Working Of Vapor Compression Refrigeration Cycle / VCRS

In an actual vapor refrigeration cycle, an expansion engine is not used since power recovery is small and does not justify the cost of the engine. A throttling valve or a capillary tube is used for expansion in reducing the pressure from p2 to p1. The basic operations involved in a vapor compression refrigeration plant are illustrated in flow diagram.

flow diagram

T-S diagram

p-v diagram

The operations represented are as follows for an idealized plant :

1. Compression :

A reversible adiabatic process 1-2 or 1-2' either starting with saturated vapor (state 1), called by dry compression, or starting with wet vapor (state 1') called wet compression. Dry compression (1-2) is always preferred to wet compression (1'-2'), because with wet compression there is a danger of the liquid refrigerant being trapped in the head of the cylinder by the rising piston which may damage the valves or the cylinder head, and the droplets of the liquid refrigerant may damage the valves or the cylinder head, and the droplets of the liquid refrigerant may wash away the lubricating oil from the walls of the cylinder, thus accelerating wear.

2. Cooling and condensing :

A reversible constant pressure process, 2-3 , first desuperheated and then condensed, ending with saturated liquid . Heat Q1 is transferred out.

3. Expansion :

A adiabatic throttling process 3-4 , for which enthalpy remains unchanged, states 3 and 4 are equillibrium points. Process 3-4 is a adiabatic (then only h3=h4 by SFEE), but not isentropic.

TdS = dh - vdp , or s4-s3 = - Integral (vdp / T)

Hence it is irreversible and cannot be shown in property diagrams. States 3 and 4 have simply been joined by a dotted line.

4. Evaporation :

A constant pressure process, 4-1 which completes the cycle. The refrigerant is throttled by the expansion valve to a pressure, the saturation temperature at this pressure being below the temperature of the surroundings. Heat then flows, by virtue of temperature difference, from the surroundings, which gets cooled or refrigerated, to the refrigerant which then evaporates absorbing the latent heat of evaporation. The evaporator thus produces the cooling or the refrigerating effect, absorbing heat Q2 from the surroundings by evaporation.

The rate of work Input to the compressor ,

Wc = w(h2-h1) kJ/s

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