PRIMARY AND SECONDARY REFRIGERANTS It is a substance in the vapour state which is used as a working fluid in refrigerators. In a refrigerating machine, heat is transferred from cold body to hot body through refrigerants. It is also a cooling agent of refrigerating system. The choice of refrigerant depends on ability to remove heat, toxicity, density, availability, cost etc. Classification Primary refrigerants Secondary refrigerants These are the refrigerants which cool the substance or space directly by absorbing latent heat. It is also known as direct expansion system Eg. Ammonia, Freon, SO2, Co2 etc.

Ammonia (NH3) Used for commercial purposes. Mainly in cold stored and ice plants. The boiling temperature of NH3 is -330C It is colorless gas with a sharp pungent smell Has good thermodynamic properties It is neutral to all metals, highly soluble in oil. It’s solubility increases with increasing pressure and decreasing temperature Used free small and medium refrigerating capacities Volatile and non toxic but in higher conc. suffocation occur due to back of O2

FREON 13 (Mono chloro-trifluromethane) (Ccl F3) Widely used Somewhat inferior in thermodynamic properties FREON 21 – (dichloro - monofluoroethane) (CHcl2F) FREON 22 – (monochloro – diflueroethane) (CHclF2) There are used in small refrigeration plants Used to obtain moderately low temperature (-500C) SULPHUR DIOXIDE (SO2) Previously used in household refrigerators Toxic, non-explosive and non-flammable, non-corrosive Irritant to human body Non mixable with oil Has pungent odour and low latent heat value

CARBON DI OXIDE (CO2) Used as dry ice (solid from of Co2) Not used as refrigerant Colourless, odourless, non-toxic, non explosive, non-corrosive, non-flammable Operating pressure is very high Not mixable with oil As it is colourless and odourless gas, leaks cannot be detected easily Safest of all Its thermodynamic properties are not desirable for refrigeration except in cryogenic applications Two forms of CO2 used to freeze foods are spraying liquid CO2 & CO2 snow.

FREON GROUP They are fluorocarbons of methane and ethane series. They contain 1 or more of these halogens (chlorine, bromine, fluorine) Non toxic, non-flammable, non-explosive, non- corrosive, non-irritant to human body and eyes. Odourless, colourless Will not react with food product stored in the refrigerated space. Will not react with lubricating oil. Has excellent thermodynamic properties Only disadvantage is ozone layer is damaged. FREON 11 (Trichloro monofluro methane – ccl3F) Has the boiling point of 74.80F Used in air conditioning plants in a larger level in factories, theatres

FREON 12 (Dichloro-difluro methane –ccl2F2) Mostly used in domestic and commercial refrigerants (in ice cream cabinets, display cabinets, deep freezer) It is very widely used, colourless gas with mild odour Heavier than air Does not dissolve in water, moisture content should not exceed 0.0025% by weigh Refrigerating effect per unit volume of ammonia is about 1.5 times that of Freon-12 It does not react with ferrous metals, aluminium, phosphor bronze It attacks copper, copper alloys, zinc and bronze and dissolves in water It does not react with lubricating oils in the absence of moisture, but oxidizes them in the presence of water vapour

The water vapour in ammonia should not exceed 0.2% According to safety regulation the use of a sulfur wick is prohibited for detection of ammonia leaks. So, use of an open flame inspection of the internal parts of an ammonia machine ammonia is used in large marine refrigeration plants to obtain temperature of -50 to -600C It is toxic It high temperature it may explode When heated it burns Irritating to human eyes, nose and throat Non-corrosive to ferrous materials but corrosive to copper, zinc and brass Has high eatent heat Cheap and easily available

Secondary Refrigerant The refrigerants are brine which is used as intermediate fluid between evaporator and the substance or space to be cooled. They cool the substance and the space by absorbing their sensible heat. Also called indirect expansion system. Eg. Brine solution made of calcium chloride or sodium chloride Water cannot be used as secondary refrigerant because at 00C itself it will become ice and circulation is not possible In brine solution CaCl2 is much preferred, it is very costly Substances that take away heat from the medium to be cooled and give it to the boiling refrigerant are called secondary refrigerants They do not change their physical condition

Cheap and harmless, chemically neutral to metals and packing material Should have a low freezing point and large heat capacity Choice of brine depends on temperature to which a material is to be cooled and industrial process in which it is to be used For lower temperature (-18 to -230C), CaCl2 is used NaCl is used when it is desired to have direct contact between brine and product. Freezing point of brine depends on its concentration Cheapest secondary refrigerants are water and air but their application is limited. Since, water has a high freezing point (00C) and air has a low heat capacity.

Physical characteristics of refrigerants Boiling and condensing temperature and pressures Freezing temperature Critical temperature Discharge temperature Latent heat of vapourisation Specific heat Density Viscosity

Boiling and condensing temperature and pressures The evaporator and condensing temperatures determine the pressures The maximum condensing temperature is largely affected by climatic condition It is desirable to select a refrigerant whose saturation pressure (at min. operating temperature) is a few pounds above atmospheric pressure. Freezing temperature Should have low freezing temperature to avoid operational obstruction by the refrigerant itself Critical temperature Should be well above the maximum condensing temperature Discharge temperature High discharge temperatures from the compressor should be avoided It causes some refrigerant breakdowns as well as poor lubrication effectiveness

Latent heat of vapourization Heat which converts the refrigerant from the liquid state to vapour It should have a higher value Specific heat Amt. of heat required to raise the temperature of unit mass of the refrigerant through 10C Higher specific heat is not desirable Density Low vapour density refrigerants are preferred Viscosity Low viscosity of the liquid refrigerant is desired to reduce the pressure drop in the lines

Chemical properties of refrigerants Toxicity Flammability and explosion hayard Refrigerant odours Rated based on its effect on human beings over specified periods Should be non toxic and non irritation Flammability and explosion Hazard Should be non-flammable and non-explosive Refrigerant Odours Can be both an asset and a hazard Makes it easy to detect the leaks but at the same time may contaminate foodstuffs in storage

Chemical properties Should be non-flammable Should be non-explosive Should be not-toxic Should not react with lubricating oil Should not react with moisture Should not contaminate the food materials kept inside the refrigerating system

Economic and thermodynamic properties Should have high value of latent heat of vapourisation Should have high thermal conductivity Should have high electrical resistance Should have low specific volume Should have low specific heat Should have low freezing point (than the maximum temperature of the refrigeration system) Should have low boiling temperature Should have low leakage tendency. Leakage should be detectable Above all the refrigerants should not be expensive