Critical Values for Condensation and Frost Onset in Air Recuperators – a Detailed Heat Transfer Evaluation

Ventilation of indoor environments is an essential requirement for residential buildings in Scandinavia. Maintaining adequate fresh air with comfortable temperatures is necessary to procure a healthy and comfortable environment for occupants. Freezing in heat exchangers is a common problem during cold winter periods that interrupts ventilation. Identifying precise freezing start provides a means of avoiding this problem using available renewables. In the present study, a detailed heat transfer model of a plate heat exchanger using a two-dimensional finite differencing scheme was developed in Matlab and validated using measurements. The goal was to investigate and map the conditions that impact condensation and frosting start in air recuperators. The simulation model was utilized to conduct a sensitivity analysis of the governing parameters affecting the heat exchanger’s condensation and frost threshold during various working conditions. The results indicated that the frosting threshold was predominantly influenced by the relative humidity of return air to the heat exchanger. The condensation in return air increased the plate temperature, which slightly increased the supply air temperature to the apartments. We have found that utilizing active monitoring of indoor relative humidity and the suggested frosting thresholds utilized the available renewable thermal resources for outdoor air preheating more efficiently. In addition to outdoor air preheating, imbalanced airflows in supply and return ducts prevented/delayed frost formation in the heat exchanger alternatively. The results are in agreement with previous investigations and provide a holistic view of the condensation/frost threshold based on the decisive parameters.