Waste heat recovery from a vented electric clothes dryer utilizing a finned-tube heat exchanger

Conventional residential clothes dryers continuously vent moist, hot air during the drying process. The vented air leaves the home but still has useful temperature and humidity that could be recovered to offset other heating demands in the home. A study is carried out to quantify the amount of heat extracted from the waste heat stream of a conventional, vented clothes dryer. To extract the heat, a water cooled, fin-and-tube heat exchanger is located within the exhaust duct. A steady state thermodynamic dry coil and wet coil model was built in Engineering Equation Solver (EES). The model accounts for the heat exchangers geometry and applies a dimensionless heat and mass transfer analogy (Colburn-j-factor) determined empirically to calculate an overall heat transfer coefficient for both dry and wet areas of the coil. Assuming water and moist air inlet temperatures and air and water side flow rates, a rate of heat transfer and outlet temperatures of both streams are predicted. Comparing the model prediction to experimental results identifies the accuracy of the model. Using energy balance, the potential heat available and the heat recovered are calculated and the effectiveness of the finned-and-tube heat exchanger are determined. It was observed that approximately 0.1 kWh of energy was recovered leading to a heat exchanges effectiveness of 55%