Design Optimization Means Money: A Step by Step Technique in Heat Exchanger Design

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Thermal equipment are used in a variety of industrial applications such as oil industry, air conditioning and refrigeration, power plant for electricity generation, food manufacturing and many other topics. Engineers who design heat exchangers are usually looking for the most efficient design with minimum manufacturing, installing and operating costs. In other words, the optimum design of thermal equipment is what the designer looking for. The word heat exchanger is a general designation used for all of the cases where there was a heat transfer processes accompanied by a single phase or a change of phase for the operating fluids, process and service fluids. In general, heat exchangers are

Figure (1): A Typical Industrial Air Cooled Heat Exchanger.

referred to the process that takes place throughout the equipment. These are known as heaters, coolers, condensers, and evaporators.

  The numerical concept has been implemented in the thermal-hydrodynamic design of heat exchangers in a numerous number of approaches. One of the most popular numerical method is the step by step technique. This concept has been implemented for the above goals where the best design can be attained for the shell and tube bundle heat exchanger without going to the most conservative design as stated by Kern or Bell-Delaware methods. Further, this technique has been implemented successfully for the thermal design of the air cooled heat exchangers.

Figure (2): A Schematic Diagram for a Shell and Tube Heat Exchanger.

   The philosophy of the technique is relied on the idea of dividing the heat exchanger into small increments where each increment was dealt with as a separate heat exchanger. The exit operating conditions such as temperature, pressure, and vapor quality for each increment are to be considered as the inlet conditions for the next one and so on until the exit design operating conditions have been attained. In this method, the accumulating error was a little or negligible when compared with that obtained with the traditional methods. The small increments of this technique has the following advantages:

- To avoid the problem of nonlinear behavior of the fluids temperature along the flow paths. Hence eliminate the effect of the potential temperature difference across the heat exchanger.

- To take into consideration the change of physical or thermal properties with temperature along the heat exchanger. This is of a vital importance on the final design of the heat exchanger especially when the fluids experience an extreme temperature difference at both ends of the flow paths.

These two issues represent the most important factors in the thermal-hydrodynamic design of heat exchanger. This is usually reflected on the final surface area required for heat exchanger and hence the overall cost of the equipment.