05-06-2012, 12:50 PM
AIRCONDITIONING
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Introduction
The primary function of an air-conditioning system is to maintain conducive conditions for human comfort. Comfort air-conditioning, is defined as the process by which simultaneously the temperature, moisture contents, movement and quality of air in enclosed spaces, intended for human occupancy may be within required limits.
In order to satisfy the stipulated requirements a complete air-conditioning system is to perform the following functions.
• Cooling and dehumidification for summer conditioning.
• Heating and humidification for winter conditioning.
• Air filteration and proper ventilation the year round.
Necessarily, these functions involve control of temp. humidity, purity and movement of air.
Refrigerant Cycle
The refrigerant cycle line diagram has been shown in fig. 1.
Mechanical refrigeration is achieved by alternatly compressing and expanding the refrigerant with the help of a compressor and pressure reducing device (such as expansion valve).
Compressor serves two purposes; first it draws the refrigerant from the evaporator (cooling coil) and forces it into the condenser and secondly it increases the pressure of the refrigerant.
By sucking the refrigerant, the compressor reduces the pressure in the cooling coil and maintains it at a level low enough to permit the refrigerant to boil or vaporize and consequently absorb heat in the process. (refrigerant boils at a relatively low temperature when pressure is reduced).
By discharging refrigerant vapour into the condenser the compressor increases vapour pressure and temperatures. The hot vapour flows to the condenser, where it is condensed into liquid at high pressure giving up heat to atmospheric air or water depending upon whether the condenser is air cooled or water cooled.
System Components
Compressor
The equipment used for compression of the refrigerant forms the heart of the refrigeration unit, and the positive displacement refrigerating compressor is the compression system most commonly used today.
All reciprocating compressors consists of one or more cylinders, suitable valves for suctions and discharge of the refrigerant gas and containing reciprocating pistons for compression. The design of the cylinders are of multicylinders type with automatic capacity control mechanism. Compressors are usually equipped with forced lubrication systems involving a separate oil pump to maintain circulation.
Condenser
It is a heat exchanger and it is in the condenser that the refrigerant must give up the heat absorbed in the evaporator plus the heat added by compressor. Good design further provide for some sub cooling of the liquid refrigerant before it leaves the condenser. The pressure of the refrigerant in the condenser is very little-less than at the compressor discharge, where it is higher than at any other part in the system. Hence the condenser and other equipment in the system between the compressor discharge and expansion valve are often referred to as high side equipment.
