$.$ A tube bank composed of tubes with $4$ m length and $1$ cm diameter is arranged in line with SL $=4$ cm

and ST $=3$ cm$.$ Water enters to the tube bank at $15$C and is heated to $65$C by passing across the

tubes which are maintained at Ts $=90$C$.$ If the mean velocity of water is $0\mathrm{.}8$ m$/$s$,$ determine the

number of tube rows $($NL$)$