Frost prediction in heat exchangers : The influence of relative humidity and outdoor temperature on frost formation in plate heat exchangers

Abstract: Frost in Mechanical Ventilation with Heat Recovery (MVHR) often occurs when indoor air is humid and outdoor air is cold. Warm and humid air from the rooms condensates or sublimates when it becomes cold enough, causing frost to grow in the heat exchanger channels. These layers of frost introduce a pressure loss over the heat exchanger and act as an insulating layer, lowering the heat recovery rate of the heat exchanger. It can also buckle the heat exchanger plates and cause long-term damage. The report outlines a method to calculate the frost limit curve for residential parallel and counter flow heat exchangers. A real heat exchanger model from company Swegon is used to provide an example of the method being used. The method involves a well-known Number of Transfer Units (NTU) equation for calculating the dry conditions temperature profile, and an approximation is used for calculation of the temperature profile under wet conditions. The lowest allowable outdoor temperature increases rapidly for increasing relative humidity up to approximately 30%, at which point it tapers off slowly. The results also show that condensation in the heat exchanger increases the lowest allowed temperature in comparison to an identical heat exchanger with no condensation. Lastly, the results show that the distance between plates change the frost limit curve considerably, with a smaller distance between plates being worse for frost risks.