Refrigerants

5/25/2017
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Md. Rahmatuzzaman Rana
Assistant Professor, Dept. of FET
Definition
 The refrigerants is a heat carrying medium which
during their cycle in the refrigeration system absorb
heat from low temperature system and discard the
heat so absorbed to a higher temperature system.

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Desirable properties of an ideal Refrigerant
1. Low boiling point
2. High critical temperature
3. High latent heat of vaporization
4. Low sp. heat of liquid
5. Low sp. volume of vapor
6. Non-corrosive to metal
7. Non-flammable, non-explosive, non –toxic
8. Low cost
9. Easy to liquefy at moderate pressure and temperature
10. Easy to locating leaks by odor or suitable indicator, and mixes well with oil.
 In industry mostly used refrigerant has an evaporating temperature -15 oC, and
condensing temperature of +30 oC.
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

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Classification of refrigerants
 The refrigerants broadly classified into
A. Primary: directly take part in the refrigeration system
B. Secondary: first cooled by primary refrigerants and then used for cooling
purposes.
 Primary refrigerants further classified into-
i. Halo-carbon refrigerants
ii. Azeotrope refrigerants
iii. Inorganic refrigerants and
iv. Hydrocarbon refrigerants.
Halo-carbon refrigerants
 Synthetically produced and developed as Freon family
trade name.

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Azeotrope refrigerants
 Azeotrope refers the stable mixture of refrigerants whose
vapor and liquid phases retain identical composition over
a wide range of temperatures.
Inorganic refrigerants
 Exclusively used before halo-carbon refrigerants.
 These are still in use due to inherent thermodynamic and physical
properties.

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Hydrocarbon refrigerants
 Though it is used successfully but are highly flammable and explosive.
Thermodynamic properties of refrigerants
1. Boiling temperature: should be low. If the BP is high at atm
pressure then the compressor need to operate at vacuum condition
which will make the operational cost high.
2. Freezing temperature: below the operating evaporator
temperature. Mostly it is below -35 oC.
3. Evaporator and condenser pressure: should be positive and should
near to atm. pressure. The positive pressure helps to prevent leakage
while too high pressure would require stronger refrigerating
equipment resulting higher initial cost.

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Thermodynamic properties of refrigerants
4. Critical temperature and pressure: It is the highest temperature at which it can
be condensed to liquid regardless of higher pressure. It should be above the
highest condensing temperature. If it is too near to condensing temperature then
excessive power consumption will be required.
5. COP and power requirements: it should be closer to reversed Carnot cycle COP
5.74 while refrigerator operating at -15 oC evaporating and +30 oC condensing
temperature.
6. Latent heat of vaporisation: should be higher at evaporation which causes high
refrigerating effect.
7. Specific volume: the sp volume at evaporator temperature indicates the
theoretical displacement of the compressor. It is depend on the type of
compressor used.
Chemical properties of refrigerants
1) Flammability: hydrocarbons are usually highly flammable and
becomes explosive when mixed with air in the ratio of 16 to 25%. The
halo-carbon refrigerant are neither flammable nor explosive.
2) Toxicity: depending upon application it should be non-toxic.
3) Solubility of water: should be less soluble in water otherwise ice
will form and chokes the expansion valve.
4) Miscibility: the ability of refrigerant to mix with oil is called
miscibility. It is advantageous from the heat transfer point of view,
lubrication and oil-separation problems.
5) Effect on perishable materials: in case of leakage it should have
no effect on perishable materials.

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Physical properties of refrigerants
1. Stability: should be stable and inert. Freon groups are stable up to 535 oC.
2. Corrosive property: non-corrosive.
3. Viscosity : low viscosity is desirable due to low pressure drop in suction line.
Also the heat transfer is improved.
4. Thermal conductivity: should have high.
5. Dielectric strength (maximum electric field can withstand without
breakdown of its insulation properties): is important due to its exposure
with electric motor.
6. Leakage tendency: low. If there is a leakage then it should be easily
detectable.
7. Cost
Secondary refrigerants (Brine)
 Used where the temperature required to maintain below freezing point of
water
 Solution of salt in water decrease freezing point up to certain point
 Eutectic temperature
 Eutectic cocentration
 Calcium chloride commonly used as salt
 CaCl: 30% by mass and temperature -55 C. Disadvantage: dehydrating effect.
 NaCl: 23 % and -21.1 C. Corrosive.
 Both of them are corrosive to metal and provide bitter taste.
 Antifreeze: Ethylene and propylene glycol.

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Application Brine used
Diaries NaCl, CaCl, propylene glycol
Ice cream CaCl, propylene glycol
Breweries propylene glycol
Designation system of refrigeration
CmHnClpFq
 For saturated (methane and ethane base) refrigerant
 n+p+q= 2m+2
 R (m-1)(n+1)(q)
 For unsaturated refrigerant (trichloroethylene)
 n+p+q= 2m
 R 1(m-1)(n+1)(q)
 For organic refrigerant (CO2, NH3)
 R 700+molecular weight. For example: NH3 MW= 17 thus R700+17

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