Refrigeration

1. Refrigeration Introduction to Food Engineering

2. Ice Box Ice -> water, latent heat = 333 kJ/kg

3. Refrigerant : liquid -> vapor

4. Selection of Refrigerant 1. Latent heat of vaporization • High value • Small amount needed per unit time 2. Condensing pressure • High pressure needs expenses on heavy construction of condenser & piping 3. Freezing temperature • Should be below evaporator temperature

5. Selection... 4. Critical temperature • Temperature that refrigerant vapor cannot be liquefied, should be high 5. Toxicity, must be non-toxic 6. Flammability –nonflammable 7. Corrosiveness –noncorrosive 8. Chemical stability - stable

6. Selection... 9. Detection of leaks –easy to detect 10. Cost 11. Environmental impact

7. Commonly used refrigerants • Ammonia • CFCs • Freon 12 = dichloro difluoromethane • Freon 22 = monochloro difluoromethane • Stable – long life in lower atmostphere • Migrate to upper atmostphere, Cl split off by UV, reacts with ozone -> deplete • More UV

9. Alternatives • HCFs • Less stable • Hydrofluorocarbons • hydrochlorofluorocarbons

10. Components of a refrigeration system

11. Evaporator

13. Compressor

14. Compressor

15. Compressor

16. Condensor

19. Expansion Valve

25. Changes • d saturated liquid  condensation temp • Passing through expansion valve • Pressure & temp drop • Some liquid -> gas • Liquid/gas mixture enters evaporator coils at e • Completely vaporize, -> saturated vapor (gain additional heat)

26. Changes... • Vapors enter compressor • Compressed -> high pressure, temp increase • Superheated refrigerant • Superheated vapor cooled by air or water in condenser • Saturated liquid

27. Pressure-Enthalpy Charts • Enthalpy H = U + PV H = enthalpy (kJ/kg) U = internal energy (kJ/kg) P = pressure (kPa) V = specific volume (m3/kg)

28. Evaporator & Condenser • Enthalpy change, pressure constant • Compression : work done • Increase enthalpy, increase pressure • Expansion valve • Constant enthalpy • Flow from high P -> low P

29. Pressure-Enthalpy Charts

30. condenser compressor evaporator

32. Cooling Load • Rate of heat energy removal from a given space or object to lower temp. to a desired level • One ton of refrigeration = latent heat of fusion of 1 ton of ice = 288,000 Btu/24 hr = 303,852 kJ/24 hr = 3.5168 kW

33. Cooling load calculation must consider heat of respiration, walls, floor, doors, etc.

34. Example Calculate cooling load caused by heat of evolution of 2000 kg cabbage stored at 5 °C. Given heat of evolution of cabbage at 5 °C = 28 – 63 W/Mg Total heat evolution (2000 kg)(63W/Mg)(1Mg/1000 kg) = 126 W

35. Calculations • Compression qw = work done on refrigerant (kW)

36. Condenser • Evaporator Rate of heat exchanged in condenser Rate of heat accepted by refrigerant

37. Coefficient of performance • Indicate efficiency of the system. • Ratio between heat absorbed by refrigerant in evaporator to heat equivalence of the energy supplied to the compressor

38. Refrigerant flow rate • Depends on cooling load & refrigeration effect q = total cooling load (kW) m = mass flow rate (kg/s)

39. Example • Cold storage room (2 °C) uses Freon-12 as refrigerant. Evaporator temp = -5 °C, Condenser temp = 40 °C, refrigeration load = 20 tons, calculate m, compressor power requirement and C.O.P. Assume saturated conditions and compressor efficiency 85 %.

40. From chart Evaporator pressure = 260 kPa Condenser pressure = 950 kPa H1 = 238 kJ/kg H2 = 350 kJ/kg H3 = 395 kJ/kg (1 ton of refrigeration = 303,852 kJ/24 hr)

41. qw = m(H3– H2), 85 % efficiency = 18.47 kW = 4.48

42. Assume vapors leave evaporator 10 °C super-heated, liquid from condenser is subcooled 15 °C. • m = 0.54 kg/s • qw = 15.9 kW • C.O.P. = 5.2 • Slower m, less power, higher C.O.P.