Reduction of flow loss due to heat recovery in PSV systems by optimum arrangement of heat-pipe assemblies.

Natural ventilation is being applied to an increasing number of new buildings to minimise reliance on mechanical ventilation and so reduce emission of greenhouse gases. However, passive stack ventilation (PSV) systems are currently designed without incorporating heat recovery leading to significant wastage of energy. Heat recovery systems have not been used in naturally-ventilated buildings because the pressure loss caused by a conventional heat exchanger is large compared to the stack pressure and could cause the ventilation system to fail. In addition, the stack pressure decreases owing to reduction of the temperature difference associated with the heat exchange, although this problem can be lessened by appropriate siting of the heat exchanger to maximise the effective stack height. In this study, natural convective flow through PSV stacks were computed using CFD to determine the effect of the layout of heat-pipe assemblies as well as the effects of spacing and length of fins on reduction of flow rate through the stack. Among the layout patterns examined, the arrangement where the assemblies are placed in a pattern of an arrow facing the flow direction produced the least insertion flow loss. The flow loss due to the insertion of the heat pipe assemblies (IFL) was found to increase sharply with the number of fins and reached over 30% when only 4 fins were used. IFL also increased with fin length but the rate of increase reduced for larger fin lengths. Therefore, for a given total surface area of fins, using fins with a larger length causes less flow loss than fins with a smaller spacing.