Abstract
In this study theoretical analysis of the heat and mass transfer in counter-flow recuperators used for energy recovery in air handling units (AHU) under sub-zero outdoor air temperature operating conditions is presented. The most probable variants of year-round heat exchanger operation performance, which characterized by existence of three active heat and mass transfer zones ( “dry”, “wet”, “frost” ), and effect of the latent heat of water vapour condensation on the realization of these variants was determined. It was established, that the frost tends to take place with increasing temperature effectiveness of the heat exchanger. Two main techniques of the frost prevention (preheating and bypassing the outdoor airflow) were described and analysed. The values of critical outdoor temperatures and outdoor-to-return airflow rate ratio were determined on the base of parametric frosting limits analysis conducted under different inlet return airflow conditions. The comparison of the heat recovery efficiency and additional energy consumption for the air treatment in the AHU is presented.
Highlights
Nowadays the increased global energy consumption, many legal requirements and environmental protection, resulted in the rational management of energy resources
Some actions are used to improve the effectiveness of existing and designed air conditioning and ventilation systems, well-known as the major consumers of electricity and heat [1]. For this reason, running costs of air conditioning system can be significantly reduced by implementing heat recovery
In this study a theoretical analysis of the heat and mass transfer in the counter-flow plate heat exchanger used for energy recovery in air handling units (AHU) under sub-zero outdoor air temperature operating conditions was presented
Summary
Nowadays the increased global energy consumption, many legal requirements and environmental protection, resulted in the rational management of energy resources For this reasons, some actions are used to improve the effectiveness of existing and designed air conditioning and ventilation systems, well-known as the major consumers of electricity and heat [1]. Nasr et al [6] studied the performance of two cross-flow heat exchangers under different operating conditions They presented the values of frosting limit and defrosting time ratio. The authors evaluated the effects of two defrosting methods on energy consumption of ventilation in three cold cities They concluded, that the air preheating method performs better than the outdoor air bypassing method for frost prevention. The problem of frost formation occurrence inside the counter-flow heat exchangers is still not solved and the most popular frost control techniques based on the outdoor air preheating or adjusting the outdoor-to-return airflow rate ratio are still facing challenges under extreme outdoor air temperature conditions
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