流体力学与传热（伍钦）08chapter4-7.pptVIP

The fluid stream having the lower coefficient is brought into contact with the extended surface and flows outside the tubes , while the other fluid, having high coefficient, flows through the tubes. When the resistance of the tube wall is neglected, the overall coefficient can be written in the following form If ho is small, hi large, the value of Ui will be small, but if the area Ao is made much larger than Ai, the resistance Ai/(Aoho) becomes small, and Ui increases just as if ho were increased. Provision for thermal expansion because of the differences in temperature existing in condensers, expansion strain may be set up which are sufficiently severe to buckle the tubes or pull them loose from the tube sheets. The most common method of avoiding damage from expansion is the use of the floating-head construction, in which one of the tube sheets is structurally independent of the shell. Various types of expansion joints are also used to eliminate excessive stresses caused by expansion Several factors must be considered in deciding which fluid to put in the tubes and which to put in the shell of a shell-and-tube heat exchanger. Choice of tube-side fluid If one of the fluids is quite corrosive it should be put in the tubes, which can be made of a corrosion resistant metal or alloy, rather than in the shell which would require that both the shell and tubes be made of the more expensive material. If corrosion is not a problem but one of the fluids is dirty and likely to form deposits on the wall, that fluid should be inside the tubes since it is much easier to clean the inside of the tubes than the outside. Very hot fluids are placed in the tubes for reasons of safety and heat economy. Finally, the decision might be based on which arrangement gives higher overall heat transfer coefficients or lower pressure drop. Very viscous liquids are often placed on the shell side, because flow across the tubes promotes some turbulence and gives better heat trans