Optimising heat exchanger approach temperatures

The amount of heat recovered in a heat exchangers increases the smaller the approach temperature but this makes the exchanger increasingly expensive.

Before a project is finalised, the exchanger approach temperatures should be optimised with regard to energy costs savings, capital cost and return on investment required.

Because optimisation may require a significant amount of work, it is normal practice to assume representative (rule of thumb) approach temperatures for preliminary investigation.

For individual heat exchange duties, economical approach temperatures may vary from as low as 5 deg C for easy to handle liquids, eg clean water etc, up to 50 deg C or more for difficult fluids.

Exchanger costs tend to be higher for gases (lower heat transfer coefficients than liquids), dirty and/or corrosive fluids, high temperature and high pressure duties.
Operating cost savings are higher for higher cost energy, especially very high temperature duties and low temperature duties where refrigeration is needed.

Optimising exchangers in a network where several process streams and a number of heat transfer duties are involved is also feasible but it is a specialised task (For help optimising multiple-stream heat recovery systems see author's web site)

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