where dM/dt is the rate of moisture removal per unit drum
surface (kg H
2
O/hr m
2
), V is the velocity of ambient air
(m sec
21
), and DP ¼ p
s
2 p
(atm) is the difference
between the vapor pressure at product surface p
a
and
the vapor pressure in the ambient air p
. In period III, the
drying rate is controlled by moisture diffusion as well as
heat transfer.