J chilled-water cooling hydronic systems

Chilled-Water Systems

Used for large central air-conditioning applications and industrial process cooling
• Chilled water circulates throughout the building, absorbs heat, then returns to the chiller for cooling

45°F

for water supplied to a building

55°F

for water returned to chiller

Types of compressors:

Reciprocating, scroll, screw and centrifugal

Compressor

lowers suction pressure and increases discharge pressure

38°F

Normal boiling point of the refrigerant

105°F

Normal condensing temperature of the refrigerant

Reciprocating Compressors in High-Pressure Chillers

Multiple small compressors commonly used instead of one large
compressor
Note: Protects against system failure
• Classified as positive displacement pumps
• May have up to sixteen cylinders
• Large chillers must have capacity control:
• Prevents compressor short cycling and smooths out temperature
fluctuations

Scroll Compressors in High-Pressure Chillers

Welded, hermetic, positive displacement compressor
• Advantages: Efficiency, quietness, fewer moving parts, size and weight, and ability to pump small
amounts of liquid refrigerant without compressor damage
• Offers little resistance to refrigerant flow
• Equipped with check valves to prevent backward flow in the off cycle

Uses scroll compressors to provide both heated and chilled water for many applications: Swimming
pool heating, commercial aquariums, radiant heating, ice/snow-melting, and conventional chilled-
water comfort cooling
• Multiple scroll compressors allow them to function efficiently and effectively during both high- and
low-load period

Rotary Screw Compressors in High-Pressure
Chillers:

Can handle large volumes of refrigerant with few
moving parts
• Positive displacement pump able to handle some liquid
refrigerant without damage
• Range from 50 to 700 tons
• Two types: semi hermetic rotary screw compressors and
open-drive compressors
• Capacity control done with a slide valve
• Usually equipped with oil separators

Centrifugal Compressors in High-Pressure
Chillers

moves refrigerant from the low- to
the high-pressure side of the system
• Gearboxes enable the compressor to reach speeds of
about 30,000 rpm
• Cause slight loss of efficiency
• Typically two gears
• When the head pressure becomes too high or the
evaporator pressure becomes too low, the
compressor stops pumping is lubricated by a separate motor and pump:
• Lubrication system is sealed
• Oil cooler is used to prevent overheating
• Capacity control is accomplished by the use of guide
vanes
• Load limiters prevent compressor overload
• Centrifugal compressors can be hermetically sealed or can
have open drives

Evaporators For High-Pressure Chillers

Liquid refrigerant boils when it absorbs heat from the circulating water
• Most commonly made of copper
• Best heat exchange is between water and liquid refrigerant
• 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
• Used on smaller chillers
• Refrigerant enters the chiller barrel from the end

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°F and leaves at 45°F
• Refrigerant is usually about 7°F cooler than the leaving water (approach temperature)
• Flooded chillers usually have permanently mounted thermometers and pressure gauges

Condensers for High-Pressure Chillers

• Used to transfer heat from the system
• Can be air-cooled or water-cooled
• Heat can be recovered for use in other applications

Water-Cooled Condensers

Usually shell-and-tube type
• Water circulates in the tubes
• Refrigerant fills space around the tubes
• Shell must have:
• Working pressure to accommodate
refrigerant
• Water-side working pressure of 150-300
psig
• Can be equipped with standard water boxes
or marine water boxes

Air-Cooled Condensers

• Usually constructed of copper tubes and
aluminum fins
• Require less maintenance than water-cooled
condensers
• Multiple fans are used for head pressure control
purposes
• Head pressures are typically higher on air-
cooled systems

Metering Devices for High-Pressure Chillers Four types of metering devices may be used for large chillers

1. Thermostatic expansion valves
2. Orifices
3. High and low-side floats
4. Electronic expansion valves

Thermostatic Expansion Valves (TXV)

• Maintain constant evaporator superheat
• The more evaporator superheat, the more vapor
will be in the evaporator, and less heat exchange
will occur
• TXV valves are not used except on smaller
chillers

Orifices

• Fixed bore metering device that is
merely a restriction in the liquid line
between the condenser and the
evaporator
• No moving parts
• Flow rate determined by the pressure
drop across it
• Overcharging the unit will cause liquid
refrigerant to enter the compressor

Low-side float

Rises to reduce refrigerant flow
when the liquid refrigerant level
rises in the low side of the chiller
• Maintains a constant liquid level in
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 evaporato

Electronic Expansion Valves

• Operate with a thermistor to monitor the refrigerant
temperature
• The large liquid handling capacity allows maximum
liquid refrigerant flow
• Often called step-control valves:
• Many increments of modulated flow with their
controls

Low-Pressure Chillers

Typically use R-11, R-113, or R-123
• Equipped with the same components as high-pressure chillers
• Many features and operations are the same or similar to those found on or with high-pressure units

Compressors for Low-Pressure Chillers

• Use centrifugal compressors; run up to 30,000
rpm
• Suction line fastened to housing
• Compressed refrigerant is trapped in the volute
and guided to the condenser
• Can be operated in series
• Can have up to three stages of compression
• Can have refrigerant working pressure as low
as 15 psi

Condensers for Low-Pressure Chillers

• Water-cooled
• Usually shell-and-tube type:
• Water is circulated through the tubes
• Refrigerant circulates around the tubes
• Located above the evaporator, so liquid leaving
the condenser flows by gravity
• Can also have a subcooling circuit

Metering Devices for Low-Pressure Chillers

• Hold liquid refrigerant back
• Meter the liquid into the evaporator at the correct rate
• Two types commonly used:
• Orifice
• High or low-side float

Purge Units

Low-pressure chillers operate with the suction pressure in a vacuum
• If a leak occurs, air will enter the system:
• Air can cause system problems
• Air can be removed by the purge unit
• Several types of purge systems:
• Pressure switch and solenoid valve
• Difference in pressure from the high-pressure side to the low-pressure side