The effect of external recycle on the collector efficiency in solar air heaters has been investigated theoretically. The application of external-recycle 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 decreasing the driving force (temperature difference) of heat transfer, due to the remixing at the inlet. It is found that considerable improvement in collector efficiency is obtainable if the operation is carried out with an external recycle, where the desirable effect overcomes the undesirable effect. The enhancement increases with increasing reflux ratio, especially for operating at lower air flow rate with higher inlet air temperature.
