Frost formation and freeze protection with bypass for counter-flow recuperators

- A numerical study of heat and mass transfer in counter-flow exchanger is presented.
- The e-NTU model of counter-flow plate heat exchanger is developed.
- The most probable variants of year-round operation performance are established.
- The values of critical outdoor temperatures are determined for frost-free operation.
- The frost control strategies using face and bypass dampers are proposed.

An accurate and efficient model based on the modified ε-NTU method was developed for numerical simulations and analysis of coupled heat and mass transfer inside the counter-flow plate heat exchanger under frosting operating conditions. The proposed model was validated with experimental data. The successful comparison between simulated and experimental data indicates that the developed model is capable to predict adequately operating performance of the counter-flow plate heat exchanger under sub-zero outdoor air temperature conditions. Three active heat and mass transfer areas were established in a counter-flow plate heat exchanger equipped with bypass damper and face and bypass dampers. The detail analysis of these particular heat and mass transfer zones creation revealed the most probable variants of year-round operating conditions of the counter-flow heat exchanger. The implementation of the particular variant of heat and mass transfer depends upon the relations of the temperatures in two decisive zones on the return air channel surface (in the “cold” and “hot” zone) and the value of the inlet return airflow dew point temperature. It was established, that the most unfavourable operating conditions at sub-zero outdoor air temperature occur at the value of inlet return air dew point temperature equalled to 0 °C. Unfortunately, such value of dew point temperature corresponds to the normal indoor air conditions in winter season. The values of critical outdoor temperatures were determined on the base of parametric frosting limits analysis conducted under different inlet return airflow conditions for different values of heat recovery efficiency of the counter-flow plate heat exchanger at different opening levels of face and bypass dampers. It was established, that the frost tends to take place with increasing temperature effectiveness of the heat exchanger. It was established, that the fully open bypass technique does not provide complete frost protection under sub-zero outdoor air temperature operating conditions.