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The purpose of this paper is to analyse the dynamic behaviour of a three-fluid heat exchanger subjected to a step change in the temperature and velocity of the fluids at the inlet.

The analysis is carried out using the finite element methodology, adopting the Galerkin’s approach, using implicit method for transient behaviour.

The effect of step changes in the inlet temperature of hot and cold fluids show that an increase in the fluid inlet temperatures leads to increased outlet temperatures of all fluids and decreased hot fluid effectiveness. The exit temperatures of the fluids do not show any response initially for a certain period of time with the step changes. The time to reach steady state is independent of the step change in inlet temperature of the hot and the cold fluids.

The findings of this paper is limited to constant property situations.

The findings will be useful in designing control and regulation systems of heat exchangers used in different industrial processes and operations, such as in nuclear reactors, cryogenic and petrochemical process plants.

The analysis provides a time frame in which the controls and regulation systems work, so that the necessary safety precautions for the people working in the surrounding area can be taken care of.

As per the best knowledge of the authors, none of the papers so far have discussed the effect of the change in the inlet temperature and velocity of both the fluids. Performance parameters such as effectiveness, time to reach steady state, etc. have not been studied so far.