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Optimal Operation of Parallel Heat Exchanger Networks

Optimal operation of parallel heat exchanger networks is desirable for many processes aiming to achieve increased supply and potentially higher profit. The aim is to control the final outlet temperature within a certain range, which in many cases includes a trade off between maximum outlet temperature and minimum operating costs. The goal with this study has been to investigate the performance of the selfoptimizing Jaschke temperature control variable, proposed by post doctor Johannes Jaschke. The Jaschke temperature approach seeks to achieve near optimal operation of parallel heat exchanger networks, exclusively by manipulation of the bypass selection - only based on simple temperature measurements. The method has been demonstrated for several different cases and investigated both at steady state and dynamically. For balanced heat exchanger networks, with evenly distributed hot stream heat capacities throughout the network, the Jaschke temperature showed good performance for all cases studied. The simulations revealed satisfactory disturbance rejection and very close to optimal operation. For cases suffering a more uneven heat capacity distribution, the method did not give near optimal operation. Also, exposed to major, non-realistic disturbances the Jaschke temperature control configuration gave poor performance due to singularities in the control variable when certain temperatures achieved equal values. In the presence of such incidents, a modified control variable was implemented by re-writing the expression controlling the Jaschke temperatures to a denomiator-free form. This gave slightly better performance and was concluded to operate the system satisfactory.