Effective mean temperature difference

The effective mean temperature difference will usually be higher with the plate heat exchanger. 

A further advantage of the plate heat exchanger is that the effective mean temperature difference is usually higher than with the tubular unit. Since the tubular is always a mixture of cross and contra-flow in multi-pass arrangements, substantial correction factors have to be applied to the log mean temperature difference (LMTD). In the plate... 

Logarithmic mean tcmperaruie diffnence is calculated by Eq. (5). Logarithirtic mean tiemperaturc difTerence multiplied by correction factor (F> is the effective mean temperature difference. It is a measure of effective heat transfer driving force in a heat exchanger. 

The LMTD, ie, logarithmic mean temperature difference, is an effective overall temperature difference between the two fluids for heat transfer and is a function of the terminal temperature differences at both ends of the heat exchanger. 

Finally, there is an interesting article" that shows how to use Taylor s series to generate shortcut methods from established theory. Examples are given for developing a criterion for replacing log mean temperature differences with average differences and for estimating the effect of temperature on reaction rate. 

The value of T is calculated from the logarithmic mean temperature difference multiplied by a correction factor. With single-pass operation, this factor is about 1 except for plate packs of less than 20, when the end effect has a... 

The effectiveness—NTU method is a procedure for evaluating the performance of heat exchangers, which has the advantage that it does not require the evaluation of the mean temperature differences. NTU stands for the Number of Transfer Units, and is analogous with the use of transfer units in mass transfer see Chapter 11. 

The shell-side leakage and bypass streams (see Section 12.9) will affect the mean temperature difference, but are not normally taken into account when estimating the correction factor Ft. Fisher and Parker (1969) give curves which show the effect of leakage on the correction factor for a 1 shell pass 2 tube pass exchanger. 

As can be seen from the nonlinear temperature profiles, the temperature difference between the fluids varies from one end of the heat exchanger to the other. To find an effective temperature difference between the two fluids, a logarithmic mean temperature difference (LMTD) is defined as... 

In units of BTU per hour per square foot of surface per °F mean temperature difference. U-values for steel pipe are quite high, but the addition of a thick concrete coating plus the effect of burying the pipe under several feet of bottom sediments could result in a very low U-value. Experimental data under controlled conditions is difficult If not impossible to obtain, but we can calculate U-values as low as 0.15. In actual practice we may experience average U-values as high as 1.0 to 2.0, particularly if some 1 sections of the pipeline do not remain completely buried. 

TABLE 8.3. Formulas for Mean Temperature Difference and Effectiveness in Heat Exchangers... 

In the convection zone of the heater, some heat also is transferred by direct radiation and reflection. The several contributions to overall heat transfer specifically in the convection zone of fired heaters were correlated by Monrad [Ind. Eng. Chem. 24,505 (1932)]. The combined effects are approximated by item 10 of Table 8.16, which is adequate for estimating purposes. The relation depends on the temperature of the gas film which is taken to be the sum of the average process temperature and one-half of the log mean temperature difference between process and flue gas over the entire tube bank. The temperature of the gas entering the convection zone... 

Heat transfer coefficient Ratio of the rate of heat transfer in a heat exchanger (in watts) to the product of the heat transfer area of the heat exchanger (in square meters) and the mean temperature difference between the hot and cold streams (in kelvins). The higher the heat transfer coefficient, the more effective the heat transfer process. 

Mean temperature difference Effective difference between the temperature of the hot stream and the temperature of the cold stream in a heat exchanger. This is the driving force for the heat transfer process. 

The log-mean-temperature-difference and effectiveness approaches are presented in heat-exchanger analysis since both are in wide use and each offers its own advantages to the designer. A brief introduction to diffusion and mass transfer is presented in order to acquaint the reader with these processes and to establish more firmly the important analogies between heat, mass, and momentum transfer. 

The expression to the right of Ao is an effective logarithmic-mean temperature difference. Thus,... 

At)LM logarithmic-mean temperature difference (At)LM,eff effective logarithmic-mean temperature difference 0 annual mmiber of hours of operation... 

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