1. CHILLED WATER AIR CONDITIONING SYSTEMS
SECTION 10
CHILLED WATER AIR CONDITIONING SYSTEMS
UNIT 48
HIGH-PRESSURE, LOW-PRESSURE, AND ABSORPTION CHILLED-WATER SYSTEMS

2. 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

3. 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

4. 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)

5. Chilled water coils
Centrifugal pump
Chiller barrel
Metering device
Condenser

6. 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

7. 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

8. 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

9. 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)

10. Solenoid valve (closed) Common suction line
Common discharge line
Compressor cylinders and pistons (the left cylinder is not pumping)

11. 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

12. 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

13. 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

14. Centrifugal compressor housing
Impeller
Suction (inlet)
Discharge (outlet)
Centrifugal compressor housing

16. 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

17. 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

18. 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

19. Water to remote cooling coils
One-pass chiller barrel
Suction line
Water to remote cooling coils
Chiller barrel
Liquid line
Centrifugal pump

20. Water to remote cooling coils
Two-pass chiller barrel
Suction line
Chiller barrel
Water to remote cooling coils
Liquid line
Centrifugal pump

21. Water to remote cooling coils
Three-pass chiller barrel
Suction line
Water to remote cooling coils
Chiller barrel
Liquid line
Centrifugal pump

22. Water to remote cooling coils
Four-pass chiller barrel
Suction line
Chiller barrel
Water to remote cooling coils
Liquid line
Centrifugal pump

23. 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

24. 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

25. 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

26. 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

27. 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

28. Directs water through the tubes Hot discharge gas from compressor
Access to tubes for cleaning
Subcooled liquid from condenser
Water box

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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°

43. 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

44. 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

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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

51. 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

52. 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

53. 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

54. 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

55. 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

56. MOTORS AND DRIVES FOR COMPRESSION CYCLE CHILLERS
Motor starting methods
Part-winding
Autotransformer
Wye-delta (star-delta)
Electronic start

57. 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

58. 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

59. 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

60. 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

61. 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

62. 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

63. 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

64. 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

65. 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

66. 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

67. 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%

68. 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

69. 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

70. 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

71. 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

72. 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

73. 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

74. 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

75. 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

76. 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