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FOOD1360/1577 Principles of Food Engineering Robert Driscoll

Topic 8 Heat Exchangers. FOOD1360/1577 Principles of Food Engineering Robert Driscoll. Introduction. Ref: S&H 4.4 Workhorses of the food industry. Main job: heating and cooling products. Ideal: continuous steady state. out. in. Heat. Hot fluid. Cold product. in. out. Concept.

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FOOD1360/1577 Principles of Food Engineering Robert Driscoll

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  1. Topic 8Heat Exchangers FOOD1360/1577 Principles of Food Engineering Robert Driscoll FST 2010

  2. Introduction • Ref: S&H 4.4 • Workhorses of the food industry. • Main job: heating and cooling products. • Ideal: continuous steady state. FST 2010

  3. out in Heat Hot fluid Cold product in out Concept • Example: countercurrent heating: FST 2010

  4. Example www.yourdictionary.com/ahd/h/h0112300.html

  5. Output energy: • Sensible ? • Dryness ? • Input energy: • Sensible ? • Latent ? • Transfer energy: • Heating or cooling ? • Resistances Looking at components: • HEATING OR COOLING FLOW: FST 2010

  6. Output: T=50oC Input: T=80oC Example 1: Sensible • Flow is 25 kg/s, cp=3.85 kJ/kg.C FST 2010

  7. Condensate out (hC, TC) Steam in (hS, TS) Example 2: Steam • Use steam tables • For hS, saturated vapour. For hC, saturated liquid TC=85oC TS=120oC FST 2010

  8. Transferred energy: • Cooling or heating ? • Output energy: • Sensible ? • Dryness ? Input energy Looking at components: • PRODUCT FLOW: FST 2010

  9. Looking at components: • TRANSFERRED ENERGY: Heating (cooling) fluid Product FST 2010

  10. Overall Heat Transfer Coefficient • Define U as overall htc: • Q is heat. • ∆Tavg is average ∆T between the two fluids. • A is contact area. FST 2010

  11. Hot fluid Surface Cold fluid Resistances to heat flow • What opposes heat flow? • Hot fluid to surface – convection • Through surface – conduction • Surface to product - convection FST 2010

  12. From heat transfer notes Resistance terms • Hot fluid to surface: • Through surface: • Surface to product: Thus: FST 2010

  13. Practise • Data: • Hot fluid: h1=1800 W/m2.K • Cold fluid: h2=120 W/m2.K • Surface: dx=0.13 mm, k=210 W/m.K • Surface area: A=3.7 m2 SOLUTION FST 2010

  14. Methods for finding U • Empirical: run water, measure T. • Manufacturer: test data. • Calculate: use HT theory. FST 2010

  15. Estimating ∆Tavg • Where to measure ∆T? • Inlet? • Outlet? • Halfway? • Best method (for no phase changes): Use ∆Tlm FST 2010

  16. Log Mean Temperature Difference (LMTD) • For a heat exchanger, • Tlm means log mean temperature difference T1 T2 T3 T4 FST 2010

  17. Example: • Find LMTD for water at 16oC cooling milk at 68oC in a countercurrent HE, if the exit temperatures are 25oC for the water and 33oC for the milk. FST 2010

  18. Diagram Milk out 33oC Milk in 68oC Hot fluid Cold product Water in 16oC Water out 25oC FST 2010

  19. Solving FST 2010

  20. Bulk and wall temperatures • Bulk temperature Tb is the average temperature over a given flow cross-section. • Wall temperature Tw is temperature near a wall or boundary. • Often we assume Tb. FST 2010

  21. The Main Energy Balance FST 2010

  22. Example calculation • A parallel flow tubular heat exchanger is used to chill water (flowing at 1.05kg/s) from 32oC to 3oC using a brine solution at -8oC. The brine exits at 10oC from the chiller. • If the heat exchange area is 1m2, determine: • The heat transfer rate • The overall heat transfer coefficient Hint: check for cross-overs by making a quick plot of all T’s. FST 2010

  23. WATER 32 10 3 BRINE -8 Plot • Is this possible? Cross-over: physically impossible T FST 2010

  24. Try again ! WATER 32 10 3 BRINE -8 FST 2010

  25. Now get correct Tlm FST 2010

  26. Calculations • First calculate log mean temperature difference (Ans = 15.9oC) FST 2010

  27. Second example • After running the plant for a year, the water chills to only 70C. The brine and water flow rate are unchanged. What is the change in the overall heat transfer coefficient of the heat exchanger? • (solution left as exercise: Ans = 6.0 kW/m2.K) FST 2010

  28. Convective Heat Transfer in Boiling Liquids • Heated surface to boiling liquid: convection. • Heat flux varies with temperature difference. • Two types of boiling: • Pool boiling (evaporation at air surface, T<5oC). • Nucleate boiling (evaporation at heating surface). FST 2010

  29. q/A T 5 50 500 Graph FST 2010

  30. Summary

  31. Modelling h (for water) • For nucleate boiling conditions (up to 20oC) FST 2010

  32. The Two KPI • Key Performance Indicators (KPI) of heat exchangers: • Overall heat transfer coefficient U • Heat exchange effectiveness • Unit thermal efficiency for a heat exchanger: the average heat transfer per unit length. FST 2010

  33. U • Would like to maximise U: • More product through equipment, OR • Smaller equipment • So promote turbulence, fast flow. • U is affected by flow conditions on both sides of the H.E. surface. FST 2010

  34. Effectiveness • How uniformly does it heat? FST 2010

  35. Unit Thermal Efficiency • Average heat transfer per unit length. FST 2010

  36. Area/Volume ratio Liquid heats fastest if A/V is high. • Cylinder: • So decrease radius to get better A/V. FST 2010

  37. Radius and Pressure We know that: • Laminar: f=16/Re • Turbulent: f=0.0791/Re1/4 FST 2010

  38. Example: water at 20oC FST 2010

  39. Comparing the two: Since Re  R, we have a problem: • Best HT – decrease radius. • Reduce pressure drop – increase radius. • Also, narrow tubes foul easily and are hard to clean. Design is a compromise. FST 2010

  40. Types of Heat Exchangers FST 2010

  41. Main Types • Jacketted pans • Parallel plate • Shell and tube, double tube • Tube and fin • Scraped surface FST 2010

  42. 1) Jacketted pans or kettles • heating/cooling liquid in thin jacket around stirred food vessel. FST 2010

  43. 2) Plate HE • racks of rectangular plates • gaskets between plates • pressure packed • food / heating fluid alternate between plates • high efficiency, due to large surface to volume ratios • plates are patterned to increase turbulence • require careful cleaning FST 2010

  44. Pics FST 2010

  45. Heating in a plate heat exchanger Orange – heating medium Blue – heated product FST 2010

  46. Peclet Number • Define Peclet number: • If Pe<100, axial conduction significant • No single equation for h • complex configurations • manufacturer’s equations: Nu = f(Re, Pr) FST 2010

  47. 3) Shell and tube exchangers • Uses: • heating/cooling liquids • condensing vapours • Many designs: • double tube • triple tube • multi-tube FST 2010

  48. Double tube • Solve using equation for flow in pipe • Same equations for both CON and COUNTER current flow • Use LMTD in calculations • Use same equations as for shell and tube (unless boiling) • To increase HT, rippled pipe walls. FST 2010

  49. product heatingmedium Diagram for Double Tube FST 2010

  50. Example • Used for cooling wine. FST 2010

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