3. THEORY

A refrigeration cycle works to lower and maintain the temperature of a controlled space by the process of heat transfer from a low to a high temperature region.

High Temperature Reservoir, TH

QH

E

.Wnet

QL

Low Temperature Reservoir, TL

Another term for the cooling effect of the refrigeration system is Refrigeration duty, which is the rate of heat being removed from the low temperature region with specified evaporation and condensation temperatures. The unit for duty measurements is in Watts (for 1 ton of refrigeration = 3517W).

3.1 The Vapor Compression Cycle

The theoretical Reversed Carnot Cycle process are been followed by Ideal refrigeration systems. In practical refrigerators, compression and expansion of a gas and vapor mixture presents practical problems in the compressor and expander. Hence, in practical refrigeration, compression usually takes place in the superheated condition and a throttling process is substituted for the isentropic expansion.

The cycle :

1 2Condensation of the high pressure vapour during which heat is transferred to the high temperature region.2 3Adiabatic throttling of the condensed vapour from the condensing to the evaporating pressure.3 4Evaporation of the low pressure liquid during which heat is absorbed from the low temperature source.4 1Isentropic compression of the vapour, from the evaporating to the condensing pressures.

Energy Transfers Analysis

Compressor

q4-1 = h4 h1 + w4-1

If compression is adiabatic, q4-1 = 0, and w4-1 = h1 h4 = wcomp

Power requirement, P = (h1 h4), where is the flow rate of working fluid per unit time.

Condenser

q1-2 = h2 h1 + w

w = 0, therefore q1-2 = h2 h1 and rate of heat rejection Q1-2 = ( h2 h1 )

Expansion Valve

q2-3 = h3 h2 + w

w = 0, therefore q 2-3 = h2 h3 and process is assumed adiabatic

therefore h2 = h3

Evaporator

q34 = h4 h3 + w

w = 0 therefore q34 = h4 h3 and rate of heat absorbed Q34 = ( h4 h3 )

Coefficient of Perfomance, COPref:

COPref = =