Handbook of Solvents - George Wypych - ChemTech - Ventech!

392 Seung Su Kim, Jae Chun Hyun
where:
Sh average Sherwood number of a dryer zone
Nu average Nusselt number of a dryer zone
Sc Schmidt number of air
Pr Prandtl number of air
Le Lewis number of air
km average mass transfer coefficient of a dryer zone
heat conductivity of air
k a
Table 7.3.1. Procedure for calculating h in dryer. [Adapted, by permission, from
Cohen, E. D. and E. B. Gutoff, Modern Coating and Drying Technology, VCH
Publishers, Inc., New York, 1992]
1 Set dryer air conditions and measure air inlet temperature
2 Run heavy gauge polyester web through dryer
3 Install two infrared thermometers relatively close together at initial part of drying
zone
4 Run web at varying line speeds and measure temperature rise in web
where:
T −T
T −T
a x
hx
= −
CtU
⎛
⎜
200 ⎞
exp ⎟
⎜ ⎟
⎝ ρ ⎠
a o p
Cp heat capacity,
h heat transfer coefficient
t thickness of web
Ta temperature of hot air
To initial temperature of web
Tx temperature of web at distance x
U line speed,
x distance between infrared thermometers
ρ density of web
The typical value of heat transfer coefficient of modern industrial dryer ranges from
50 to 150 J/m 2 sec o C. Figure 7.3.5 shows the calculated heat transfer coefficient of an impingement
dryer with varying nozzle exit velocity of air and fraction open area (nozzle
spacing), calculation was done according to the Martin’s correlation.
7.3.2.4 Prediction of drying rate of coating
The drying rate of coating and the subsequent residual solvent amount along with dryer
length can be found by applying above equations. In magnetic media manufacturing pro-