Analysis of the frost formation and freeze protection with bypass for cross-flow recuperators

A theoretical analysis of coupled heat and mass transfer inside the cross-flow plate heat exchanger used for energy recovery under frosting operating conditions was presented. Three selected frost control techniques were analysed on the base of original ε-NTU model. Frost-free operating conditions were determined for the cross-flow plate heat exchanger under different opening levels of bypass damper and face and bypass dampers. It was found that the implementation of the particular variant of heat exchanger performance and the values of critical outdoor temperatures for different operating conditions in which the onset of frosting occurs in the cross-flow flat plate heat exchanger depend upon the heat and mass transfer conditions realized in two decisive points on the return air channel surface known as the “cold corner” and “hot corner”. It was concluded, that the fully open bypass technique does not provide complete frost protection under sub-zero outdoor air temperature operating conditions.