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Heat Exchanger for Husk Burner

Heat Exchanger for Husk Burner. 吳柏青 Poching Johnny Wu. Outline. Basics of Heat Transfer External Forced Convection Flow across Cylinders and Spheres Flow across Tube Bank Types of Heat Exchangers The Log Mean Temperature Difference Method Selection of Heat Exchangers.

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Heat Exchanger for Husk Burner

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  1. Heat Exchanger for Husk Burner 吳柏青 Poching Johnny Wu

  2. Outline • Basics of Heat Transfer • External Forced Convection • Flow across Cylinders and Spheres • Flow across Tube Bank • Types of Heat Exchangers • The Log Mean Temperature Difference Method • Selection of Heat Exchangers

  3. Heat Transfer Mechanisms • Conduction • Convection • Radiation • Mechanisms of heat loss from the human body and relative magnitudes for a resting person,

  4. Thermal ConductionFourier’s Law of Heat Conduction • k = thermal conductivity, W/m℃ • A = cross section area, m2 • dT/dx = temperature gradient, ℃/m

  5. The Mechanisms of Heat Conduction in different phase of a substance

  6. Thermal Conductivity, k

  7. The variation of the thermal conductivity of various solids, liquids, and gases with temperature.

  8. Thermal Diffusivity, a

  9. Thermal Convection

  10. Newton’s Law of Cooling • h = convection heat transfer coefficient, W/m2℃ • As = surface area, m2 • Ts = surface temperature, ℃ • T∞ = temperature of the fluid, ℃

  11. Thermal Radiation

  12. Stefan-Boltzmann Law • e = emissivity of the surface • s = Stefan-Boltzmann Constant, 5.67×10-8 W/m2 K4 • As = surface area, m2 • Ts = surface temperature, ℃

  13. The Net Rate of Radiation Heat Transfer between two Surfaces

  14. Heat Loss from a personConsider a person standing in a breezy room at 20℃. Determine the total rate of heat transfer from this person if the exposed surface area and the average outer surface temperature of the person are 1.6 m2 and 29℃, respectively, and the convection heat transfer coefficient is 6 W/m2℃.Assume the emissivity of a person is e = 0.95.

  15. Reynolds Number, Re Reynolds number is the ratio of the inertial forces to viscous forces, and it serves as a criteria for determining the flow regime.

  16. Prandtl Number, Pr The Prandtl number is a measure of the relative magnitudes of the diffusivity of momentum and the diffusivity of heat. The Pr is a fluid property, and thus its value is independent of the type of flow and flow geometry.

  17. Nusselt Number, Nu Nusselt number is the dimensionless convection heat transfer coefficient, and it represents the enhancement of heat transfer through a fluid layer as a result of convection relative to conduction across the same fluid layer. A Nusselt number of Nu = 1 for a fluid layer represents heat transfer across the layer by pure conduction.

  18. Nu >> 1 • Nu > 1 • Nu = 1

  19. External Forced Convection

  20. Drag Force Acting on a Car in a Wind Tunnel

  21. Drag Force = Friction Drag + Pressure Drag Viscosity of Fluid Density of Fluid Velocity Shape and Orientation of Body

  22. Drag Force acting on a flat plate normal to flow depends on the pressure only and is independent of the wall shear, which acts normal to flow.

  23. Drag Force, FD • CD = drag coefficient • r = the density of the fluid, kg/m3 • V = the upstream velocity, m/sec • A = the frontal area (projected area), m2

  24. Average drag coefficient for cross flow over a smooth circular cylinder and a smooth sphere.

  25. Effect of Surface Roughness

  26. Variation of the Local Heat Transfer Coefficient

  27. Flow Across Tube Banks

  28. Flow Patterns (a) In-line (b) Staggered

  29. For cross flow over tube banks(In-line Arrangement, N > 16, and 0.7 < Pr < 500) (Staggered Arrangement, 2AD > AT) (Staggered Arrangement, 2AD < AT)

  30. For NL < 16 and ReD > 1,000

  31. The Logarithmic Mean Temperature Difference ΔTln • NL = the number of rows • f = the friction factor • c = the correction factor • r = density, kg/m3

  32. Pressure drop, ΔPbetween the inlet and the exit of tube bank • NL = the number of rows • f = the friction factor • c = the correction factor • r = density, kg/m3

  33. Friction factor and correction factor for tube banks (In-line arrangement)

  34. Friction factor and correction factor for tube banks (Staggered arrangement)

  35. In an industrial facility, air is to be preheated before entering a furnace by geothermal water at 120℃ flowing through the tubes of a tube bank located in a duct. Air enters the duct at 20℃ and 1 atm with a mean velocity of 4.5 m/sec, and flows over the tubes in normal direction. The outer diameter of the tubes is 1.5 cm, and the tubes are arranged in-line with longitudinal and transverse pitches of SL = ST = 5 cm. There are 6 rows in the flow direction with 10 tubes in each row. Determine the rate of heat transfer per unit length of the tubes, and the pressure drop across the tube bank.

  36. Double-Pipe Heat Exchanger

  37. Cross-flow Heat Exchanger

  38. Shell-and-Tube Heat Exchanger

  39. Typical Temperature Distribution for the Cross-flow Heat Exchanger

  40. Precipitation Fouling of Ash Particles on Superheater Tubes

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