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
- Primary refrigerants : Cools the substance by absorbing latent heat.
- 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.
Last updated on December 17, 2003
, 08:08 PM