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Mechanical Vapor compressiontype of refrigeration
Has four basic elements:
an evaporatora compressor,
a condenser, anda refrigerant flow control (expansion
valve).
A refrigerant circulates among the fourelements changing from
liquid to gas andback to liquid.
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I n the evaporator, the liquid refrigerantevaporates (boils)
under reduced pressure and indoing so absorbs latent heat of
vaporization andcools the surroundings.
The evaporator is at the lowest temperature inthe system and
heat flows to it.
This heat is used to vaporize the refrigerant.
The temperature at which this occurs is afunction of the
pressure on the refrigerant: forexample if ammonia is the
refrigerant, at -18 oCthe ammonia pressure required is 1.1 kg/sq.
cm.
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R efrigerant recycling
This process is necessary only for therefrigerant to be returned
to the evaporatorto continue the cycle.
The refrigerant vapour is sucked into acompressor, a pump that
increases thepressure and then exhausts it at a higher
pressure to the condenser.
For ammonia, this is approx. 10 kg/sq. cm.
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To complete the cycle, the refrigerant must be
condensed back to liquid and in doing this itgives up its latent
heat of vaporization to somecooling medium such as water or
air.
The condensing temperature of ammonia is29 oC, so that cooling
water at about 21 oC couldbe used.
I n home refrigerators, the compressed gas(not ammonia) is sent
through the pipes at theback, which are cooled by circulating air
aroundthem.
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O ften fins are added to these tubes toincrease the cooling
area
The gas needs to be compressed so that itcould be condensed at
these highertemperatures, using free cooling from water orair.
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The refrigerant is now ready to enter the
evaporator to be used again.I t passes through an expansion
valve to
enter into the region of lower pressure,
which causes it to boil and absorb moreheat from the load.
By adjusting the high and low pressures,
the condensing and evaporatingtemperatures can be adjusted as
required.
