Normally, the length to diameter ratio of rotary dryers varies from $4$ to $10$ and for its economic operation; the number of transfer units $($NTU$)$ for such dryers varies from

$1\mathrm{.}5-2\mathrm{.}5$

$3\mathrm{.}5-5\mathrm{.}5$

$7\mathrm{.}5-10\mathrm{.}0$

$42278$