INTRODUCTION
1 TR = Rate of removal of heat from 1 tonne of water to freeze it into ice in 24 hr at 0ºC = 50.4 K Cal/min
Specific heat of water =4.18 KJ/kgK
Specific heat of ice = 2.11 KJ/kgK
Specific heat of vapur = 1.99 KJ/kgK
Latent heat water
in fusion = 335 KJ/kg (at 0ºC)
in vapourization = 2260 KJ/kg (at 100ºC)PRODUCTION OF LOW TEMPERATURE
(i) Throttling expansion of liquid with flashing
(ii) Reversible adiabatic expansion of a gas(iii) Irreversible adiabatic expansion of real gasIn contrast to the reversible expansion, an irreversible expansion is not performed continuously at equilibrium by means of infinitesimal changes in pressure. In the limiting case, the value of the pressure p abruptly changes from p1 and p2. The following expansion proceeds in one step against a constant pressure p2. Hence,
(iv) Thermoelectric effect or peltier effect
SATURATION PRESSURE VS. SATURATION TEMPERATURE LINE
Units in Refrigeration
1 cal = 4.1868 Joule (J)
1 horse power = 746 Watt (W)
1 unit of power = 1 KWH
= 3600 kJ = 860 K cal
1 TR (ton refrigeration) = 50 K cal/min
= 3.5167 kW = 211 KJ/min.
Properties of Air
CP = 1.005 KJ/kgK
Cv = 0.718 KJ/kgK
R = 0.287 KJ/ kgK
g = 1.4
M = 28.966
REFRIGERATIN MACHINE/HEAT PUMP
COEFFICIENT OF PERFORMANCE (COP)
COP = Energy Ratio =
(COP)pump =1+(COP)ref
COP of Different Vapour Compression SystemType of vapour compression systemCOPWater cooled3Air Cooled2Domestic Refrigerator1Vapour absorption system< 1• Power consumption of a refrigerator /heat pump
1 H.P. Refrigerating capacity
where, W is power consumption (KW)
Q0 is refrigerating effect (KW)HEAT PUMP VS. ELECTRIC RESISTANCE HEATER
Qk = (COP)pump W = (1 + COP ref) W
VAPOUR AS REFRIGERANT IN A REVERSED CARNOT CYCLEFor per Unit Mass of Vapour
Refrigeratinrg Effect = q0 = h1 – h4
Heat reject qm = h2 – h3
Compressor work Wc (Consumed )= h2 – h1
Expander work We (gained ) = h3 – h 4
Net Work = Wc – We = (h2 – h1) – (h3 – h4)GAS AS REFRIGERANT IN REVERSED CARNOT CYCLE
Process 12: Reversible Adiabatic CompressionThis process is isentropic. The engine is perfect insulated so that no heat is lost and absorbed. Gas is compressed slowly until the temperature rises from TL to TH.Process 23: Reversible Isothermal Compression (TH=constant)During this process, heat is rejected. Gas is compressed reversibly at the constant temperature TH.Process 34: Reversible Adiabatic ExpansionThis process is isentropic. The engine is perfect insulated so that no heat is lost and absorbed. Gas expands slowly until the temperature drops from TH to TL.Process 41: Reversible Isothermal Expansion (TL=constant)During this process, heat is absorbed. Gas is compressed reversibly at the constant temperature TH.The coefficient of performance (COP) of any refrigerator or heat pump, reversible or irreversible, can be calculated with the general expression:
Therefore
The net work done can be calculated by the area enclosed in 1234.Draw back of using Air as Refrigerant in Reversed Carnot Cycle
Draw Back of Using Vapour as Refrigerant in Reversed Carnot Cycle
1. Liquid refrigerant may be trapped in heat of cylinder and damage the compressor valves.
2. Liquid droplets may wash away the lubricating oil from the walls of compressor cylinder.
3. Expander is costly and the work gained in expander is not significant.
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