Heat Exchanger Performance Assessment

Maintaining good performance of the heat exchanger is a major part of an energy efficiency program. However, how do we identify the root causes of inefficiency and what options do engineers have to resolve in the design and operation These are the focus of discussions in this chapter. 

exchangers do not perform as they should, and their performance deviates from optimum. Sometimes, they do not accomplish what they are capable of, and other times they are asked to perform what they are not capable of. The primary purpose of heat exchanger assessment is to identify the root causes if they are due to poor design, excessive fouling, or mechanical faUiure, and determine the required actions to improve the performance. 

Energy and Process Optimization for the Process Industries, First Edition. Frank (Xin X.) Zhu. 

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The purpose of any fouling model is to assist the designer or indeed the operator of heat exchangers, to make an assessment of the impact of fouling on heat exchanger performance given certain operating conditions. Ideally a mathematical interpretation of Equation 4.6 would provide the basis for such an assessment but the inclusion of an extensive set of conditions into one mathematical model would be at best, difficult and even impossible. 

There are in general, two ways in which plant data on fouling may be directly obtained. They involve an assessment of heat exchanger performance and the use of a side stream of process fluid. The use of probes may provide an indirect method of fouling evaluation. 

An improvement on the assessment of individual heat exchanger performance is to pass a sidestream through some kind of monitor. In general monitors resemble the test equipment used in the laboratory. The use of monitors allows a more careful control of the fouling conditions to be exercised particularly in terms of temperature, temperature changes and flow rates, using actual plant streams. The data obtained are more reliable than those obtained directly from the operation of full scale plant. 

One of the most useful methods of evaluating the performance of an existing heat exchanger or to assess a proposed design is to determine its effectiveness rj, which is defined as the ratio of the actual rate of heat transfer Q to the maximum rate Qmm that is thermodynamically possible or ... 

The concept of a transfer unit is useful in the design of heat exchangers and in assessing their performance, since its magnitude is less dependent on the flowrate of the fluids than the heat transfer coefficient which has been used so far. The number of transfer units N is defined by ... 

The performance of a micro structured plate and fin heat exchanger (0.25 dm3 reactor volume), wash coated with 2.0 g of catalyst, was assessed in comparison with similar alternative technologies by Dudfield et al. [88], The first was a shell and tube heat exchanger filled with 4.66 g of catalyst microspheres (0.25 dm3 reactor... 

When an evaluation is performed to assess the suitability of an existing heat exchanger for given process conditions or for new conditions, this exercise is called... 

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