(249i) Dynamic Heat Exchanger for Flow Transients

Authors: 
Ebrahimzadeh, E., Brigham Young University
Wilding, P., Brigham Young University
Fazlollahi, F., Brigham Young University

This work describes a dynamic heat exchanger process that can decrease or eliminate heat exchanger performance and efficiency losses during transient changes in flow rates or other properties of the streams passing through them. For example, when the flow rate of any one stream entering an optimized and balanced heat exchanger changes, it degrades the heat exchanger performance, creating larger temperature differences in the entering and exiting streams than would otherwise exist and increasing the total amount of entropy generation associated with heat transfer. This performance degradation leads to both direct and indirect consequences. The direct consequence involves energy consumption and stream exit temperatures outside of their design ranges. These direct results frequently create indirect consequences, such as missing quality control and assurance standards, creating safety or environmental hazards, and disposing or flaring compromised material. Both the direct and indirect consequences can be significant.

Heat exchangers also limit on the rates at which process loads can change. Heat exchanger constraints may represent the most restrictive transient response constraints in a process and thereby compromise the agility and responsiveness of processes. These constraints include thermal stresses, expansion, or temperature changes in the devices or their near neighbors in the flow process. The device described in this work can also reduce or eliminate the constraints heat exchangers place on the rate at which loads or other stream properties can change, thus greatly increasing the rate that process changes can occur.

The dynamic heat exchanger comes in several embodiments. The first embodiment, involves two streams and is most applicable to a system that involves streams with relatively high volumetric heat capacities. The second embodiment, introduces one additional stream in the heat exchanger and is most appropriate if one or more of the streams have low volumetric heat capacities, such as gases, or if the heat exchanger process more than two streams. These embodiments are called the two-stream and the multi-stream (multi meaning more than two in this case) configurations, respectively.

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