4463 (RP-862) -- Air Crossover in Rotary Wheels Used for Air-to-Air Heat and Moisture Recovery PDF

In this paper, crossover between the exhaust and supply air is measured and analyzed for a commercially available energy wheel with typical sliding seals on the rotary wheel faces and perimeter. It was found that a purge section could reduce, but not eliminate, carryover of exhaust gases to the supply due to wheel rotation only when the supply air pressure is greater than the exhaust. Increasing pressure differences between the supply and the exhaust decreased the ventilation air flow rate compared to the maximum supply side flow rate. This reduction was much larger for a wheel with a purge section than one without. Since a pressure drop across an energy wheel always occurs for supply and exhaust airflows, we need to know only the carryover due to wheel rotation and identify the pressure differences between the supply and exhaust airstreams to characterize its crossover characteristics. Tests were done for the case of P_supply - P_exhaust > 0 on both sides of the wheel, and the crossover from the exhaust to the supply was found to be small for all tests. On the other hand, when P_supply - P_exhaust< 0 beside the wheel, the crossover from the exhaust to the supply was significant. For each test, the supply air outlet flow was equal to the exhaust air inlet flow. The cases when one side of the exchanger experienced a positive pressure difference from the supply to the exhaust while the other side experienced a negative pressure difference were not investigated because they did not occur during the tests where both the pressure differences and the supply outlet and exhaust inlet flows were preselected for standard values. Equations and data are presented to show that, when the pressure differences between the supply and exhaust measurements of the air flow rate could supplement or even replace tracer gas measurements when evaluating crossover from the exhaust to the supply air. Equations are also presented to calculate the sensible, latent, and total energy transfer rates of an energy wheel when crossover occurs between the exhaust and supply air irrespective of the cross-flow direction.

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