TIP 0404-07
Paper machine drying rate / 4
where:
T
avg
=
average saturation steam temperature, qC (qF)
T
i
=
saturation steam temperature, of dryer (i), qC (qF)
N
=
number of steam-heated dryers that contact the sheet
Saturation steam temperatures can be determined from measured dryer steam pressures using common steam tables
(1, 2). Abbreviated excerpts from these references are included in this TIP.
Note that a weighted average steam pressure is not necessarily equal to the corresponding weighted average
steam temperature. This is because steam pressure and steam temperature are not linearly related. Also note that the
temperature of the saturated steam in the dryer is higher than the temperature of the condensate leaving the dryer.
Drying rate for coated papers
Equations 1 and 2 can also be used to compute the drying rate for coated papers. However, entering dryness E
must generally be calculated from information on coating weight and moisture, while reel basis weight B can be
either used directly or computed from the raw stock basis weight. The equations for these calculations are as
follows:
)
100
/
(
)
100
/
(
L
W
P
B
B
c
(4)
»
»
¼
º
«
«
¬
ª
C
W
c
C
P
c
B
W
B
E
)
100
(
100
100
)
1
(
)
1
(
100
100
(5)
in which the variables are defined as:
B
c
=
basis weight of the sheet entering the coater (wet basis), kg/m
2
(lb/ream)
W
=
dry coating weight applied, kg/m
2
(lb/ream)
P
=
percent dryness of sheet entering the coater
C
=
percent coating solids in coating solution as applied to the sheet (wet basis).
Equations 4 and 5 can also be used to calculate the drying rate after a size press, where W is the dry weight per ream
of the starch picked up and C is the percent starch solids in the size solution.
An example of these equations follows in both customary and SI units:
Customary units
S
=
2000 ft/min
B
c
=
43 lb/ream into coater (includes moisture)
L
=
96% dryness (fiber and coating) leaving the dryer section
P
=
94% dry (wet basis) entering the coater
A
=
3300 ft
2
/ream
D
=
5.0 ft diameter
N
=
6 coater dryers
W
=
8 lb of dry coating applied per ream
C
=
60% coating solids