Parallel-flow heat exchangers with ambient thermal interaction

A mathematical model is developed to study the performance of a parallel-fl ow heat exchanger in which both fluid streams are interacting thermally wit h the surroundings. The fluid temperatures are found to be dependent on the magnitude of the ambient temperature relative to fluid inlet temperatures, the ratios of conductances between the fluids and the ambient and the inte rfluid conductance, the ratio of minimum to maximum fluid capacities, and t he number of transfer units, NTU, for the heat exchanger. Two heat exchange r effectiveness criteria, one each for the hot and cold fluids, are used to study performance. The effectiveness is found to be adversely affected by increasing conductance ratios, increasing NTU, and increasing temperature d ifference between the ambient and the fluid of interest. For very high valu es of the conductance ratios, the heat exchanger will not perform as expect ed and both fluid temperatures will approach that of the ambient. The paral lel-flow arrangement is compared to counterflow and is found to be less eff ective under the external heat transfer condition.