3176 -- The Performance of a Run-Around Heat Recovery System Using Aqueous Glycol as a Coupling Liquid PDF

The performance of a typical commercial run-around heat recovery system was investigated to determine if the theoretical equations given in the literature for’ the optimization of performance are valid and to discover some of the important operating and design considerations. The system monitored was typical of those used in the HVAC industry, composed of finned-tube coils with an aqueous solution of ethylene-glycol as the coupling liquid. It was found that the thermal model given in the literature to predict the overall effectiveness of the system is valid, but the equation for the optimum coupling liquid thermal capacity rate is in error by a factor of four" or five for the finned-tube coil system monitored. The assumption that the liquid flow rate could be decoupled from the overall conductance of each heat exchanger was found to be invalid for this type of system, because, with ethylene-glycol, the liquid-side thermal resistance, which is very dependent on the liquid’s flow rate, is of the same order of magnitude as the air-side thermal resistance. The performance of the system was found to be more dependent on the liquid Reynolds numbers in the coil tubes than on the liquid thermal capacity rate, as implied by the literature. The results of this study imply that fully developed turbulent flow in the coil tubes is essential if the maximum overall effectiveness is to be realized. The data show that too high a concentration of ethylene-glycol can result in a 50% drop in system effectiveness under some conditions.

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