Drying of Coated Webs

James Y. Hung, Richard J. Wimberger, and Arun S. Mujumdar

CONTENTS

40.1 Introduc tion ........ ............... .......... .......... ............... .......... ............. ............... .......... .......... ............... ...... 931
40.2 Drying Curv es ..... ............... .......... .......... ............... .......... ............. ............... .......... .......... ............... ...... 932
40.3 Drying Tec hniques for Coat ed Webs ..... ............... .......... ............. ............... .......... .......... ............... ...... 934

40.4 Applicati on Example s ..... .......... ............. ............... .......... ............. ............... .......... .......... ............... ...... 944
40.5 Safety Aspec ts ..... ............... .......... .......... ............... .......... ............. ............... .......... .......... ............... ...... 945

40.5. 1 The Expl osion-Pr oof Stan dard ..... ............... .......... .......... ............... .......... .......... ............... ...... 946

4 0.5.1.1 Lower Explosi ve Limi t Regul ations ............... .......... .......... ............... .......... .......... .. 946
4 0.5.1.2 Inert Gas Drying Pro cess ............. ............... .......... ............. ............... .......... .......... .. 946

40.5. 2 The Dryer Emission Cont rol ..... ............... .......... ............. ............... .......... .......... ............... ...... 947

4 0.5.2.1 Solvent Recove ry ..... .......... .......... ............... .......... ............. ............... .......... .......... .. 948
4 0.5.2.2 Inciner ation ..... .......... ............... .......... ............. ............... .......... .......... ............... ...... 948

40.6 Conclu sion .......... ............... .......... .......... ............... .......... ............. ............... .......... .......... ............... ...... 951
Biblio graphy ..... ............. ............... .......... .......... ............... .......... ............. ............... .......... .......... ............... ...... 951

40.1 INTRODUCTION

The objec tive of this chap ter is to revie w briefly the
drying process , drying equipment , drying strategi es,
and web hand ling avail able for co ated webs . Based
on the substr ate mate rials, coated webs can be divide d
into three types: (1) coated pa per and pa perboard; (2)
coated plastic films (e.g ., photo graphic films) and
tapes (e.g ., ad hesive tapes, magnet ic tapes, pressur e-
sensitiv e tapes, and phot osensitive tapes); and (3)
coated meta llic sheets. Pape r and pape rboard are
coated on machi ne or off machin e, while plast ic
films, tapes, or meta llic sheets are general ly coated
off machi ne. ( On machine indica tes the coati ng ope r-
ation that is done on the web before it is remove d from
the origina l manu facturing machi ne, wher eas off ma-
chine impl ies the coatin g ope rations done on a free-
standin g machi ne remote from the original machi ne.)

During the coating process, some coated webs

require a single coating; other webs require more

than one coating layer either by passing a web of
material through a single coating station more than
one time or by coating a web with a multiple-station
coating machine. In the converting industry, paper,
films, and foils can be combined together to form
multiple-layer structures in a process called laminat-
ing. In the graphic arts industry, the coated papers are
further coated with ink to generate the desired images
through a single printing station or multiple-color-
printing units .

Figure 40.1

shows a finished Polaro id

instant color picture containing polyester supports on
the top and bottom with active layers sensitized to the
three primary colors (blue, green, and red), timing
layers, and spacing layers to display the image be-
tween the supports.

Coatings, inks, and adhesives contain more than

one component: (1) a binder that may include par-
ticles to give it a useful function (e.g., pigments for
color and opacity, silver halide particles for photo-
graphic activity, or iron and chromium particles for

2006 by Taylor & Francis Group, LLC.

magnetic activity); (2) a variety of additives (e.g.,
surfactants that aid the coating process, plasticizers
for flexibility, biocides that prevent bacterial growth,
cross-linking agents for toughness, and other addi-
tives to minimize static buildup); (3) surface particles
to control reflectivity and transport; and (4) a liquid
solvent to dissolve or suspend all particles. These
components can be reduced to two basic elements:
(1) coating liquids and (2) coating solids. The solids
can be concentrated on the web surface, such as in
coating and printing, or can be distributed through-
out a fibrous web, as in saturating or encapsulation,
or can be located at the interface between two webs,
providing adhesion, as in laminating.

Coatings can be applied in three forms: (1a) solv-

ent borne; (2) water based; and (3) 100% solids. Both
solvent and water types must pass through an evap-
oration dryer to remove the diluent (e.g., solvent or
water) so that a dry film remains on the web or sheet.
Organic solvents require use of a closed-cycle oper-
ation and special care in design and operation owing
to the potential fire. A 100% solids coating is a liquid
that does not contain any solvent or water. It changes
into a solid through a chemical action (i.e., exposure

to a catalyst). The catalysts used in the printing and
converting industries are moisture, heat, and ultravio-
let (UV) and electron beam (EB) energy. The web
coated with solventless liquids usually passes through
a hot-air dryer for heat-active liquids, or a UV or an
EB emitter for UV and EB liquids. The antipollution
regulations have caused the publication, commercial,
and packaging printers to seek a replacement for
solvent types of printing inks. The three most com-
mercially acceptable replacement printing inks are:
(1) aqueous; (2) high-solids (heat-curable); and (3)
solventless (100% solids) inks.

Most coating systems involve drying as well as

curing of the coating. The drying of the coated webs
involves a combined transfer of heat and mass. Dur-
ing the transfer process, water or solvent vapor is
removed from the web, leaving nonvolatile solids
behind. Heat transfer resulting from temperature dif-
ference between a coated web and its surrounding
media can be accomplished by conduction, convec-
tion, radiation such as infrared (IR), or a combin-
ation of these methods. Mass transfer occurs within
the coated web primarily through capillary force and
vapor diffusion, and from coating surface to its sur-
rounding air by diffusion or forced ventilation. While
conductive and radiative heating are useful tech-
niques for some applications, convective heating is
by far the most common means of supplying the
energy needed to evaporate water or solvent, and is
the only form of heating that also provides a means of
enhancing the transport of water or solvent vapor
away from the coating surface. Curing involves the
cross-linking mechanism of UV- or EB-curable ma-
terials, or drying and hardening of any film coating by
chemical reaction, especially polymerization.

Air-knife coating poses a special consideration in

drying system design because of the low solids con-
tents (14 to 20% solids) of the coating applied, e.g., in
carbonless copy coating, in which coating applied is
in the 40 to 50 g/m

range and the coat weight (dried

product) is in the 2 to 5 g/m

range. The high water

content causes extension of the web between the
coater and the dryer and contraction during the dry-
ing. In on-machine coating of board substrates, this
problem is not encountered. Air-knife coating sys-
tems operate with web speeds up to 600 m/min; in
most cases 450 m/min is the maximum speed. Other
coating techniques, e.g., blade or roll coating, may
operate at speeds up to 1200 m/min.

40.2 DRYING CURVES

In order to design an efficient drying system for coat-
ings, the individual drying curves showing their dry-
ing mechanism must be taken into consideration.

Expose/view

Clear polyester

Image-receiving layer

Clearing layer

Antiabrasion layer

Spacer

Antifoggant

Interlayer

Green-sensitive emulsion

Yellow dye developer

Blue-sensitive emulsion

PDD

Magenta dye developer

Spacer

Cyan dye developer

Timing layer

Acid polymer

Opaque polyester

Red-sensitive emulsion

FIGURE 40.1 Makeup of a typical Polaroid film.

2006 by Taylor & Francis Group, LLC.

Because of the product quality problems associated
with the drying process, the proper quantity of heat
and mass transfer has to be applied to the coating at
the proper time in the drying curve. There are two
basic types of drying curves that can be easily con-
structed from drying data. The first type of drying
curve is shown in Figure 40.2, representing percent
solvent remaining in the coating vs. drying time for a
typical solvent-based coating of about 0.1 mm wet
thickness. Although this curve shows that the dry-
ing rate decreases with drying time, there are no
drying rate values at various points in time for the
effective drying equipment or optimal drying process
control.

information

given

the

second

type of drying curve (Figure 40.3) constructed
from the same drying data. Here, the drying curve
is a plot of drying rate as a function of drying time
and reveals three distinctive drying rate periods:
(1) the adjustment stage or the cooling-down period
as represented by the segment A–B; (2) the con-
stant drying rate period represented by the segment
B–C; and (3) the falling drying rate period by the
segment C–D.

Figure 40.3 also represents many solvent-based

coatings. A rather short duration of cooling down
and constant drying rate period is then followed by
a very long period of falling drying rate. In contrast,
the drying curve of water-based latex coatings has a
longer constant drying rate than solvent-based coat-
ings. Paper has a drying curve with a short warm-up
period and a constant drying rate period even longer
than latex coatings (Figure 40.4). Once the drying
mechanism or drying characteristics of each drying
rate stage are identified for each coated web, the
proper design and control of the drying system can

be offered to obtain the best quality products as each
drying rate stage has its special drying mechanism to
affect the finished product quality. For instance, Fig-
ure 40.3 shows a large amount of drying solvent
evaporates during the constant drying rate period;
it is desirable to design a drying system to extend
this constant drying rate period for optimal drying
operation.

The success of a drying system for coatings re-

quires not only a means to remove water and solvent
vapor from the coated webs, but also proper drying
operation to accomplish the quality requirements ne-
cessary for further application. For example, most
coated papers are intended for printing, and binder
migration causing printing mottles should be elimin-
ated or minimized during drying operation. Binder
migration can be affected by the rate in the preheating
stage. The time in the constant drying rate zone and
the temperature in the falling drying rate zone
also affect many of the final sheet properties. Over-
drying or rapid drying can cause brittleness, blister-
ing, curl, and wrinkles. Brightness, ink receptivity,

Drying time (s)

Percent solvent in the adhesive

FIGURE

40.2 Drying

curve

(solvent

remaining

vs.

drying time).

Drying time

(s)

Drying rate (g/s)

B C

FIGURE 40.3 Typical drying rate curve (drying rate vs.
drying time).

Warming-up

stage

Constant-

rate stage

Falling-

rate stage

Drying rate

Time or position through dryer section

FIGURE 40.4 Typical drying rate curve for paper.

2006 by Taylor & Francis Group, LLC.

and varnisha bility can be reduced . Nonunif orm mois -
ture profile, whi ch is normal ly caused by raw stock or
coatin g, can somet imes also be attribut ed to drying
conditi ons.

40.3 DRYING TECHNIQUES

FOR COATED WEBS

The follo wing are the princi pal hardw are available for
drying of coated webs :

1. Steam-heat ed cylin ders
2. High-veloc ity air ca p
3. Impinging jet tunne l dryers
4. Air flotation dryers
5. Air turns
6. IR dryers
7. Ultravio let curers /dryers
8. EB curers /drye rs

40.3.1 S

TEAM

-H

EATED

YLINDERS

In the pap ermaking process , the steam-heat ed cyli n-
ders have been tradi tionally placed in staggered posi-
tions for drying of pa per web because of their ea se
and econo mics in ope ration . How ever, coated webs
cannot be dried on the same steam-heat ed cylind er
configu ration wi thout special precaut ions becau se of
picking of the coatings. To prevent pick ing, the dry-
ing cyli nder surface need s a specia l finish, pa rticular ly
on the fir st few cylinde rs. The CIS (coat ed one-sid ed)
web is made to wra p arou nd a series of steam -heated
cylind ers (1 to 2.5 m diame ter, steam pressure s of 3 to
5 kg/cm

), yielding a rather low evaporat ion capacity

of 3 to 6 kg/m

h. One of the new solut ions to this

picking prob lem includes the use of circular flotation
dryers ’ jet foil cyli nder in the locat ion of the first and
second steam cylind ers (F igure 40.5) . This initial co n-

vection air drying woul d allow en ough evap oration to
prevent picking on downst ream cylinders. (The jet foil
cylind er is a uniqu e co ntactless drying syst em designe d
in a circul ar configu ration. Cont actless drying is
achieve d using co mbination of airfoil and pressur e
pad air flotatio n techn iques descri bed in

Secti on

40.3.4

and

Se ction 40.3.5

. High- tempe rature c irculat-

ing air is the he at and mass trans fer med ium, whi le
total circulati on with a control led exh aust ensures
economic operati on.)

M any tim es a steam -heat ed cyli nder ha s a felt on

the oppos ite side of the web from the steam-heat ed
cylind er to insure good co ntact wi th the cylind er at
elevated web sp eeds and low web tensions. Drying
rates woul d range from 2.4 (ass uming full wrap ) to
7.3 kg/m

h, dep ending upon whether the steam -

heated cyli nder is operate d with or witho ut a felt and
whether or not the c oated side is tow ard the cylinder or
away from it. Also, the drying would need to vary for
coatin g quality reasons , depend ing on whi ch poin t in
the drying cu rve is being add ressed. Dryi ng rate en -
hancement can be accompl ished through poc ket ven -
tilation syst ems that evacuat e evap orated co mpound s
between steam -heat ed cylind ers. Also, heat trans fer
rates will be increased through enhanced design of
steam delivery and removal within the cylinder.

40.3.2 H

IGH

-V

ELOCITY

Air cap dryers resemble the Yankee dryer, wrapping
around a steam-heat ed cylin der as shown in

Figu re

40.6

. The Yankee dryer was designe d for drying of

tissue and towel, whereas the air cap was intended for
drying of paper coating when the blade coaters were
introduced. Air caps consist of a series of circular or
slot nozzles with a pressure and exhaust plenum
hood, providing high-velocity hot air for rapid drying
by penetration of the vapor barrier on the coated side
of the web. Both circular and slot nozzle spacing
typically range from 10 to 25 mm and the nozzle
clearance from the steam cylinder surface varies
from 6.5 to 50 mm. The air cap must be retractable
to permit easy threading and broke clearance. One air
cap is generally adequate to immobilize the coating
(75% dryness) if the web speed is no more than 500
m/min. Air cap systems can be used as long as the
drying or curing load can be handled by a 1.5 to 2.4 m
diameter cylinder or air cap assembly.

Air cap drying rates range from 35 to 95 kg/m

with jet air at 50 to 60 m/s at temperatures from 150
to 3158C. Higher drying rates are attainable with
higher jet velocities (above 60 m/s). However, very
high jet velocities can cause flow of the coating; the
typical operating range is 40 to 60 m/s. Lower veloci-
ties, 20 to 40 m/s, are needed when the solid content

Jet foil cylinders

machine floor level

FIGURE 40.5 Steam-heated dryer configuration.

2006 by Taylor & Francis Group, LLC.

of the coati ng form ulation is low . For accep table
therma l effici ency, signi ficant recir culation of the air
(70 to 80%) is needed.

Bin der migratio n is a common prob lem that de-

pends on the coati ng form ulation , substrate charac-
teristics, and coatin g rheology . Upper drying rates are
impos ed by the onset of binder migrati on. The use of
natural starch bin ders tends to impos e a lower dr ying

rate lim it to avoid binder migrati on, and synthet ic
binders (e.g., latex) permi t drying rates of up to
35 kg/m

h. Little migrati on occurs once the coating

is about 75% dryness . Further, higher drying rates
can be a ccomplished with little migr ation problem if
the therma l en ergy is sup plied from both sides of the
web. How ever, the high -velocit y air impi nging dir-
ectly on the co ating surface tends to dry the surface
coatin g a nd accele rates the binding migrati on in the
coatin g. ‘‘Rail road tracki ng’’ will take place when the
dryer tempe rature exceed s the wet bulb tempe rature
of the constant drying rate period.

40.3.3 I

MPINGING

UNNEL

RYERS

If the impi ngement length of 4 m is inadequat e in an air
cap dryer, lon ger dwel l tim es (or lengt hs) can be pr o-
vided in a tunnel dryer (Figure 40 .7). In the 20 0 to 250
m/min ran ge, the web is sup ported on bars carried
through a tunnel on conveyor chains whi le hot air is
impinge d on the coated surfa ce. In impro ved version s
of this impingemen t tunnel dryer, the web is suppo rted
on roll s (driven , undriven, or tenden cy driven) . To
avoid sag and flutter , slot noz zle impi ngement is
typicall y applie d above eac h web support roll. For
lightw eight webs run ning at speeds up to 450 m/min,
a fabric supp ort is commonl y employ ed.

Bot h round jet and slot jet arrays are used. In the

early drying period, slots may be preferred to mini m-
ize distu rbance to the web coatin g. Jet veloci ties up to
75 m/s are used. Air-re cyclin g rates up to 92% may be
needed for optim al therm al effici ency. To avoid the
possibili ty of conden sation on the cold web as it
enters the dryer, the web may be prehe ated using IR
lamps. The a ir jet humidi ty is typic ally in the range of
0.016 to 0.25 kg water/ kg dry a ir.

40.3.4 A

LOTATION

RYERS

There are two types of air flotation dryers: (1) the
single-slot airfoil dryer and (2) the double-slot air-
bar dryer.

Figure 40.8

shows the pressur e dist ribution

Key

Damper control pneumatic actuator

Pneumatic actuator with positioner

Manual damper

Damper control

FIGURE 40.6 Air cap dryer layout.

Insulation

Rail for chain

and sticks

Heating coils

and fans

Cross section

Plenum

Web

Chain and

sticks

Longitudinal section

FIGURE 40.7 Conveyor tunnel dryer schematic.

2006 by Taylor & Francis Group, LLC.

on the fla t face of an air ba r or airfoil . The trip le-slot
air ba r, intende d for higher heat trans fer, has posit ive
pressur e distribut ion sim ilar to that of the double-s lot
air bar.

The single-s lot airfoil s (w hich ha ve both a posit ive

and negati ve pressure on the face of the airfo il) a re
mounted above the web, whi le the double-s lot air
bars, having only pos itive pre ssure, are placed on
both sides of the web in staggered pos itions. The
moving co ated web in the flotation dryer is suppo rted
on a cushion of he ated air issue d from the slot s (

Fig-

ure 40.9

). The key requir ement s in slot design are high

heat an d mass transfer rates an d web stabili ty. Heated
air emerg es out of the slot s at veloci ties up to 80 m/s

and impinges on the moving web. The double-s lot air
bars hav e a more stabl e flotati on than the single-s lot
airfoil s partl y be cause the posit ive pressur e pad of the
double-s lot air bars reacts agains t the vertical com-
ponen ts of web tensi on and creat es an eq uilibrium ,
and partl y because the staggered co nfigurati on form s
a sine wave in the web that elim inates ed ge curl and
provides the best possibl e web trans port without co n-
tact. Beside, the doubl e-slot air bars can be ope rated
over a wi der rang e of pressur e and clearan ces.

The standar d size of c onventi onal airfoil s an d air

bars ranges from 6 to 8 cm in wi dth. Typi cal flot ation
clearan ces of a singl e-slot, standar d-size airf oil would
be 2 to 3 mm as co mpared wi th a double-s lot air ba r
having a typical flotati on height of 6 to 7 mm. The
double-s ize air bar would creat e twice the clear ance of
the single-s ize air bar. Air bars are typic ally spaced 25
to 30 cm apart but can be as close as 18 cm for singl e-
size air bars or as far as 1 m for air bars of four tim es the
size, depending on web wei ght, porosit y, tension ,
needed clear ance from the air ba r, and a ir-bar pressur e.

The basic airflow system for flotation dryers is

shown in

Figure 40.10

. It consists of three basic

components: (1) supply fan; (2) exhaust fan; and (3)
heater. The supply fan is sized for the air volume re-
qu ir ed by t he o p en a re a o f t he dr ye r n oz zl es a nd t he
maximum nozzle outlet velocity. It blows hot air
through the air-bar nozzles onto the coated web sur-
face. The spent air is exhausted by an exhaust fan. The
heater can be a direct-fired gas burner for air temperat-
ures up to 4008C or high-steam coils for air temper-
atures up to 2008C. When drying aqueous coatings,
the exhaust is led to a recirculation system where part
is bled to the atmosphere and the remainder reheated
for recirculation. When solvents are involved, this spent
air may be incinerated or led to a solvent-recovery unit.

Air floaters used in the graphic arts indust ry ope r-

ate at web tension s on the or der of 5 to 6 kg /cm; in the
coatin g application, tensio ns as low as 0.03 kg/cm
may be nee ded. Highe r tension stabili zes the sheet.
Drying rates up to 60 kg/m

h are attained in commer -

cial install ations . Web speed s of up to 1100 m/min can
be handled with web wid ths of up to 8 m.

An important asp ect of any flotation system is the

stabili ty of the web as it passes ov er an air bar.
Airflow instabil ities ne ar the web can indu ce web
flutter and sub sequent web con tact with mechani cal
parts of the dryer, resul ting in coati ng disturban ce.
Web flutter can co me in a multit ude of form s, ranging
from a violent flapping of the web to a high-frequency
drumming. Some designs will allow stable flotation
near the center of the web but cannot accommodate
proper flotation at the web edges.

In order to avoid these types of instabilities, an air

bar should be manufactured to tight tolerances, contain

Cushion pressure

Web

–1

Pressure

cushion

area

Web

FIGURE 40.8 Pressure distribution of an air bar/airfoil.

2006 by Taylor & Francis Group, LLC.

‘‘Coander’’ radius, and have a cushion pressure sup-
pression plate placed at a higher elevation than the
impingement nozzles. Also, distance between impinge-
ment nozzles and air-bar alignment should be opti-
mized. By following these basic rules and delivering
the supply air evenly to the air bar, web flutter can be
avoided. Also, problems with web shift, such as move-
ment of the web toward the drive or operator side, and
air-bar noise can be eliminated.

No w flotatio n a ir bars come in differen t sizes. The

historical size would place the air bars on approxi-
mately 2 5- or 30-cm center lines, whi le one of the large
sizes would have air-bar ce nter distances in the 50- to
60-cm ran ge. Eve n higher sizes are avail able but a re
mostly used on nonwoven or coil-coati ng operati ons.
When adequate heat trans fer is maint ained by the air-
bar design, the need for a longer dryer is avoided
and the ad vantage s of the large -scale air bars can be

realized. A doubl e-size air bar woul d require only one
half of the number of air bars of a singl e-size air-ba r
dryer. This benefits the user during cleani ng and
mainten ance. Dou ble-size air bars also have wider
slots and are therefore less likely to become plugged
from deb ris or foreign mate rials. The doubl e-size air
bar also pr ovides twice the flotation clear ance whi le
maintaini ng the same air horsepow er as the smal ler
air ba r. Flo tation clear ance and stabili ty of flot ation
are impor tant consider ations when produ cing quality
products that requir e no co ntact an d avoidance of
markin gs on the coati ng during drying. Double- size
air ba rs make lot of sense when consider ing all ope r-
ating co ndition s. Equi pment maintenan ce, flot ation
clearan ce, and cap ital co sts seem to be benefited.

The drying capabil ity of a flotati on dryer is often

measur ed in term s of heat trans fer coeffici ent h (kcal /
h/m

/ 8 C or Btu/h/ ft

/8 F). Every air ba r or foil ha s an h

value that shou ld ha ve been measur ed as well as
calculated unde r special cond itions.

Fi gure 40.11

shows heat transfer values for the single-, doubl e-,
and triple- slot air bars. Air veloci ty, air tempe ratur e,
web temperatur e, orifice coeffici ent, open area, and
spacing play an impor tant role. In some air- bar types,
clearan ce is also a major consider ation. For exampl e,
single-s lot airf oils have a dramat ic de crease in both
heat transfer and flotati on stabili ty as clear ance is
increased. This compares with double- and triple-
slot air bars that have fairly stable heat transfers up
to a clearance of 25 mm on the single-size air bar and
50 mm on the double-size air bar.

When comparing air bars, there should be a logical

basis. Equal power consumption is one such basis for
comparison. Under equal air horsepower, single-,
double-, and triple-slot air bars have very distinctive

Web

Airfoil

Web

Two-slot
coanda

Web

Tri-slot
coanda

FIGURE 40.9 Flotation air-bar/airfoil arrangement.

Comb. air

Gas

Makeup air

Exhaust

Dryer

Web

Supply

Burner

chamber

FIGURE 40.10 Flotation dryer airflow system.

2006 by Taylor & Francis Group, LLC.

−5

120

130

140

150

100

110

−4

−3

−2

−1

Position (in.)

Coanda

Air-bar heat transfer

Tri-coanda

Air-bar heat transfer

Airfoil

Air-bar heat transfer

Position (in.)

h (ave) 27.8

h (ave) 32.2

h (ave) 37.5

Average corrected local ‘n’

(Btu/h/ft

/⬚

Average corrected local ‘n’

(Btu/h-ft

⬚F)

Average corrected local ‘n’

(Btu/h/ft

/⬚

−5

120

130

140

150

100

110

−4

−3

−2

−1

−5

120

130

140

150

100

110

−4

−3

−2

−1

FIGURE 40.11 Heat transfer coefficient comparison.

2006 by Taylor & Francis Group, LLC.

heat trans fer pro files. As seen in

Figure 40.11

, this

TRI-FLO AT has the highest average heat trans fer
coeffici ent. The air-bar type uses mult iple smal l orifices
to accompl ish the same task as the double-s lot air bar.
Total open area is the same on both.

Heat transfer wi th doubl e- or triple-slot air ba rs

can be co ntrolled by air veloci ty over a very broad
range since these air-bar types are not nearly as sen-
sitive to flotation clearan ce as single-s lotted air bar.
Whene ver air can be tolerated on both sides of the
web, either the double- or triple- slot air bar is by far
the be st choice. W hen flotation must be acc omplished
with air only on one side, the singl e-slot airfo il is most
efficien t.

The impact of spent air on flotatio n an d he at

transfer is often overlook ed. Deli vering the air in a
useful fashi on to the web is of paramount importance
but, if the air afte r impi ngement to the web is not
remove d properl y from the web an d air-bar area, it
can significantl y erod e an otherwis e exc ellent he at
transfer. Drying streak s, flotati on problem s, or diffi-
culty with web tracking also might occur.

Spe nt air shou ld be remove d from the web and

air-bar inter face area quickly , evenly, an d at substa n-
tially reduced veloci ty as compared wi th the air im-
pingem ent veloci ty. Spe nt air veloci ties usually shou ld
not exceed 10% of the air-b ar outlet veloci ties. On
narrow webs , spent air is exhau sted at the web edges.
When deali ng with wid e webs , spent air is remove d at
the web center to avoid overdryi ng the web edges.

Air -bar pe rformance is not only depende nt on the

air-bar de sign but also is signifi cantly affe cted by
aerodynam ical ly correct supply and return air be-
tween air bars an d wi thin a given air bar. Air veloci ty
enterin g the air bar should not exceed 15 to 25% of
the air-b ar outlet velocity. As with spen t air, sup ply
air is often delivered correct ly only in one porti on of
the dryer and incorrectly delivered in another portion.

All successful air bars have a cushion pressure

between the face of the air bar and the web. The

cushion pressure profile in the web direction often
looks very similar to the heat transfer profile. The
profile of a successful positive-pressure air bar
would have most, if not all, points between the web
and air-bar face at a positive pressure as shown in

Figure 40.8

. This sounds simple enough but, too often,

air-bar designs have pressure profiles with negative
pressure just outside the impingement slots. Worse
yet are the differing pressure profiles at the web center-
line vs. the web edges. The profile at the web edge is of
great concern if web flutter is to be avoided at that
point. Swings of positive or negative pressure at the
web edge will cause the web to act like a flag in
heavy wind.

Often overlooked in cushion pressure evaluations

is the angular flow from the air-bar slot. If the air
emanates from the slots at a very uniform angle to-
ward the drive side or operator side of the web, a
steering effect will take place that will hamper good
web transport. Usually, proper internal construction
of the air bar will eliminate this possibility and allow
angularity confined within +58 and in a random
fashion. Finally, the amount of air translated from
supply pressure to cushion pressure is a sign of an
air bar that is efficient. Therefore, the ratio of cushion
pressure to supply pressure Pc/Ps is important
and worth study at the desired flotation clearance.
Figure 40.12 shows a typical curve of this type.

40.3.5 A

URNS

Air turns combine the features of a circular web path
with a flotation dryer. They use single-size air bars
and support a web without contact on a cushion of air
while the web follows a circular path and is simultan-
eously heated and dried. When using an air turn to
float and dry, recirculation air is required. Usually,
the jet foil cylinder approach provides this option. Air
turns were initially proposed for retrofitting paper
machines by replacing steam-heated cylinders located

.60

.50

.40

.30

.20

.10

.2 0.3 0.4 0.5

Clearance (in.)

Ratio

/Ps

Double-slot air bar clearance curve

.6 0.7 0.8 0.9 1.0

FIGURE 40.12 Clearance curve.

2006 by Taylor & Francis Group, LLC.

immediately after coating or sizing stations. Quality
problems frequently occur in these applications be-
cause wet coating is sensitive and sometimes subjected
to picking when contacted by a steam dryer surface.
Today, air turns are often combined with straight
path flotation systems allowing web paths that have
a U shape as well as an S shape (Figure 40.13 and
Figure 40.14, respectively).

The web-supporting cushion pressure on an air

turn is proportional to the web tension and inversely
proportional to the turning radius. Assuming a uni-
form cushion pressure, then

Pc(cushion pressure)

¼ T(web tension)=R(radius)

In air turns with double-slot air bars, the cushion
pressure is not uniform, but is made up of two separ-
ate and distinct pressures. The first is developed over
the face of the air bar and is referred to as the pressure
pad, while the second is a back pressure component
that occurs as a function of the spent air condition, if
any, between air bars. The two pressure fractions are
weighted by the area according to the proportion of
wrap area they each represent; the ‘‘weighted’’ pres-
sure, in total, must be equal to the equivalent uniform
cushion pressure required. Pressure combinations as
well as pressure and area combinations suitable for
any given tension are virtually unlimited.

Air turn configurations are typically available in

0.5 to 2.0 m diameters. Volumetric flow rates, pres-
sure requirements, and physical layouts would be
considered when sizing an air turn for a particular
process. The interrelationship among cushion pres-
sure, web tension, and clearance results in an oper-
ational system that is forgiving and self-adjusting.
Close attention should be given to the design of web
entry and leaving air bars so that touching of the web
can be avoided at these points. Heat transfer of the
selected air bar will not change when utilized in a
circular configuration as compared with a straight
path configuration.

40.3.6 I

NFRARED

RYERS

Figure 40.15 shows the electromagnetic energy spec-
trum ranging from the gamma ray of short wavelength
to the radio waves of long wavelength. The IR wave-
band falls between the wavelengths of 0.76 and 100 mm,
which can be divided into three regions: (1) short-wave

FIGURE 40.13 Air turns in a U shape.

FIGURE 40.14 Air turns in an S shape.

Gamma

ray

Wavelengths in angstroms

Microns

Millimeter Centimeter

Meters

Kilometers

X-ray

Ultraviolet

Infrared

Medium-

wave

Short-

wave

Long-wave IR

0.76

0.1

100

1000

100

Radio waves

Visible

FIGURE 40.15 The electromagnetic energy spectrum.

2006 by Taylor & Francis Group, LLC.

IR (0.76 to 2 mm); (2) medium-wave IR (2.1 to 4 mm);
and (3) long-wave IR (4.1 to 100 mm).

Ther e are two basic types of IR heater s: electric IR

and gas-fired IR (Figure 40.16). The most impor tant
elemen t of the IR dr ying system is the IR emitter .
These two types of IR heater s fit into three tempe ra-
ture ran ges: (1) 343 to 538 8 C (e.g., gas and elect ric
IR); (2) 538 to 1100 8 C (e.g ., gas and elect ric IR); and
(3) 1100 to 2200 8 C (e.g., electric IR only). Electrical ly
heated or natural-ga s-heat ed units are used mainl y to
preheat the web, althoug h these can be used in prin-
ciple to carry out the entire drying process . IR tem-
peratur es are typicall y in the 650 to 1200 8 C range.
Improv ed drying rates can be obtaine d by a combined
radiation–co nvectio n syst em.

M ost high -temperat ure units have a coo ldown

time of 1 to 1.5 min. Occur rence of a break means a
signific ant fire risk if the hot surface is below the web.
Since air-k nife an d blade co aters usu ally coat the
surface , this is not a problem .

In gen eral, the ope rating effici ency of an elect ric

IR heater ranges from 40 to 70%, while that of gas-
fired IR hea ter ranges from 30 to 50%. These fig ures
depend upon the de sign of the IR units and mois ture
content in the web, and repres ent conversi on effici en-
cies for drying of thin films , c oatings, and paper.

The penetra tion of IR rad iation into a mate rial is

a function of the wave lengt h. Highe r emitter sou rce

tempe ratures pro duce a sho rter wave lengt h, which
has the ab ility to pene trate more than lower wave
tempe ratures, whi ch prod uce a longer wave length .
The maxi mum dep th of pe netration is approxim ately
1.5 mm. Good penetra tion into the substrate of coat-
ing, film, or ink heats the substrate rather than skin-
ning and blisterin g it.

Color is sensitive to the IR source temperature. The

darker the color, the more heat absorption there is.

The short- wave IR radiat ion can only be gener-

ated by an electric IR he ater employ ing a tun gsten
heatin g element. A tungst en filame nt has small mass
as wel l as low electrica l resi stance , which allow s heavy
current flow and very rapid heatup and co oldown .
The rate of response of various types of radiant he at
sources can be an important c riterion in the selec tion
of a prop er source for coating drying applic ations.

The useful IR wave lengt hs for indust rial applic a-

tions range from 1 to 10 m m (

Figur e 40.17

) . For this

reason, the IR radiant heatin g produced by short-
wave or medium -wave IR heater s is often used in
drying thin coati ng, films , an d webs . The primary
criteri on for determ ining whi ch type of IR system
and wave length is most effective for drying of inks,
coatin gs, and webs is the IR abso rptivity charact er-
istics of various materials.

Radiation that strikes a material surface must be

reflected, absorbed, or transmitted. Only the absorbed
radiation raises the temperature of the material. The
absorption of incident radiation into a homogenous
material increases with the thickness of the material,
while the transmission of incident radiation exponen-
tially decreases with the thickness of the material. If the
coating or substrate is very thick or opaque to IR
radiation, then there is no transmission and the inci-
dent radiation is fully absorbed by the coating or
substrate.

Figu re 40.18

shows the IR ab sorption spectru m of

a 4-mil water film, which is a common ingredient of
paper and latex coating. The first peak at 3 mm
matches the radiation of medium-wave IR heaters.
The second peak at 6 mm and the longer wavelength
corresponds to the radiation spectrum of long-wave
IR heaters. In order to be effectively heated, the water
film requires radiation energy concentrated between
2.6 and 3.3 mm and between 6 and 8 mm. Thus, the
selection of an IR unit with properly tuned character-
istics becomes important in order to raise the tem-
perature of the target material quickly and effectively.
The absorption and radiation curves should be
matched as closely as possible, and the absorption
rate of the material must be examined. (The absorp-
tion of a material is a function of the material’s phys-
ical, chemical, and color properties, and its moisture
content.)

Radiating surface

Glass

wool

Air–gas

mixture

Porous

catalyst

bed

(a)

(b)

FIGURE 40.16 Infrared radiation heaters: (a) electric and
(b) gas fired.

2006 by Taylor & Francis Group, LLC.

Many times, IR drying is combined with con-

vection or conduction dryers to provide special
surface or product properties. Electric IR drying
has been very successfully employed as a moisture

profiling system in the paper industry. The IR bulbs
are oriented in the web direction and are cycled on
or off locally to correct moisture profile differences
resulting from the primary drying process and/or

37008F

Dotted line,

λ max

18008F

10008F

max for radiation = 0.5 μm

Relative radiation intensity

Visible spectrum

0.4 0.7

Wavelength (

μm)

100

FIGURE 40.17 Infrared radiation energy distribution.

100

Absorption (percent)

Wavelength (

μm)

Water

0.002

⬙

thick

Water

0.0004

⬙

thick

Visible

light

Paper

0 0.4 0.7 1

FIGURE 40.18 Absorption of paper and water.

2006 by Taylor & Francis Group, LLC.

uneven app lication of water. Thes e systems are
almost always conn ected to sophist icated mois -
ture measur ement and elect ronic devices . Some suc-
cess ha s be en found in the drying of coated webs ,
e.g., paper co ating, capsula ted coati ng, and therm al
papers.

40.3.7 U

LTRAVIOLET

URERS

RYERS

UV dryers are intende d for curing of coa tings. UV -
curable coati ngs con sist of a blend of reactive mon o-
mers or oligom ers cap able of free-radi cal-init iated
polyme rization. Phot oinitiat ors are used wi th UV -
curable coati ngs an d they are the source of free
radica ls produ ced on irradiat ion.

UV radiation , whi ch compri ses less than 1% of the

sun’s incide nt energy, can be selec tively generate d to
promot e the elect ronic excit ation of molec ules, resul t-
ing in chemic al changes. UV wave lengt h ranges from
100 to 400 nm (nanome ters) and can be divided into
four regions : (1) UV A (315 to 400 nm); (2) UV B (280
to 315 nm); (3) UVC (200 to 280 nm); an d (4) vacuu m
UV (100 to 200 nm ). The ba sic too l used for cu ring
UV-sens itive mate rials is the mercur y vapor lamp,
which provides the UV flux activati ng the photo -
initiato rs in the cu rable mate rials. Since diff erent
photoini tiators requir e different UV wave lengt hs,
the radiation from the source and the phot oinitiat ors
should be match ed to effe ct polyme rization .

Fiv e basic lamp syst ems are a vailable to produce

UV radiation: (1) medium -pressu re mercur y vapor
lamps; (2) elect rodeles s lamps ; (3) pulsed xenon
lamps; (4) hyb rid xenon /mercury vapor lamps ; and
(5) low-pres sure germi cidal lamps. Medium- pressure
mercur y lamps that emit a wide range of wave length s
are by far the most important radiation sources for
curing of coati ngs.

The UV dryer consis ts of lamp hous ing and

reflect or assem bly. The reflecto r itself may be of eithe r
paraboli c or ellip tical geomet ry. Elliptica l reflect ors
are most often used wi th medium -pressure mercury
lamps. Parab olic reflect ors provide a parallel beam
of radiation, while the elliptical reflect ors produce a
focused be am of radiat ion on the substr ate (Figure
40.19). UV-cura ble coatin gs are 100% solids and
solvent remova l equipment is not require d.

W hile this type of UV-curi ng process doe s indeed

have uniqu e applic ation, there are also hazards intr o-
duced, includi ng biological effects on the skin and
eyes, alon g with the g eneration of ozo ne, oxides of
nitroge n (NO

), and other by -product s from the UV -

curing process . In order to employ this type of light
energy, all UV equ ipment must be designe d to meet all
safety requir ement s of personnel and environm ent.

40.3.8 E

LECTRON

EAM

URERS

RYERS

Earlier work in EB curing was done with rela-
tively high-e nergy (150 to 1000 kV) , scann ed-beam -
type equipment . The advent of the modern linea r
cathode unscann ed accelerator has revolutioni zed
the radiation curing/ drying indust ry. This equ ipment
generat es a co ntinuous ‘‘waterfa ll’’ of high-e nergy
electron s. As a result, the greatly simp lified de sign
provides excellen t reli ability. Beam energy gen erated
is in the range of 100 to 300 kV, allow ing com-
pact sh ielding, whi ch is usuall y built integral to the
accele rator itself .

Figu re 40.20

shows a schema tic of an electron

process or. The electron pro cessor is a stai nless steel
vacuum tube in whi ch one or more longitu dinal
heated- filame nt e lectron ‘‘guns ’’ are rais ed to a po-
tential of severa l kilovolts. The electron s are then
accele rated as they move from the filame nts to the
perimeter of the tube. A slot in the tube defining
the wid th of the process ing zon e permi ts the acce-
lerated electrons to reach a metallic foil window that
is thin at ground potential. This foil window is thin
enough to permit efficient electron transmission. The
electrons emerge into a controlled processing envir-
onment, where they are absorbed by the products to
cause curing of the coating. The accelerated or ener-
getic electrons as carriers of energy are somewhat like
bullets. The higher the voltage through which they are
accelerated, the deeper they can penetrate into the
products.

EB curing/drying systems have been successfully

integrated into multicolor web offset printing pre-
sses for printing of liquid packaging and folding
carton stock. Other commercial applications include

(lamp)

Irradiator

Substrate

Direction of
travel

FIGURE 40.19 Elliptical reflector assembly.

2006 by Taylor & Francis Group, LLC.

EB-curing intaglio ink s and EB-curing high-g loss
overpri nt varnis hes over heat-set inks in-l ine on roto-
gravure presses.

EB process ing is a v ery practical and wi dely used

commer cial process for carrying out ch emical reac-
tions such as cross- linking and cu ring ster ilizatio n on
web material s. In most cases, it is more energy effici ent
than normal process ing methods such as: (1) high-
tempe rature chemical cro ss-linkin g; (2) hot air dry ing
in the case of laminati ng or print ing; a nd (3) high-
tempe rature or chemi cal steriliz ation. The use of radi-
ation curing for lami nating adhesiv es and printing ink s
eliminat es or minimiz es the costl y solvent s. This in
turn reduces or eliminat es the need for an emission
control system to meet environm ental standar ds set
by governm ent agen cies. In the case of curing or
cross-li nking, it general ly provides a clear-cut reaction
with no resid ual promot ers or cross- linking agents that
might cause degrad ation of the pro duct or pr oblems
with governm en tal agen cy regula tions on toxic res-
idues in products for foo d and drug app lications .

UV an d EB curing may be used as practi cal, effi-

cient, and safe methods to cure coati ngs and print ing
inks in the con verting and graphic arts indust ries. The
most apparen t advantag e is the cu tting of produ ction
time by elim inating steps and the redu ction of labo r
cost, making the end produ ct more co mpetitive. A
second adva ntage is the special curing of unique
produc ts that c annot be made an y other way as effi-
ciently. Typical exampl es include stat ic co ntrol of
electric compone nts, textu red release surfa ce coat-
ings, graphic s on food pa ckaging (becaus e of the
very low extra ctabl es achieva ble), and curing of mag-
netic media binders (process control and durabil ity).

With the increa sing popul arity of higher-int ensity
(120 W/cm or more) UV lamps and the develop ment
of curing form ulations, espec ially in the high-s peed
wide-web ap plications , UV eq uipment , lower in cost
is much easie r to justify than EB equipment .

40.4 APPLICATION EXAMPLES

This section present s a few typic al cases of flot ation
drying systems app lied to co ated papers and films
because flotati on dryers have been wid ely used in
the convert ing and graphic arts industries . They
have numerous advantag es over other co mpeting
techni ques.

W ithin the last 10 y, the use of IR heaters has been

widely accepte d for the drying of coated pa per and
paperboa rd. For exampl e, when drying coati ng was
applie d to commer cial- or publ ication-g rade pap ers
during the paperma king proc ess, the techni que used
IR, flotati on air, an d steam-heat ed cyli nders. The
degree of dr ying with each mechani sm varie s depen d-
ing on co ating form ulation and use.

Figure 40 .21

shows a typic al system that uses elect ric or ga s-fired
IR to heat the web to 66 8 C whi le continui ng the
drying to an 80% soli d level by air flotatio n. The
balance of the drying is conducted through the use
of 4 to 6 steam-heated cylinders with an evaporation
rate restriction of 20 kg/h/m

. There are some differ-

ences in the opinion regarding the amount of IR, air,
and contact drying necessary and whether or not gas-
fired or electric IR is most beneficial. Often, IR is
placed as close as possible to the coating source to
enhance product quality as it relates to solids migra-
tion and adhesion of the coating to the paper web.

Structure terminal (T)

Chamber (W)

Vacuum

Shielding

Moving product (P)

Electron gun (G)

Filament (F)

Electron beam (B)

Metallic foil (F)

FIGURE 40.20 Schematic of electron processor.

2006 by Taylor & Francis Group, LLC.

Drying of publication- and commercial-grade

printed products is usually done with a system having
an air velocity between 61 and 76 m/s at web speeds
from 6 to 18 m/s. Web temperatures range from 93 to
1778C with multizone drying systems employed. The
printing process typically includes both evaporation
of hydrocarbon and solidification of a thermal plastic
solid material; therefore, postevaporation conduction
cooling is usually employed. Some special problems in
this drying area include excessive shrinkage and dam-
age to the paper product during the tortuous drying
curve, environmental problems associated with the
evaporated hydrocarbon, and special quality de-
mands imposed by the consumers.

When

drying

adhesives,

including

pressure-

sensitive adhesives, a dry coat weight of 20 g/m

/mil

(1 mil

¼ 40 mm) of coat weight thickness usually is

employed. Solid contents range from 20 to 50%. Solv-
ent removal rate limitations are at 18 kg/h/m

. Drying

systems are usually multizone with outlet velocity
ranging from 20 to 40 m/s and air temperatures ran-
ging from 100 to 1608C. Typically, a two- or three-
zone drying system is employed.

Magnetic media webs typically range from 15 to

66 cm in width and employ a multizone (generally
three) drying system. Impingement velocities are usu-
ally limited to 20 m/s in zone 1, 30 m/s in zone 2, and 40
m/s in zone 3, with operating temperatures ranging
from 60 to 1208C. Evaporation rates are generally in
the 64 to 80 kg/h/m

range. Airstream components in

the dryers should be stainless steel with filters for
maintaining a clean airstream. Varying substrate and
coat weight thicknesses are employed depending on
whether the magnetic media is audio, video, computer,
or floppy disk. Solvents generally are tetrahydrofuran
(THF), methyl ethyl ketone (MEK), toluene, methyl
isobutyl ketone (MIBK), or cyclohexanone.

Carbonless coatings or encapsulated coatings

would typically be used for carbonless carbon paper,
cosmetic or fragrance samples, or other special appli-
cations. Coatings could range from 1.5 to 5 g/m

dry

at a solid range of 20 to 40%. Solvent removal rates,
assuming no damage to the capsules, would range

from 30 to 50 kg/h/m

. Outlet velocities typically are

40 to 60 m/s with operating temperatures of 204 to
2608C.

Polyvinylidene chloride (PVDC) is typically used

as a vapor barrier in food packaging. In these applica-
tions, evaporation rates are usually 10 to 15 kg/h/m

with air velocities of 20 to 40 m/s when utilizing paper
substrates and 20 to 40 m/s when using plastic films.
Dryer construction is usually of stainless steel because
of the high corrosivity of the coatings and airstream.

Other applications include drying of aluminum

printing plates, 100% solid silicone coatings, steel
coils, and publication-grade paper products.

40.5 SAFETY ASPECTS

In the converting and graphic arts industries, the
drying of coated webs involves the evaporation of
solvents, water, and oils from the solvent-based coat-
ings, adhesives, and printing inks into the air. These
evaporated solvent vapors and ink oil mixing with
dryer air will cause a fire and explosion hazard if an
excessive amount of solvent vapors is accumulated in
the dryer without monitoring. In addition, the ex-
haust gases combined with traces of carbon monoxide
from the dryer burner due to incomplete combustion
are toxic to humans, wildlife, and vegetation if emit-
ted to the atmosphere without treatment. Further-
more, odor of the exhaust stream is objectionable.

The role of government agencies and insurance

companies is becoming greater in safety, product for-
mation, raw material selection, and all environmental
affairs of all industries. In the United States, the
federal Environmental Protection Agency (EPA)
will gradually tighten up air pollution control by
enforcing existing and new environmental regulations
under the Clean Air Act, including the regulations of
emissions from all adhesive, coating, and ink pro-
cesses. Also, the Factory Mutual (FM) and Fire In-
surance Association (FIA) will require more safety
plants, equipment, and operations, e.g., all machines
using a solvent-based or a water-based liquid with
approximately 20% alcohol mixed with water must

Steam

cylinders

Flotation

FIGURE 40.21 Paper coating with infrared radiation, flotation, and steam cylinders.

2006 by Taylor & Francis Group, LLC.

be built to co nform to exp losion-proo f regula tions .
Therefor e, the design and operation of dryers for the
solvent -based ad hesive, coatin g, and ink print ing pr o-
cesses are confi ned by two facto rs: (1) the explosio n-
proof standar d requ ired by MF and FIA an d (2) the
dryer emis sion con trol e nforced by the EPA . These
two facto rs are briefly discus sed below .

40.5.1 T

XPLOSION

-P

ROOF

TANDARD

It is of adva ntage to recir culate as much heated air
inside the dryer as possibl e for the purpo se of energy
conserva tion. However, accumul ation of an excess ive
amount of flamma ble solvent vap or will not only
retard the evapo ration rate of solvent but a lso
create a dr yer fir e or explosio n hazard. To avo id the
potenti al fire or e xplosion hazard, the dryers must be
designe d to meet the explosi on-proof standar d that
requir es that the mixtu re of so lvent vapo rs an d air
cannot burn if expo sed to an open fla me.

Solv ent vapors can not burn if the amount of air in

the mixt ure is ab ove or below the correct comb ustion
ratio. If the amount of ox ygen in the mixture is too
small to sup port combust ion due to a surplus of the
fuel vapo rs, it is at the uppe r explosi ve limit (UEL) of
the mixtu re. W hen the volume of the fuel in the
mixture is too low to sup port combust ion due to a
lack of burnable vapors, it is at the low er explosive
limit (LE L) of the mixture. It is necessa ry to ensu re
that these vap or–air mixtures are alw ays below LEL
or above UEL ratio s.

40.5.1 .1 Lo wer Explos ive Limit Reg ulations

Convent ionally, LEL s are alw ays used as the basis for
making a dryer system safe. In the Unit ed State s,
LEL regula tions stipu late an exhaust volume of
10,000 standard c ubic feet (SCF) per U.S. gallon
(gal) are requ ired, while in Eur ope the regula tions
specify 90 standard cubic mete rs (SC M) per kilogr am
(kg) of solvent evap orated, if prop er inst rument ation
such as the combust ible gas analyze r or the IR an a-
lyzer for measur ing pe rcentage of the flamma ble solv-
ent vapors in the dryer or exhaust ing air streams is
used. All agenc ies permit a maxi mum solvent vapo r
concen tration of up to 25% of LEL of the solvent s
being eva porated, providi ng the operator wi th a 30 0%
safety fact or before the mixtu re will reach combust -
ible level s. However, if one specific solvent is used on
a machi ne that does not have the LEL measur ing
system, the legal limit is 25% of LEL for that parti cu-
lar solvent rather than the standar d of 10,000 SFM/
U.S. gal or 90 SCM/kg.

The LEL regula tions also allow the fla mmable

solvent vapor con centration up to 50% LEL when

the LEL measur ing syst em is provided. This effect-
ively doubl es the amount of solvent that can be eva p-
orated in the same amou nt of air. How ever, the LEL
measur ing syst em must be properl y c alibrated for
each specific solvent or combinat ion of solvent s,
must be fitted with audible and visible alarm s to
warn the operator s when the mixture approaches
50% LEL rati o, and most impor tantly, must automa t-
ically shut down the machi ne if the lim it is exceeded .

40.5.1 .2 Inert Gas Drying Process

Since the LEL regula tions permi t the con vention al
dryers to maintain the maxi mum flamma ble solvent
concentra tion of 25% LEL, or up to 50% LEL if
proper inst rumentati on for monit oring the solvent
vapor co ncentra tion is applie d, the LEL dr yers
allow qui te limite d recir culation, emit a large vo lume
of heated air to the atmos phere, and introd uce an
equall y large volume of makeup air. Alternat ely, if
the oxygen concen tration in the dr ying gases is de -
crease d below 12%, whi ch is too smal l to supp ort
combust ion, then the solvent vapo r con centration
and the recir culation can be large ly increa sed witho ut
maintaini ng LEL for avo iding e xplosion or fires. In-
stead of air, whi ch co ntains ap proxim ately 21% ox y-
gen, the inert gas (e.g., carbo n dioxide or nitr ogen),
contai ning only a small amount of oxygen , can be
used as the drying medium. This inert gas drying
process allows an increa se in recir culation beca use a
flamma ble mixture cannot be developed in a low -
oxygen atmos phere regardless of how high the solvent
concentra tion is.

Table 40.1

sho ws maximu m permi s-

sible oxygen percentage to prevent ignition of flam-
mable gases and vapors using nitrogen and carbon
dioxide for inerting.

Figu re 40.22

shows an example of a low-oxyge n

dryer with a solvent-recovery system, meeting Na-
tional Fire Protection Association (NFPA) standards.
As in the conventional case, the coated web enters and
leaves the dryer chamber through the web openings.
The dryer atmosphere consists of an inert carrier gas
that is simultaneously recirculated through the dryer
enclosure (line 1). A gas seal is provided around web
openings to restrict air from entering the drying cham-
ber and to prevent the inert gas from escaping. In
actual operation, oxygen levels are monitored and
controlled within the dryer to maintain <5% oxygen.
This allows a higher solvent vapor concentration in the
dryer environment without risking explosion and fires.
A bleed stream (line 2) is processed by a solvent-
recovery system. A coolant (line 3) acts to condense
the solvent, which is discharged to storage (line 4).
With solvent removed, the gas stream is returned to the
dryer instead of being discharged to the atmosphere.

2006 by Taylor & Francis Group, LLC.

40.5.2 T

RYER

MISSION

ONTROL

The evaporated solvents, ink oils, and toxic gases
such as carbon monoxide in the dryer exhaust are
mixtures of hydrocarbons, i.e., carbon bonded to
each other and/or to hydrogen in various lengths
and configurations. The general term for these mater-
ials is volatile organic compounds (VOCs). Re-
gulations for the total reduction of VOC emissions
is a combination of destruction efficiency and capture
efficiency. The United States is the only country

following a path to cleaner air. Most often, 90, 95,
or 99% destruction efficiency will be seen, depending
on whether it is the local, state, or federal level.
Germany and the Scandinavian countries already
have regulations limiting emissions from a source to
50 mg/Nm

(normal cubic meter) in many areas.

Some require as low as 20 mg/Nm

depending on

the type of VOCs. Many other industrialized coun-
tries are following this lead.

Basically, there are two methods of solvent re-

moval from the dryer exhaust: (1) solvent recovery

TABLE 40.1
Oxygen Percentages Relating to Ignition

–Air

Percent above

Which Ignition Can

Take Place

Maximum

Recommended O

Percent

Percent above

Which Ignition Can

Take Place

Maximum

Recommended O

Percent

Acetone

13.5

15.5

12.5

Benzene (Benzol)

Butadiene

10.5

Butane

9.5

14.5

11.5

Butene-1

11.5

Carbon disulfide

6.5

Carbon monoxide

5.5

4.5

Cyclopropane

11.5

Dimethylbutane

9.5

14.5

11.5

Ethane

13.5

11.0

Ether

—

10.5

Ether (diethyl)

10.5

8.5

10.5

Ethyl alcohol

10.5

8.5

10.5

Ethylene

11.5

Gasoline

11.5

Gasoline
73–100 Octane

9.5

100–130 Octane

9.5

115–145 Octane

9.5

14.5

11.5

Hexane

9.5

14.5

11.5

Hydrogen

Hydrogen sulfide

7.5

11.5

Isobutane

9.5

Isopentane

9.5

14.5

11.5

JP-1 fuel

10.5

8.5

JP-3 fuel

9.5

JP-4 fuel

11.5

Kerosene

Methane

14.5

11.5

Methyl alcohol

13.5

Natural gas (Pittsburgh)

9.5

Neopentane

12.5

n-Heptane

11.5

Pentane

11.5

14.5

11.5

Propane

11.5

Propylene

11.5

2006 by Taylor & Francis Group, LLC.

and (2) solvent incineration. Solvent recovery requires
a high solvent concentration operation or the use of a
very expensive solvent to be economically worth-
while, while solvent incineration is mostly used for
the dryer effluent around or below 25% LEL.

40.5.2.1 Solvent Recovery

Solvent vapor can be recovered by adsorption on
activated carbon. The exhaust stream is passed
through an activated carbon bed and the hydrocar-
bon vapors are adsorbed. When the carbon bed be-
comes saturated with vapor, the exhaust stream is
directed to another fresh bed and the saturated bed
is recovered by stripping the solvent by either steam
or vacuum. Steam stripping is the often-used method
for most coating operations. The solvent is removed
along with the condensate and it is separated by
decantation if the solvent is water insoluble or by
distillation if it is soluble.

The efficiency of activated carbon adsorption

units is 90 to 99%, depending on the specific solvent
vapor used and the design of the carbon bed. If the
gas adsorption system is equipped with an IR ana-
lyzer for monitoring the solvent vapor concentration,
the discharge effluent can be automatically switched
from one saturated bed to another fresh bed by the IR
sensing device, and the recovery process is started.

Solvent vapor can also be recovered by using an

inert gas (e.g., nitrogen) dryer atmosphere and gas
condensation. Essentially, the recovery unit attached
to an inert gas dryer (Figure 40.22) contains a heat
exchanger, a condenser, and a separator. The dryer
exhaust stream is passed through a heat exchanger

and a condenser that cools the exhaust, causing the
solvent vapors to condense. The liquid solvents flow
to a solvent separator and are pumped into solvent
collection tanks. The cleaned nitrogen then returns to
the dryer to pick up more solvent vapors. Heat re-
moved from the exhaust stream by the heat exchanger
preheats the cleaned nitrogen before it returns to the
dryer.

40.5.2.2 Incineration

Incinerators are designed to heat the dryer exhaust
for proper time and temperature required to decom-
pose hydrocarbons into carbon dioxide and water
through the process of oxidation. Temperature,
time, and turbulence are the three factors contribut-
ing to the conversion efficiency. Thermal and catalytic
incinerators are the two major incinerators in use.
Thermal incinerations are used for the solvent vapor
concentration around or below 25% LEL, while cata-
lytic incinerators should be considered for applica-
tions in which there is a low concentration (below
15% LEL).

Thermal incinerators process dryer discharges by

burning them at high temperatures (between 760 and
8008C). They are designed with 0.5 s or greater total
residence time, although hydrocarbons generally oxi-
dize in 0.3 s. Residence time that the exhaust stream is
maintained at the oxidation temperature is critical not
only for proper oxidation but also for proper mixing.
Turbulence assures proper mixing of the airstream.
Improper or unsatisfactory mixing will require higher
temperatures and/or longer residence time to achieve
proper oxidation.

Coolant

Oven

Web

Bleed stream
without solvent

Solvent-
recovery
system

Recovered
solvent

Bleed
with

Stream
solvent

Inert
gas

FIGURE 40.22 Inert gas dryer layout.

2006 by Taylor & Francis Group, LLC.

Fig ure 40 .23 shows a schema tic diagra m of a ther-

mal incine rator contai ning two basic elements, a heat
exchanger and a burner (combu stion chamber) . A
heat ex changer is designe d to trans fer therm al en ergy
from one airstrea m to the other for the purpo se of
reducing the bur ner input ne eded for reachi ng oxida-
tion tempe ratur e. Typi cal VOC reduction of therm al
incine rators is in the ran ge of 99 %, but the cleanup
rates can be greater than 99%, depen ding on the
design and the ope rating condition s (e.g., ope rating
tempe rature, residence time, and turbulen ce) to ob-
tain goo d mixing.

Car bon monoxide can be eithe r created by par-

tial incine ratio n of ink oils or destro yed by complete
incine ration of carbo n mon oxide to carbon di-
oxide in a thermal incinerator , de pending on the ope -
rating temperatur e. Carb on mono xide produc tion
tends to rise wi th increa sing temperatur e until it
reaches a maxi mum at abo ut 650 8 C. Then, the carbon
monoxide content tends to decreas e wi th increa s-
ing tempe rature until it reaches a negli gible level at
about 760 8 C.

Bur ners creat e NO

(e.g ., NO and NO

) at high

tempe ratures. As the tempe rature of the incine rator
rises, NO

concentra tion in its exhaust will increa se.

Unfor tunate ly, VOCs and NO

in the atmosp here, in

the presence of sunligh t, en ter into a chemi cal reac-
tion that prod uces ozone (O

). Ozone is the sub stance

measur ed as the criterion for ‘‘smog alerts’ ’ in cities.
Even extre mely low levels of ozon e in the air can
cause signifi cant respirator y difficul ties in a sizable
portio n of the populati on.

Ther mal incine rators typic ally use two major

types of he at exchangers : (1) envelope type and (2)
shell an d tube exchan gers (

Figure 40.24

). A regener a-

tive system is a special type of therm al incine rator
that utilizes a ceram ic stonew are as the heat excha n-
ger med ium and ope rates at ve ry high tempe ratur es
(above 800 8 C). The ceram ic stonew are is mou nted in
at least three columns for better automa tic control
purposes.

Figu re 40.25

sho ws a two-col umn regener a-

tive system for discus sion purpo ses.

The process airs tream is passed through the stone-

ware as it enters an d exits the cen tral incine ration
chamber. By con stantly cyclin g the airstrea m between
two columns , the incomin g airs tream is heated by
stoneware, which in the previous cycle was absorbi ng
heat from the airstrea m exiting the centra l c hamber.
As this co lumn loses heat to the incoming airstrea m,
it cycles and beco mes the recep tor of he at, repeating
the cycle. The cleanup rates are great er than 90%.
Regene rative syste ms requir e high capit al/install ation
costs, but their operati ng c ost is low if ru n c ontinu-
ously due to large stoneware mass.

Cat alytic incine rator s are an alternati ve to ther-

mal incine rator s as a mean s of oxidiz ing exhau st
hydrocarbo ns into carbon dioxide and water at
lower tempe ratur es (315 to 480 8 C). The contact time
with the catalyst bed is about 0.3 s. Cat alytic inci-
nerator s typicall y destr oy 90 to 95% of VO C, depen -
ding on the design of the unit and catalyst activity.

Figure 40.26

shows a schema tic diagra m of a cata-

lytic unit. The basic elements of the catalytic units
are a preheating/mixing section, designed to achieve
a uniformly preheated and distributed exhaust stream
flow, and the catalyst bed and catalytic matrix,
where the major portion of the oxidation reactions
take place.

Two types of catalyst systems are commonly used:

(1) packed beds and (2) monolith blocks. The first type
is a perforated container containing spherical beads
coated with catalytic ingredients; it allows the react-
ants to flow through the catalytic beds. It has high
external surface area and high mass transfer efficiency
between reactants and catalyst. The second type has
the walls of monolithic honeycomb wash coated with
the catalyst ingredients and allows the reactants to
flow through the channels in the monolith.

Catalytic systems are limited to applications in

which the exhaust stream has the lower particulate
loading and/or the exhaust stream has negligible ‘‘poi-
sons.’’ The poisons are primarily silicon and phos-
phorus that coat the catalyst. Halogens such as
chlorine and sulfur can decrease the effectiveness of
the catalyst.

Exhaust

Heat

exchanger

Exhaust

fan

Process

Burner

FIGURE 40.23 Thermal incinerator schematic.

2006 by Taylor & Francis Group, LLC.

Hot air flow

Metal plates

Warm air

outlet

Cold air

inlet

Envelope

Shell and tube

Hot air sleeve

Hot air flow

through sleeve

Metal tubes

Cold air

tube inlets

Warm air

tube outlet

FIGURE 40.24 Heat exchanger types.

Auxillary air

Ceramic

packing

chamber

Ceramic

packing

chamber

Ceramic

packing

chamber

Ceramic

packing

chamber

Combustion

chamber

Combustion

chamber

Auxillary

fuel

Auxillary

fuel

Emission

source

Emission

source

Stack

Mode “A”

Mode “B”

FIGURE 40.25 Regenerative system of thermal incinerator.

2006 by Taylor & Francis Group, LLC.

40.6 CONCLUSION

Various commercially employed drying systems for
coated webs are summarized. The reader is referred
to the cited references in the bibliography for further
details.

BIBLIOGRAPHY

Arganbright, D.G. and Resch, H., Review of basic aspects

of heat transfer under impinging air jets. Wood
Science and Technology, 5:78–94 (1971).

Bennett, R.A., Air foil dryers: principle of operation and

application. In Drying ’80, Vol. 2, Mujumdar, A.S.
(Ed.), Hemisphere, New York, 1980, pp. 308–314.

Cohen, E.D., Coatings: going below the surface, Chemical

Engineering Process, 19–23 (September 1990).

Cohen, E.D. and Lightfoot, E.J., A primer on forming

coatings, Chemical Engineering Process, 30–36
(September 1990).

Eldred, N.R. and Fadner, T.A., Challenges of drying the

printed web. In Drying ’80, Vol. 2, Mujumdar, A.S.
(Ed.), Hemisphere, New York, 1980, pp. 343–346.

Hardistry, H., Drying printed ink coatings by impinging air

jets. In Drying ’80, Vol. 2, Mujumdar, A.S. (Ed.),
Hemisphere, New York, 1980, pp. 422–430.

Hung, J.Y., Fundamentals of IR Energy, presented at 1989

TAPPI Coating Conference, Chicago, IL, 1989.

Martin, H., Heat and mass transfer between impinging gas

jets and solid surfaces. In Advances in Heat Trans-
fer, Vol. 13, Academic Press, New York, 1977.

Mujumdar, A.S., Drying of coated webs. In Handbook of

Industrial Drying, 1st ed., Mujumdar, A.S. (Ed.),
Marcel Dekker, New York, 1987.

Mujumdar, A.S., Huang, P.G., and Douglas, W.J.M., Pre-

diction of heat transfer under a plan turbulent im-
pinging jet including effects of cross-flow and wall
motion. In Drying ’82, Mujumdar, A.S. (Ed.),
Hemisphere, New York, 1982, pp. 91–98.

National Fire Protection Association (NFPA), Ovens and

Furnaces, NFPA, Boston, MA, 1990.

Obot, N.T., Mujumdar, A.S., and Douglas, W.J.M., Design

correlation for heat and mass transfer under various
turbulent impinging jet configurations. In Drying
’80, Mujumdar, A.S. (Ed.), Hemisphere, New
York, 1980, pp. 388–402.

Rie, J. and Berejka, A.J., Generation of UV and EB radi-

ation. In Radiation Curing, AFP/SME, Dearborn,
MI, 1986.

Saad, N.R., Mujumdar, A.S., and Douglas, W.J.M., Heat

transfer under multiple turbulent slot jets impinging
on a flat plate. In Drying ’80, Vol. 2, Mujumdar,
A.S. (Ed.), Hemisphere, New York, 1980, pp. 422–
430.

Scheuter, K.R. and Dosdogru, G., Factors influencing the

physical drying of printing inks in drying systems.
In Advances in Printing Science, Vol. 6, Pergamon
Press, Oxford, 1970.

Spooner product literature on jet foil, air turn drying sys-

tems, Spooner Industries, Inc., Green Bay, WI,
1991.

TEC product literature on HI-FLOAT Coanda air bars and

air flotation drying systems, TEC Systems, De Pere,
WI, 1983.

Exhaust

Burner

Exhaust

fan

Process

Catalyst

chamber

Heat

exchanger

FIGURE 40.26 Catalytic incinerator.

2006 by Taylor & Francis Group, LLC.

Document Outline

Table of Contents
Chapter 040: Drying of Coated Webs

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040 Drying of Coated Webs

Document Outline