Experimental Measurement of Frosting Limits in Cross-Flow Air-to-Air Energy Exchangers

摘要

Frosting in air-to-air energy exchangers is a common problem when the outdoor air temperature is very low.Membrane-based air-to-air energy exchangers that are capable of moisture transfer as well as sensible heat transfer may assist in overcoming frosting. To understand the effect of water vapor transfer on frosting, laboratory experiments were conducted to investigate the frosting conditions for two geometrically identical air-to-air cross-flow plate exchangers. One exchanger was made with a water vapor permeable membrane (energy exchanger), while the other exchanger was made with an impermeable polymer film with similar thickness (heat exchanger). Tests for heat and energy exchangers were conducted in several operating conditions to detect the conditions that resulted in frosting in the exchangers. The laboratory test conditions for the exchangers were 0°C (32°F) to -32°C (-26°F) for the supply inlet (simulated outdoor air) temperature and 5% to 55% exhaust inlet (simulated indoor air) relative humidity, while exhaust inlet temperature was≈22 °C (72 °F). Flow rates in the supply and exhaust air streams were maintained at 20.8 L/S (≈40 cfm) to provide a balanced mass flow rate of dry air between the two air streams. Experimental results confirmed that the energy exchanger was more frost resistant than the heat exchanger. For example, when the indoor relative humidity was 30%, frost formed in the energy exchanger for outdoor temperatures of-10°C (14°F) or lower, whereas the heat exchanger frosted at -5°C (23°F). Additionally, the lower the indoor relative humidity, the lower the frosting limit temperature. Both heat and energy exchangers experienced frosting for almost all indoor relative humidities tested when the supply temperature was less than -25°C (- 13°F). Deter mining the operating conditions in which frost first begins to form (frosting limit) will help designers and engineers to select a suitable exchanger and apply proper frost protection techniques.