C

CO2

1.310 20°C Cl2 1.340

1000°C 1.358 20°C 1.300 −115°C

CH4

1.410

2000°C 1.318 100°C 1.281 −74°C 1.350

20°C He 1.660 400°C 1.235 20°C 1.320

20°C

H2O

1.330 1000°C 1.195 15°C NH3 1.310

100°C 1.324 20°C CO 1.400 19°C Ne 1.640

200°C 1.310 −181°C

O2

1.450 19°C Xe 1.660

−180°CAr

1.760 −76°C 1.415 19°C Kr 1.680

20°C 1.670 20°C 1.400 15°C SO2 1.290

0°C

Dry Air

1.403 100°C 1.399 360°C Hg 1.670

20°C 1.400 200°C 1.397 15°C C2H6 1.220

100°C 1.401 400°C 1.394 16°C C3H8 1.130

Carnot's heat engine

In this diagram, abcd is a cylindrical vessel, cd is a movable piston, and A and B are constant–temperature bodies

The vessel may be placed in contact with either body or removed from both (as it is here)

Mechanics HVAC

is the work done by the system (energy exiting the system as work), is the heat put into the system (heat energy entering the system), is the absolute temperature of the cold reservoir, and is the absolute temperature of the hot reservoir.

Carnot's theorem is a formal statement of this fact: No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs

Carnot realized that in reality it is not possible to build a thermodynamically reversible engine, so real heat engines are less efficient than indicated by Equation (1)

Although Carnot's cycle is an idealization, the expression of Carnot efficiency is still useful. Consider the average temperatures,

Mechanics HVAC

Different measurements of heat capacity can therefore be performed, most commonly either at constant pressure or at constant volume

Measurements under constant pressure produce larger values than those at constant volume because the constant pressure values also include heat energy that is used to do work to expand the substance against the constant pressure as its temperature increases

This difference is particularly notable in gases where values under constant pressure are typically 30% to 66.7% greater than those at constant volume

Hence the heat capacity ratio of gases is typically between 1.3 and 1.67

Mechanics HVAC

The piston is locked

The pressure inside is equal to atmospheric pressure

This cylinder is heated to a certain target temperature

Since the piston cannot move, the volume is constant

The temperature and pressure will rise

When the target temperature is reached, the heating is stopped

The amount of energy added equals: , with representing the change in temperature

The piston is now freed and moves outwards, stopping as the pressure inside the chamber equilibrates to atmospheric pressure

To understand this relation, consider the following thought experiment.

A closed pneumatic cylinder contains air

Mechanics HVAC

We are free to assume the expansion happens fast enough to occur without exchange of heat (adiabatic expansion)

Doing this work, air inside the cylinder will cool to below the target temperature

To return to the target temperature (still with a free piston), the air must be heated

This extra heat amounts to about 40% more than the previous amount added

In this example, the amount of heat added with a locked piston is proportional to

whereas the total amount of heat added is proportional to .

Therefore, the heat capacity ratio in this example is 1.4

Mechanics HVAC

Another way of understanding the difference between and is that applies if work is done to the system which causes a change in volume

(e.g. by moving a piston so as to compress the contents of a cylinder),

or if work is done by the system which changes its temperature

(e.g. heating the gas in a cylinder to cause a piston to move)

Mechanics HVAC

There are three primary classifications of heat exchangers according to their flow arrangement

In counter-flow heat exchangers the fluids enter the exchanger from opposite ends

The counter current design is the most efficient, in that it can transfer the most heat from the heat (transfer) medium due to the fact that the average temperature difference along any unit length is greater

In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger

For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger

The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence

Mechanics HVAC

The plate heat exchanger, is composed of multiple, thin, slightly separated plates that have very large surface areas and fluid flow passages for heat transfer

This stacked-plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger

Online monitoring of commercial heat exchangers is done by tracking the overall heat transfer coefficient. The overall heat transfer coefficient tends to decline over time due to fouling

U=Q/AΔTlm

By periodically calculating the overall heat transfer coefficient from exchanger flow rates and temperatures, can estimate when cleaning the heat exchanger is economically attractive

Mechanics HVAC

Temperature Glycol -45°C (-50°F) to 121°C (250°

°C °F Vol% -7 20 12

-12 10 20

-18 0 24

-23 -10 28

-29 -20 30

-34 -30 33

<-40 <-40 35

Recommended Range

Heat transfer fluids, propylene glycol is mainly used to reduce the freezing point of the liquid, thus preventing the cooling system and the engine from corrosion, overheating and freezing

In burst protection liquids, propylene glycol, with its inherently high boiling point, lowers vapor pressure:

When fluids freeze they expand in volume, which can cause pipes or other containment vessels to rupture

When a water-glycol mixture becomes colder, it retains its flow ability and does not create added pressure in pipes or vessels. This makes it an ideal solution for burst protection in pipe and containment systems

Applications include pipes and tubes

Mechanics HVAC

    Boiling point at atmospheric pressure 14.7 psia, 1 bar abs

  Freezing Point at atmospheric pressure 14.7 psia, 1 bar abs

        

Refrigerant

Name

   No.   (oF) Centigr (oF) CentigrR-22 Monochlorodifluoromethane3) -41.3    (40.72) -256  (160.00)R-134a Tetrafluoroethane6) -15    (26.11) -142    (96.67)

R-404aR125(44%)/R143a(52%)/R134a(4%) -55.4    (48.56)             -   

R-717 Ammonia -28    (33.33) -107.9    (77.72)

Freezing, the process of changing a liquid into a solid by cooling below a certain temperature called the freezing point

The freezing point is the temperature at which the liquid and solid forms of a substance are in equilibrium (balance)

At this point, if heat is neither added nor taken away, the liquid will not change into a solid, nor the solid into a liquid. Thus, the freezing point and the melting point of a substance are the same

A heat exchanger is used for more efficient heat transfer or to dissipate heat

One common example of a heat exchanger is a car's radiator, in which the hot coolant fluid is cooled by the flow of air over the radiator's surface

Mechanics HVAC

About 80 calories of heat are required to convert a gram of ice into water. Conversely, water liberates about 80 calories of heat per gram into the air when it freezes

This heat is called the heat of fusion, or latent heat. The temperature of both the ice and the water from melting ice is 32 F., or 0 C

The freezing point of a substance often determines how it can be used. Ethyl alcohol, for example, freezes at -179 F. (-117 C.), mercury at -38 F. (-39 C.)

A familiar example of changing the freezing point of a liquid by adding another substance is the use of antifreeze. Automobile radiators are commonly protected against freezing during winter by adding ethylene glycol or other antifreezes to the water to lower its freezing point

Most substances contract and become denser upon freezing, but water expands and becomes less dense. It is this expansion that causes pipes and bottles to crack when their contents freeze, and rocks to split open when water freezes in their crevices. Icebergs and blocks of ice float in water because they are less dense than the water from which they were frozen

Mechanics HVAC

The fundamental principle in freeze-drying is sublimation, the shift from a solid directly into a gas. Like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it

Water will sublime from a solid (ice) to a gas (vapor) when the molecules have enough energy to break free but the conditions aren't right for a liquid to form.

You can see from the chart that water can take a liquid form at sea level (where pressure is equal to 1 atm) if the temperature is in between the sea level freezing point (32 degrees Fahrenheit or 0 degrees Celsius) and the sea level boiling point (212 F or 100 C)

But if you increase the temperature above 32 F while keeping the atmospheric pressure below .06 atmospheres (ATM), the water is warm enough to thaw, but there isn't enough pressure for a liquid to form. It becomes a gas

Mechanics HVAC

With most machines, you place the material to be preserved onto the shelves when it is still unfrozen

When you seal the chamber and begin the process, the machine runs the compressors to lower the temperature in the chamber

The material is frozen solid, which separates the water from everything around it, on a molecular level, even though the water is still present

The machine turns on the vacuum pump to force air out of the chamber, lowering the atmospheric pressure below .06 ATM

The heating units apply a small amount of heat to the shelves, causing the ice to change phase. Since the pressure is so low, the ice turns directly into water vapor

The water vapor flows out of the freeze-drying chamber, past the freezing coil. The water vapor condenses onto the freezing coil in solid ice form, in the same way water condenses as frost on a cold day

This continues for many hours (even days) while the material gradually dries out. The process takes so long because overheating the material can significantly change the composition and structure

Mechanics HVAC

1. Basics of HVAC control system

● Function of control system

● Basic control loop

● Sensor

● Controller

● Actuator, valves and dampers

2. Control fundamentals

● Set point, control variable, input/output

● Control modes- Two-position control- Floating control- Modulating control

3. Typical application

● Usual application of subsystems

● Examples:- Discharge temperature control- Static pressure control- Ventilation control- Chilled water system control- Fan pressure optimization

4. O&M of control system

● Common operation procedures and checkout items

● Common maintenance procedures and issues

Mechanics HVAC

Function of control system

An HVAC control system operates the system and equipment (chillers, pumps, fans, AHUs, boilers, etc.) to maintain a comfortable environment, Inclusive of temperature, humidity, pressure and ventilation etc.

Mechanics HVAC

Function of control system

Control loop Classification Description

Ventilation

Basic Coordinates operation of the outdoor, return, and exhaust air dampers to maintain

the proper amount of ventilation air.

Better Measures and controls the volume of outdoor air to provide the proper mix of

outdoor and return air under varying indoor conditions

Cooling

Chiller control Maintains chiller discharge water at preset temperature or resets temperature

according to demand.

Cooling tower control Controls cooling tower fans to provide the coolest water practical under existing wet

bulb temperature conditions.

Coil control Adjusts chilled water flow to maintain temperature.

Direct expansion (DX) control

Cycles compressor or DX coil solenoid valves to maintain temperature. If compressor is unloading type, cylinders are unloaded as required to maintain

temperature.

Fan

Basic Turns on supply and return fans during occupied periods and cycles them as

required during unoccupied periods.

Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces system

operating cost and improves performance (essential for variable air volume systems).

Heating

Coil control Adjusts water or steam flow or electric heat to maintain temperature.

Boiler control Operates burner to maintain proper discharge steam pressure or water

temperature. For maximum efficiency in a hot water system, water temperature should be reset as a function of demand or outdoor temperature.

Mechanics HVAC

Basic control loop

The intent of this control, as an example, is to maintain a desired supply air temperature.

It is called a control loop because information flows in a circle

• The senor measures the temperature and the signal is sent to the controller where the actual value is compared to the setting

• The controller then makes control decision to adjust position of the valve; this then has an effect on the current temperature.

Mechanics HVAC

Basics of HVAC control system

Temperature sensor

Dew-point sensor

Differential pressure sensor

Flow sensor

Sensors

Components & functions

• Sensing element, Transducer, Transmitter

Standard signals of transmission

• 0 ~ 10V, 4 ~ 20mA, Pulse, Free contact

Typical sensors in HVAC application

• Temperature sensor

• Humidity or dew-point sensor

• Pressure/differential pressure sensor

• Velocity/flow sensor

• Others: various gas detectors

Basics of HVAC control system

Controller

Controller provides a signal to the controlled device in response to feed back from the sensor.

Controller could be a hardware device in a simple application or a software function in a large system, for example DDC or PLC/field bus

Examples of controllers

Example of us systemExample of us systemDDC with centralized controller and hardwiringDDC with centralized controller and hardwiring

Basics of HVAC control system

Actuators, valves and dampers

An actuator responds to the output signal from a controller and provides the mechanical action to operate the control device – usually a valve or damper.

Types of actuators• Electrical, Pneumatic, Hydraulic, etc

Characteristics of valves• Equal percentage is suitable for continuous volume

control

Control fundamentals

Set point, Control variable, Input/output

Set point is the desired condition of a variable that is to be maintained, such as room temperature

Control variable is the actual process value being sensed

Input/output signal to/from the controller

• Digital input: fan status (on/off), dirty filter• Digital output: start/stop fan or pump, open/close damper• Analog input: temperature, pressure, airflow• Analog output: control valve or damper position

Control mode

Controllers maintain the process value at the desired set point, through output signal to the controlled device.

The output signal is a function of the difference between the control variable and the set point.

The action that the controller takes is called control mode or control logic, of which there are three basic types:

• Two-position control

• Floating control

• Modulating control

Control fundamentals

Control fundamentals

Two-position (on/off)

Applies to systems that have only 2 states, for example On/Off of equipment, Close/Open of solenoid valves, etc

Balancing between frequent start/stop of equipment and tighter tolerance of the control variables

Control fundamentals

Floating control

Similar to two-position control but not limited to two states. The controller has three modes – Open/Idle/Close

A modulating-type controlled device is needed, typically a valve or damper driven by a slow moving bi-directional actuator

More stable control: smaller variation on process and less cycling…

Control fundamentals

Modulating control

PID (proportional – integral – derivative) algorithm has been widely applied for majority of the industrial processes .

Best tuning of PID parameters depends on a clear understanding of behavior of the process responding to the changes; fortunately there are lots of techniques helpful for selecting appropriate values.

Loop tuning is some what of an art and is usually done empirically by trial-and-error……

Typical applications

Usual application of subsystems

System start/stop control

Air filter section control

Mixed air section control

Cooling/heating coil control

Dehumidification/humidification control

Air distribution control

Fan capacity modulation and static pressure control

Terminal units control

Pumping systems control

Chilled water system control

Boiler plant control

Etc…

Typical applications

Example: discharge temperature control

The controller compares the measured temperature to the set point

The valve is adjusted to change chilled water flow rate through the coil to achieve the set point.

Typical applications

Example: static pressure control

The controller compares the measured static pressure to the set point

Capacity of the supply fan is adjusted to maintain the static pressure and deliver required air volume to the space.

Common methods to modulate fan capacity

• Inlet vanes control• Fan speed control• Discharge damper control• Variable pitch blade control

Typical applications

Example: ventilation control

Fresh air (outdoor air) entering the AHU is monitored and compared to the desired value

The controller compares the difference and adjusts the damper to bring in the proper amount of fresh air.

Acceptable IAQ……

Typical applications

Example: chiller water system control

Control strategy inclusive of sequencing and staging is defined to respond to load change and equipment failure

Heat-load, as an example of load indicator, is calculated on-line based on measured supply/return temperature and flow rate

The controller determines operation of ancillary equipment based on pre-defined philosophy for specific situation.

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Typical applications

Example: fan pressure optimization

Each VAV terminal unit is positioned based on individual demand

Positions of all dampers are monitored, and the control system re-sets its set point so that at least one damper is near fully open

Beneficial to energy efficiency

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O&M of control system

O&M is required for control system

Control systems & devices are made of components with highly complex mechanisms, circuits and/or software

Proper operation is required to assure functioning of the control systems. The devices must be commissioned, programmed, and adjusted to incorporate with M&E equipment

Proper maintenance is required to maintain reliability of the devices to provide operation as designed. Maintenance should be carefully planned and carried out properly

O&M of control system

Operation procedures usually include

Initial set-up of control components in the system

Operational checkout of control system

Functional checkout of control system

Common checkout items for a elect. control system

Power supply• Verify requirement, supply installation, wiring inter-connection, etc

Controller• Verify parameters set-up, calibration, signal transmission, etc

Controlled devices• Verify input/feedback signals, motion linkage, limit switches, valve/damper positioning, etc

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O&M of control system

Maintenance procedures usually include

Routine maintenance• Re-calibration, functioning test, tightening connections, cleaning, etc

Breakdown maintenance• Rectification, repair or replace damaged parts

Common issues

Loosening of Damper Linkage Fastenings

Loosening of Control Wiring Connections

Damaged due to vibration, wear and tear, etc

Content

1. Basic of Refrigeration

2. Servicing valves and gauges

3. Fundamentals of servicing

● Using proper tools and techniques

● Basic rules of refrigerant piping

● Maintaining cleanliness

● Brazing tubes

● De-contamination

● Leak detection

4. Standard operation design data

● Air-cooled packaged units

● Water-cooled packaged units

5. Safety precautions

● Pump down

● Evacuation

● Charging refrigerant

● Compressor replacement

● Condenser cleaning

Definitions of Heat

First of all, did you know that there is no such thing as cold?

You can describe something as cold and everyone will know what you mean, but cold really only means that something contains less heat than something else.

The definition of refrigeration is the removal and relocation of heat.

Definitions of Heat

Let us take an example:

If you have a warm can of drink at say 25°C and you would prefer to drink it at 15°C, you could place it in your fridge for a while, heat would somehow be removed from it, and you could eventually enjoy a less warm drink. But lets say you placed that 15ºC pop in the freezer for a while and when you removed it, it was at 5ºC. Even "cold" objects have heat content that can be reduced to a state of "less heat content".

So if something is to be refrigerated, it is to have heat removed from it.

The limit to this process would be to remove all heat from an object. This would occur if an object was cooled to Absolute Zero which is -460 ºF or -273 ºC. They come close to creating this temperature under laboratory conditions and strange things like electrical superconductivity occur.

Energy

Units

There are many domains where there is the notion of energy but there are also many units to express it. The Si unit is the joules (J) but there several other units:

Electron volt. We use this unit for the energy gained by a single electron –

1 eV = 1.60217646 × 10-19 J

Kilowatt hour. We often find the kilowatt hour in electric utilities –

1 kWh = 3.6 MJ

Calorie. We use this unit for heat, it is the pre-Si unit –

1 cal = 4.182 J

Kilo-calorie or large calorie. As the calorie, it is the pre-Si unit –

1 Cal = 1 kcal = 4182 J

British thermal unit is for steam generation, heating or air conditioning –

1 Btu = 1054.35 J

Defining Heat

Sensible heat

When an object is heated, its temperature rises as heat is added. The increase in heat is called sensible heat.

Similarly, when heat is removed from an object and its temperature falls, the heat removed is also called sensible heat.

Heat that causes a change in temperature in an object is called sensible heat.

Defining Heat

Latent heat

All pure substances in nature are able to change their state. Solids can become liquids (ice to water) and liquids can become gases (water to vapor) but changes such as these require the addition or removal of heat. The heat that causes these changes is called latent heat.

Latent heat however, does not affect the temperature of a substance. The heat added to keep the water boiling is latent heat.

Heat that causes a change of state with no change in temperature is called latent heat.

Dynamics of Heat

Superheat occurs when all the phase changes have been completed and the temperature rises in proportion to the heat energy added.

Phase changes

Transitions between solid, liquid and gas phases typically involve large amounts of energy.

If heat were added at a constant rate to a mass of ice to take it through its phase changes to liquid water, the heat is called the latent heat of fusion.

If heat were added at a constant rate to liquid water to take it through its phase changes to steam, the heat is called latent heat of vaporization.

Definition of Heat

Superheat What is superheat?

Superheat refers to the number of degrees a vapor is above its saturation temperature (boiling point) at a particular pressure.

How do I measure superheat?

Superheat is determined by taking the low side pressure gauge reading, converting that pressure to temperature using a PT chart, and then subtracting that temperature from the actual temperature measured (using an accurate thermometer or thermocouple) at the same point the pressure was taken.

Refrigerants

A refrigerant is a chemical that is used to provide cooling in a heat transfer system.

Chlorofluorocarbons (CFCs)

This refrigerant contains: Chlorine, Fluorine and Carbon. Positive points: non-toxic, non-flammable, and non-reactive with other chemical compounds. Negative points: Chlorine atom is a catalyst for ozone depletion.

Refrigerants

Hydrochlorofluorocarbons (HCFCs)

This refrigerant contains: Hydrogen, Chlorine, Fluorine, and Carbon. Positive points: energy-efficient, low-in-toxicity, cost effective and can

be used safely. Negative points: HCFCs are greenhouse gases, despite their very low atmospheric concentrations.

Hydrofluorocarbons (HFC's).

This refrigerant contains: Hydrogen, Fluorine, and Carbon. Positive points: do not contain any ozone depleting Chlorine.Negative points: targets of the Kyoto-Protocol because of an activity in

an entirely different realm of greenhouse gases.

Refrigerants

Ozone Depletion Potential (ODP)

The ozone layer is damaged by the catalytic action of chlorine and bromine in compounds, which reduce ozone to oxygen when exposed to UV light at low temperatures. The ODP of a compound is shown as an CFC-11 equivalent (ODP of CFC-11 = 1).

Global Warming Potential (GWP)

The greenhouse effect arises from the capacity of materials in the atmosphere the heat emitted by the Earth back onto the Earth. The direct GWP of a compound is shown as a CO2 equivalent (GWP of CO2 = 1)

Pressure Temperature Charts

Refrigeration Circuit

Refrigeration Circuit

Refrigeration Cycle

System Components

Subcooling

What is meant by subcooling?

Subcooling is the condition where the liquid refrigerant is colder than the minimum temperature (saturation temperature) required to keep it from boiling and, hence, change from the liquid to a gas phase.

The amount of subcooling, at a given condition, is the difference between its saturation temperature and the actual liquid refrigerant temperature.

Subcooling

Why is subcooling desirable?

Subcooling is desirable for several reasons:

You pump less refrigerant through the system to maintain the refrigerated temperature you want. This reduces the amount of time that the compressor must run to maintain the temperature. The amount of capacity boost which you get with each degree of subcooling varies with the refrigerant being used.

It prevents the liquid refrigerant from changing to a gas before it gets to the evaporator. Pressure drops in the liquid piping and vertical risers can reduce the refrigerant pressure to the point where it will boil or "flash" in the liquid line. This change of phase causes the refrigerant to absorb heat before it reaches the evaporator.

Superheat settings

Why is it important to know the superheat of a system?

Superheat gives an indication if the amount of refrigerant flowing into the evaporator is appropriate for the load.

If the superheat is too high, then not enough refrigerant is being fed resulting in poor refrigeration and excess energy use.

If the superheat is too low, then too much refrigerant is being fed possibly resulting in liquid getting back to the compressor and causing compressor damage.

Superheat settings

Superheat?

The superheat should be checked whenever any of the following takes place: • System appears not to be refrigerating properly. • Compressor is replaced. • TXV is replaced. • Refrigerant is changed or added to the system.

Note: The superheat should be checked with the system running at a full-load, steady-state condition.

How do I change the superheat?

Turning the adjustment stem on the TXV changes the superheat. • Clockwise - increases the superheat. • Counterclockwise - decreases the superheat

Chilling & Freezing Process

Heat energy is transferred by temperature difference.

The coldest point of a refrigeration system is the evaporator, called the evaporation temperature. The temperature of the surface of an evaporator is almost the temperature of the boiling refrigerant liquid within it.

Examples:

If a coldstore is required to be at a temperature of +5°C, then the temperature of the evaporator surface needs to be colder and typically minus -3°C.

If a frozen food coldstore is required to be at minus - 25°C, the evaporator temperature is likely to be at minus -32°C.

This temperature gradient is necessary for all steps in the chilling and freezing process.

Chilling & Freezing Process

Another practical case:

If a spiral freezer is required to freeze product quickly to a deep temperature of -25°C typically an air temperature needs to be -35°C, and the evaporator temperature needs to be even colder, likely to be -40°C. Air temperatures and product temperatures will tend to converge only during light refrigeration demand.

Freezing and chilling times are dependent on two properties:

1. The weight of the product – heat energy contained is in direct proportion to the weight.2. The size of the product – thicker product takes time for the temperature to penetrate.

Super heat and the TX Valve

Servicing Valves and Gauges

System service valves

Almost all refrigeration and air conditioning systems have service valves for operational checking and maintenance access.

Servicing Valves and Gauges

Service gauge manifold

Important tool used for checking system pressures, charging refrigerant, evacuating the system, purging non-condensable and adding oil, etc

Carry out service properly

Use proper tools / techniques

Carry out service properly

Minimize contamination

Air and water/moisture can cause corrosion, copper plating, acid formation, sludge, and other harmful reactions

Basic rules of refrigerant piping

Drying -- make sure no moisture in the pipes

Cleaning -- make sure no dirt in the pipes

Air tight -- make sure no leakage of refrigerant

Constraint on piping/equipment arrangement

• Max allowable piping length/height for split type

• Oil return

An example of proper riser to prevent oil from being trapped in the

horizontal portion of the pipe.

Maintaining Cleanliness

Materials handling

Use only copper tubing especially cleaned and dehydrated for refrigeration usage.

Soft copper tubing is available in rolls with the ends sealed, and hard drawn tubing is available capped and dehydrated.

Keep the tubing capped or sealed until ready for installation, and reseal any tubing returned to storage

Maintaining Cleanliness

Care must be taken during service or installation

Use only refrigeration grade copper tubing; properly sealed to keep tubing clean and dry.

Pass an inert gas (N2 usually ) through the tubing when brazing tubes.

Evacuate the system if exposed to the environment during service.

Replace the filter-drier each time the system is opened for service.

Do not leave filter-driers open to the atmosphere.

Refrigerant pipe flushing

Flushing removes foreign particles from the inside of pipes by means of gas pressure

Mount a pressure regulator on the N2 cylinder; connect the charge line to the pipes

Open the main valve of the N2 cylinder, and adjust the pressure regulator to 0.5 MPa.

Flushing

Maintaining Cleanliness

Brazing tubes

Tubing should be cleaned and burnished bright before brazing.

Particular attention should be given to preventing metal particles or abrasive material from entering the tubing

Follow soldering procedures:

Cut the tubing to length and remove the burrs.

Clean the joint area with sandpaper.

Clean inside the fitting. Use sandpaper or wire brush.

Brazing tubes

.

Apply flux to the inside of the fitting.

Apply flux to the outside of the tubing.

Assemble the fitting onto the tubing.

Obtain proper tip for the torch and light it. Adjust the flame for the soldering being done.

Brazing tubes

Apply heat to the joint.

When solder can be melted by the heat of the copper (not the torch), apply solder so it flows around the joint.

Clean the joint of excess solder and cool it quickly

Brazing tubes

Prevention of oxidation during brazing

Oxidization layer is formed on the inside surface of the pipe during brazing if no preventive measure

Supply N2 (Nitrogen) into the pipe to replace the air during brazing

Oxidation can cause clogging of solenoid valve, capillary tube, accumulator's oil return or compressor's internal oil inlet, etc.

De-contamination

Drying is needed in case of leakage at water-refrigerant heat-exchangers

The system is dried by decontamination, evacuation, and driers.

Transfer refrigerant into a separate storage tank. Parts of the system may have to be disassembled and the water drained from system low points.

Perform decontamination before re-installation of compressors. After reassembly, dry the compressor further by passing N2 through the system and by heating and evacuation. Using internal heat, by circulating warmed water on the water side of water-cooled equipment, is preferred.

Drying may take an extended period and require frequent changes of the vacuum pump oil; filter-driers need to be changed often.

Leak Detection

Leak test is a critical process for installation, trouble-shooting or repair.

Leakage results in loss of the refrigerant charge, reduces equipment capacity, and allows air & moisture to enter the system and can cause major breakdown.

Methods

• Bubble test using water/soap solution

• Pressure test using nitrogen

• Electronic leak detector

Pump down

If leakage is found to be minor and on the low side: Pump-down is necessary before carrying out the repairs.

If leakage is found on the high side: Removal of all the refrigerant is required.

Evacuation

Purpose

Needed whenever the system is exposed for prolonged periods to atmospheric air, or if the system is contaminated and removal of the refrigerant charge is necessary

The only effective way of removing air and moisture to required low level prior to charging refrigerant

Methods

Perform leak test before evacuation

Utilize vacuum pump

Charging Refrigerant

Methods

Vapor charging

Liquid charging

Determine proper amount

Weighing the Charge

Using A Sight Glass

Using A Liquid Level Indicator

Checking operational parameters

Compressor Replacement

Possible causes of a hermetic compressor motor burn-out

Prolonged operation at high discharge pressures and temperatures

Excessive motor starting

Fluctuating voltage

Shortage of refrigerant charge

Shortage of oil in the compressor

……

The system must be cleaned thoroughly to remove all contaminants.

Or, a repeat burnout will likely occur !

Compressor Replacement

Applicable for positive-displacement hermetic compressors only

Compressor Replacement

Applicable for positive-displacement hermetic compressors only

Condenser Cleaning

Water-cooled condenser: scaling and corrosion

Water treatment must be carried out properly

Scaling could be removed manually or through chemical clean carried out by qualified specialists

Chemical handling procedure and EHS requirement must be followed

Standard Operation Design Data

Standard design value, R22 system

Air-cooled packaged unit

Based on piping length of 5m and level difference of 0m

Outdoor air temp. 35˚C DB, Indoor air temp. 27˚CDB/19.5˚C WB

sodexo.com 163

Standard Operation Design Data

Standard design value, R22 system

Water-cooled packaged unit

Based on tower water temperature 32˚C / 37˚C

Indoor air temp. 27˚CDB/19.5˚C WB

Safety precautions

Wear proper PPE to protect eyes and to prevent direct contact of refrigerant with the skin which can cause burns, especially when charging or discharging refrigerant.

Make sure that the service cylinder is not over filled.

Do not expose cylinders to direct sunlight, or other heat sources.

Avoid discharge near naked flames.

Avoid direct contact with refrigerant/oil solutions from hermetic systems in the case of motor burn-out, which can be very acidic.

Always perform vapor charge from low-pressure side.

Always check that the refrigerant is correct for the system being charged.

Ensure that the working area is well ventilated. Breathing apparatus should be at hand in case of ammonia system

Safety Precautions

Ammonia Systems

What is Ammonia ?

Ammonia is made up of one atom of nitrogen and three atoms of hydrogen, with the chemical symbol NH3.

Ammonia is a key element in the nitrogen cycle.

Ammonia can be found in water, soil, and air, and is a source of much needed nitrogen for plants and animals.

In fact, ammonia is among the most abundant gasses in the environment.

What is Ammonia ?

Ammonia is the refrigerant with the demonstrably best thermodynamic properties.

Ammonia is also unbeatable in ecological terms:

• no ozone depletion potential

• no global warming potential (ODP and GWP = 0)

• a favourable energy consumption to load balance

thanks to the high COP of ammonia systems.

Refrigerants

Ozone Depletion Potential (ODP)

The ozone layer is damaged by the catalytic action of chlorine and bromine in compounds, which reduce ozone to oxygen when exposed to UV light at low temperatures. The ODP of a compound is shown as an CFC-11 equivalent (ODP of CFC-11 = 1).

Global Warming Potential (GWP)

The greenhouse effect arises from the capacity of materials in the atmosphere the heat emitted by the Earth back onto the Earth. The direct GWP of a compound is shown as a CO2 equivalent (GWP of CO2 = 1)

Ammonia as a Refrigerant

Ammonia is an efficient refrigerant used in food processing and preservation.

Ammonia has desirable characteristics:• corrosive• hazardous• irritating odor

Restrictions on chlorine and fluorine containing refrigerants have focused attention on ammonia to emerge as one of the widely used refrigerants that, when released to the atmosphere, do not contribute to ozone depletion and global warming.

Ammonia is difficult to ignite and will not support combustion after the ignition source is withdrawn.

in large quantities.

What is Ammonia ?

Ammonia is extremely soluble in water, it chemically combines with water to form ammonium hydroxide.

Household ammonia is a diluted water solution containing 5 to 10 % ammonia.

Preserving the purity of the ammonia is essential to ensure proper function of the refrigeration system.

On the other hand, anhydrous ammonia (without water) is essentially pure (over 99%) ammonia.

Ammonia as a Refrigerant

In industrial systems with capacities exceeding 500 kW, ammonia is simply unsurpassed in terms of energy and cost efficiency.

Ammonia Systems

Ammonia is one alternative refrigerant for new and existing refrigerating and air-conditioning systems. Ammonia has

a low boiling point (-28°F at 0 psig)

an ozone depletion potential (ODP) of 0.00 when released to atmosphere

a high latent heat of vaporization (9 times greater than R-12)

the atmosphere does not directly contribute to global warming

These characteristics result in a highly energy-efficient refrigerant with minimal environmental problems.

Ammonia Refrigeration Systems

Ammonia Refrigeration Systems – Ways They Differ

In a Direct Expansion halocarbon unit, oil is continuously returned to the compressor. 

Oil is not returned to the compressor in ammonia refrigeration systems but is instead drained out of the system periodically. 

Ammonia Refrigeration Systems

Ammonia Refrigeration Systems – Ways They Differ

Oils used in ammonia refrigeration systems are essentially insoluble in NH3 (some slight solubility exists at high pressures); oil is heavier than liquid ammonia, making it easy to drain out. 

On the other hand, oil solubility is absolutely essential with the halocarbons in order to facilitate oil return.

Paraffinic-based oils are commonly used with ammonia.  These oils do a good job of cleaning out old welding slag and dirt.  Oil, once drained from the system, is no longer usable.

Chiller Water System

Lube Oil System

The machines we are looking at are oil flooded compressors. These machines require large amounts of lube oil to:

provide sealing between the rotor lobes and the casing

provide sealing between the male and female lobes where the compression occurs.

for lubrication of the bearings and shaft seals

reduce the heat of compression in the machine.

Chiller Water System

Quick overview of the mechanism

The lube oil system on a screw compressor is a closed loop system.

The oil is injected into the machine in several places. The main oil injection port feeds the rotors directly with smaller lines feeding various points on the machine for seals and bearings.

Once the oil is injected, passages within the machine will drain all the bearing and seal oil into the rotors where it combines with the gas.

The gas and oil mixture is then discharged out of the machine. The oil that is injected must be removed from the gas down stream of the compressor.

Chiller Water System

Oil separator

A separate oil separator vessel is required to remove this oil from the gas. This vessel can be either vertical or horizontal in design. The vessel will require coalescing type elements to remove as much oil as possible.

The oil separator also acts as a reservoir for the lube oil system. The lube oil will flow from the bottom of the separator, through an oil cooler where it is cooled from discharge temperature down to 140-160ºF, through an oil filter and then back to the machine.

Chiller Water System

Lube oil pumps

Depending on the compressor manufacturer and the operating conditions, some machines require lube oil pumps to circulate the oil.

Other manufacturers will use the differential pressure from discharge to suction to move the oil around the system.

Chiller Water System

The oil cooling

The oil cooling can be done using two different methods.

The direct cooling method simply uses a section in the after cooler to cool the oil using the ambient air.

The indirect method cools the oil in a shell and tube or plate and frame cooler. A water/glycol mixture is pumped through the other side of the exchanger and then circulated through a section in the gas after cooler. This method requires an additional exchanger and water pump.

Draining Oil from the System

Guidelines for Changing Oil in Ammonia Refrigeration Systems

The draining of oil from and addition of oil to ammonia refrigeration systems are routine maintenance procedures. The following guidelines have been developed in an effort to reduce the risk associated with these procedures, and to ensure compliance with the OH&S Act and Regulations:

1. Personal protective equipment for the protection of respiratory system, skin and eyes must be worn while carrying out these procedures.2. Have a second person nearby observing the procedure and prepared to give assistance. Appropriate safety equipment must be readily accessible.3. Ensure that workers have received education and training in the Workplace Hazardous Materials Information System (WHMIS) requirements, Material Safety Data Sheet (MSDS) and exposure limits for anhydrous ammonia.4. Schedule these procedures for times when the facility is normally not in use or occupied by patrons. Keep a log of the quantity of oil drained and added to the system.5. Oil should not be drained while the refrigeration system is running. The refrigeration system should be shut down to allow the oil to settle.6. Prepare written procedures for the draining and addition of oil and post them in the machinery room. Each system has its own unique operating and installation characteristics and site specific procedures should be developed for each installation. 7. Personnel responsible for the maintenance of the plant must be trained in these procedures. Check lists should be developed to guide personnel and provide a record of the work.8. An emergency response plan must be developed and emergency instructions, contact names, addresses and phone numbers posted in a conspicuous location.

Ammonia as a Refrigerant

Refrigerant management

The refrigeration units are built using the simplest designs – no pressure vessels, no solenoid valves. 

Any excess liquid becomes stored inside the condenser – and any excess liquid decreases system refrigerating capacity. 

A few systems will have suction traps (a small vessel) to detain a liquid surge; some traps have an internal heat exchanger to facilitate liquid boil-off.

Piping materials

Copper, brass, bronze cannot be used with ammonia . Metal choices are mild steels, stainless steels, nickel.

Compressors

Difference halocarbon / ammonia

The major difference between the halocarbon series and ammonia with respect to compressors has to do with the motor – open drive versus hermetic. 

With only one exception (Japan), all ammonia compressors are open-drive design due to the incompatibility of copper and NH3.

The most commonly applied compressor design is twin rotary screw in today’s industrial marketplace. 

Screw Compressors

There are no valves, pistons, rings, or connecting rods that require regular maintenance.

With the elimination of the pistons, rings and valves, annual maintenance costs are also reduced on screw machines.

The rotary screw compressor is a positive displacement machine that operates without the need for suction or discharge valves. It has the ability to vary suction volume internally while reducing part load power consumption.

The only significant moving parts in a screw compressor are the male and female rotors.

Screw Compressors

The twin-screw type compressor consists of two mating helically grooved rotors, one male and the other female.

Generally the male rotor drives the female rotor.

Twin-screw compressor with 4 male lobes and 6 female gullies

The male rotor has lobes, while the female rotor has flutes or gullies.

The frequently used lobe-gully combinations are [4,6], [5,6] and [5,7].

male female

Screw Compressors

Discharge takes place at a point decided by the designed built-in volume ratio, which depends entirely on the location of the delivery port and geometry of the compressor. However, different built-in ratios can be obtained by changing the position of the discharge port.

Both under-compression and over-compression are undesirable as they lead to loss in efficiency.

As, the flow is mainly in the axial direction. Suction and compression take place as the rotors unmesh and mesh.

When one lobe-gully combination begins to unmesh the opposite lobe-gully combination begins to mesh.

male female

Twin-screw compressor with 4 male lobes and 6 female gullies

Chiller Water System

Lubrication and sealing between the rotors is obtained by injecting lubricating oil between the rotors.

The oil also helps in cooling the compressor.

The capacity of the screw compressor is normally controlled with the help of a slide valve.

As the slide valve is opened, some amount of suction refrigerant escapes to the suction side without being compressed.

This yields a smooth capacity control from 100% down to 10% of full load. It is observed that the power input is approximately proportional to refrigeration capacity up to about 30%, however, the efficiency decreases rapidly, there after.

Chiller Water System

do not suffer from vibration problems

As the rotor normally rotates at high speeds, screw compressors can handle fairly large amounts of refrigerant flow rates compared to other positive displacement type compressors.

compact

reliable

rugged

Advantages of the screw compressor

Compressors

Chiller Water System

Water and Refrigeration Systems

Ammonia is different from its halocarbon counterparts has to do with a water inclusion. 

The desiccant used adsorbs water.  A high pressure drop across a dryer core is a sure sign that water has accumulated. 

If this goes uncorrected, water will freeze inside distributor tubing if the evaporating temperature is < 32 ºF.

Purgers

Ammonia Remediators & Foul Gas Purgers

One way to remove water from ammonia is to use a batch remediator – a tall, skinny vessel with a couple of float switches and usually one or more belly-band resistance heaters. 

"Weak aqua" is then manually drained out. 

If desired (or required), neutralize the aqua with a little citric acid before sending it down a sewer.

Purgers

These two devices - a remediator and a purger – make operating an ammonia refrigeration system low side below atmospheric pressure feasible. 

Ammonia Remediators & Foul Gas Purgers

To get air out, we use a "foul gas" (non-condensable gas) purger – remove collected air, trapped during servicing.

Water in the system

Effects of Water Contamination

Water contamination in an industrial ammonia refrigeration system can lower system efficiency, and increase the electrical costs required to run the system’s refrigeration compressors.

At typical suction pressures, the addition of 10% water by weight will increase the evaporator temperature by about 4°F (2°C).

Water in the system

Therefore to maintain an evaporator temperature of -4°F (-20°C) in a system with 10% water contamination, the suction pressure would have to be run at 12.9 – 10.2 = 2.7 psi (0.18 bar) lower than if the water were not present.

Let us take an example

Pure ammonia at -4°F (-20°C) has a saturation pressure of 12.9 psig (0.90 bar). An ammonia-water solution of 10% water and 90% ammonia by weight, at the same -4°F (-20°C) has a saturation pressure of 10.2 psig (0.72 bar).

Water in the system

Lost compressor capacity due to water

For each percent of water in the ammonia, we are losing about 1% in compressor capacity

Water in the system

Energy cost due to water

With water in the system, the evaporator pressure must be lowered to maintain the desired temperature. So the compressor must work harder and uses more energy.

Maintenance

Benefits of Proper Maintenance of Refrigerant Quality

Removing air and water from ammonia refrigeration systems certainly saves energy, but there are other benefits that improve system operation.

On the high side of the system, operating at the lowest condensing pressure permissible means less wear and tear on compressor bearings and other mechanical parts.

The lower discharge temperatures also reduce oil and refrigerant breakdown, and extend gasket life.

Maintenance

Benefits of Proper Maintenance of Refrigerant Quality

Air removal means there is less oxygen in the system, which is sometimes associated with corrosion of piping and vessels.

In the same way, removing water from the refrigeration system may reduce corrosion.

Air in the system

Where does air collect?

Air collects at various locations on the high pressure side of the system. These locations are typically the lowest gas velocity and coolest temperature areas.

Air can enter a system in a number of ways:

leaky gaskets and shaft seals allow air into the system

during repairs and service

adding refrigerant to the system

through the chemical breakdown of refrigerant.

lubricating oils can breakdown under heat and high

pressure to create non-condensable gases.

Air in the system

Why there needs purger?

A refrigeration system without a purger or with an inadequate purger may allow fluctuations in condensing pressure or may not be able to maintain the minimum possible condensing pressure.

AUTO-PURGERs remove more air and over a shorter period of time than other purging methods or units to maintain the minimum possible condensing pressure.

Ammonia Purges

Cost savings and payback

The presence of air in a refrigerant system increases the condensing pressure. As a result, the power requirement of the compressor also increases.

An AUTO-PURGER quickly and effectively removes all air from the system. For every 10 psi decrease in condensing pressure, there is a 6% decrease in power consumption by the system.

Air in the System

Air as an insulator

Air tends to act as insulation in refrigeration systems. A layer of air forms a blanket on the walls of the condensing surface, preventing refrigerant from making contact with the lower-temperature heat exchanger surface. This results in greatly reduced system efficiency.

Air acts as an insulator between the refrigerant and the cooling surface, greatly reducing condensing efficiency.

Air in the system

The amount of air can also be measured by condensing pressure of pure refrigerant at a given temperature.

How much air in the system

The presence of air in a refrigeration system is indicated by excessively high head pressure. This may be indicated by a pressure gauge or by system compressors shutting down due to the high pressure.

Ammonia Safety Principals

Basic principles

When operating, servicing and maintaining screw compressor units and chillers keep in mind, in particular, the following instructions:

• It is forbidden to weld or use open flames unless special safety instructions are observed. • Smoking is not allowed in the refrigeration machinery room. • Escape routes must be free from obstacles. • Store suitable personnel protective equipment and respirators at an accessible point of the refrigeration machinery room (acc. to EN 378-3, appendix A). • Store fire extinguishers at an accessible point of the refrigeration machinery room (acc. to EN 378-3, 5.1.j). • Any work on units and chillers may only be carried out by appropriately trained and instructed staff. • Intimate knowledge of the complete delivered documentation is a prerequisite for operating the equipment correctly and safely. • The refrigeration units must not be operated unless full functional and operational safety and reliability of all components, safety devices and circuits (refrigerant and oil circuits, secondary refrigerant and cooling water circuits) and of the electrical switchgear is ensured. • The elements of the safety chain, the sensors and controllers shall be adjusted according to the designed values and must not be set out of operation, not in part either.

System components

Maintenance of the System

Visual testing

Visual inspections are relatively inexpensive and provide a great deal of valuable information to the system operator. To monitor the condition of the ammonia refrigeration system, the person inspecting the system should:

• note any corrosion of piping, valves, seals, flanges and other pertinent equipment.

• inspect insulation for breeches in its integrity.

• keep a log, including photographs of all findings.

Here we can see external corrosion under insulation (CUI) attacks

Maintenance of the System

Leak testing

Recommended practice involves leak testing all piping, valves, seals, flanges and other pertinent equipment at least 4 times a year. Some methods that can be used for leak testing are sulfur sticks, litmus paper, or a portable meter equipped with a flexible probe.

Operators, maintenance personnel and other facility workers should be encouraged to immediately report ammonia odors. Facilities should immediately investigate all reports of ammonia leaks and take corrective actions without delay.

Maintenance of the System

Vibration Testing

Depending on the nature of equipment at the site, some facility operators may elect to perform vibration testing on rotating equipment .

Excessive vibration can lead to potential equipment damage which could increase the probability of an ammonia release.

The equipment manufacturer should be consulted to provide guidance on the usefulness of vibration monitoring for their particular equipment.

Maintenance of the System

Thermal imaging

A growing trend preventive maintenance is the use of infrared imaging. Infrared thermography helps locate many problems in their early stages often before they can be seen or found in any other way.

A temperature difference, usually in abnormal hot spot, is typically associated with these problems due to high electrical resistance or excessive friction.

Ammonia Safety

Characteristics of anhydrous ammonia

Very corrosive, exposure may result in chemical-type burns to skin, eyes and lungs

Flammable

Explosive

High affinity for water and migrates to moist areas

Ammonia Safety

Released anhydrous ammonia will rapidly absorb moisture from air and form a dense visible white cloud. Do not enter a visible cloud of ammonia, it will damage your lungs.

Detect the presence of anhydrous ammonia

Ammonia release has a pungent odor.

Ammonia Safety

Effects of anhydrous ammonia

Exposure to anhydrous ammonia can cause:

headaches

loss of the sense of smell

nausea

vomiting

irritation to the nose, mouth and throat

lung irritation

coughing

shortness of breath

pulmonary edema

Maintenance

Maintenance

Daily Maintenance:1.Check Refrigerant levels2.Check Oil levels3.Check operation of Purge unit4.Check operating conditions

Weekly1.Purge air from condenser2.Drain Oil from the system and record3.Top up oil as required4.Check refrigerant levels5.Check condenser water for dosing levels6.Check operating conditions

Six monthly1.Carry out vibration analysis2.Carry out Refrigerant analysis3.Check motor4.Infra red scan the electrical panels5.Check the integrity of the insulation

Oxygen Acetylene Brazing

What are the benefits of brazing when we are in a cold room ?

With acetylene or other gas such as butane, brazing has advantages. Indeed, the potential distortion of the metal, created by heating or in our case cooling, can be predicted and controlled and even minimized or eliminated.

Thank You

L | C | LOGISTICS PLANT MANUFACTURING AND BUILDING FACILITIES EQUIPMENT

Engineering-Book

ENGINEERING FUNDAMENTALS AND HOW IT WORKS

MECHANICS HVAC