All about CENTRIFUGAL CHILLERS

What is a chiller

A chiller is an equipment which uses either the vapour compression system or vapour absorption system to cool a liquid. A chiller uses the reverse rankine cycle also refered as the Vapour compression cycle for producing the cooling effect. Basic design of a chiller system working in the vapour compression referegeartion cycle includes

1) Compressor     2) Condensor  3) Expansion valve  4) Evaporator

Four types of compressors are widely being used for the purpose of compression viz the rciprocating compressors, the scroll compressors, the screw compressors and the centrifugal compressors.

The Vapour Compression Refregeration Cycle

 All chillers uses the vapour compression refregeration cycle to chill the water passing in the evaporator by rejecting the heat from the chilled water and the heat from the compressor to the water that is fed to the cooling tower.

The main parts of a Vapour compression refergeration cycle are

1) Compressor : The compressor is the main part of the chiller system. It is the heart of the system. Here the refergerant is being sucked from the evaporator and is compressed to such high pressures which make the word Vapour compression refrigeration a reality. The compressor incorporated in a centrifugal chiller is of the type of Centrifugal compressor.The centrifugal compressor is a non positive displacement compressor which converts the kinetic energy of the gas to pressure energy using the vanes and volute casing present in its design. The compressor is mainly driven by a primover which may be a motor. Phase1-2 in the fig:.

2) Condensor : Condensor is a heat exchager,generally of the shell and tube type. Here the high pressure high temeprature refrigerant leaving the compressor will be cooled to the liquid state at high pressure.The high temeprature high pressure refrigerant will exchage the heat with a water source passing through the tubes, thereby giving out its superheat and condenses to liquid at high pressure.Phase 2-3 -4 in the fig:.

3) Expansion valve  : The high presuure low temeprature refergerant is passed through an expansion device.Here the refrigirant will expand suddenly to low pressure thorugh a controlled orifice producing a low temperature. The low temperature liquid vapour mixture will reach the evaporator. Phase 4-5 in fig:.

4) Evaporator :  The evaporator is a shell and tube heat exchanger as like the condensor. Here the refrigerant is in the state of low pressure low temperature gas and liquid. Through the evaporator tubes the liquid to be cooled is passed. The refregerant will change its phase to vapour by absorbing the required latent heat from the liquid passing through the tubes.

5) The Refregerant :  The refrigerant is the blood of the chiller system. The desired effect is produced due to the special properties of the refrigerant.  In olden days most of the refrigerants used were of CFC type(Chloro Fluro Carbons) which has got a high ozone depleting potential. Now a days more environment friendly types such as HFC(Hydro Fluro Carbons are used). They are having low Ozone depleting potential.The basic properties of Refrigerants are

1) The refrigerant should have low boiling point and low freezing point.

2) It must have low specific heat and high latent heat. Because high specific

heat decreases the refrigerating effect per kg of refrigerant and high latent

heat at low temperature increases the refrigerating effect per kg of

refrigerant.

3) The pressures required to be maintained in the evaporator and condenser

should be low enough to reduce the material cost and must be positive to

avoid leakage of air into the system.

4) It must have high critical pressure and temperature to avoid large power

requirements.

5) It should have low specific volume to reduce the size of the compressor.

6) It must have high thermal conductivity to reduce the area of heat transfer in

evaporator and condenser.

7) It should be non-flammable, non-explosive, non-toxic and non-corrosive.

8) It should not have any bad effects on the stored material or food, when any

leak develops in the system.

9) It must have high miscibility with lubricating oil and it should not have

reacting properly with lubricating oil in the temperature range of the system.

10) It should give high COP in the working temperature range. This is

necessary to reduce the running cost of the system.

11) It must be readily available and it must be cheap also.