Overall heat-transfer coefficient definition

From this relationship it is clear that the rate of heat transfer is the product of three factors the overall heat transfer coefficient the area of the hot surface and the temperature drop. If instead of Sj either S2 or Sm had been chosen, one would have obtained the heat transfer coefficients based on these areas, namely, U2 or Um. It follows that a definite area must be chosen and that the overall heat transfer coefficient is automatically based on the chosen area. The choice is, in general, arbitrary. 

Thermal resistance due to fouling has to be included in the calculation of the overall heat transfer coefficient. By definition, the overall heat transfer coefficient is the reciprocal of the overall thermal resistance. Thus, including the thermal resistance due to fouling, it follows... 

The rate of heat transfer q through the heating surface of an evaporator, by the definition of the overall heat-transfer coefficient given in Eq. (11.9), is the product of three factors the area of the heat-transfer surface A, the overall heat-transfer coefficient U, and the overall temperature drop AT or... 

In order to outline how to take into account the temperature and length effects, specific definitions of local and mean overall heat transfer coefficients are summarized in Table 17.8 [18]. The three mean overall heat transfer coefficients are important (1) the traditional Um defined by Eq. 17.6 or 17.25, (2) U that takes into account only the temperature effect and (3) U that takes into account both effects, with k providing a correction for the length effect. Note that Um(T) is traditionally (in the rest of this chapter) defined as... 

TABLE 17.8 Definitions of Local and Mean Overall Heat Transfer Coefficients... 

U l/= dq dAAT Local heat flux per unit of local temperature difference This is the basic definition of the local overall heat transfer coefficient. 

Now by definition, q = CM(0a - 0b) and so the value of U, the overall heat transfer coefficient, will depend upon the value of A chosen. This does not matter, provided that the area to which U is referred is stated. There are two possibilities in this case and we will use A = 2 irrinL. Thus ... 

Overall Coefficient of Heat Transfer In testing commercial heat-transfer equipment, it is not convenient to measure tube temperatures (t,3 or t4 in Fig. 5-6), and hence the overall performance is expressed as an overall coefficient of heat transfer U based on a convenient area dA, which may be dAi, oi an average of dAi and dA whence, by definition,... 

The thermal duty here is the opposite of sohdification operations. The indirect heat-transfer equipment suitable for one operation is not suitable for the other because of the material-handhng rather than the thermal aspects. Whether the temperature of transformation is a definite or a ranging one is of little importance in the selection of equipment for fusion. The burden is much agitated, but the beds are deep. Only fair overall coefficient values may be expected, although heat-flux values are good. 

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