Stephen Hall discusses the golden rules for design
HEAT exchangers are critical elements in every process plant. Project engineers are responsible for determining the performance requirements and specifications. Specialty manufacturers then design and fabricate the exchangers in accordance with the specifications. I enthusiastically recommend to specifying engineers that they expend the effort to fully understand their heat transfer applications before engaging equipment manufacturers. This reduces the chances that the manufacturer provides a suboptimal design or, worse, that the engineer specifies the wrong type of exchanger.
Use Table 1 to make a preliminary selection of the exchanger type. Answer the questions listed below, if applicable. Tabulate the performance criteria (I recommend the use of a datasheet, see Table 2). Then, you are well-prepared to engage vendors.
I enthusiastically recommend to specifying engineers that they expend the effort to fully understand their heat transfer applications before engaging equipment manufacturers
Here are guidance questions (Hall, Rules of Thumb for Chemical Engineers, 6th Ed, Elsevier, 2018).
Table 2 lists the minimum data that the engineer’s specifications should include. The manufacturer will provide physical properties unless the fluids are unusual.
Determine the “allowable” pressure drop from the context into which the exchanger and its two streams are installed. For example, if a cooling water stream is returned to a common header, the pressure drop through the exchanger may be constrained.
The fouling factor accounts for performance degradation over time. Various mechanisms may deposit material on the tube or plate wall and inhibit heat transfer. The manufacturer will calculate a “clean” and “dirty” duty; these values relate to the condition with no fouling and complete fouling as specified.
The more common fouling mechanisms are: crystallisation, sedimentation, biological growth, chemical reaction coking, and corrosion. In tubular exchangers, the fouling factor may range from 0 for extremely clean fluids, such as pure steam, to 0.001 m2K/W for heavy oils. If in doubt, start with 0.0002 if the velocity is greater than 1 m/s or 0.0004 for lower velocities. For similar services, a plate exchanger will have a fouling factor that is about one-tenth of the factor for a tubular exchanger.
An alternative to specifying a fouling factor is to specify excess area. For this approach, the designer will establish the optimum exchanger configuration with no fouling, and then add extra heat transfer area – up to 20% - without changing critical design choices. This is typically accomplished by adding length to a shell-and-tube design (to maintain velocity while adding extra area) or by providing for adding extra plates to a plate-and-frame exchanger.
Due to the many independent variables, the design of a heat exchanger is sometimes considered to be an art form. But much of the mystery will be removed when you have answered the questions that I have posed
With the guidance given in this article, you should be positioned to confidently specify a heat exchanger, and intelligently discuss design alternatives and trade-offs with the exchanger manufacturer. Due to the many independent variables, the design of a heat exchanger is sometimes considered to be an art form. But much of the mystery will be removed when you have answered the questions that I have posed.
This is the twentieth in a series that provides practical insights into on-the-job problems. To read more, visit the series hub at https://www.thechemicalengineer.com/tags/rules-of-thumb
Disclaimer: This article is provided for guidance alone. Expert engineering advice should be sought before application.
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