REFRIGERATION AND AIR CONDITIONING

           Refrigeration system is a mechanical system which circulates the coolant or refrigerant to absorb the surrounding heat.  Refrigeration is the withdrawal of heat from a substance or space so that temperature lower than that of the natural surroundings is achieved.  Refrigeration may be produced by

• Thermoelectric means
• Vapor compression systems
• Expansion of compressed gases
• Throttling or unrestrained expansion of gases.

            Vapour compression systems are employed in most refrigeration systems. Here, cooling is accomplished by evaporation of a liquid refrigerant under reduced pressure and temperature. The fluid enters the compressors at state 1 where the temperature is elevated by mechanical compression (state 2). The vapor condenses at this pressure, and the resultant heat is dissipated to the surrounding. The high pressure liquid (state 3) then passes through an expansion valve through which the fluid pressure is lowered. The low-pressure fluid enters the evaporator at state 4 where it evaporates by absorbing heat from the refrigerated space, and reenters the compressor. The whole cycle is repeated.

Refrigerant:

           It has its boiling point below the atmospheric temperature, hence when subjected to atmospheric temperature it absorbs heat and becomes vaporized.  Some of the most commonly used refrigerants are Ammonia, Carbon dioxide, Sulphur di oxide, F - 12 and F - 22.  The atmosphere also gets coooled.  There are two types of refrigerants.  They are

1. Primary refrigerants : Cools the substance by absorbing latent heat.
2. Secondary refrigerants : Cools the substance by absorbing their sensible heat ( E.g. Air, water )

COP:

            It is nothing but, Coefficient of Performance.  It is the ratio of actual refrigeration obtained to the Work done in the system.  But Relative COP is the ratio of actual to Theoretical COP.

Capacity of Refrigeration:

           It is expressed in Tons, the rate at which refrigeration is produced.  One ton of refrigeration is the heat rate for melting one ton of ice in 24 hours.

Air Conditioning:

Absolute Humidity:

           Ratio of weight of water vapor per unit volume.

Relative Humidity:

           The actual amount of moisture in air at any temperature divided by the greatest amount of moisture the air could hold without condensation.
          
Psychrometry:

           It is the branch of science, which deals with the study of mixture of dry air and water vapor.  Dry air contains Nitrogen, Oxygen, Carbon dioxide, Water vapor and traces of other gases.

Moist Air:

           Mixture of dry air and water vapor.  The quantity of water vapor present in air depends upon the temperature.

Moisture:

           The water vapor present in air is called as moisture.

Saturated Air:

           When moist air contains the maximum amount of water vapor, that it can hold, then the air is said to be saturated.  If any more water is added to the saturated air, it remains in suspension and makes the air foggy.  Moist air that is not saturated is called as unsaturated air.

Humidity Ratio:

           It is the weight of water vapor per unit weight of dry air in vapor air mixture.

Degree of Saturation:

           Ratio of Humidity ratio of moist air to the Humidity ratio of saturated air at the same conditions of Temperature and Pressure.

Dew Point Temperature:

           It is the temperature at which moist air just becomes condensed when cooled at constant pressure.

Sensible Heat Factor = Sensible heat / ( Sensible heat + Latent Heat )

Cooling Loads:

           The total quantity of heat that has to be pumped out of a space to maintain a level of temperature using a refrigeration equipment is called as cooling load.