For ordinary heat exchanger Nusselt number is validated using (9)and friction factor using(10). The Nusselt number and friction factorfor tube side flow in double pipe heat exchanger with twisted tape inserts is validated using(6) and (7). Fig.3 shows the variationof tube side Nusselt numberand friction factorwith Reynolds number in various models of heat exchanger.Figs.3a and 3b shows the variation of tube side Nusselt number and tube side friction factor with Reynolds number.