Basic Refrigeration Cycle

Made by

Engr. Zubair Latif

Comsats institute of information technology, Lahore

IntroductionA major application area of thermodynamics is refrigeration,

which is the transfer of heat from a lower temperature region to a

higher temperature one.

Devices that produce refrigeration are called refrigerators, and the

cycles on which they operate are called refrigeration cycles.

The most frequently used refrigeration cycle is the vapor

compression refrigeration cycle in which the refrigerant is

vaporized and condensed alternately and is compressed in the

vapor phase. Another well-known refrigeration cycle is the gas

refrigeration cycle in which the refrigerant remains in the gaseous

phase throughout

Principles of Refrigeration

Liquids absorb heat when changed from liquid to gas(latent heat of vaporization)

Gases give off heat when changed from gas to liquid.(latent heat of condensation)

Four basic processes

1. 1-2 isentropic compression (in a compressor)

2. 2-3 constant pressure heat rejection ( in a condenser )

3. 3-4 isentropic expansion ( in a metering device such as

throatling valve)

4. 4-1 constant pressure heat addition ( in a evaporator)

The cycle of cooling

Compressor

The refrigerant comes into the

compressor as a low-pressure gas, it is

compressed and then moves out of the

compressor as a high-pressure gas.

Condenser

The gas then flows to the condenser.

Here the gas condenses to a liquid, and

gives off its heat to the outside air.

Expansion Valve/Metering Device

The liquid then moves to the expansion

valve under high pressure. This valve

restricts the flow of the fluid, and lowers

its pressure as it leaves the expansion

valve.

Evaporator

The low-pressure liquid then moves to

the evaporator, where heat from the

inside air is absorbed and changes it

from a liquid to a gas.

Compressor

As a hot low-pressure gas, the

refrigerant moves to the compressor

where the entire cycle is repeated.

Coefficient of performance (COP)• The performance of refrigerators and heat pumps is expressed

in terms of coefficient of Performance, defined as

COP = desired output / required input

Or

COP = Cooling effect/ work input

= QL/Wnet,input

• A rule of thumb is that the COP improves by 2 to 4 percent for

each °C the evaporating temperature is raised or the condensing

temperature is lowered.

Selecting the right refrigerant• There are several types of refrigerants such as CFCS, ammonia,

hydrocarbons,( propane, ethane, ethylene etc.), Carbon dioxide,

air( in the air conditioning of aircraft) and even water ( in the

applications above the freezing point).

• The right choice of refrigerant depends on the situation at hand

• The industrial and heavy-commercial sectors were very satisfied

with ammonia, and still are, although ammonia is toxic but due

to its several advantages like higher COPs, low cost, no effect on

ozone etc.