The following
articles will consider refrigerants and compressors which are two of the more
complex components of vapour compression chillers.
Refrigerants. Refrigerants are liquids that evaporate very
easily at relatively low temperatures. Refrigerants are so volatile that if a
liquid refrigerant was spilled in a room at normal temperatures it would very
quickly disappear by evaporation. Refrigerants must posses good thermodynamic
properties but also have low toxicity and low flammability. Refrigerants are
also the subject of environmental concerns as described (CAREFUL USE OF
REFRIGERANTS).The evaporation and condensation of refrigerants in a chiller
is controlled by lowering and increasing the pressure using the expansion valve
and compressor respectively.
Compressors are electrically driven pumps of which there
are three main types. These are; reciprocating, rotary and centrifugal
compressors.
Reciprocating
compressors work by allowing refrigerant
to flow into a
chamber on the down stroke of a piston.
The refrigerant is
then forced out of this chamber towards
the condenser as the piston moves upwards once
more.
Rotary compressors have two interlocked helical screws
which when rotated move refrigerant which is trapped between
the two screws
along the line of the thread.
Centrifugal
compressors have a rotating impeller which
forces the refrigerant outwards
against the casing.
This force is sufficiently strong to drive the refrigerant
towards the condenser.
Compressors are further classified as either
hermetic, semi hermetic or open depending on the seals between the motor and
the compressor it drives. Hermetically sealed compressors have the motor and compressor
together inside a shell whose seams are sealed by welded joints. Refrigerant is
in contact with the motor and compressor. Semi hermetic compressors are similar
but the joints are bolted rather than welded allowing servicing to take place.
Open compressors have an external motor connected via a shaft to the pumping
mechanism. A seal around the shaft stops refrigerant escaping. The different
forms of compressor are suitable for different cooling load ranges.
Reciprocating up to 180kW, Rotary up to 2MW and centrifugal in the range 180kW
to 3.5MW. Comfort cooling tends to be of a lower cooling capacity and so uses reciprocating
compressors. Rotary and centrifugal compressors are used for large capacity centralized
cooling systems.
