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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
CHILLED
WATER
OUT
CHILLED
WATER
IN
SENSING
BULB
HEADER
CHILLED
WATER OUT
CHILLED
WATER IN
SHELL
REFRIGERANT
LIQUID INLET
Fig. 3. Flooded Shell and Tube Chiller Evaporator.
REFRIGERANT
SHELL
VAPOR OUTLET
BAFFLES
CHILLED
REFRIGERANT
WATER IN
LIQUID INLET
Fig. 4. Direct Expansion Chiller Evaporator.
The direct expansion chiller evaporator is the reverse of the
flooded shell and tube chiller evaporator, water in the shell and
the refrigerant in the tubes.
The compressor can be reciprocating, centrifugal, or screw
type. The centrifugal and screw types are generally found on
the larger systems.
The chiller condenser is usually water cooled but may be air
cooled or evaporative cooled. The most common water cooled
condenser is the shell and tube type (similar to Figure 3). The
cooling (condenser) water flows through the tubes and the
refrigerant vapor condenses on the cool tube surface and drops
to the bottom of the shell where it flows into the liquid line to
the receiver or evaporator.
EXPANSION VALVE
EVAPORATOR
(HEAT EXCHANGER)
COMPRESSOR
SUCTION
(LOWSIDE)
SUCTION LINE
PRESSURE
LIQUID LINE
Fig. 2. Typical Vapor-Compression Cycle Water Chiller.
REFRIGERANT
HEADER
VAPOR OUTLET
TUBES
C2687
CHILLED
WATER OUT
U-TUBES
C2688
HOT GAS LINE
HEAD
(HIGHSIDE)
PRESSURE
CONDENSER
(HEAT EXCHANGER)
RECIEVER
An air cooled condenser is a series of finned tubes (coils)
through which the refrigerant vapor flows. Air is blown over
the coils to cool and condense the refrigerant vapor.
An evaporative condenser is similar to the air cooled
condenser where the refrigerant flows through a coil. Water is
sprayed over the coil and then air is blown over the coil to
evaporate the water and condense the refrigerant. Evaporative
condensers are rarely used because of the additional
maintenance compared with an air cooled condenser.
CENTRIFUGAL COMPRESSOR
Centrifugal compressors are available in a wide range of sizes.
Compressor capacity can be modulated from maximum to
relatively low values. Centrifugal chiller systems can be
designed to meet a wide range of chilled liquid (evaporator)
and cooling fluid (condenser) temperatures.
Operation of the compressor is similar to a centrifugal fan or
pump. Gaseous refrigerant enters the inlet (Fig. 5) and passes
through inlet vanes into the chambers or blades radiating from
the center of the impeller. The impeller, rotating at a high rate
of speed, throws the gas to the outer circumference of the
impeller by centrifugal force. This increases the velocity and
pressure of the gas. The gas is then thrown from the impeller
into the volute where most of the velocity (kinetic energy) is
converted to pressure.
Use of a larger evaporator and condenser decreases the energy
needed by the compressor for a given cooling load. Typical
single stage high speed compressor construction is shown in
Figure 5. The prerotation vanes (inlet guide vanes), located in
the suction side of the compressor, control the gaseous
refrigerant flow by restricting flow. As the vanes vary the flow,
the compressor pumping capacity varies. In the open position
the vanes give a rotating motion to the refrigerant in a direction
opposite to the impeller rotation. This allows the chambers or
blades to pick up a larger amount of gas.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
298
CONDENSER
WATER
OUT
CONDENSER
WATER
IN
C2685
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