1. 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.

2. 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

3. 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

4. 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.

5. 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

6. 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 % 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

7. 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

8. 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

9. 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.

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

11. 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

12. 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

13. 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

14. 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

15. 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