3/31/2023 0 Comments Paint fx heat transferThe resulting temperature profile of the steam and secondary fluid is typically as shown in Figure 2.5.1. This is simply because the temperature difference between the steam and secondary fluid reduces with the rise in secondary temperature. However, as the secondary fluid passes over the heat transfer surface, the highest rate of heat transfer occurs at the inlet and progressively decays along its travel to the outlet. The result is that the temperature profile is no longer dependent on the direction of flow. When saturated steam is used the primary fluid temperature can be taken as a constant, because heat is transferred as a result of a change of phase only. The primary and secondary fluids may flow in the same direction (parallel flow/co-current flow), in the opposite direction (countercurrent flow), or perpendicular to each other (crossflow). The determination of the mean temperature difference in a flow type process like a heat exchanger will be dependent upon the direction of flow. Therefore, the use of established typical values of overall heat transfer coefficient will be suitable for practical purposes. A starting point for this procedure usually involves selecting typical values for the overall heat transfer coefficient of various types of heat exchanger.Īn accurate calculation for the individual heat transfer coefficients is a complicated procedure, and in many cases it is not possible due to some of the parameters being unknown. The magnitude of the individual coefficients will depend on the nature of the heat transfer process, the physical properties of the fluids, the fluid flowrates and the physical layout of the heat transfer surface.Īs the physical layout cannot be established until the heat transfer area has been determined, the design of a heat exchanger is by necessity, an iterative procedure. The fouling factor represents the additional thermal resistance caused by fluid impurities, rust formation or other reactions between the fluid and the wall. The deposition of a film or scale on the heat transfer surface will greatly reduce the rate of heat transfer. The overall heat transfer coefficient may also take into account the degree of fouling in the heat transfer process. The overall heat transfer coefficient is the reciprocal of the overall resistance to heat transfer, which is the sum of the individual resistances. This takes into account both conductive and convective resistance between two fluids separated by a solid wall. The overall heat transfer coefficient (U)
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