In one design of a direct$-$contact crossflow gas$-$solid heat exchanger, the

solids are conveyed horizontally on a screen conveyor while gas percolates

upward through the solid. Suppose that hot solids $(\frac{?}{\text{b}\text{a}\text{r}}(d{)}_p=10(mm),=0\mathrm{.}4)$ are

conveyed at $0\mathrm{.}2\frac{m}{s}$ in a layer $0\mathrm{.}2m$ thick while cold air flows upward

through the solid mass from a high$-$pressure chamber $($pressure is $2$kPa

above atmospheric$)$ to atmospheric pressure, the whole exchanger being at

an average temperature of $100C.$ Find the direction of airflow through the

layer of solid, and give this as the angle from the vertical $,$ as shown below.