Flow rate optimization in run-around heat recovery systems

Abstract Heat recovery technologies are used to reduce the energy use and the operating costs for ventilation systems in buildings. Run-around heat recovery systems for ventilation are commonly used in buildings when cross-contamination between the air streams is not acceptable, in buildings with complex ducting and in retrofit projects with space limitations. The design and operation of run-around systems are rather complex, especially in ventilation systems with variable air flow rates since the coupling liquid flow rate must be adjusted with respect to the air flow rate. This paper presents a mathematical model of a run-around heat recovery system. The model is validated with lab measurements and used further in parametric studies to evaluate how the overall thermal effectiveness of a system is influenced by different heat exchanger configurations, coupling liquids and operating conditions. Important findings suggests that the thermal effectiveness is highly sensitive to the coupling liquid flow rate, particularly for systems designed for high thermal effectiveness and for variable air volumes. The optimum liquid flow rate cannot only be determined by the air flow rate as it is influenced by the heat exchanger configuration and the liquid properties and not always found within the turbulent flow regime.