$\backslash$Delta H for coal $=6$kW$*($h$)/($k$)$g $50\%,$ the temperature of the exhaust gas from the coal combustion is $125\backslash$deg C higher than

the ambient air temperature, and the cooling water should be warmed by only $20\backslash$deg C compared to its

ambient temperature? "Exhaust gas to fuel mass" ratio is about $20.$ Water specific heat, c$\_($p$)=4\mathrm{.}184$

k$($J$)/($k$)($g$)/(\backslash$deg $)$C$.$ Air specific heat, c$\_($p$)=1$k$($J$)/($k$)($g$)/(\backslash$deg $)$C and c$\_($v$)=0\mathrm{.}75$k$($J$)/($k$)($g$)/(\backslash$deg $)$C$.$ Water density $\backslash$rho $=1$k$($g$)/($l$).$