1. INTRODUCTION TO FOOD ENGINEERING
Lecture 5
HEAT TRANSFER IN FOOD PROCESSING

2. Objectives Calculate convective heat transfer coefficient
Calculate overall heat transfer coefficient
Calculate heat transfer area in tubular heat exchanger

3. Estimation of Convective Heat-Transfer Coefficient
h is predicted from empirical correlation for Newtonian fluids only
Forced convection

4. Forced Convection NNu = Nusselt number NRe = Reynold number
NPr = Prandtl number

5. Larminar flow in pipes
NRe < 2100
For
(4.38)
b = bulk, w = wall

6. For
(4.39)

7. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING

8. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING
(4.41)
(4.42)

9. Free Convection
(4.43)

10. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING

11. Example
Water flowing at 0.02 kg/s is heated from 20 to 60 C in a horizontal pipe (D = 2.5 cm). Inside T = 90 C. Estimate h if the pipe is 1 m long.
Average T = (20+60)/2 = 40 C
 = kg/m3, cp = kJ/kg C
k = W/m C,  = x 10-6 Pa.s
NPr = cp/k = 4.3, w is  at 90 C

12. = laminar flow
= > 100
NNu = 11.2

13. = 284 W/m2 C

14. Turbulent flow in pipes

15. Estimation of Overall Heat-Transfer Coefficient
Conduction + Convection

16. If temperature of fluid in pipe is higher
Heat flows to outside
Ti > T
Ui = overall heat transfer coefficient
based on inside area

17. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING
Convection from inside
Conduction
Convection to outside

18. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING
(4.48)
(4.49)
(4.50)

19. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING
(4.51)
(4.52)
(4.53)
(4.54)

20. Example
A steel pipe (k = 43 W/mC) inside D = 2.5 cm, 0.5 cm thick, conveys liquid food at 80 C. Inside h = 10 W/m2C. Outside temp = 20 C, outside h = 100 W/m2C. Calculate overall heat transfer coefficient and heat loss from 1 m length of pipe.

21. ro = m
Ri = m
rlm = m
1/Ui = m2 C/W
Ui = 9.32 W/m2 C
Heat loss
q = UiAi(80 – 20)
= 43.9 W
Uo = 6.66 W/m2 C
q = 43.9 W

22. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING
6. Role of Insulation in Reducing Heat Loss from Process Equipment
(4.55)
(4.56)

23. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING
(4.57)
(4.58)

24. Design of a Tubular Heat Exchanger
Determine desired heat-transfer area for a given application. Assuming
Steady-state conditions
Overall heat-transfer coefficient is constant throughout the pipe length
No axial conduction of heat in metal pipe
Well insulated, negligible heat loss

25. Design of Tubular Heat Exchanger
Heat transfer from one fluid to another
Energy balance for double-pipe heat exchanger
(4.59)
(4.60)
(4.61)

26. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING

27. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING
Slope of T line
(4.62)
(4.63)

28. CHAPTER 3  HEAT TRAMSFER IN FOOD PROCESSING
(4.64)
(4.65)

29. Example
A liquid food (Cp = 4.0 kJ/kgC) flows in inner pipe of heat exchanger. The food enters at 20 C and exits at 60 C. Flow rate = 0.5 kg/s. Hot water at 90 C enters and flows countercurrently at 1 kg/s. Average Cp of water is 4.18 kJ/kgC.
Calculate exit temp of water
Calculate log-mean temperature difference
If U = 2000 W/m2C and Di = 5 cm calculate L.
Repeat calculations for parallel flow.

30. Liquid food
Inlet temp = 20 C
Exit temp = 60 C
Cp = 4.0 kJ/kg C
Flow rate = 0.5 kg/s
Water
Inlet temp = 90 C exit temp = ?
Cp = 4.18 kJ/kgC
Flow rate = 1.0 kg/s

31. q = mcCpc Tc = mhCph  Th
Tc = 70.9 C
Tlm = 39.5 C
q = UA(T)lm = UDiL(T)lm
= mCp T = 80 kJ/s
L = 6.45 m
For parallel flow L = 8 m