A Testing and HVAC Design Methodology for Air-to-Air Heat Pipe Heat Exchangers

Abstract

Air-to-air heat pipe heat exchangers were tested using ASHRAE Standard 84-1991 as a guide. Some changes are introduced for the test facility and methods of calculating effectiveness. ASME PTC 19.1-1985 is used as a guide for uncertainty analysis. Tests were done for a range of mass flux [1.574 to 2.912 kg/(m²·s)], ratios of mass flow rates (0.6 to 1.85), supply air temperatures (−10°C to 40°C), and heat exchanger tilt angles (−8.9° to 11.2°). Because humidity changes in the exhaust and supply air streams were negligible, only the effectiveness of sensible and of total energy was considered. Measured and calculated results show significant variations in the effectiveness of sensible and of total energy, and uncertainties with each independent variable. For balanced exhaust and supply flow rates at −10°C supply air temperature and 1.574 kg/(m²·s) mass flux, the measured effectiveness for sensible and total energy was calculated to be 0.48 and 0.44, respectively, with uncertainties of 0.057 and 0.052. These measurements decreased to 0.42 and 0.37, with uncertainties of 0.016 and 0.018 for a mass flux of 2.912 kg/(m²·s). Because water vapor condensation effects were small or negligible, the difference between the effectiveness for the sensible and total energy was within the overlapping uncertainty range of each. Based on counterflow heat exchanger theory and convective heat transfer equations, expressions are presented to extrapolate the effectiveness data between and beyond the measured data points. These effectiveness equations, which represent the variation in effectiveness with several independent operating variables, are used for HVAC design that is aimed at achieving minimum life-cycle costs.