the maximum temperature limit for CPVC pipes is $93\mathrm{.}3$C$.$ A double$-$pipe heat

exchanger is located upstream of the pipe system to reduce the hot water

temperature before it flows into the CPVC pipes. The inner tube of the heat

exchanger has a negligible wall thickness, and its diameter is $25$ mm$.$ The

convection heat transfer coefficients inside and outside of the heat exchanger

inner tube are $3600$ W$/$m$2$

K and $4500$ W$/$m$2$

K$,$ respectively. The hot fluid $($cp $=$

$4216$ J$/$kgK$)$ enters the heat exchanger at $105$C with a flow rate of $0\mathrm{.}75$ kg$/$s$.$ In

the cold fluid stream, water enters the heat exchanger at $10$C and exits at $80$C$.$

To ensure that the hot water exiting the heat exchanger is $93\mathrm{.}3$C or lower,

please determine the length for counter flow

and the length of parralle flow