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CHILLED WATER AIR CONDITIONING SYSTEMS
SECTION 10 CHILLED WATER AIR CONDITIONING SYSTEMS UNIT 48 HIGH-PRESSURE, LOW-PRESSURE, AND ABSORPTION CHILLED-WATER SYSTEMS
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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
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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
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75° air from occupied space
55° air to occupied space 75° air from occupied space 55° water from coil (to chiller) 45° water to coil (from chiller)
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Chilled water coils Centrifugal pump Chiller barrel Metering device Condenser
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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
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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
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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
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Compressor cylinders and pistons (both cylinders are pumping)
Solenoid valve (open) Common suction line Common discharge line Compressor cylinders and pistons (both cylinders are pumping)
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Solenoid valve (closed) Common suction line
Common discharge line Compressor cylinders and pistons (the left cylinder is not pumping)
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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
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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
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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
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Centrifugal compressor housing
Impeller Suction (inlet) Discharge (outlet) Centrifugal compressor housing
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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
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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
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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
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Water to remote cooling coils
One-pass chiller barrel Suction line Water to remote cooling coils Chiller barrel Liquid line Centrifugal pump
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Water to remote cooling coils
Two-pass chiller barrel Suction line Chiller barrel Water to remote cooling coils Liquid line Centrifugal pump
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Water to remote cooling coils
Three-pass chiller barrel Suction line Water to remote cooling coils Chiller barrel Liquid line Centrifugal pump
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Water to remote cooling coils
Four-pass chiller barrel Suction line Chiller barrel Water to remote cooling coils Liquid line Centrifugal pump
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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
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Refrigerant boiling temperature: 38°F
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
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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
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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
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Hot discharge gas from compressor Water Tubes Shell
Warm water out Cool water in Subcooled liquid from condenser Bottom of the condenser acts as the receiver
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Directs water through the tubes Hot discharge gas from compressor
Access to tubes for cleaning Subcooled liquid from condenser Water box
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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
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Hot gas from compressor Refrigerant condensing temperature: 105°F
Liquid line Water to cooling tower 85°F Approach temperature = 105°F - 95°F = 10°F Water from cooling tower
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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
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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
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METERING DEVICES FOR HIGH-PRESSURE CHILLERS
Float-type metering devices Low-side float Located at the inlet of the chiller barrel Maintains a constant liquid level in the barrel High-side float Located in the liquid line before the evaporator Opens when the level of liquid refrigerant is higher in the liquid line than the evaporator Electronic expansion valves
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LOW SIDE FLOAT Suction gas to compressor Water box
Liquid level in the evaporator Water box 55°F water 45°F water Float ball Liquid refrigerant from condenser Float valve seat
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Suction gas to compressor
HIGH SIDE FLOAT From condenser Suction gas to compressor Water box Liquid level in the evaporator Float ball 55°F water 45°F water
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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
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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
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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
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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
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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
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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
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ABSORPTION AIR-CONDITIONING CHILLERS
Water is impractical as a refrigerant in a compression system For water to boil at 40°, the pressure must be 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°
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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
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BASIC ABSORPTION CYCLE
Evaporator section Water (refrigerant) metered into the evaporator Water experiences a pressure drop to 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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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MOTORS AND DRIVES FOR COMPRESSION CYCLE CHILLERS
Motor starting methods Part-winding Autotransformer Wye-delta (star-delta) Electronic start
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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EXAMPLE OF A 460-V SYSTEM Phase 1 to phase 2 = 475 V
Average voltage = V Maximum deviation from average = 475 – V = 13.7 V Unbalance = 13.7 V ÷ V = = 2.97%
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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
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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
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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
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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
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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
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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
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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
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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
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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
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