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SECTION 10
CHILLED WATER AIR
CONDITIONING SYSTEMS
UNIT 48
HIGH-PRESSURE, LOW-PRESSURE,
AND ABSORPTION CHILLEDWATER 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
55° air to occupied space
55° water from
coil (to chiller)
75° air from occupied space
45° water to coil
(from chiller)
Chilled water coils
Centrifugal
pump
Chiller barrel
Metering
device
Condenser
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
Solenoid
valve (open)
Common suction line
Common
discharge line
Compressor cylinders and pistons (both cylinders are pumping)
Solenoid
valve (closed)
Common suction line
Common
discharge line
Compressor cylinders and pistons (the left cylinder is not pumping)
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
Impeller
Suction (inlet)
Discharge
(outlet)
Centrifugal
compressor
housing
CENTRIFUGAL COMPRESSOR
CHILLERS (HIGH-PRESSURE)
• 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
EVAPORATORS FOR HIGHPRESSURE 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
Suction line
Water to
remote
cooling coils
Chiller barrel
Liquid line
Centrifugal pump
Two-pass chiller barrel
Suction line
Chiller barrel
Water to
remote
cooling coils
Liquid line
Centrifugal pump
Three-pass chiller barrel
Suction line
Water to
remote
cooling coils
Chiller barrel
Liquid line
Centrifugal pump
Four-pass chiller barrel
Suction line
Chiller barrel
Water to
remote
cooling coils
Liquid line
Centrifugal pump
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
Suction line
Refrigerant boiling
temperature: 38°F
45°F
Liquid line
Water to
remote
cooling coils
55°F
Approach temperature =
45°F - 38°F = 7°F
CONDENSERS FOR HIGHPRESSURE 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
Shell
Water Tubes
Hot discharge gas from
compressor
Warm
water out
Cool water in
Bottom of the condenser
acts as the receiver
Subcooled liquid from condenser
Directs water
through the
tubes
Hot discharge gas from
compressor
Access to tubes
for cleaning
Water box
Subcooled liquid from condenser
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
Hot gas from compressor
Refrigerant
condensing
temperature: 105°F
95°F
Liquid line
Water to
cooling tower
85°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 HIGHPRESSURE 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 HIGHPRESSURE 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 float
1. Located in the liquid line before the evaporator
2. Opens when the level of liquid refrigerant is higher in
the liquid line than the evaporator
•
Electronic expansion valves
LOW SIDE FLOAT
Water box
Suction gas to
compressor
Liquid level in the evaporator
Water box
55°F water
45°F water
Float ball
Liquid refrigerant from
condenser
Float valve seat
HIGH SIDE FLOAT
Water box
55°F water
45°F water
Suction gas to
compressor
Liquid level in the evaporator
From
condenser
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 highpressure 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 LOWPRESSURE 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 LOWPRESSURE 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 AIRCONDITIONING 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 AIRCONDITIONING 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 AIRCONDITIONING 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 lowpressure 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 partwinding start, Part-winding, autotransformer, wye-delta
(star-delta) and the electronic start