Prepared by:- B.S.Bhandari Faculty HNBGU

 Refrigeration is a science of producing and maintaining temperature below that of the surrounding temperature. this means removing of heat from a substance to be cooled.  In simple, refrigeration means cooling of or removal of heat from a system. The equipment employed to maintain the system at low temp. Is termed as refrigerating system. The system which is kept at low temp. Is termed as refrigerated system. Refrigeration is generally produced in one of the following ways 1. By melting a solid 2. By sublimating a solid 3. By evaporation of a liquid

 Most of the commercial refrigeration is produced by the evaporation of liquid called refrigerant.  Mechanical refrigeration is depends upon the evaporation of liquid refrigerant and its circuit include the equipment naming evaporator, compressor, condenser and expansion valve.  Important refrigeration application : 1. Ice making 2. Transportation of food above and below freezing 3. Industrial air-conditioning 4. Comfort air-conditioning 5. Chemical and related industries 6. Medical and surgical aids etc.

 Elements of refrigeration system-: A. A low temp. Thermal sink B. Means of extracting energy from the sink, raising the temp. level C. A receiver to which heat will be transferred from the high temp. High pressure refrigerant D. Means of reducing of pressure and temperature of the refrigerant  Various refrigeration system:- i. Ice refrigeration ii. Air refrigeration iii. Vapour compression refrigeration system iv. Vapour absorption refrigeration system v. Specific refrigeration system

 Start with reversed Carnot cycle  Process 1-2-: The refrigerant absorbs heat isothermally from a low temp. Source at T L in the amount of Q L.  Process 2-3-: Refrigerant compressed isentropically (temp. Rises to the T H )  Process 3-4-: Heat rejected isothermally to the high temp. Sink at T H in the amount of Q H  Process 4-1-: Refrigerant expand isentropically and temp. Drop to T L  Schematics diagram for reversed Carnot cycle

 in process 2-3 and 4-1 can not be approximated closely in practice. This is because of 1. process 2-3 involves the compression of liquid-vapour mixture. Which require a compressor that will handle two phases. 2. Process 4-1 involves the expansion of high moisture content refrigerant in the turbine.

The ideal vapour-compression refrigeration cycle -: Many of impracticalities associated with reversed Carnot cycle can be eliminated by vaporising the refrigerant completely before it is compressed and by replacing the turbine with a throttling device, such as an expansion valve. The cycle that results is called vapour-compression refrigeration cycle. It consists the following four processes-: Process 1-2 -: isentropic compression in compressor Process 2-3 -: constant-pressure heat rejection in a condenser Process 3-4 -: throttling in an expansion device Process 4-1 -: constant-pressure heat absorption in an evaporator

 In an ideal vapour-compression refrigeration cycle, the refrigerant enter the compressor at state 1 as saturated vapour and is compressed isentropically to the condenser pressure. The temp. Of the refrigerant increases during this compression to well above the temp. Of the surrounding.  The refrigerant then enter to the condenser as superheated vapour and leaves as saturated liquid as a result of heat rejection to the surroundings.

 The saturated liquid refrigerant at state 3 is throttled to the evaporator pressure by passing through an expansion valve. The temp. Of the refrigerant drops below the temperature of refrigerated space.  Refrigerant enter the evaporator as low-quality mixture and completely evaporates by absorbing heat from the refrigerated space.

Factors affecting the performance of vapour compression system :- 1. Suction pressure 2. Delivery pressure 3. Effect of superheating 4. Effect of sub-cooling of liquid 5. Effect of suction temperature and condenser temperature

 Actual vapour compression cycle is different from the theoretical cycle in several ways due to following reason -: 1. Frequently the liquid refrigerant is sub-cooled before it is allowed to enter the expansion valve, and usually the gas leaving the evaporator is superheated a few degrees before it enters the compressor. 2. Compression may usually neither isentropic nor polytropic. 3. Both the compressor suction and discharge valves are actuated by pressure difference. 4. isentropic compression contains heat transfer 5. Pressure drop in long suction.

 Process this process represents passage of refrigerant through the evaporator  Process this process represents the passage of the vapour refrigerant from the entrance to the discharge of the compressor  Process this process represents the passage of refrigerant through the condenser.  Process this process represents passage of the refrigerant through the expansion valve.

 In vapour absorption system, the function of a compressor is accomplished in a three step process by the use of the absorber, pump and generator as follows-:  Absorber : absorption of the refrigerant vapour by its weak or poor solution in a suitable absorbent, forming a strong or rich solution of the refrigerant in the absorbent.  Pump : pumping of the rich solution raising its pressure to the condenser pressure.  Generator : distillation of vapour from the rich solution leaving the poor solution for recycling.  A simple vapour absorption system, therefore, consists of a condenser, as expansion device and an evaporator as in the vapour compression system, and in addition an absorber a pump, a generator and a pressure reducing valve.  The fig. Is given as follows-:

 Introduction -: a refrigerant is define as a substance that absorbs heat through vaporisation and loses heat through condensation in the refrigeration system.  Classification of refrigerants -: 1. Primary refrigerants 2. Secondary refrigerants

 Primary refrigerants are those working mediums or heat carriers which directly takes part in the refrigeration system and cool the substance by the absorption of latent heat.  e.g., Ammonia, Carbon dioxide, sulphur dioxide, alkyl chloride, Freon group  Secondary refrigerants are those circulating substances which are first cooled with the help of primary refrigerants and are then employed for cooling purpose.  These refrigerants cools substances by absorption of sensible heat.  e.g., ice, solid carbon dioxide, etc.

 R-10 Carbon tetrachloride (CCL 4 )  R-11 Trichloro-monofluoro methane (CCl 3 F)  R-12 Dichloro-difuloro methane (CCl 2 F 2 )  R-13 Mono-bromo trifuloro methane (CBrF 3 )  R-22 Mono chloro difuloro methane (CHCLF 2 )  R-100 Ethyl chloride (C 2 H 5 Cl)

 Thermodynamic properties : 1) low boiling point 2) Low freezing point 3) High latent heat of vaporisation 4) High saturation temperature  Chemical properties : 1) non-toxicity 2) Non-flammable and non-explosive 3) Non-corrosiveness 4) Chemical stability in reacting

 Physical properties : 1) Low specific volume of vapour 2) Low specific heat 3) High thermal conductivity 4) Low viscosity  Other properties : 1) Availability and low cost 2) Ease of handling 3) High C.O.P 4) Low power consumption per tonne of refrigeration

 In Vapour compression system  energy supplied is low,  wear and tear is more  performance of part load is poor  leakage of refrigerant more chances and  liquid traces in suction line may damage the compressor vapour absorption system  In vapour absorption system  energy supplied is high,  wear and tear is less,  system not affected by variation of load,  no chance of leakage of refrigerant,  no damage to the compressor.

1.Thermal engineering by “ R K RAJPUT ” 2.Refrigeration and Air Conditioning by “ C.P.ARORA ” 1.Thermal engineering by “ R. K. RAJPUT ” 2.Refrigeration and Air Conditioning by “C. P. ARORA” 1.