ABSTRACT
Because water evaporates on the material surface during the constant drying-rate period, the drying rate R
R W A
dw d
k H H h H H Cc m m
u ( ) ( )
(10.3)
When the heat is supplied from the hot air only, the material temperature t m is equal to the wet-
bulb temperature of the air t w . Hence,
R h t t
( )
(10.4)
On the other hand, when the heat is supplied by convection, conduction, and/or radiation, the constant drying rate can be estimated by considering that the heat input to the material surface is consumed only for the evaporation of water. The material temperature sometimes deviates from the wet-bulb temperature in the case of other liquids, although a constant material temperature and drying rate are observed. 1-3
This rate, R d , is given as a function of water content w. The drying time u d during the decreasing
drying rate period can be estimated by
u u
dw R
d dw
w ⎛ ⎝⎜
(10.5)
Many types of dryers have been developed and operated, as listed in Table 10.1. 4 First, dryers should be selected based on shape, size, water content of initial and fi nal target wet material, the production rate, and the mode of drying operation. Then, the properties of the material (e.g., stickiness) are taken into consideration, and the selection of dryer type is made. Finally, the volume of the dryer selected should be roughly estimated.
