Centrifugal chillers are used for cooling large buildings in
a centralized air conditioning system. There are other types
of chillers using screw compressors and reciprocating
compressors. These are usually smaller in size. Today's topic
is about centrifugal chillers - the ultimate choice for any
air conditioning installations higher than 500 RT
(refrigeration tons)
Basically, centrifugal chillers use centrifugal fans to move
the refrigerant within the chiller circuit. As with any
refrigeration circuit, there must be a compressor, a
condenser, an expansion device, and the evaporator.
In the case of the centrifugal chiller, the fan is the
compressor. Rotating at very high speeds, it is capable of
pressurizing the refrigerant gas so as to increase its
temperature. Because the pressure is so dependent on the
efficiency of the impeller of the compressor, the impeller
is very carefully designed to match the system.
The clearances between the impeller and the housing at the
mouth ring (impeller suction) are made very small so as to
reduce leaks back to the discharge.
Usually inlet vanes that throttle the gas control the flow
of the gas, and thus the chiller load. The expansion of
the liquid to gas is usually made through a fixed orifice.
The lubrication of the shaft bearings must be done within
the chiller itself. Oil passages and shaft driven pumps
(with motor driven pumps at startup) is made for
lubrication.
The oil will find their way to the condenser and has to be
extracted back to the lubrication system. Usually there
will be an eductor system to suck back the oil to the oil
sump.
Centrifugal chillers are usually designed for low-pressure
refrigerant like R-123a.
These chillers sometimes surge when running, and they are
very sensitive to sudden changes in loads. Therefore
modern centrifugal chillers make use of electronic
microprocessors to control the various parameters and
timing in order to run smoothly.
Indeed, when everything is perfect, the centrifugal
chiller is capable of extremely smooth operation.
Folks, be cool!
Until next time…
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