Frost Growth in Regenerative Wheels

An experimental investigation was carried out for frost growth in a desiccant-coated regenerative wheel. The test facility was set up following ASHRAE Standard 84-1991R. Temperature, relative humidity, mass flow rate, and pressure drops were measured at each measuring station. Photos of frost within energy wheel flow channels show frost accumulation. The problem of frost growth within the narrow parallel flow passages of a regenerative heat or energy rotary wheel is formulated for a very cold-temperature ventilation application. Frost growth is assumed to grow as a porous media while the wheel is exposed to warm humid airflow on the exhaust side. While the wheel is exposed to cold dry airflow on the supply side, the frost is cooled but no frost grows. This cyclic frost growth and cooling process is continued with each wheel rotation. An analytical/numerical model is developed to simulate these frost properties over the depth of the wheel and as a function of time. Simulation results are used to interpret experimental data for the early stage of frost growth on a typical energy wheel with a cold supply air temperature of $−40°C$⁠, a warm exhaust temperature of 20 °C and 40% relative humidity. Pressure drop measurements across a wheel taken for constant mass flow conditions revealed some very significant fluctuations of up to 100% of original pressure drop with a period ranging from 2 to 4 min for a wheel speed of 20 rpm. Each fluctuation in pressure drop is interpreted to imply a catastrophic failure of the outer frost layer sequenced over 1–2 min throughout the wheel followed by another frost growth period on top of a slightly thicker frost base.