A. TRANSIENT BEHAVIOR OF STEADY STATE HEAT EXCHANGERS

Although not emphasized, it has been implied above, and is true, that all the heat exchangers mentioned in Section 92 are designed for steady state operation. However, every heat exchanger operation must have a beginning and an end, and the requirements of industrial use necessitate changes from one steady state to another. Each such change occupies a finite amount of time, and it may be that the sum of the transitional periods forms an appreciable fraction of the whole time of operation of the heat exchanger.

This is one reason why the quantitative study of the unsteady behavior of heat exchangers is often desirable, for performance predicted solely on the basis of a succession of steady states may be considerably at variance with reality.

There is another reason: sometimes the start-up and shut-down performance of a heat exchanger has implications for the safety of a plant, especially when the transient is unexpected, resulting perhaps from a power failure. Thus, thermal stresses may result from rapid changes of temperature and water hammer effects, associated with the sudden stopping of slugs of liquid, can cause fractures of pipes and fittings.

B. PERIODICALLY OPERATING HEAT EXCHANGERS (REGENERATORS)