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CHILLER DESIGNS
Other Major
Components in a Chiller Design
- CPR Valve. The CPR or crankcase pressure regulating valve is an
inline valve placed in the suction or return line to the compressor. This
valve regulates the maximum allowable pressure to return to the
compressor. Without this device, during a warm fluid startup of the
chiller unit, a high pressure and temperature gas may return to the
compressor and cause a failure of the compressor.
- Desuperheating Valve. The desuperheating valve is used when the
fluid being cooled will be used at a temperature higher than what would
return a cool gas to the compressor. The DSH valve is similar to an
expansion valve in that it takes the liquid refrigerant, drops its
pressure and temperature, and injects it into the suction line rather than
a heat exchanger. The design is a little bit different in that it does not
allow as much liquid refrigerant pass through to the suction line. As the
DSH valves allows this cold refrigerant mist to enter the suction line, it
cools this return gas to the compressor. (How is this different from the
CPR valve? The CPR is mainly designed for startup conditions of high
pressure overload. The DSH is for constant suction line cooling.)
- Hot Gas Bypass. HGB is used for capacity reduction and
sometimes a defrost cycle for a refrigeration system. As I stated earlier
in this course, a chiller must be designed for its load. If you have a
varying load, you need the chiller that will adjust accordingly. Say, for
example, you have a demand of 10 tons of cooling at 100%, but only needed
7 tons of cooling for a few hours of the day. A system without the HGB
would do the job for the full load task, but when the load decreased for
the 7 ton task, the compressor would "short cycle" or turn off and on
constantly. This off and on routine of the compressor can greatly reduce
its lifespan or damage it immediately because of starting up when the
refrigerant pressures are not equalized. What the HGB does is simple. It
takes the discharge hot gas from the compressor and "bypasses" the
expansion valve and sends hot gas to the heat exchanger. This simulates a
load in the heat exchanger and the compressor stays running. the
compressor amps go down because it is not working as hard, but at the same
time it is running at a reduced capacity. The hot gas also defrosts the
heat exchanger and compressor because of the raised heat exchanger
temperature. With the HGB, one should always use a Desuperheating valve to
insure that the hot gas is properly cooled before it is returned to the
compressor.
- Low Ambient Controls. LAC is a control used when ambient air
temperatures are below the temperature of which the refrigerant is able to
condense properly in the condenser. In lay terms, below about 70 deg F for
R22 refrigerant. The LAC controls can be in a variety of ways. first is
the fan cycle control. What this does is turn the fan on and of in
relation to the pressure of the liquid refrigerant leaving the condenser.
Another control is the fan speed control. This is a somewhat better
control (and costly) than the fan cycle control. This device raises and
lowers the speed of the condenser fan according to the temperature of the
liquid refrigerant leaving the condenser coil. This device provides less
wear on the fan motor by not starting and stopping as often, only changing
the fan's speed. The third design of LAC is the flooded condenser control.
this is a valve that will restrict some liquid refrigerant in the
condenser coils to create less surface area for the refrigerant to cool
within the condenser coils. this is similar to partially blocking a
radiator in your car in the winter months so you car will warm up more
quickly. This control will allow the refrigerant pressures to be
maintained without the fan control devices.
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