1. Introduction to Food Engineering
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 5. Toxicity, must be non-toxic
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 Compression : work done Expansion valve
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 = 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
= kW
= 4.48

42. Slower m, less power, higher C.O.P.
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.