Fig.4a, it is understood, that both twisted tapes have OER around unity when the tube side flow isconsidered. This shows that the penalty in pressure drop is justified by the increase in heat transfer. A trend ofincreasing OER with Reynolds number can be seen for both twist ratios. From Fig.4b, it is seen that twisted tapesconsiderably increased heat transfer. Nusselt number in annulus with twisted tape inserts were about 1.4 (with tapeof twist ratio 3) and 1.3 (with tape of twist ratio 5) times greater than that of annulus of ordinary heat exchangers.