Refrigerants , Refrigeration and air conditioning (RAC), GATE & ESE , Mechanical engineering

 Types of refrigerant

1. Primary refrigerant : they are the working fluid which undergo the cyclic process and produces lower temperature. There is a latent heat of transformation occurs for the refrigerant and the examples are R-11, R-12, R-22, R-134(a)
2. Secondary refrigerant : These are the working fluid which are first cooled with the help of primary refrigerant and than used for cooling at the desired location . eg: H2O, Brine

Designation of refrigerant

1. If the refrigerant is saturated hydrocarbon

Note:

R-134a is known as ecofriendly refrigerant because of the absence of chlorine element which is present in the commonly present refrigerant. The chlorine element attacks the ozone layer which is located in stratosphere. The purpose of the ozone layer is to filter out the harmful ultravoilet radiation which was emitted by the sun . Therefore we have to prefer those types of refrigerant which are having a lower value of ozone depletion potential (ODP).  The other alternatives are flourocarbon , etc.

2. If the refrigerant is unsaturated hydrocarbon:

3. If the refrigerant is inorganic compound then:

Properties of refrigerant 

a) Thermodynamic properties

1. Critical temperature

It should be as high as possible or at least above the condensor temperature and pressure.

Note:

• The critical temperature of CO2 and ethylene are almost undesirable for the indian summer ambient condition.
• The highest value of critical temperature is for water.

2. Specific heat

The specific heat of vapour refrigerant should be as high as possible in order to limit the degree of superheat whereas the specific heat of liquid refrigerant should be low in order to minimize the irreversibility i.e. lower value of ds.

3. Enthalpy of vapourisation

It should be as high as possible , as it helps in reducing the mass flow rate of refrigerant for the same capacity.

Note : Among the commonly used working fluid ammonia is having higher enthalpy of vapourisation.

4. Conductivity

It should be as high as possible as it helps in reducing the size of evaporator and condencer.

5. Evaporator pressure and condensor pressure

Both should be positive if the evaporator pressure is less than atmospheric pressure then their is a possibility for leakage of air . Therefore evaporator pressure should be kept almost equal to atmospheric pressure whereas condensor pressure should have some moderate values.

6. Compression ratio

It is defined as the ratio of volume before compression to the volume after compression , lower value of compression ratio is desirable because higher value of compression ratio results increase work input and decrease volumetric efficiency.

7. Freezing point

It should be low.

Note: The freezing point of water is 0 degree celcius and below it , it is converted into solid state and its flow is not possible .

8. Compressor discharge temperature

Ammonia compressors are water cooled because of its higher compressor discharge temperature whereas R-11, R-12 compressors are air cooled.

9. COP

It represent the running cost of equipment , lower the running cost higher the cop.

Note:

• Almost all the refrigerant are having similar value of cop when operating between same  temperature limits.
• Even though the latent heat of vapourisation is having higher value of ammonia refrigerant but it does it not help in any ways to improve its cop because of its higher work input to the compressor.
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b) Chemical properties

1. Toxicity : It should be non toxic in nature.

2. Flammability : It should be non flammable in nature.

Note : Ammonia is both toxic as well as non flammable .

3. Action with oil

• ref. + fully miscible with oil ------- R-11, R-12
• ref. + fully immiscible with oil ------- NH3, CO2
• ref. + partially miscible with oil ------- R-22

There are some refrigerants which are fully miscible with the oil like R-11, R-12.

Therefore the mostly chocking problems are occurs with R-22. Therefore synthetic oil is used in place of mineral oil.

Note : 

• Oil seprator is installed between compressor and condensor.
• Oil seprator is not required to be installed if the refrigerant and oil are immiscible at condensor pressure and temperature.
• Sensing bulb is placed at exit of evaporator.
• Action with material of construction

c) Physical properties

1. Cost : It should be low.

2. Viscosity : It should be low for easy flow of refrigerant 

3. Leak detection method : First of all there should not be leakage of refrigerant at any cost but if it leaks out then its detection should be as fast as possible and by the simplest method. eg: 

• Freon leaks or halocarbon compound leaks are detected by halide torch test , soap bubble method.
• Ammonia leaks are detected by sulphur stick method 
• SO2 leaks are detected by ammonia swab test.

Application of refrigerant 

R-11 > large centralised AC plants

R-12 > domestic refrigerator , water cooler

R-22 > window AC

NH3 > cold storage plants 

Brine > milk chilling plants

CO2 > direct contact freezing of food 

Air > gas liquification , air craft refrigeration system

Azeotropes

It is a mixture of refrigerant which behaves like a pure substance . 

Their designation started with R-500