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 he at 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 ef fect having more influence than the undesirable residence-time effect for h igh Graetz numbers, and hence the increase in preheating effect by increasi ng the reflux ratio leads to improved performance in heat transfer rate com pared with that in single-pass operation (without an insulation sheet inser ted). (C) 2000 Elsevier Science Ltd.