Components In Vapor Compression Cycle

1. Condenser :

It must desuperheat and then condense the compressed refrigerant. Condensers may be either air cooled or water cooled. A air-cooled condenser is used in small self - contained units. Water cooled condensers are used in larger installations.

2. Expansion Device :

It reduces the pressure of the refrigerant, and also regulates the flow of the refrigerant to the evaporator. Two widely used types of expansion devices are : Capillary tubes and throttle valves ( Thermostatic expansion valves).Capillary tubes are used only for small units. Once the size and length are fixed, the evaporator pressure. etc., gets fixed. No modification in operating conditions is possible. Throttle valves are used in larger units. These regulate the flow of the refrigerant according to the load on the evaporator.

3. Compressor :

Compressors may be of three types:

i) reciprocating , ii) rotary, iii) centrifugal.

When the volume flow rate of the refrigerant is large, centrifugal compressors are used. Rotary compressors are used for small units. Reciprocating compressors are used in plants up to 100 tonnes capacity. For plants of higher capacities , centrifugal compressors are employed.

In reciprocating compressors, which may be single - cylinder or multi - cylinder ones, because of clearance, leakage past the piston and valves, and throttling effects at the suction and discharge valves, the actual volume gas drawn into the cylinder is less than the volume displaced by the piston. This is accounted for in the term volumetric efficiency, which is defined as,

N vol = Actual volume of the gas drawn at evaporator pressure and temperature / Piston displacement

Volume of the gas handled by the compressor , = w. v1 (m3/s) = (3.14 / 4) * (D^2)*L *(N / 90)n * vol. eff

where w is the refrigerant flow rate,

'v1 is the specific volume of the refrigerant at the compressor inlet,

D and L are the diameter and stroke of the compressor,

n is the number of cylinders in the compressor,

N is the r.p.m

The clearance volumetric efficiency is given by,

vol. eff. = 1+ C-C (p2 /p1)^ (1/n)

where C is the clearance.

4. Evaporator :

A common type of evaporator is coil brazed on to a plate, called a plate evaporator. In a flooded evaporator the coil is filled only with the liquid refrigerant. In an indirect expansion coil, water (up to 0*C) or brine (for temperatures between and 21*C) may be chilled in the evaporator, and the chilled water or brine may then be used to cool some other medium.

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