A pulp containing $40$ per cent by weight of moisture is fed into a counter$-$

current drier at a rate of $1000k\frac{g}{h}.$ The pulp enters at a temperature of

$20C$ and leaves at $70C,$ the moisture content at the exit being $8$ per cent.

Air of humidity of $0\mathrm{.}007kg$ water per $kg$ dry air enters the drier at $120C$

and the temperature of the air leaving the drier is $80C.$ What is the rate

of air flow through the drier and the humidity of the air leaving the drier?

Data: heat losses from drier $=5{10}^{6}J,$ specific heat of water vapour.

$c_p=2.{10}^3\frac{J}{k}gC,$ specific heat of dry pulp $c_p={10}^3\frac{J}{k}gC,$

$H_v$ at $80C=2\mathrm{.}3{10}^6\frac{J}{k}gC$

Answer: ${}_g=2\mathrm{.}30{10}^{4}k\frac{g}{h},$ water content $=0\mathrm{.}021kg$ water per $kg$

dry air