BO-75-03-2 -- The Effectiveness and Rating of Air-to-Air Heat Exchangers

For a number of years air-to-air heat exchangers have been rated by what has been termed an efficiency expression. In some applications this expression has been interpreted incorrectly which has lead to considerable confusion. It also tends to indicate that the unit is a better. heat transfer device when the exhaust air mass flow is less than the supply air mass flow when it is equally effective for reciprocal values of exhaust/supply air flow ratios. This paper will introduce a rating method which has been in use in the theoretical heat transfer field for some time and is being included in the 1975 Equipment Volume of ASHRAE HANDBOOK & Product Directory (Chapter 34, 11Air-to-Air Heat Recovery Eq4ipment. 11) It is hoped that this paper will remove some of the mystique surrounding these heat exchangers, and the discussion on the efficiency rating will clarify problems which have been encountered.