REFRIGERATION LAWS CLASS

NOTES FOR MECHANICAL

ENGINEERING

Evaporator abstracts heat

from the products/space to be

cooled. Compressor adds energy to raise

its temperature and pressure. This enabled

the refrigerant to lose heat in the

condenser equal to the sum of

cooling effect and work input.

High pressure helps the refrigerant

to re-circulate. Re-circulation causes

the cooling again and again. Boiling

point increases with the increase of

pressure.

 Refrigeration laws

1. Work input in compressor converts into heat. Condenser loses heat equal to sum of heat gained in the evaporator  and work input in the compressor.

Q _condenser =Cooling effect +work input to compressor=N+W

2. Heat causes refrigeration in vapor absorption refrigeration. Heat is a substitute  compressor in the vapor compression refrigeration.

3.On increase of pressure, the boiling temperature increases. It is applicable for all refrigerants in the evaporator & condenser.

Boiling point of ammonia=–33.5⁰ C

Normal Boiling point of R 22          =–40.9⁰ C

Boiling point of R 134 a       =–26.5⁰ C

The temperature in the evaporator, in most of the applications, is–15⁰ C. Hence boiling point is to increased from –33.5 to -15⁰ C. Thus, pressure in the evaporator becomes more than one atmospheric pressure.

Similarly pressure in the condenser is many times more. The condensation of ammonia is at +30⁰ C. Thus there is increase of pressure both in the evaporator and condenser. It is due to the fact that increase of the boiling point increases with increase of pressure.

4. On decrease of pressure, the boiling temperature decreases. It applies in the evaporator and condenser of steam jet refrigeration system. Pressure in the flash chamber is vacuum. It  applies in the expansion valve of the vapor refrigeration system. As the pressure decreases, the boiling point decreases  in the expansion. In the expansion valve, only evaporation decreases the temperature of the refrigerant.

5. Heat naturally flows higher temperature to low temperature. This is applicable in the evaporator. The temperature of the refrigerant in the evaporator is less than the temperature  of the cooled material. The temperature of the refrigerant in the condenser is much higher than the temperature of surrounding  air or water.

6. Clausius statement of second Law of Thermodynamics

  Heat cannot flow from lower temperature to higher temperature without the help of an external agent. This is applicable in all the systems of refrigeration.  Heat  transfers from low temperature  to  high temperature.  External agent used is compressor. Compressor helps to transfer heat from low to high temperature.

7. Cooling effect in vapor refrigeration is very large with small amount of refrigerant. It is possible that liquid changing into vapor in the evaporator requires latent heat. Latent heat effect in vapor refrigeration is very large. Sensible heat effect in air refrigeration is less.

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