Experimental methods for detecting frosting in cross-flow air-to-air energy exchangers

Abstract Frost formation in air-to-air heat/energy exchangers is a challenging problem in regions with very cold weather conditions, when energy recovery is most needed. Membrane-based energy exchangers may assist in overcoming frosting. In this paper, an experimental facility was developed to enable researchers to test air-to-air exchangers under frosting conditions. The test setup, sensors and data acquisition system, uncertainty bounds for major parameters, and experimental procedure are described. Two geometrically identical plate exchangers: one with a water vapor permeable membrane (energy exchanger), and one with an impermeable plate (heat exchanger) were tested under frosting conditions. The first step in frosting experiments is to find a reliable and cost-efficient method to detect frosting in the exchangers. Four methods were used to determine the onset of frost growth: visual inspection, change in the effectiveness (Δ e ), change in the pressure drop across the exchanger (Δ p ), and change in the outlet temperature (Δ T ). The main contributions of the paper are the detailed comparison of these methods and the introduction of a new frost detection method based on temperature measurements only (Δ T method). It is concluded that the Δ p and Δ T methods were more reliable and practical than the other methods for both heat and energy exchangers. The Δ T method detected frosting sooner and with lower uncertainties than the other methods. Furthermore, the Δ T method was least affected by the operating conditions. On the other hand, the Δ P method gave a better indication of the severity of frosting in the exchanger as more frost resulted in a higher pressure drop across the exchanger.