《传热学》HEAT EXCHANGERS.ppt

by Prof. Liu Zhongliang by Prof. Liu Zhongliang Chapter 10 Heat Exchangers Overall Heat Transfer Coefficient Fouling factors Types of Heat Exchangers Log Mean Temperature Difference Effectiveness-NTU Method §10-1 Introduction Application to design: technical and economic considerations: Process requirements; Technically reliable; Economic gain; Extreme situations: space or aeronautical applications. Heat Exchange Equipment: Transfer the heat from one fluid to another; Heat transfer process §10-2 The Overall Heat Transfer Coefficient Heat Transfer through a solid wall: Heat Transfer Equation: Plane Wall Heat Transfer Mathematical Model: Hollow Cylinder Heat Transfer Questions: Wall temperatures: T1 and T2 =? Composite cylindrical walls? Remember the critical thickness of insulation? The thermal analog? Enhancing Heat Transfer Be important for engineering Principle: For a given heat transfer process, Example 1: Enhancing Heat Transfer Heat transfer through a pipe. Re-arranging one can have, If hi=12000W/m2.oC, ho=25W/m2.oC, k=111W/m?oC , then Uo is found to be, Uo=24.9331 W/m2.oC If hi=6000W/m2.oC, ho=25W/m2.oC, k=383W/m.oC , then Uo is found to be, Uo=24.8716 W/m2.oC If hi=6000W/m2.oC, ho=50W/m2.oC, k=111W/m.oC , then Uo is found to be, Uo=49.4807 W/m2.oC Why?More detailed calculations give: The thermal resistances per unit pipe length are: The Engineering Concepts: The metal wall conduction resistance is small; The resistance of the gas side is much greater than that of the liquid if no phase change is involved; The resistance of the free convection side is much bigger than that of the forced convection if no phase change is involved; The resistance of the phase changing side is usually much smaller than that of the single phase side; If both sides are forced convection of liquids or phase changing happens, then the fouling resistance may dominate. Finned walls are an efficient way to enhance heat transfer, however, the following should be noted: The fi