Heat exchanger operating temperature
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The operation of a heat exchanger is generally determined by the maximum temperature of the fluid, or by the range between the inlet and outlet temperatures, or by the logarithmic average or the arithmetic average. Which value or which range should be used?
The design temperature of the tube (shell) side in a shell-and-tube heat exchanger is the same as the design temperature of the tube box on the tube side (or the shell on the shell side). It is a key parameter for determining the materials and allowable stresses for the tube (shell) side, and it is different from the average temperature of the tube (shell) side in such heat exchangers – which refers to the average value along the length of the heat transfer tubes and the cylindrical metal of the shell side during the heat transfer process. This average value depends on factors such as the flow rates and physical properties of the fluids on either side of the heat transfer tubes, and it is used to calculate the thermal expansion difference between the heat transfer tubes and the cylindrical shell.
Taking extreme values is quite common; range values between the minimum and maximum values are rarely used. This is because the temperature of the material directly affects the determination of parameters such as the allowable stress of the material, and it is crucial for the accuracy of equipment design
We generally use the arithmetic mean for the shell side and the logarithmic mean for the tube side, based on empirical formulas
Heat exchange tubes, which are subjected to the temperatures in both the tube side and the shell side, should have their design temperature determined based on the metal temperature. This metal temperature is determined through heat transfer calculations (refer to Appendix B of GB/T151-2014 for the value of the heat transfer coefficient α); it can also be determined by testing on similar heat exchangers that are already in use, or based on established design experience；
The operating temperature of the heat exchanger indicated on the diagram should be the temperature at the inlet and outlet; for cases where it is necessary to calculate the metal temperatures in the tube side and shell side, this value serves as the basis for such calculations.