The influence of recycle on double-pass parallel-plate heat and mass exchangers with uniform wall temperature has been studied analytically. The heat or mass transfer problem is solved for fully developed laminar velocity profiles in parallel-plate channels by ignoring axial conduction, and with fluid properties of temperature independence. Analytical results show that recycle can effectively enhance the heat or mass transfer rate compared with that in an open conduit (without an impermeable plate barrier inserted for double-pass and recycle operation) . The desirable preheating effect and the undesirable effect of decreasing residence time are the two conflict effects produced by the recycle operation. It was found that the increase in preheating effect by increasing the reflux ratio can generally compensate for the decrease of residence time, leading to improved performance, exceptionally for very small Graetz numbers.
International Journal of Heat and Mass Transfer 42(9), pp.1707-1722