Freezing in residential air-to-air heat exchangers: an experimental study

Abstract

Mechanical ventilation of residences, with heat recovery in air-to-air heat exchangers, is an increasingly common practice. When this technique of ventilation is used in cold climates, however, freezing can occur in the air-to-air heat exchanger and substantially reduce its performance.A laboratory investigation was conducted to determine the indoor and outdoor environmental conditions that lead to freezing, the impact of freezing on performance, and the effect on performance of a common freeze protection strategy based on periodic defrosts. In experiments with three different models of air-to-air heat exchangers, the temperature of the inlet cold airstream at which freezing was initiated ranged from _3 0 C to _12 0 C and varied with the humidity of the inlet warm airstream. Freezing caused the temperature efficiency of a cross-flow heat exchanger to decrease at a rate that ranged from 1.5 to 13.2 percentage points per hour. Small rates of decrease in efficiency (0.6 to 2.0 percentage points per hour) resulted from freezing in a counterflow exchanger. The rate of decrease in efficiency depended on the airstream temperatures and humidities, and the duration of the period of freezing. The amount of time required to defrost the heat exchanger's core was 6 to 26% of the total operating time. The average temperature efficiency for freeze-defrost cycles ranged from 48 to 64 percent in tests of the cross-flow exchanger and 70 to 82 percent in tests of the counterflow exchanger. When the frequency and duration of defrosts were nearly optimal, approximately a ten to fifteen percentage point decrease in average temperature efficiency was attributed to the freezing and required defrosts. The results' suggested that the rate of performance deteriorations due to freezing can be reduced by avoiding small airflow passages that can easily be plugged with ice and by designing the exchanger so that condensed water does not drain toward the cold regions of the core. Based on this investigation, suggestions are made for future experimental studies of freezing and for improved control of freeze-protection systems.iii 9