What is a Refrigerant ?
1) A refrigerent is a substance that carries heat from the refrigerated space or body to the outside surrounding.
2) Therefore a refrigerent may be defined as any substance that absorbs heat either by expansion or vapourisation and rejects it through condensation.
3) In Mechanical refrigeration systems refrigerant is subjected to cyclic thermodynamic processes, so that it is forced to vapourise in the evaporator by absorbing heat from refrigerated space and it is forced to condense and convert into liquid in the condenser by rejecting heat to surrounding.
4) The blood of any Refrigerating system is the refrigerant circulating inside the system.
Desirable Properties of a Good Refrigerant
1) Thermodynamic Properties :-
a) Boiling Point :- It should have Low Boiling Point.
b) Freezing Point :- It should be below the Evaporator temperature.
c) Evaporative Pressure :- It should be above Atmospheric pressure.
d) Condensing Pressure :- It should have LOW Condensing pressure.
e) Latent Heat of Vapourisation :- It should have HIGH Latent heat of Vaporisation.
f) Critical Temperature & Pressure :- It should be above the condensing Temperature & Pressure.
2) Chemical Properties :-
a) Toxicity :- It should not be Poisonous or injurious . It should not be non-irritating to eyes.
b) Corrosiveness :- It should not be corrosive & should not have any effect on materials used in equipment.
c) Leak Detection :- It should have less tendency to leak & if it is leaking it should be easily detectable.
d) Flammability :- It should not be Inflammable.
e) Miscibility with Oil :- It should be immiscible with oil & should not have any effect on the properties of Oil used for Lubrication.
f) Effect on Foodstuff :- It should not affect on food articles Or make them poisonous or unportable.
g) Stability :- It should be capable of withstanding high pressure & temperature.
3) Physical Properties :-
a) Specific Volume :- It should be LOW in Vapour state.
b) Viscosity :- It should have LOW viscosity.
c) Thermal Conductivity :- It should have HIGH Thermal Conductivity.
d) Di-Electric Strength :- It should have High strength.
4) Other Properties :-
a) Handling & Maintainance :- It should be easy & safe to handle.
b) Cost & Availability :- It should be readily available at LOW cost.
c) Performance of the System :- It should have high COP & LOW power requirement.
1) Water :-
i) Working temp. :- above 3 degree celcius.
ii) It is the cheapest, less corrosive & have high specific heat.
2) Brines :-
i) They are the solutions of Nacl & Cacl2 .
ii) Used below the freezing point of water.
3) Ethylene glycol & Propylene glycol :-
i) Used as anti-freeze mixtures.
ii) Mixture with water makes colourless & odourless.
iii) Capacity to lower freezing point.
4) Halocarbon compounds :-
i) These contains one or more of 3 halogens i.e., chlorine, bromine or fluorine. These are sold under names Freon, Genetron, Isotron, Aretron etc.
ii) Have low freezing points.