The effect of recycle on laminar countercurrent heat exchangers with insulation sheet inserted for double-pass operation

The effects of recycle at the ends on double-pass laminar countercurrent heat exchangers with uniform wall temperatures are investigated analytically by an orthogonal expansion technique. Analytical results show that recycle can enhance the heat transfer efficiency due to the desirable preheating effect having more influence than the undesirable residence-time effect for high Graetz numbers, and hence the increase in preheating effect by increasing the reflux ratio leads to improved performance in heat transfer rate compared with that in single-pass operation (without an insulation sheet inserted).