Refrigeration in detail

Refrigeration :-

A refrigeration system is an apparatus or combination of mechanical equipment in which a refrigerant is circulated for cooling, or extracting heat from spaces or bodies. Refrigeration system could be of two types, viz compression and absorption. Compression system, also sometimes known as mechanical system is the more popular of the two. A refrigerant having its boiling temperature less than atmospheric temperature will start boiling immediately if subjected to atmospheric conditions, absorbing heat from its surroundings, thereby producing cooling effect To be economical it is essential that means must be available to capture and reuse the refrigerant vapour.

(¡) The refrigerant gas is compressed by the compressor and discharged into condenser, (ii) In conenser it is cooled and condensed to liquid; atmospheric air or water being used for abstracting heat from condenser, (iii) Condensed, and warm high pressure refrigerant then travels through strainer/ drier which prevents plugging of the flow control device by trapping scale, dirt and moisture, (iv) Flow control device controls the flow of refrigerant to and refrigerant is throttled, (v) in the evaporator, refrigerant is subjected to a much lower pressure due to the combined suction of the compressor and the pressure drop across the flow control device. As a result, refrigerant tends to expand and evaporate thereby cooling the air passing over evaporator.

Refrigeration takes place from the absorption of heat during vaporisation in the cooling coils (vi) The gas is again drawn into compressor and the cycle is repeated. Refrigeration systems may be classified as (i) flooded type and (ii) dry type depending on the condition of the refrigerant in the evaporator. In flooded system, there is a pool of refrigerant in the evaporator. Dry systems have only droplets of liquid in the evaporator, there being almost all f refrigerant in vapour condition in evaporator.
Flooded systems are further classified as follows depending on the type of flow control device used (i) low side float system, (ii) high side float system, (iii) capillary tube or restrictor systems. Dry type systems are of two types (i) automatic expansion systems and (ii) thermostatic expansion valve systems. In flooded systems, the refrigerant level in the evaporator is maintained through the action of the refrigerant flow control device. Flooded system has following advantages over dry type 
(i) As system operates at higher average suction pressure, the operating cost is low, and efficiency is high 
(ii) It provides closer control of temperature.
(iii) More liquid on low pressure side of the system provides a greater area ofwetted surface, and allows a higher rate of heat transfer through the evaporator walls and tubing.
 (iv) Number of starts and stops of compressor are low .
(v) operating cost is lower. 

Fig:- low side float system. 

In dry system, the refrigerant passing through the valve is partially evaporated immediately after passing the orifice. The fine suspended droplets of liquid refrigerant are completely evaporated as they flow through the balance of the evaporator. Evaporator is generally made from one continuous length of tubing which ensures better control of liquid refrigerant when the flow control device is properly adjusted. 
The working of which is self-explanatory. Important point to note is that refrigerant charge used in system is not critical (which is so in other systems) as any excess or over-charge will remain in the liquid receiver.

As the flow of liquid refrigerant in this system is controlled by both the high side float in the liquid receiver and the pressure reducing valve at the evaporator to maintain differential pressure between the refrigerant in the liquid line and the refrigerant in the evaporator, the refrigerant charge in the high side float system is very critical. Excess charge will result in flood back and liquid refrigeran in that case will flow into suction line of compressor causing frost to appear and may damage compressor. Insufficient charge may result in a starved evaporator and loss of refrigeration.

Fig:- High side float system. 

capillary tube or restrictor system is widely used because of low cost, simplicity, trouble-free operation and achieving of hermetically sealed system. The amount of refrigerant allowed to flow is dependent upon the length and inside diameter of capillary tube provided between the filter and evaporator. This system also requires an accurate charge of refrigerant. Overcharge may result in floodback. Automatic expansion valve system uses a pressure operated refrigerant flow control device which when properly adjusted, keeps the evaporator fully refrigerated. In thermostatic expansion valve system, thermostatic control is used to maintain evaporator outlet temperature in combination flow of with evaporator pressure by automatic expansion valve. It provides more accurate control of refrigerant in the evaporator.

The capacity of a refrigerator is the rate at which refrigeration is produced and is expressed in tons of refrigeration. One ton of refrigeration is defined as the heat rate corresponding to melting of 1 ton of ice in 24 hours Assuming latent heat of fusion of ice as 80 kcal/kg one ton of 1 ton of ice in 24 hours


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