Effect of Pass Number on Collector Efficiency in Downward-Type Multipass Solar Air Heaters

The effect of pass number on the collector efficiency in downward-type multi-pass solar air heaters has been investigated theoretically. The application of multi-pass operation to solar air heaters actually has two conflict effects. One is the desirable effect of increasing fluid velocity to decrease the heat transfer resistance. The other is the undesirable effect of increasing the hydraulic dissipated power. It is found that considerable improvement in collector efficiency is obtainable if the operation is carried out with multi-pass operation, while the undesirable effect is still small and may be ignored. The enhancement increases with increasing pass number, especially for operating at lower air flow rate with higher inlet air temperature.