SECTION 10

CHILLED WATER AIR CONDITIONING SYSTEMS

UNIT 48

HIGH-PRESSURE, LOW-PRESSURE, AND ABSORPTION CHILLED-

WATER SYSTEMS

UNIT OBJECTIVES After studying this unit, the reader should be able to

• List various types of chilled-water air conditioning systems

• Describe the operation of a typical chilled-water system

• Describe the compressors typically used on chilled-water systems

• Describe the difference between direct expansion and flooded chillers

• Explain the concept of approach temperature in chiller systems

• Explain the application and purpose of the purge unit

• Describe the absorption cooling system process

• Describe the motor types used in chiller systems

CHILLERS• Refrigerate circulating water

• Chilled water is circulated and used to absorb heat from the building

• 45° water is supplied to the building (design)

• 55° water is returned to the chiller (design)

• Water is cooled from 55° to 45° in the chiller

• Compression type and absorption type

45° water to coil (from chiller)

55° water from coil (to chiller)

75° air from occupied space55° air to occupied space

Centrifugal pump Chiller barrel

Chilled water coils

Condenser

Metering device

COMPRESSION CYCLE CHILLERS

• Compression (vapor pumps): reciprocating, scroll, screw and centrifugal

• Normal boiling point of the refrigerant is 38°

• Normal condensing temperature of the refrigerant is 105°

• Classified as high-pressure or low-pressure systems

RECIPROCATING COMPRESSOR CHILLERS

• Multiple small compressors are commonly used instead of one large compressor

• If the large compressor fails, the entire system is off line• If one small compressor fails, the others continue to

operate, providing some backup • Large chillers must have capacity control

– Prevents compressor short cycling

– Reduces compressor wear

CYLINDER UNLOADING• Provides means for controlling compressor capacity• As the capacity is reduced, the power needed to operate

the compressor is also reduced• Compressors operate with lower compression ratios

when unloaded• Blocked suction

– Utilizes a solenoid valve– When the valve closes, no refrigerant enters the cylinder

• Suction valve lift unloading– Accomplished by lifting the suction valve from its seat– Prevents the cylinder from pumping

Compressor cylinders and pistons (both cylinders are pumping)

Common discharge line

Common suction lineSolenoid valve (open)

Compressor cylinders and pistons (the left cylinder is not pumping)

Common discharge line

Common suction lineSolenoid valve (closed)

SCROLL COMPRESSOR CHILLERS

• Positive displacement compressor

• Efficient, low noise levels, fewer moving parts can pump small amounts of liquid refrigerant without compressor damage

• Compressor uses two nested scrolls

• Equipped with check valves to prevent backward flow in the off cycle

ROTARY SCREW COMPRESSOR CHILLERS

• Can handle large volumes of refrigerant with few moving parts

• Positive displacement compressor• Can handle small amounts of liquid refrigerant

without compressor damage• Compressors range from 50 to 700 tons • Capacity control is accomplished with a slide valve• Usually equipped with oil separators

CENTRIFUGAL COMPRESSOR CHILLERS (HIGH-PRESSURE)

• Refrigerant is moved from the low side of the system to the high side by centrifugal force

• Gear boxes are used to enable the compressor to reach speeds of about 30,000 rpm

• When the head pressure becomes too high or the evaporator pressure becomes too low, the compressor stops pumping

Discharge (outlet)

Suction (inlet)

Centrifugal compressor

housing

Impeller

• Compressor is lubricated by a separate motor and pump

• Capacity control is accomplished by the use of guide vanes

• Load limiters are used to prevent compressor overload

• Centrifugal compressors can be hermetically sealed or can have open drives

CENTRIFUGAL COMPRESSOR CHILLERS (HIGH-PRESSURE)

EVAPORATORS FOR HIGH-PRESSURE CHILLERS

• Liquid refrigerant boils when it absorbs heat from the circulating water

• Most commonly made of copper

• Chillers have a water-to-liquid heat exchange in the evaporator

• Can be direct expansion type or flooded type

DIRECT EXPANSION EVAPORATORS

• Also known as dry-type evaporators

• Operate with a predetermined superheat

• Thermostatic expansion valves are normally used to control refrigerant flow to the evaporator

• Water is piped to the shell of the chiller barrel• Refrigerant enters the chiller barrel from the end

One-pass chiller barrel

Liquid line

Suction line

Chiller barrel

Centrifugal pump

Water to remote

cooling coils

Two-pass chiller barrel

Liquid line

Suction line

Chiller barrel

Centrifugal pump

Water to remote

cooling coils

Three-pass chiller barrel

Liquid line

Suction line

Chiller barrel

Centrifugal pump

Water to remote

cooling coils

Four-pass chiller barrel

Liquid line

Suction line

Chiller barrel

Centrifugal pump

Water to remote

cooling coils

FLOODED EVAPORATOR CHILLERS• Refrigerant enters the barrel at the bottom• Water boxes are used to direct water flow through the

tubes • By design, water enters the chiller at 55° and leaves at 45° • The refrigerant is usually about 7° cooler than the leaving

water (approach temperature)• Flooded chillers usually have permanently mounted

thermometers and pressure gages• Freeze protection may be required

Liquid line

Suction line

Water to remote

cooling coils

55°F

45°F

Refrigerant boiling temperature: 38°F

Approach temperature = 45°F - 38°F = 7°F

CONDENSERS FOR HIGH-PRESSURE CHILLERS

• Used to transfer heat from the system

• Can be air cooled or water cooled – Air-cooled condensers require less maintenance

• Heat can be recovered for use in other applications – Can be used to heat domestic water

WATER-COOLED CONDENSERS

• Usually shell and tube type (for high-pressure chillers)

– Water circulates in the tubes

– Refrigerant is piped into the shell

– Bottom of the shell acts as a receiver

• Can be equipped with water boxes or marine water boxes

Hot discharge gas from compressor

Subcooled liquid from condenser

Cool water in

Warm water out

Water Tubes

Bottom of the condenser acts as the receiver

Shell

Hot discharge gas from compressor

Subcooled liquid from condenserWater box

Directs water through the

tubes

Access to tubes for cleaning

CONDENSER SUBCOOLING • Design condensing temperature is about 105° when 85°

water is supplied to the condenser• Subcooling the refrigerant adds to the system capacity• One degree for subcooling can increase capacity by 1%• Normal approach temperature is about 10°• Head pressure must be controlled • Head pressure can be maintained by a bypass valve• Bypasses water during a startup when the condenser

water is too cold

Liquid line

Hot gas from compressor

Water to cooling tower

85°F

95°F

Refrigerant condensing

temperature: 105°F

Approach temperature = 105°F - 95°F = 10°F

Water from cooling tower

AIR-COOLED CONDENSERS• Usually constructed of copper tubes and

aluminum fins

• Multiple tans are used for head pressure control purposes

• Head pressures are typically higher on air-cooled systems

• Air-cooled condensers require less maintenance than water-cooled condensers

METERING DEVICES FOR HIGH-PRESSURE CHILLERS

• Thermostatic expansion valve– Maintains constant evaporator superheat– The more evaporator superheat, the slower the heat

exchange

• Orifice– Fixed bore metering device– Flow rate is determined by the pressure drop across it

METERING DEVICES FOR HIGH-PRESSURE CHILLERS

• Float-type metering devices– Low-side float

1. Located at the inlet of the chiller barrel 2. Maintains a constant liquid level in the barrel

– High-side float1. Located in the liquid line before the evaporator2. Opens when the level of liquid refrigerant is higher in

the liquid line than the evaporator

• Electronic expansion valves

Suction gas to compressor

Liquid refrigerant from condenser

45°F water

55°F water

Water boxWater box

Liquid level in the evaporator

Float valve seat

Float ball

LOW SIDE FLOAT

Suction gas to compressor

From condenser

45°F water

55°F water

Water boxLiquid level in the evaporator

HIGH SIDE FLOAT

Float ball

LOW-PRESSURE CHILLERS

• Typically use R-11, R-113, or R-123– CFC refrigerants are no longer available– Manufacture of CFC refrigerants has been completely

halted

• Equipped with the same components as high-pressure chillers

• Newer chillers use R-123

COMPRESSORS • Low-pressure chillers use centrifugal compressors• Centrifugal compressors run at speeds up to

30,000 rpm• Suction line fastened to the housing in the center• Compressed refrigerant is trapped in the volute

and guided to the condenser• Can be operated in series with each other• Low-pressure chillers can have refrigerant

working pressure as low as 15 psig

CONDENSERS FOR LOW-PRESSURE CHILLERS

• Low-pressure chillers have water-cooled condensers• Usually are shell and tube type

– Water is circulated through the tubes

– Refrigerant is piped into the shell

• Located above the evaporator• The liquid leaving the condenser flows to the evaporator

by gravity• Can also have a subcooling loop

METERING DEVICES FOR LOW-PRESSURE CHILLERS

• Controls the flow of refrigerant to the evaporator

• Orifice and the float type are typically used– Same as those used on high-pressure chillers– High side float– Low side float

PURGE UNITS • Low-pressure chillers operate with the

suction pressure in a vacuum

• R-113 systems operate with both high- and low-pressure sides in a vacuum

• If a leak occurs, air will enter the system

• Air can cause system problems

• Air can removed by the purge unit

ABSORPTION AIR-CONDITIONING CHILLERS

• Very different from compression process • Uses heat instead of a compressor • Has many piping connections

– Chilled water piping

– Condenser water piping

– Steam or hot water piping

• Equipped with oil or gas burners• Usually range from 100 to 1,700 tons

ABSORPTION AIR-CONDITIONING CHILLERS

• Water is impractical as a refrigerant in a compression system – For water to boil at 40°, the pressure must be

0.122 psia– Volume of rising vapor from boiling water is

excessive– 2,444 cubic feet of water vapor would have to be

removed for each pound of water that boils at 40°

ABSORPTION AIR-CONDITIONING CHILLERS

• Water is used as the refrigerant in absorption systems– Does not use a compressor

– Uses salt solutions

– Lithium Bromide (LiBr) is commonly used to attract the water (called absorbent)

– Lithium Bromide is usually mixed with distilled water to create a 60% LiBr/40% water solution

• Absorption means to attract moisture

BASIC ABSORPTION CYCLE

• Evaporator section– Water (refrigerant) metered into the evaporator– Water experiences a pressure drop to 0.122 psia– This cold water is sprayed over the evaporator

tube bundle system circulating water– The cold water then evaporates, absorbing heat

from the system water

BASIC ABSORPTION CYCLE

• Absorber section – The LiBr solution attracts the water vapor– The LiBr solution then becomes diluted by the

vapor– The LiBr has the water vapor removed in the

concentrator (condenser)– The diluted LiBr solution is called the weak

solution

BASIC ABSORPTION CYCLE• Concentrator and condenser section

– The diluted weak solution is boiled– The heat used to boil the solution is either

steam or hot water– The water vapor then condenses to a liquid– It is gathered and metered back to the

evaporator– The concentrated solution is drained back to the

absorber

SOLUTION STRENGTH• The greater the difference between the weak and

strong solutions, the greater the system capacity• An over-concentrated strong solution can become

rock salt• The start-up technician is responsible for the trim

(proper adjustment of the charge) • When the system is initially started up, samples

of the strong and weak solutions are taken and compared

SOLUTIONS INSIDE THE ABSORPTION SYSTEM

• Corrosion occurs when air is introduced to the system

• Systems must be kept as clean as possible• Filters are used to stop solid particles• Magnetic devices are used to remove steel

particles • Solutions may appear to be rusted, but this is

normal

CIRCULATING PUMPS FOR ABSORPTION SYSTEMS

• Centrifugal type• Shaft and impellers are made of non-corrosive materials• Motors

– Hermetically sealed– Operate within the system atmosphere– Cooled with cold refrigerant water from the evaporator (closed

loop)

• Manufacturer’s recommendations should be followed when servicing the pump motors

CAPACITY CONTROL

• Can be accomplished by throttling the heat supply in the concentrator– 12 to 14 pounds of steam at full capacity– 6 pounds of steam pressure at half capacity

• Can be accomplished by controlling the flow of the weak solution to the concentrator

CRYSTALLIZATION• Occurs when the solution becomes concentrated• Rock salt forms • Crystallization can be detected automatically by a

pressure drop in the strong solution across the heat exchanger

• Some units have a “dilution cycle” when over concentration occurs

• Crystallization can occur– When the condenser removes too much water– When the unit shuts down while operating at full load– When air is introduced to the system

PURGE SYSTEM• Removes non-condensable during the operating cycle• Non-motorized units

– Use system pumps to move non-condensable to a chamber

– Non-condensable are then bled off by the machine operator

• Motorized units – Essentially a two-stage vacuum pump– Pumps absorber gas to the atmosphere

ABSORPTION SYSTEM HEAT EXCHANGERS • Chilled water heat exchanger

– Removes heat from building water and adds it to the refrigerant (water)

– Approach temperature is usually 2° or 3°

• Absorber heat exchanger– Exchanges heat between the absorber solution and the water

returning from the cooling tower • Heat exchange between the refrigerant and the heat source • Thermometer wells are provided to check the heat

exchangers

DIRECT-FIRED SYSTEM

• Use gas or oil as the heat source

• Can be dual-fuel systems

• Range in size from 100 to 1,500 tons

• Can provide heating or cooling by furnishing hot or chilled water

MOTORS AND DRIVES FOR COMPRESSION CYCLE CHILLERS

• High-efficiency, three-phase motors • Open-type compressors • Suction-cooled compressors• Electrical connections must be leak-free• Compressors are energized and controlled by starters• Start-up amperage is about five times full-load

amperage

MOTORS AND DRIVES FOR COMPRESSION CYCLE CHILLERS

• Motor starting methods– Part-winding– Autotransformer– Wye-delta (star-delta)– Electronic start

PART-WINDING START

• Used on motors over 25 horsepower

• Normally have nine leads

• Actually two motors in one

• Motors can be wired to operate at two different voltages– Low voltage (208/230 V): motors are wired in

parallel– High voltage (460 V): motors are wired in series

AUTOTRANSFORMER START• Reduced voltage start• Coils are connected between the motor and starter

contacts• When motor is energized, the voltage being

supplied to the motor is reduced• When the motor is up to speed, the coils are

electrically removed from the circuit• The voltage supplied to the motor now increases

to line voltage • The motor has low starting torque

WYE-DELTA • Also called star-delta• Used on large motors with six leads• Motor initially starts as a Wye circuit• After motor is up to speed, the circuit switches to a delta

configuration • Motor draws less amperage on start-up• The changeover from Wye to delta uses three contactors• When the motor is up to speed, the Wye connection is

disconnected

ELECTRONIC STARTERS

• Also called soft starters• Reduce the voltage on motor start-up• The frequency of the power being

supplied is changed• Once the motor is started, the voltage

is restored

MOTOR PROTECTION • Large motors are expensive and should be

protected

• Many newer motor protectors are electronic devices

• Load-limiting devices– Used to control the motor amperage– Throttles refrigerant entering the compressor

MECHANICAL-ELECTRICAL MOTOR OVERLOAD PROTECTION

• All motors must have overload protection

• Dashpot type of overload– Operates on electromagnetic concepts with time delay– When excessive current is sensed, the contactor or

starter coil will be de-energized– Tolerates amperages about 5% above full-load rating– Overloads on large motors are usually manually reset

ELECTRONIC SOLID-STATE OVERLOAD DEVICE PROTECTION

• Wired in the control circuit

• Monitors the full-load amperage of motors

• Usually installed close to the motor for accurate operation

ANTI-RECYCLE CONTROL

• Prevents motor from short cycling • Allows the motor to restart after it has had

enough run time or enough off time to cool off

• Many centrifugal compressors have a 30 minute time setting

• If the motor has not run or tried to run in 30 minutes, it is ready for a start

PHASE FAILURE PROTECTION

• Large motors use three-phase power

• All three phases must be supplied or the motor will overload

• Electronic phase protectors ensure that all three phases are present

VOLTAGE UNBALANCE

• All three phases must be balanced

• Most manufacturer’s accept 2% as the maximum allowable voltage unbalance

• Voltage unbalance = Maximum deviation from average voltage

average voltage

EXAMPLE OF A 460-V SYSTEM

• Phase 1 to phase 2 = 475 V

• Phase 1 to phase 3 = 448 V

• Phase 2 to phase 3 = 461 V

• Average voltage = 461.3 V

• Maximum deviation from average = 475 – 461.3 V = 13.7 V

• Unbalance = 13.7 V ÷ 461.3 V = 0.0297 = 2.97%

PHASE REVERSAL• Rotation of a three-phase motor can be reversed

• Reciprocating compressors can usually function either way (bi-directional oil pumps)

• Scrolls, screw, and reciprocating compressors must be phased correctly

• Some compressors are equipped with safety devices that prevent operation when the phasing is incorrect

SUMMARY - 1• Chillers refrigerate circulating water to absorb heat from

the building• Water is cooled from 55° to 45° in the chiller• Compression chillers are classified as high-pressure or

low-pressure systems • Commonly used compressors include the reciprocating,

scroll, screw and centrifugal • Multiple small compressors are commonly used instead

of one large compressor• Large chillers must have capacity control• Blocked suction of suction valve lift unloading

SUMMARY - 2• Rotary screw compressors can handle large volumes of

refrigerant with few moving parts• Capacity control on rotary screw compressors is

accomplished with a slide valve • In centrifugal chillers, refrigerant is moved from the

low side of the system to the high side by centrifugal force

• High speed fans are used to move the refrigerant• Capacity control on centrifugal chillers is accomplished

with guide vanes

SUMMARY - 3• Chillers have a water-to-liquid heat exchange in

the evaporator• Dry type evaporators operate with superheat• Water is piped to the shell of the chiller barrel• The refrigerant is usually about 7° cooler than the

leaving water (approach temperature)• Condensers for high pressure chillers can be air

cooled or water cooled

SUMMARY - 4• Condensers on high-pressure chillers are usually

shell and tube type• Design condensing temperature is about 105°

when 85° water is supplied to the condenser• Air-cooled condensers require less maintenance

than water-cooled condensers• Metering devices used on high pressure chillers

include the orifice, TXV, high side float, low side float and the electronic expansion valve

SUMMARY - 5• Low pressure chillers typically use R-11, R-113, or R-123• Low pressure chillers are equipped with the same

components as high-pressure chillers• Low-pressure chillers use centrifugal compressors• Low-pressure chillers can have refrigerant working

pressure as low as 15 psig• Low-pressure chillers have water-cooled condensers• Orifice and the float type are typically used on low-

pressure chillers

SUMMARY - 6• Low-pressure chillers operate with the suction pressure in

a vacuum (air can enter system)• Air is removed from the system with a purge unit• Absorption systems use heat instead of a compressor• Absorption systems are made up of an evaporator,

concentrator, condenser and absorber• Absorption systems have a strong and a weak solution• The greater the difference between the weak and strong

solutions, the greater the system capacity

SUMMARY - 7• When the system is initially started up, samples of

the strong and weak solutions are compared• Corrosion occurs when air is introduced to the

system • Centrifugal pumps are used on absorption systems• Can be accomplished by throttling the heat supply

in the concentrator or by controlling the flow of the weak solution to the concentrator

SUMMARY - 8• Crystallization occurs when solution becomes concentrated• Crystallization can be detected automatically by a pressure

drop in the strong solution across the heat exchanger• The purge system removes non-condensable during the

operating cycle• Three-phase motors are used on compression chillers• Common starting methods for these motors are the part-

winding start, Part-winding, autotransformer, wye-delta (star-delta) and the electronic start