Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.3 Drying of coated film 391
Figure 7.3.4. Geometry of slot nozzles for the Martin’s correlation [After reference 37].
Nu
Pr
042 .
2
= f
6
34 /
O
⎛ ⎞
⎜ ⎟
⎜ 4Re
⎟
⎜f
f ⎟
O
⎜ + ⎟
⎝fO
f ⎠
23 /
[7.3.6]
where:
Nu average Nusselt number over a zone (2hB/κa) Re Reynolds number (2wB/νa) Pr Prandtl number of air (νa/αa) f Fraction open area (B/LT) fO [60 + 4(H/2B - 2) 2 ] -1/2
B Nozzle slot width (Figure 7.3.4)
LT Nozzle spacing (Figure 7.3.4)
H Nozzle to coating surface distance (Figure 7.3.4)
Range of applicability is
1,500≤Re≤40,000
0.008≤f≤2.5fO 2≤H/B≤80
The heat transfer coefficient of the arrays of round jet nozzles and the other shapes of
nozzles can be found elsewhere. 37,45 In general the heat transfer coefficient can be written in
the following form 9,10
h =Kw n
[7.3.7]
where:
K a constant that depends on the physical properties of the air and geometric properties of
the dryer
n 0.6 ~ 0.8 (depending on the nozzle geometry)
One can easily calculate the actual heat transfer coefficient of a zone by running a
heavy gauge web through a dryer and measuring temperature rising of web using non-contacting
infrared thermometer in the early part of the dryer where the web is heating up. 10 The
procedures of measuring and accompanied calculation are illustrated in Table 7.3.1.
The mass transfer coefficient is related to the heat transfer coefficient through the
Chilton-Colburn analogy. 16 Thus,
042 .
Sh Sc
h
( Le) or k
Nu
k Le
= m
⎛ ⎞
⎜ ⎟ = =
⎝ Pr ⎠
042 . 042 .
a
[7.3.8]