$10000$ kg$/$h solvent will be heated from $20$ to $80\backslash$deg C in a shell and tube heat exchanger. The heating is to be accomplished by passing oil, which is available at $160\backslash$deg $C,$ through the shell side $of$ the counter current exchanger. The oil provides $an$ average convection coefficient $ohh0=420\frac{W}{m}2K.$ There are $20$ tubes $(Di=25mm$ ; $D0=27mm)$ and each tube makes two passes through the shell. The oil leaves the exchanger $at90\backslash$deg $C.$ What $is$ the mass flow rate $of$ the oil? How long tubes should $be$ used $to$ supply desired heating?