Drying

35 Drying of Pulp and Paper

� 2006 by Taylor & Francis Grou
p
Osman Polat and Arun S. Mujumdar

CONTENTS

35.1 Introduction ......................................................................................................................................... 793

35.2 Drying of Paper ................................................................................................................................... 794

35.2.1 Drying Process ........................................................................................................................ 794

35.2.2 Types of Dryers....................................................................................................................... 795

35.2.2.1 Cylinder Dryers....................................................................................................... 795

35.2.2.2 Air Drying............................................................................................................... 801

35.2.2.3 Radiant Drying ....................................................................................................... 807

35.2.2.4 Recent Developments in Paper Drying ................................................................... 809

35.3 Drying of Pulp ..................................................................................................................................... 815

35.3.1 Conventional Pulp Drying ...................................................................................................... 815

35.3.2 Flash Drying ........................................................................................................................... 816

35.3.3 Steam Drying .......................................................................................................................... 816

35.4 Conclusion ........................................................................................................................................... 817

Acknowledgment............................................................................................................................................ 817

Nomenclature ................................................................................................................................................. 817

References ...................................................................................................................................................... 818

35.1 INTRODUCTION

Ts’ai-Lun apparently produced a sheet of paper in

about A.D. 100 in China and became the first recorded

papermaker in the world. However, it took about

1000 y for this new art to reach Europe. In the medi-

eval era, the progress of papermaking was very slow

and the major ingredient of paper was old rags. By

the beginning of the 19th century, the progress of this

industry was enormously accelerated. The first prac-

tical paper machine was produced in the early 1800s;

then continuous drying techniques were introduced to

the industry by means of cylinder drying in 1817 by

John Dickinson; later, in the mid-1840s, the extensive

use of wood as a cellulose-fiber source began by the

advent of the first wood grinder. Today, papermaking

has become one of the major industries in the world.

The production of paper increased enormously, over

60 million tons per year in the United States alone.

The machine speeds also increased up to 10 to 15 m/s

and even higher for tissue products, to keep pace with

the increased production rates.

, LLC.

Papermaking is essentially a massive dehydration

operation. A fiber–water suspension with initial con-

sistencies of 0.2 to 1.0% (consistency ¼ grams of fiber

per gram of fiber–water suspension) is delivered to a

screen, where, with the application of vacuum, much

of the free water is drained off and the consistencies

rise to about 18 to 23%. Then, more of the free water

is removed by mechanical ‘‘squeezing’’ at the press

section. The sheet is then transferred to the drying

section, with a consistency of 33 to 55% to remove the

remaining excess water to obtain the final product

with 6 to 9% moisture content. In the United States,

the production of over 60 million tons of paper per

year entails the removal of over 80 million tons of

water by thermal dryers. Considering that for a typ-

ical newsprint machine the water removed in the

dryers is less than 1% of the original water, one can

easily realize the amount of water that must be re-

moved in the papermaking process.

Although drying removes the least amount of

water in absolute terms, it still remains the most

costly and energy-intensive step in the papermaking

process. Thus, improvement of the efficiency of water

removal before drying and improvement of the drying

system and its thermal efficiency without adversely

affecting the product quality are of great importance

to the pulp and paper industry.

In this chapter, pulp drying and paper drying are

discussed under separate headings.

t, dr

y ba

sis

35.2 DRYING OF PAPER

35.2.1 DRYING PROCESS

Paper is hygroscopic. The transfer of moisture

between the paper and the surrounding atmosphere

takes place unless the sheet is in equilibrium with the

surroundings. However, the amount of water present

in the paper at equilibrium conditions depends upon

whether it has been taken up or given off by the paper.

This hysteresis phenomenon is known for many other

hygroscopic substances. The sorption behavior of a

paper sample (Figure 35.1) shows that an equilibrium

moisture content reached by wetting and drying will be

different at the same humidity. There is no satisfactory

theory to explain the hysteresis. Luikov [1] suggested

two explanations for this phenomenon. One is that the

hygrothermal equilibrium sets in slowly, as a result of

which the observed equilibrium is not a true equilib-

rium. The other hypothesis suggests that evaporation

and condensation phenomena are irreversible. In dry-

ing (desorption), full wetting of the capillary walls

occurs. On the other hand, during wetting (sorption),

the capillary walls are gradually covered with a layer of

liquefied vapor, but the meniscus is not formed until

the adsorption layer is sufficiently thick to close the

pore at the narrowest point.

20

40

60

80

100

2010Equilibrium moisture

Rel

ativ

e hu

mid

ity

1

2

FIGURE 35.1 Sorption behavior of paper sample (1, drying;

2, wetting).

� 2006 by Taylor & Francis Group, LLC.

The drying cycle of paper is divided into three

fairly distinct stages, as for most materials. The initial

warming-up stage is followed by the constant-rate

stage, which is followed by one or more falling-rate

stages. This idealized concept is shown in Figure 35.2.

The heat is supplied to the sheet to increase its

temperature up to a certain value at which the

heat demand for evaporation and losses comes into

equilibrium with the heat supply. At this point,

constant-rate drying begins. During this period, water

evaporates from the paper surface and the diffusion of

moisture from inside the sheet is rapid enough to keep

up with the vapor-removal rate from the surface.

When the rate of diffusion cannot keep up with the

rate of evaporation, the drying plane recedes into

the sheet and a falling-rate stage begins. In his drying

studies on various paper and paperboard products,

Montgomery [2] observed that the drying rate is fairly

constant up to 10 to 15% moisture content, after

which it decreases rapidly. The falling-rate period

can be divided into three phases. The behavior of

moisture movement in these phases is not yet well

understood. It is generally accepted that the capillary

action and diffusion inside the fibers are responsible

for the first and second falling-rate periods in drying,

respectively. At the end of these two stages, the sheet

is almost ‘‘oven-dry.’’ The last stage is to break up

the strong chemical bonds and to remove the final

a b c

c

dTime

Moi

stur

e co

nten

ab

d

Moisture content, dry basis

Dry

ing

rate

FIGURE 35.2 Typical drying curves.

TABLE 35.2Paper Industry Dryer Distribution by Application(Pulp Dryers Excluded)

Dryer Distribution (%)

Tissue Paper Board Coating

Cylinder 5 95 95 35

Impingement — 4 — 50

Yankee 84 — 3 —

Infrared — 1 1 15

Through 11 — — —

Condebelta — — 1 —

aCondebelt is a trademark of Metso Paper Oy, Finland.

Source: From Kuhasalo, A., Niskanen, J., Paltakari, J., and

Karlsson, M., in Papermaking, Part 2, Drying, M. Karlsson, Ed.,

Fapet Oy, Helsinki, Finland, 2000.

molecules of water, which is not important for paper-

makers as the drying ends at the oven-dry stage for

almost all paper products.

35.2.2 TYPES OF DRYERS

The types of dryers used in the paper industry can be

classified by the basic means of transferring heat to the

paper. For conventional steam-heated cylinders, the

predominant mechanism of heat transfer is conduc-

tion; convection is for air dryers, such as impingement

and through-dryers; radiation is used for infrared

dryers; and dielectric heating is used for microwave

and radio frequency (RF) dryers. On the other hand,

some of the dryers use more than one means of heat

transfer, for example, Yankee dryers are a combin-

ation of cylinder and impingement dryers.

An estimate of the distribution of dryer types used

in the paper industry is shown in Table 35.1. This

table shows that the conventional steam-heated cylin-

der dryers are still dominant in the industry. The

others have application to only one type of product,

as shown in Table 35.2.

35.2.2 .1 Cyli nder Dryers

The multicylinder dryer section of a paper machine

consists of a number (up to 70) of large, hollow, cast

iron or steel cylinders over which the web passes. These

cylinders are used to alternately heat the two sides of

the sheet. The major mode of heat transfer is the

conduction through the steam-heated cylinder shells.

Part of the multicylinder drying section of a news-

print machine is illustrated in Figure 35.3. The sheet is

tightly pressed against the cylinders by a dryer felt to

enhance heat transfer. This figure also shows some of

the important elements of a dryer section such as felt

rolls, felt dryers, felt stretchers, and felt guides. The

other auxiliary equipments, lead dryer, breaker stack,

TABLE 35.1Paper Industry Dryer Distribution

Dryer Types Industry Share (%)a

Cylinder dryer 85–90

Impingement dryer 2–3

Yankee dryer 4–5

Infrared dryer 3–4

Through-dryer 1–2

aPulp dryers excluded.

Source: From Kuhasalo, A., Niskanen, J., Paltakari, J., and

Karlsson, M., in Papermaking, Part 2, Drying, M. Karlsson, Ed.,



sweat dryer, size press, spring roll, and dryer doctors,

are not shown in the figure.

The number of dryers and the auxiliary equipment

used in a dryer section depend on the particular grade

to be produced and the speed of the machine. Up to

70 cylinders may be used in a Kraft dryer section;

50 to 55 would be adequate for a typical newsprint or

fine paper dryer section. On the other hand, although

the breaker stack and size press are necessary for

Kraft bag and the hot press, size press, and interca-

lender dryers for linerboard, none of these is used for

corrugating medium [4].

Typically, the dryer cylinders are made of cast iron

and have diameters of 0.91 to 1.83 m (mostly 1.52 to

1.83 m in modern machines). The length of the cylin-

ders ranges up to 9.1 m for the largest paper machines.

The shell thickness varies with diameter but is gener-

ally around 25.4 mm or more. The design and manu-

facture of these cylinders have important effects on the

quality of the finished product. The outer cylinder

surface must be highly finished and free of any imper-

fections to avoid marking the paper; the wall thickness

must be uniform throughout the periphery to provide

uniform heat transfer. As the machine runs at very

high speeds (up to 3000 fpm; 15 m/s), the head, journal

bearings, and other parts must also be carefully

designed for smooth operation. Cast iron is a widely

accepted material for dryer cylinders because of its low

cost, corrosion-resistance, and ability to take a fine

finish. However, its relatively low resistance to thermal

shock may cause some problems [4].

35.2.2.1.1 Mechanism of Heat and Mass Transfer

Drying involves simultaneous heat and mass transfer.

In conventional machines using steam-heated cylin-

ders, the temperature of the paper entering the dryer

A

D

C

E

F G

Archs

C

B

FIGURE 35.3 A typical newsprint dryer section (A, dryers; B, felt dryers; C, felt rolls; D, paper; E, felt; F, felt guides; G, felt

stretchers).

section generally varies between 5 and 30 8C. The web

must be heated to a temperature at which significant

evaporation can take place, which is normally be-

tween 77 and 93 8C. The first two or three cylinders

are generally used for this warming-up period, and

beyond these the temperature of the sheet is assumed

to be the evaporation temperature [5].

The drying process on felt-covered cylinders can

be divided into four phases, as illustrated in Fig-

ure 35.4 [6]. Nissan et al. described the mechanism

of water removal in those phases in a series of papers

published between 1954 and 1962. They summed up

their conclusions as follows [7]. There are three pri-

mary mechanisms for water removal on a cylinder

dryer:

FIG

� 20

1. Direct evaporation in phases 1, 3, and 4

2. Partition of water between the sheet and the

cylinder as the sheet leaves the cylinder

3. Extraction by the felt, as both vapor and liquid

13

4

2

URE 35.4 Phases of felt-covered cylinder drying.

06 by Taylor & Francis Group, LLC.

Nissan et al. [7] also proposed a mechanism of

moisture transfer from the sheet to the felt:

1. Liquid transfer due to capillary suction

2. Liquid transfer due to the force exerted by

expanding gases

3. Evaporation from the sheet followed by con-

densation in the relatively cooler felt

However, the first hypothesis of Nissan et al. that

the dryer felt removed liquid water from paper by

capillary suction was disproved by Kirk [8]. A more

detailed discussion on this subject can be found in the

literature [9]. A more detailed summary of various

mathematical models proposed to describe multicy-

linder drying is given in a recent review [10].

The heat flux for steady-state conduction can be

written as

q ¼ kA

LDT (35 :1)

The individual resistances to the heat transfer on a

steam-heated cylinder are shown in Figure 35.5. In

practice, the overall heat-transfer coefficient, which is

the combination of all conductive and convective

resistances in the system, is used for calculation of

the heat flux. The overall heat-transfer coefficient for

the system illustrated in Figure 35.5 is

1

hoAo

¼ 1

hsAs

þX6

i¼1

Lf

kiAl

1

haAa

(35:2)

The overall transfer area Ao, which is the dryer sur-

face area, must be clearly defined, and all other areas

A B C D E F G H

hs hak1 k2 k3 k4 k5 k6

FIGURE 35.5 Resistances to heat transfer on a cylinder

dryer (A, steam; B, condensate; C, scale; D, dryer shell; E,

dirt and air; F, paper; G, dryer felt or fabric; H, air bound-

ary layer).

should be adjusted to reflect the dryer diameter. Some

suggested overall heat-transfer coefficients for various

grades of paper are shown in Table 35.3.

In actual practice, paper drying is a transient pro-

cess. For more realistic results, the following transient

heat-transfer equation must be used:

@2T

@x2þ 1

u2

@2T

@t2¼ 1

a

@T

@t¼ 0 (35:3)

which assumes very thin and isotropic material. Fur-

ther, curvature is neglected, as is conduction through

the edges. For the fast speeds of commercial ma-

chines, the second term of Equation 35.3 is negligible;

so, with little error one may write [11]

@T

@t¼ a

@2T

@x2(35:4)

TABLE 35.3Overall Heat-Transfer Coefficients for VariousGrades of Paper

Grade of Paper ho (W/m2 K)

Felt paper 45–85

Corrugating medium 140–230

Linerboard 170–230

Kraft sack paper 230–255

Fine paper 255–285

Newsprint 285–315


Nissan and Hansen [12] solved this equation numer-

ically for the conditions of hot-cylinder drying with

the following initial and boundary conditions. Initial

conditions:

t ¼ f (x) for t ¼ 0 (35:5)

Boundary conditions, phases 1 and 3:

� @T@x

��x¼0

¼ hcs

k(Tcyl � Tx¼0)

� @T@x

��x¼X

¼ hsa

k(Tx¼X � Ta)þ

l

k

@w

@t

� �x¼X

Phase 2:

� @T@x

��x¼0

¼ hcs

k(Tcyl � Tx¼0)

� @T@x

��x¼X

¼ hsf

k(Tx¼X � Tf )þ

l

k

@w

@t

� �x¼X

Phase 4:

� @T@x

��x¼0

¼ hsf

k(Ta � Tx¼0)þ

l

k

@w

@t

� �x¼0

� @T@x

��x¼X

¼ hsa

k(Tx¼X � Ta)þ

l

k

@w

@t

� �x¼X

The calculated temperatures agreed well with their

own experimental results for a three-layer muslin

sheet on an unfelted cylinder.

Water evaporates at the hot-cylinder interface,

and the resulting vapor diffuses through the paper

at a rate modeled by Fick’s law:

w ¼ �Ddc

dx(35:6)

At the air–paper interface, the mass-transfer equation

may be expressed in the following form for simplicity:

w ¼ bDc (35:7)

where b is a mass-transfer coefficient, which embodies

both the flow conditions and the fluid properties, and

Dc is the concentration difference. The partial pres-

sure of water vapor is the most commonly used con-

centration term in the paper industry; therefore, Dc

becomes the difference between the partial pressures

of water vapor at the interface and in the bulk air.

There are two main factors that affect the rate of

mass transfer: (1) the sheet temperature, which deter-

mines the vapor pressure of the evaporating water at

the surface; and (2) the partial pressure of water

(a) (b)

(c) (d)

FIGURE 35.6 Condensate behavior at different machine

speeds: (a) puddle; (b) climbing puddle; (c) cascading; (d)

rimming.

vapor in the air near the sheet, which is kept at a low

value by replacing the air, i.e., pocket ventilation.

As there are so many different types of drying

media in use today to supplement cylinder dryers,

TAPPI has outlined the following procedure to cal-

culate the drying rate. The calculation taken from the

TAPPI technical information sheet [13] is

w ¼ (0:318)Sm

BM

DN(35:8)

In this method, the drying rate is expressed in kilo-

grams of water evaporated per second per square

meter of the drying surface, where drying surface is

the circumferential area of the dryer cans multiplied

by the sheet width at the reel.

35.2.2.1.2 Steam Supply and Condensate Removal

All cylinder dryers are heated by the steam condensing

inside the sheet because of its efficiency (heat-transfer

coefficient for film condensation of steam ranges from

5,700 to 17,000 W/m2 K (1000 to 3000 Btu/h ft2 8F). In

the early days of papermaking, steam pressures as low

as 14 to 21 kPa (2 to 3 psi) were used; after World War

II, the new machines for Kraft papers were made for

586 kPa (85 psi). Today, after the introduction of

fabricated steel dryers, mills use 827 to 1016 kPa (120

to 150 psi), the Yankee machines have operated at

pressures up to 1206 kPa (175 psi) [5].

In general, the dryer steam can be produced by

central steam electric stations, heating plants, or any

industrial steam systems. The rotating joints to intro-

duce steam into the dryers and the condensate-removing

devices must be specially designed, however.

In a paper machine, the dryers are arranged in

groups that have a common steam-supply header.

Each group has to operate in such a condition that

neither the condensate nor the noncondensables accu-

mulate in the dryer. There are basically two approaches

to design the dryer steam and the condensate system.

One is the recirculation system in which most of the

blow-through is recirculated and the other is the cas-

cade system in which the blow-through steam from

higher-pressure sections is used in lower-pressure

sections. The recirculation system is more flexible

whereas the cascade system is simpler and cheaper [4].

Dryer steam should be free of superheat. Although

the condensate inside the dryer desuperheats the steam,

a separate desuperheater is used in some cases. The

effect of superheating above 1008C is controversial.

The incidence of noncondensable gases (usually

air or dissolved gases in feed water) in the steam

supply is one of the problems that must be considered

when designing the steam system. In most cases, only

trace quantities are involved; but if they are allowed


to accumulate inside the cylinder, they cause trouble.

The accumulated noncondensable gases affect the

vapor pressure of water inside the cylinder, especially

near the condensing surface, which in turn affects the

condensation temperature at a given total pressure.

As a result, the drying capacity and uniformity are

reduced. For example, a 5.58C drop in steam tem-

perature on dryer-limited linerboard grades will result

in an approximate 8% production loss [14]. In order

to prevent this effect, special steam-supply arrange-

ments are suggested; but the common way to deal

with the problem is to simply bleed off sufficient

steam with the condensate so that the accumulation

of noncondensable gases is prevented [15]. However,

recent studies [14] have showed that this practice

(bleeds) is very poor and unnecessary.

Another problem that must be considered when

designing the steam-heated dryers is the removal of

the condensate. If the condensate is not removed from

the cylinders continuously, the heat-transfer rate

drops and the power load on the drive increases be-

cause of the buildup of a heavy load.

At slow machine speeds, the condensate forms a

puddle in the dryer. As the speed increases, the cen-

trifugal and viscous forces cause the condensate to

climb the side of the cylinder; then the cascading from

the top starts and a rim of condensate forms suddenly

at higher speeds, as illustrated in Figure 35.6. At

speeds higher than 5 m/s (1000 fpm), if the condensate

layer is thin, there is a definite centrifugal action and

the condensate will form a rim around the inner

circumference. It is much better to maintain the

rimming condition because of its relatively lower

power requirement, more stable heat transfer, and

steady machine speed [5].

Buckets (dippers) are used to remove the conden-

sate in low-speed dryers. No pressure differential is

required to lift the condensate above the dryer center-

line, where it flows through a pipe to the outside.

Buckets can be used in dryers up to the rimming

speed [15]. Modern higher speed machines provide a

pressure differential to remove the condensate.

Syphons are used to pick up the condensate by ap-

plying a pressure differential. These can remove the

condensate from both a puddle and a rim. A syphon

is basically an open-ended pipe leading from the bot-

tom of the dryer through the steam supply, opening

to a condensate line. A simplified diagram of a sy-

phon is illustrated in Figure 35.7. The extracted con-

densate flow depends on the velocity of blow-through

steam, the pressure differential, and the distance be-

tween the condensate and the syphon inlet. The flow

of the extracted condensate can be calculated by using

the simplified equation

Qe ¼ QsR(PA � PB)1=2 (35 :9)

There are fixed and rotary syphons. Different types of

tips have been designed to increase the efficiency of

fixed syphons. The vibration of the syphon, the length

of the cantilever, and the position of syphon for opti-

mum heat-transfer uniformity are the basic problems

that must be considered when using fixed syphons.

Rotary syphons may have one or more tips that

move with the dryer shell. The basic problems here

are the need for a rotary seal at the junction of

the rotary pipe and the fixed condensate line and the

extra force (centrifugal force) that must be overcome

inside the pipe.

A more detailed discussion of steam-supply sys-

tems, condensate behavior, and condensate removal

can be found in the literature [5,15].

2

1

PB

PA

FIGURE 35.7 A simplified diagram of a syphon (1, syphon;

2, condensate).


35.2.2 .1.3 Air Sy stems

The dryer air systems, namely, the dryer hoods and

ventilation systems, are designed to pick up and carry

the water vapor evaporated from the paper web.

These systems affect not only the uniformity and

rate of drying and ultimately the quality of paper

but also the working conditions and the operating

and capital costs.

To remove the large quantities of water vapor

involved in paper drying and to keep the dew point

of the exhaust air below the wall temperature of

the hood, large volume of air is required. As the

handling and heating of air is expensive, there are a

variety of commercially available hood designs and

heat-recovery systems.

There are basically two types of hoods: open and

closed vapor hoods. In the open type, only a roof panel

is placed on the top of the dryer bank. This panel can be

either insulated or uninsulated. Open hoods are only

suitable for limited production capacity paper ma-

chines. Modern paper machines are usually built with

a fully closed vapor hood. The supply air can be heated

to elevated temperatures so that the dew point, and thus

the vapor-intake capacity of air, increases. The air can

be heated to 1808C indirectly in the heat exchanger by

saturated steam or directly to 4508C with gas or oil

burners. Modern air hoods allow faster drying rates

and a more uniform drying profile owing to the high

dew point and efficient means of pocket ventilation.

The most common dew point for these hoods is about

55 to 608C. However, these high temperatures require

additional insulation of the hood walls. A well-designed

heat-recovery unit, however, permits an amortization in

less than 2 y. Closed hoods operate on the principle of

two-stage recirculation. The function of a heat-recovery

system with a closed hood is shown in Figure 35.8. The

fresh air supplied to the machine room is exhausted

through stage 1 of the heat-exchanger system, where it

is heated for the ventilation of the hood. If required, as

during winter, the room-supply air can be preheated in

stage 2.A comparison between the heat requirements of

an open hood ventilated by convection and a totally

enclosed unit with heat-recovery system is shown in

Figure 35.9. With the open hood ventilated by convec-

tion (Figure 35.9a), the loss of heat in the exhaust air is

denoted as 100%. With the totally enclosed hood (Fig-

ure 35.9b), the heat requirements of the drying steam [1]

are substantially lower from the outset. The fluctuation

of figures for unused waste heat [3] is attributable to the

difference in the heat required for machine-room heat-

ing between winter and summer.

The space-bounded dryer cylinder, felt roll, and

the sheet approaching and leaving the cylinder, as

illustrated in Figure 35.10, are called the pocket.

2

14

8

91011

12

4

5

3

15

7

6

13

1

FIGURE 35.8 Function of a heat-recovery unit with closed

vapor hood (1, vapor hood; 2, exhaust fan; 3, supply air heat

exchanger; 4, supply air booster heater; 5, fan for supply air;

6, air distributor; 7, pocket ventilation; 8, room air heat

exchanger; 9, room air booster heater; 10, temperature-

control flaps; 11, room-supply air fan; 12, ceiling air dis-

tributor; 13, air outlet louvers; 14, warm-water unit; 15,

warm-water discharge). (Courtesy of J.M. Voith GmbH.)

The moist air trapped in these pockets retards the

drying of the sheet in the draws and causes nonuni-

form moisture distribution across the width of the

sheet. However, conditions in the pockets can be

5

3

2

1

8

1

4D

(a) (b)

FIGURE 35.9 Heat-flow diagram of (a) an open hood ventilated

(Courtesy of J.M. Voith GmbH.)


improved by introducing hot, dry air evenly across

the width. Pocket ventilation systems can increase the

drying efficiency up to 20% depending upon the pre-

vious conditions [16].

There are different designs to achieve ventilation

in the pockets. Low-pressure crossblow pipes, ar-

ranged alternately on tender and drive sides, induce

crossflow of air through the pockets. Low-pressure

blow pipes, extending across the full width, have per-

forated nozzles that introduce hot air into the pockets

at low velocities. These pipes are commonly used in

low-speed machines. The Grewin system, which is a

high-pressure blowing unit, is positioned similarly to

blow pipes. The Grewin system uses a comparatively

small quantity of hot air. However, because of its

injection principle—alternately arranged nozzles—

this system provides effective crossventilation and is

applicable for heavier-basis weight products. Hot-air

blowing rolls are also used for pocket ventilation

purposes. Hot air is introduced into the pocket

through the perforated shell of the felt roll. This

system requires sufficiently permeable felts and is the

most effective equipment for this application. How-

ever, these rolls are also the most expensive equip-

ment for ventilation.

The application of very high–permeability dryer

fabrics in the paper industry made it possible to intro-

duce air through the fabrics. Therefore, the above-

mentioned blowing rolls and the hot air ducts below

(or above) the felt rolls between the cylinders became

very popular. These systems provide better crossflow

ventilation, which in turn results in more uniform and

higher mass-transfer rates.

2

F

A

C

G

B

D

H

5

4

7

6

E

3

by convection; (b) a unit with totally enclosed vapor hood.

AC

D

B

FIGURE 35.10 Dryer section pocket (A, paper dryer; B, felt

roll; C, felt; D, paper).

A more detailed discussion on air- and heat-

recovery systems can be found in the literature

[4,5,17–20].

35.2.2.1.4 Dryer Felts and Fabrics

Dryer clothings are normally used for cylinder dryers

only. The term fabric represents the highly permeable

structure of synthetic materials, whereas by felt is

understood a woven, comparatively low-permeability

structure of both natural and synthetic fibers.

The basic function of dryer felts or fabrics is to

improve the contact between the cylinder and the web

by reducing the air gap between them. Other advan-

tages of using drying felts can be summarized as: (1)

prevent cockling, (2) control shrinkage of the web,

and (3) support and guide the sheet [21]. The conven-

tional dryer felts are very costly, however, and hinder

mass transfer and pocket ventilation. On the other

hand, felt tension variations, wet streaks, felt seam,

and other problems also affect product quality. After

high-permeability synthetic fabrics were introduced

to the industry, most of these drawbacks were over-

come. However, the use of high-permeability felts

caused sheet-flutter problems on high-speed ma-

chines. This problem is solved by repositioning the

lead rolls and by using less-permeable fabrics. The

recent introduction of serpentine (or slalom) wire,

which is an endless fabric and eliminates the bottom

fabric, to the industry made it possible to run the

newsprint machines at speeds higher than 20 m/s

without flutter in the sheet and with no change in

the drying capacity [22].

The advantages of dryer fabrics can be summar-

ized [23] as: (1) increased drying capacity, (2) uniform

moisture profile, (3) improved runnability and easier

cleaning, (4) increased running life, (5) no fiber shed-

ding, and (6) elimination of felt dryers.


35.2.2.1.5 Problems in Cylinder Dryers

At the beginning of the drying process, the fibers are

free to slide over one another; but as moisture is

removed they tend to come together, and at the end

fiber–fiber bonding takes place. Then the sheet tends

to shrink. The restraint owing to dryer felt and the

tension due to draw tend to prevent shrinkage. This

restraint results in a high tensile strength and very

little ability to stretch. Otherwise, the net effect of

those two forces is to reduce the bursting strength

and basis weight and to increase compactness [24].

A very good discussion of the control of shrinkage

has been presented by Nuttall [25].

The most common quality problem in papermak-

ing is the nonuniform cross-machine (CM) moisture

profiles. The traditional approach taken for wet

streaks is to overdry the sheet. However, a 1982

study [26] showed that overdrying from 6 to 4% reel

moisture for a typical 800 t/d capacity linerboard

machine results in $189,000 additional steam cost

and 11,200 t production loss per year because of the

reduction in machine speed. Uniform CM moisture

profile is obtained by improving dryer pocket venti-

lation systems and using supplemental drying, such as

air impingement or radiant drying.

Cockling of paper results mainly from imperfect

sheet formation. The difference in felt tension may

also cause cockling and uneven drying. Uniformity of

moisture content and basis weight in CM direction

prevents cockling.

Another common dryer-related defect is curl,

which results mainly from the differences in stress–

strain characteristics through the thickness of the sheet.

However, moisture distribution through the sheet and

the drying process may also cause the sheet to curl.

Low sheet tension, excessive sheet flutter, loose

draws, and misaligned, cold, or undersize (due to

wear resulting from dryer doctors) dryers caused

dryer wrinkles. Other than these problems there are

various defects related to dryer section, such as blis-

tering due to loose dryer felt or a hot or dirty cylinder;

linting (accumulation of solid particles); and dryer felt

marks, especially felt seam marks.

35.2.2.2 Air Drying

In the early days of papermaking, the only means of

paper drying was natural convection—air—and this

practice continued until the advent of cylinder dryers.

Today, the old loft, festoon, barber, and tunnel dryers

are obsolete (or used in a few cases in which certain

special low-production paper is made) because of

their very low drying rates. However, it is very difficult

to operate conventional cylinder dryer sections at

speeds much higher than 1000 m/min. Therefore, an

alternative method of drying must be found to keep

pace with increasing machine speeds. Currently, one

viable alternative dryer is an air dryer. By combining

impingement and through-drying methods, a dryer

can be built to provide, at acceptable cost, not only

high-speed operation and high drying rates at all sheet

moisture contents but also safe sheet handling, im-

proved paper quality, flexibility, convenient process

control and profiling, and comfortable working con-

ditions [27]. Of course, through-drying can be used for

permeable grades only.

35.2.2 .2.1 Imp ingement Drying

Early types of air dryers were all designed to operate

at low air velocities and relatively low air temperat-

ures. The drying rates were quite low for those dryers.

The application of high-temperature, high-velocity air

jets impinging on the wet web results in very high

drying rates. The average heat and mass transfer

under single or multiple jets can be represented func-

tionally by

Nu(Sh) ¼ C1RemPrn(Scn)HrDsp(X

sp) (35 :10)

for single jets and by

Nu(Sh) ¼ C2ReaPrb(Scb)Hcf d (35 :11)

for jet arrays. An extensive summary of the correl-

ations proposed for the prediction of transfer rates

BC

A

(a)

FIGURE 35.11 High-capacity hot air dryer hoods: (a) MG hood


under various turbulent impinging jet configurations

can be found in Obot et al. [28] and in the relevant

parts of this handbook.

Typical high-temperature, high-velocity air hoods

used in industry are shown in Figure 35.11. The air-

flow pattern is also indicated. A variety of heat

sources, e.g., indirect heating by steam or oil, natural

gas, or direct firing, can be applied to those hoods

depending upon the temperatures sought. The air

velocities for the modern high-velocity impingement

hoods range between 60 and 130 m/s. In practice, jet

velocities around 100 m/s are commonly employed.

Air temperatures for those hoods range from 150 to

5408C, but 3008C seems to be the most commonly

used value.

A schematic showing the drying capacities attain-

able at different temperatures and jet velocities is

shown in Figure 35.12. The graphs denoting the spe-

cific energy requirements (referred to the effective

dryer area) show that the energy demand rises rapidly

with increasing air velocity and decreasing tempera-

ture. The diagram applies to an initial web dryness of

35%; when the web is simultaneously dried by contact

drying, the specific drying capacity of the contact

dryer must be added. An example for a desired spe-

cific drying capacity of 89 kg H2O/m2 h at 4208C air

temperature is shown in this figure. The diagram gives

approximately 90 m/s air velocity and 6.5 kW/m2 as

the specific power requirement.

The heat-transfer coefficient as a function of jet

velocity, the pressure drop at the nozzle, and the

power requirement are the key parameters in the

design of impingement dryers. These parameters are

BC

A

(b)

; (b) tissue hood (A, fresh air; B, heater; C, to heat recovery).

10

14

12

10

8

6

4

2

0

30 50 70m/s

90 110

kw/m

2

120°C

170°C

250°C300°C

120°C

450°C

420°

C20

40

60

80

100

kgH

2O/m

2 h

FIGURE 35.12 The drying capacities and specific energy

requirements of high-velocity hoods. (Courtesy of J.M.

Voith GmbH.)

influenced by various geometric parameters (e.g., ar-

rangements of nozzles and exhaust ports, fractional

open areas, nozzle shape and dimensions, nozzle-

surface spacing, and others). For the calculation of

the heat-transfer coefficients under various turbulent

impinging jet configurations, the reader may refer to

available reviews [28,29]. In a recent review of im-

pingement drying, Polat has given a procedure for

optimum design of such hoods [30]. There is also a

simple and useful procedure for the design of convect-

ive dryers (impingement, through-, and floater dryers)

for best efficiency and productivity [31].

Supporting deck

of floater boxes

Im

FIGURE 35.13 A multizone impingement dryer for coated boa


Impingement air drying has found only limited

application, either to increase the capacity or to

control the moisture profile, in conventional cylin-

der drying systems. In such applications, the par-

ticular dryer cylinder is left unfelted. However,

impingement dryers are widely accepted in two

important fields, drying of coatings and drying of

tissue and machine-glazed papers in conjunction

with Yankee and MG cylinders.

A typical multizone impingement dryer for coat-

ings is shown in Figure 35.13. In this particular dryer,

supporting air decks are used to convey coated paper

webs, which are dried by high-velocity impingement

hoods. Different arrangements can be used for on- or

off-machine drying of coatings. In some cases, a ra-

diant dryer, with an infrared radiation (IR) zone, is

located to accomplish the initial drying, and a couple

of impingement units, usually two to three zones with

different air velocities, follow to carry out the drying

process. The impingement systems offer not only con-

siderable improvement in drying rate but also undis-

turbed drying for coatings.

35.2.2 .2.2 Yan kee an d MG Drying

Yankee and MG dryers are large, 3 to 6.1 m (10 to

20 ft) in diameter, steam-heated cylinders that are used

on tissue and toweling grades and on machine-glazed

papers, respectively. The surface of a Yankee or MG

cylinder has to be very smooth in order to have a

shiny finish on MG papers and a uniform crepe for

tissue products. The application of high-velocity, high-

temperature air impingement on conventional Yankee

cylinders doubles their drying capacity. Therefore, it is

a common practice to use air impingement on Yankee

dryers for modern tissue machines.

Typical tissue and MG paper machines with high-

velocity impingement hoods are shown in Figure 35.14.

pingement dryer

Supply fan for supporting deck

rd. (Courtesy of Flakt Ind. AB.)

(a)

(b)

FIGURE 35.14 (a) Tissue machine with a creping dryer; (b) Yankee dryer in machine for MG papers. (Courtesy of J.M.

Voith GmbH.)

35.2.2.2.3 Through-Drying

Through-drying is a process of drying permeable webs

by the percolation of hot air through its mass. As the

heat- and mass-transfer areas are increased owing to

the intimate contact of flowing air with the fiber sur-

face, much higher drying rates than those achieved by

conventional methods can be obtained for sufficiently

permeable products (including nonwovens).

The use of the through-drying technique for por-

ous grades (e.g., tissue, toweling, filter, blotting, and

nonwovens) has increased owing to (1) the produc-

tion of soft, high-bulk products; (2) the extremely

high forming speeds achieved by today’s twin-wire

formers, outstripping the capacity of the largest Yan-

kee dryers; (3) manufacturing advantages of single-

ply products; (4) little or no press dewatering needed


in through-drying; and (5) better, increased product-

ivity coupled with high thermal performance of

through-dryers [32].

Although through-drying has become a very ef-

fective method of drying permeable grades in recent

years, there is no generally applicable approach for

the prediction of the heat- and mass-transfer rates. It

is known that there is a minimum pressure difference

(threshold pressure, which can be related to the sur-

face tension forces) below which no flow occurs [33].

Once this pressure is reached, the through-drying

process commences. Some authors suggest that the

air exiting the sheet is at the saturation temperature,

which is valid for sufficiently thick sheets or at low

through-airflow rates [27,34,35]. Otherwise, it over-

predicts the actual drying rates. Recent works on the

transport phenomena analysis [36], on modeling [37],

and reviews [38] of both science and technology of

through-drying of paper have brought more under-

standing of this relatively new technique, but the pre-

sent knowledge of the basic transport processes is still

far from complete.

As it is still a relatively new technology, there is no

well-established through-drying system design pro-

cedure. The existing applications can be classified

into two basic configurations: rotary through-dryers

and flat-bed dryers. The rotary type has an open

(perforated) roll to allow the passage of air and a

hood arrangement to provide the pressure differential

required. Two methods of forcing air through the web

have been used for cylinder-type through-dryers. The

first method is to use a pressurized roll and apply the

hot air through the roll and the web and out to a

chamber. In the second case, the roll is under vacuum

and the hot drying air is supplied from a pressure

hood that covers the drum. The air is then pulled

through the web and roll by vacuum. There is no

need to use dryer felt for the second method, as the

sheet is held tightly by vacuum. However, for the

pressurized-roll case a very open fabric is necessary

to hold the sheet onto the cylinder.

In flatbed design, there are a fabric support and

conveying structure and top and bottom air cham-

bers. The supply air can be applied through either

chamber depending upon the design and process con-

ditions. This flatbed arrangement is particularly sui-

ted to highly permeable products in which the

pressure differential requirements are relatively low

and do not create significant mechanical problems

between the support members [39].

Through-drying is used in tissue drying because of

its very high permeability, and very high drying rates

(which exceed those on Yankee cylinders with high-

velocity hoods) are achieved even at fairly low temper-

atures. It is not recommended for newsprint, especially

at the wet end, where the permeability is relatively low

[27]. However, a recent application of a high-vacuum

through-drying unit in a roofing-felt (which has low

permeability) mill is claimed to have resulted in a 40%

production increase plus a 20% reduction in energy

use per ton when compared with the previously used

natural-gas-fired steam cylinders [40].

The high through-airflow rates for permeable

sheets make possible high drying rates, at the same

time virtually eliminating ‘‘one-sidedness’’ and CM

variations in the product. The thermal efficiencies of

through-dryers were increased up to 80% with the use

of several drying stages and air recycling or cascade

systems. Another potential advantage is the possibil-

ity of operating through-dryers at low temperatures

by using low-grade waste energy [41]. Honeycomb

vacuum cylinders of diameters up to 6.71 m (22 ft)


and widths up to 7.6 m (25 ft) operate at speeds up to

30 m/s and at air temperatures 370 to 430 8C (700 to

800 8F) with high air-recirculation ratios (e.g., 80%).

Recently published experimental data [42,43]

and calculations [44] suggest that strategically placed

through-dryers may even be used for higher basis

weight, lower air permeability grades to improve dry-

ing rates. This process basically consists of a cylinder

dryer section followed by a through-dryer. Use of

through-air drying at lower sheet moistures not only

improves the drying rates but also reduces the fan

power requirement due to increased sheet permeability.

35.2.2 .2.4 Airbor ne Drying

Use of air floater dryers is recommended especially

for the drying of paper products for which unre-

strained drying is desired.

In air floater dryers, the web floats freely while

drying by the introduction of a contact-free guide

plane. In practice, the guide plane is designed as a

blowing deck, where air is supplied through blow

boxes with specially designed perforations. Various

dryer designs are shown in Figure 35.15. The one-

sided dryer design is particularly useful for paper

and board coated on one side. The blow boxes for

perpendicular impingement are positioned facing

the coated side and for slanting impingement to the

untreated side. The uncoated side of the web can be

treated with either warm or cold air, depending upon

the process conditions. In the two-sided dryer, hori-

zontal and vertical air impingement alternate on both

sides of the web. This design affords a higher drying

capacity and is suitable for booster drying of different

kinds of paper and board, including drying of sack

Kraft paper, liner, and board. The printing dryer is a

special version of the one-sided dryer. The lower blow

boxes use cold air; the upper ones use hot air. The

noncontact dryer uses both vertical and angular air

impingement in the same blow boxes and is suitable

for drying two-sided coated webs, impregnated web

materials, and so on.

The drying decks are stacked on top of each other,

and the web is guided through the dryer in a multiple-

pass fashion as shown in Figure 35.16. Each turning

roll is driven individually, and the speed is controlled

to conform to the machine-direction shrinkage of

the web.

The internal ducting of a floater dryer is shown in

Figure 35.17. The drying air is fed through horizontal

ducts to the flow boxes and blown onto the sheet

through the perforations in the boxes, then dis-

charged through slots between the blow boxes. The

air is recirculated inside the dryer by a series of fans

along both sides of the dryer; only a small portion of

the recycled air is withdrawn and replaced by fresh

H ~2mm

(a)

(b)

One-sided dryer

Two-sided dryer

Printing dryer

Noncontact dryer

FIGURE 35.15 Air floater dryers: (a) blow-box arrangement; (b) various blow-box designs. (Courtesy of Flakt Ind. AB.)

air. The air velocity range normally used is 25 to 50

m/s. The air can be heated either by direct firing or by

steam coils.

The application of floater dryers allows the sheet

to be dried virtually tension-free in the CM direction

with only slight tension in the machine direction. It is

well known that drying under zero or moderate ten-

sion in the shrinkage range gives higher stretch and

tensile energy absorption (see Figure 35.18) to the

product whereas the modulus of elasticity and stiff-

ness decreases.


A typical application of a floater dryer on a sack

paper unit is shown in Figure 35.19. As natural

shrinkage occurs principally between 50 and 85%

dryness, the floater dryer is positioned between the

predryer and the after-dryer in order to produce high-

stretch paper. The after-dryer is used to remove the

cockles and to give a smooth surface to the paper. If

desired, a smoother surface can be obtained by calen-

dering the paper.

Air floater (or airfoil) dryers have become well

established for drying coated papers and sack paper,

Turning rolls Blow boxes

Flakt FC drying decks

FIGURE 35.16 Stacked drying decks. (Courtesy of Flakt

Ind. AB.)

for example, and operating at speeds greater than 10

m/s (2000 fpm).

The advantages claimed, other than those already

mentioned, are high drying rates, uniformity of

moisture profile across width, and favorable energy

efficiency [45].

35.2.2 .3 Ra diant Dry ing

Radiant drying has not been used in the papermaking

industry very extensively. Relatively recent applica-

tions of infrared dryers have been limited to special

applications, such as drying of coatings, in which

contact-free drying is a valuable asset. However, the

applications of high-frequency dielectric heating, i.e.,

RF and microwave dryers, are still at the preliminary

stage. The main drawback of radiant dryers is their

relatively high operating costs.

FIGURE 35.17 Internal ducting of a floater dryer. (Courtesy of


35.2.2 .3.1 Princ iples of Rad iant Heating

The transfer of heat by radiation is expressed by

qr ¼ sA1F1 �2(T41 � T4

2 ) (35 :12)

F1–2 in this equation is called the overall interchange

factor and is a function of both the geometric param-

eters, such as view factors, and the emissivities of the

radiating and receiving surfaces. As the temperature

driving force is the difference between the fourth

power of the absolute temperatures of the radiating

and receiving surfaces, very high heat-transfer rates

can be obtained by radiant dryers, given higher emis-

sivities and optimal design of the system for better

view factors.

The frequency and wavelength bands of com-

mercially available radiant dryers are shown in

Table 35.4. This table gives broad ranges for the

dryers; but in practice almost all IR dryers are in

near-infrared range (1 to 10 mm), and the specific

frequencies for RF and microwave dryers are allo-

cated by international agreement, such as 13.56,

27.12, and 40.68 MHz for RF and 433.92, 896, 915,

and 2450 MHz in the microwave region.

35.2.2.3.2 Infrared Dryers

Infrared dryers consist simply of a bank of infrared

heaters combined side by side or end to end. These

heaters are arranged to provide high-density heating

of the sheet by the geometry of the oven and the

optically designed reflectors. There are basically two

types of infrared heaters: electric infrared and gas

Flakt Ind. AB.)

Cylind

er d

ried

Flakt dried

= Higher TEA(tensile energyabsorption)

% Stretch

Load

FIGURE 35.18 Stretch of paper depending on drying con-

ditions. (Courtesy of Flakt Ind. AB.)

TABLE 35.4Frequency and Wavel ength Ranges ofRadioact ive Dryers

Dryer Type Wavelength

Range

Frequency

Range (MHz)

Overall Typicala Overall Typicala

Infrared 1–1000 mm 1–10 mm 107

Microwave 1–100 cm 10–70 cm 400–5000 900–2450

Radio frequency 1–100 m 7–22 m 1–100 10–40

aCommonly used ranges in dryer designs.

100

120

infrared. The electrical infrared heater is primarily a

metal filament in a sealed enclosure. The spectral

energy distribution of various infrared heat sources

is shown in Figure 35.20. In addition to high radiant

efficiency, the higher source temperature generates

shorter wavelength radiation. The shorter wave-

lengths are more penetrating and more sensitive to

color differential absorption. There will be a greater

difference between black-and-white products in a

high temperature–source oven. However, the greater

radiant efficiency in the high-temperature ovens com-

pensates for this drawback, as shown in Figure 35.21.

The characteristics of commercially used electric in-

frared heat sources are summarized in Table 35.5,

which shows that in some applications convection

is a very important factor for heat transfer. There-

fore, the overall oven design that allows better circu-

lation of air, of which the primary function is to

carry out the evaporated moisture, becomes very

Flakt dryer

% Possible free shrinkage

500

100

60 70 80 90 100% Dryness

~80%

Predryer After-dryer

FIGURE 35.19 The position of a floater dryer in a sack

paper dryer unit. (Courtesy of Flakt Ind. AB.)


important. The various types of oven design are

shown in Figure 35.22.

Gas infrared burners for industrial processing are

usually in two general styles: surface (Schwank)

burners and impingement burners. A surface burner

(Figure 35.23) is a gas generator consisting of a per-

forated special ceramic tile, set in a rugged cast iron

or ceramic housing, with a special alloy screen grid

that protects the tile and also serves as a reradiator

and air deflector. These burners have an operating

temperature range from 760 to 890 8C (1400 to

1650 8F) when the proper mixture and volume of gas

and air are supplied. The fuel mixture is ignited on the

surface of the ceramic tile. Higher temperatures

(~1550 8C) can be achieved by using refractory IR

burners.

1

20

40

60

80

3 5 7 9Wavelength (μm)

A

B

C

DE

Rel

ativ

e en

ergy

FIGURE 35.20 Spectral energy distribution of infrared heat

sources (A, quartz lamp coiled tungsten, 22008C; B, quartz

tube coiled nickel chrome alloy, 19808C; C, metal sheath

heater surface, 7608C; D, Schwank gas infrared burner,

9008C; E, electric panel heater, 4308C). (Courtesy of Fostoria

Ind. Inc.)

101

2

4

6

10

20

40

60

100

20 40% Voltage

60 80 100

% W

atts

and

col

or te

mpe

ratu

re (

�F)

A

B

C

DE

F

G

FIGURE 35.21 Voltage characteristics of infrared sources

(A, tungsten color temperature; B, nickel chrome color tem-

perature; C, 22008C tungsten wattage; D, quartz tungsten

radiation output; E, G-30 glass tungsten radiation output;

F, nickel chromium wattage; G, nickel chromium radiation

output). (Courtesy of Fostoria Ind. Inc.)

A newly developed, ceramic, fiber matrix, gas-

fired infrared generator [46] is claimed to have a

high gas-infrared conversion efficiency (50 to 70%)

and high power density (30 to 100 kW/m2 depending

on temperature); it can be turned off from 8508C to

touch in seconds; it can be rapidly modulated to meet

heat load or machine speed variation; and it has no

naked flame. The ceramic fiber pad of this generator

is robust, and the supporting chamber and frame are

air cooled.

The advantages of IR dryers can be summarized

as: (1) low capital cost, (2) compactness, (3) contact-

free drying, (4) instantaneous startup and shutdown

of electric infrared, and (5) better product quality

because of the possibility for zoning the electric heat-

ers across the width of the web and modulating the

heat in each zone separately. However, application of

the IR dryers is still limited to the drying of coatings,

primarily owing to the relatively high operating costs,

i.e., the cost of electrical energy or gas. There are also

safety problems related to IR dryers, especially with

gas burners, because of the very high operating tem-

peratures.

35.2.2.4 Recent Developments in Paper Drying

In this section, the more significant developments

made to increase the efficiency of multicylinder dryers


in recent years, and the new or novel concepts not

used extensively in the papermaking industry yet, will

be summarized. Some of these methods are still at a

laboratory stage but show the potential for a real

breakthrough in the paper-drying technology.

35.2.2.4.1 Innovations in Conventional Drying

of Paper

Cylinder-end insulation: The heat loss from a single

cylinder (1.5 m in diameter) over a period of a year is

found to be equivalent to 86 t of steam. Further cal-

culation shows that 90% of the heat loss can be saved

by the application of an insulator with an overall

heat-transfer coefficient of 0.7 W/m2 K to the cylinder

ends. However, a practical insulator should be fire,

oil, water, and chemical resistant, with the character-

istics such as easy to install, cost-effective, have a long

life, and should also provide access to manholes in

order to solve this problem.

35.2.2.4.2 Ribbed Dryers, Spoiler Bars

Augmentation of heat transfer through the conden-

sate layer becomes more important with increasing

machine speed. In order to obtain higher heat transfer

between the condensing steam and the wet web, the

thickness of the condensate layer should be reduced

by more efficient condensate-removal techniques or

by the use of fins so that heat can be conducted

around the condensate. This has led to such concepts

as ribbed dryers, spoiler bars, and grooved dryers.

The ribbed dryer has a series of ribs or fins

machined on the inside cylinder wall. The condensate

forms in grooves and is removed by a series of small

syphon pipes. The application of this type of dryer is

limited (only on Yankee cylinders) due to the com-

plexity of the condensate-removal system.

There is an oscillatory motion in the rimming

condensate layer and, if some restrictions (e.g., bars)

are spaced at the resonant frequency of the conden-

sate, a higher intensity of turbulence can be achieved.

This is the idea behind the dryers with spoiler bars.

There are two ways to attach the bars inside the

cylinder: (1) magnetic bars that utilize 460- to 920-

mm bar magnets laid end to end and (2) spring-

loaded hoop rings that use circumferential rings at-

tached to the longitudinal bars.

It has been reported [47] that the overall heat-

transfer coefficient can be increased up to 40 to 50%

by ribbed dryers or spoiler bars at machine speeds of

1400 m/min, as compared with plain shell dryers.

35.2.2.4.3 Profilers

Profiling steam boxes and infrared and magnetic pro-

filers are some of the devices utilized in paper ma-

chines.

TABLE 35.5Characteristics of Commercially Used Infrared Heat Sources

Characteristics Tungsten

Filament Wire

Nickel Chrome Alloy

Spiral Winding

Low-Temperature

Panel Heater, Buried

Nickel Chrome Alloy, Metallic Salt

Glass Bulb T3 Quartz Lamp Quartz Tube Metal Sheath

Source temperature (8C)

Normal maximum 2200 2200 870 650 315–430

Usual range 1650–2200 1650–2200 760–980 540–760 205–590

Brightness Bright white heat Cherry red Dull red Nonvisible light

Usual size G-30 lamp 3/8-in. diameter tube 3/8- or 5/8-in.

diameter tube

Various flat panels

Wavelength at energy

peak (mm)

Normal maximum 1.15 1.15 2.6 3.1 ~4–5

Usual range 1.15–1.5 1.15–1.5 2.6–2.8 2.8–3.6 3.2–6

Relative energy

distribution (%)

Normal maximum

Radiation 80 86 55 50a 40–30a

Convection and

conduction 20 14 45 50a 60–70a

Usual range

Radiation 65–80 72–86 55–45 53–45a 50–20a

Convection and

conduction 35–20 28–14 45–55 47–55a 50–80a

Degree of heat

penetration

Depth of penetration varies with the characteristics of the product; as a general rule, energy of shorter wavelengths penetrates deeper than energy of longer wavelengths

Relative response

to heating up Seconds Seconds Minutes Minutes Scores of minutes

Cool down Seconds Seconds Seconds Minutes Scores of minutes

Color sensitivity Bodies of different colors can be heated at more nearly the same rate by IR with long wavelengths than they can be short-wavelength IR

Ruggedness

Mechanical shock Poor Good Good Excellent Varies with panel

Thermal shock Poor Excellent Excellent Excellent Design could be quite good

aRelative energy distribution will vary with the amount of convective cooling, which can vary with the position of heater and volume of air moving by, among other factors.

Source: Courtesy of Fostoria Ind. Inc.

�2006

by

Taylo

r&

Fra

ncis

Gro

up,L

LC

.

(a)

(b)

(c)

FIGURE 35.22 Infrared designs: (a) oven with pressurized

sections to introduce heated air; (b) ovenwithpressurized sec-

tions for cooling and ventilation; (c) different design of an

oven with pressurized sections for cooling and ventilation.

(Courtesy of Fostoria Ind. Inc.)

Profiling steam boxes, which selectively heat the

web, are placed just prior to the pressure-roll nip. The

increased temperature of the web improves water

removal, thus providing a means for moisture profile

control. However, the width of profile control is typ-

ically 310 to 460 mm and the range of the moisture

control is limited between 2 and 3% [48].

The infrared profiler uses segmented infrared

heaters that are individually controlled in 150-mm

zones in the CM direction.

The magnetic profiler uses independently exited

electromagnets installed across the width of a dryer

cylinder. As the cylinder, which is a conductor, passes

through the magnetic flux lines, eddy currents (which

produce heat) are induced in the shell. This heat

provides very precise and discrete control of the mois-

A

C

D

B

FIGURE 35.23 Surface-type gas infrared burner (A, premix

gas and air; B, chamber; C, perforated ceramic tile; D,

special alloy screen grid). (Courtesy of Fostoria Ind. Inc.)


ture profile. The eddy current heating produced can

be 100 times higher than the input power to the

inductor in the 150-mm width band. Commercial

tests show that these profilers control the moisture

within þ0.3% and increase the machine speed up to 5

to 10% [48].

35.2.2.4.4 Papridryer

The Papridryer consists of two principle components:

a high-velocity hood and a vacuum cylinder. High

velocity (60 to 100 m/s), hot (250 to 4508C) air jets

impinge against the web supported on the vacuum

roll, resulting in higher drying rates at the surface of

the web. The vapor formed within the sheet is re-

moved by suction. The application of suction pro-

vides not only through-drying but also enhanced

impingement heat-transfer rates. Laboratory [49]

and mill [50] trials of the Papridryer show very high

drying rates (almost 10 times the average rate of

modern conventional dryer section for newsprint)

without significant change in quality. The preliminary

design calculations showed that only six Papridryers,

each having 1.5- to 2-m suction roll, could provide all

the necessary drying for a 15-m/s newsprint machine

with a dryer train consisting of about 60 steam-heated

cylinders. A better CM moisture profile can be ob-

tained by varying the jet velocities. Fast response to

control action and better energy efficiency are among

the other advantages of the Papridryer. Recent ex-

perimental studies published on the effect of surface

motion on slot jet impingement heat transfer [51] and

the effect of high-temperature difference (up to

3008C) on impingement heat transfer [52] will make

it possible to improve the Papridryer design.

The need for higher drying rates and controlled

paper properties has revived the Papridryer, i.e., com-

bined impingement and through-air drying, idea in

the last decade. Recent experimental studies [53,54]

reaffirmed the drying benefits, but still there is no

commercial application.

35.2.2.4.5 High-Intensity Impingement Drying

High-intensity drying is a concept using high-velocity

impingement drying in combination with cylinder

drying. It is similar to Yankee dryers on tissue ma-

chines, i.e., impingement hoods on a steam-heated

cylinder. Very smooth paper surface is achieved due

to the contact with the drying cylinder. Two dryers

can be used for printing papers to ensure that both

sides are treated in the same way [55].

The impingement-drying concept developed by

Valmet comprises a combination of conventional cy-

linder dryers and high-intensity drying units. Each

high-intensity drying unit has a large diameter roll

and two retractable air-impingement hoods. Three

of such units can be installed in a dryer section for

printing- and writing-paper grades. It is claimed that

the specific energy consumption per ton of paper is

almost exactly the same as the corresponding conven-

tional dryer case [56,57]. It is expected that this type

of dryer will play a significant role in developing

compact, high-speed machines of the future [57,58].

35.2.2 .4.5 Rad io Frequency and Microw ave Dr ying

Another alternative in maintaining higher drying rates

is the application of high-frequency dielectric heating,

covering both RF and microwave ranges. As shown in

Table 35.4, microwaves have higher frequencies and

shorter wavelengths whereas the RF radiations have

lower frequencies and longer wavelengths.

An industrial microwave heating system consists

of a dc power supply, a microwave generator (mag-

netrons are available in 915- and 2450-MHz bands

and klystrons at the higher-frequency bands) and an

applicator. A microwave heater has only one elec-

trode. On the other hand, an RF device requires two

electrodes. The power can only be drawn from an RF

generator when there is material present in the appli-

cator; therefore, the material is an essential electrical

component of the circuit and affects the electrical

characteristics [59].

In the early designs of the RF dryers, flat-plate

electrodes are used and the product is placed between

these electrodes. More recent designs use rod elec-

trodes either in a staggered through-field arrangement

in which the sheet is between the electrodes or in a

stray-field arrangement in which the electrodes are on

the same side of the sheet. These configurations have

been applied successfully to paper drying.

As the heat is absorbed only by water and as the

power-conversion efficiency is low, this mechanism is

suitable only for profile correction and for the drying

of laminated sheets.

A recently proposed system combines RF heating

with hot-air impingement. RF energy is applied to the

sheet through the air-impingement nozzles. Initial tests

have shown that the same drying rate can be achieved

up to one tenth of the time as that of the conventional

dryer, for some products, by using only 10 to 20% of the

energy required in the form of RF energy.

Jones [60] states that as the machine width in-

creases to become a significant proportion of a quar-

ter of a wavelength (i.e., 5 m at 13 to 56 Hz), the field

uniformity becomes increasingly difficult to achieve.

As paper machines are typically wider, an account

must be taken of standing waves.

35.2.2.4.6 Innovative Press-Drying Techniques

In the last 20 y, the main research effort in paper

drying has been concentrated on what we may call


the variations of wet pressing or a combination of

pressing and drying. Press, impulse, and Condebelt

drying techniques have made significant progress in

recent years. Each of these processes uses mechanical,

thermal, and interactive effects in a unique way; be-

cause of that they are sometimes called thermomecha-

nical web consolidation processes [61]. All these

techniques claim very high dewatering or drying

rates that are applicable even to sheets made from

high-yield pulps and offer opportunity for greater

energy efficiency. Detailed, comparative reviews can

be found elsewhere [61–64].

35.2.2.4.7 Press Drying

The FPL (Forest Product Laboratory, U.S. Depart-

ment of Agriculture) press-drying process combines

mechanical and thermal means of water removal for

drying stiff pulp fibers (e.g., for linerboard production

under compressive force that improves interfiber

bonding). Press drying utilizes less energy because

the sheet has lower moisture entering the drying sec-

tion. Wet web is sandwiched between two felts and

pressed between two hot surfaces.

Laboratory tests have shown that it is possible to

achieve average drying rates about 10-fold at 1778Cand 20-fold at 2888C compared with conventional

drying of linerboard [65]. The estimated dryer size

needed for paper-machine speeds ranging from 0.25

to 10 m/s (50 to 2000 fpm) based on the tests per-

formed at FPL can be found in the literature [66].

The techniques developed for press drying and the

effects of press drying, particularly changes in drying

variables such as initial moisture content, tempera-

ture, pressure, and time, on sheet properties have

recently been summarized by Mitchell [67].

Back and Anderson [68] have found that at a

surface temperature of 1508C, both the tensile index

and the modulus of elasticity remain at high levels in

the range of 39 to 70% initial solids content, but

decreases sharply at higher solids content. They used

press-dried paper made from 60%-yield softwood

Kraft pulp.

Both the studies of Back and Anderson [68] on the

strength properties of press-dried sheets at the surface

temperature of 150 to 3008C and Yang et al. [69] on

the effect of density on modulus of elasticity of 1078C,

1498C, and 2328C showed no appreciable effect of

change in temperature. Setterholm and Benson [70]

showed that increased pressure increases the consoli-

dation of the sheet, which is accompanied by in-

creased strength properties such as breaking length

and elastic modulus.

Some of the reported advantages of the press

dryers are listed below [71]:

. Effective utilization of high-yield fibers

. Better use of hardwood fibers

. Improvement in the characteristics of paper

products made from refiner and thermomecha-

nical pulps. Improvements in paper-containing waste paper

that leads to a promising future for paper recyc-

ling. Reduction in the amount of refining required to

obtain the given web characteristics. Improved dimensional stability and smooth-

surface production. Increase in the output of dryer-capacity-limited

machines

Press drying offers a very promising method for

paper and paperboard drying. However, the method

is still in the development stage and many problems

(i.e., the venting of water vapor and the need of

extended nip residence time) remain to be solved.

Pilot-scale testing, at PAPRICAN, PIRA, and so

on, of this process is still underway.

35.2.2 .4.8 Condebe lt (or Conva c) Dr yer

The Convac process is an entirely novel concept in

paper drying. In this process, the wet sheet is pressed

on a steam-heated metal surface with a permeable felt

or mat and an impermeable metal sheet that is water-

cooled [72] (Figure 35.24). Before drying starts, air is

removed from the web and felt by vacuum. Once

drying begins, water vapor evaporates from the web,

passes through the felt and condenses on the cold

metal sheet, and this continues throughout the drying

Steam, T =P =

Cooling water,

Cooling-water chest

Seal

Edge seal

Seal

Steam chest

FIGURE 35.24 Basic scheme of a Condebelt device. (Courtesy


process. Therefore, by using this mechanism, vacuum

is maintained in the dryer. The static laboratory tests

resulted in very high drying rates, exceeding 145 kg/

m2 h at a metal temperature of 170 8C. Convac-dried

paper and board products show higher stiffness and

tensile strength, and the web surface in contact with

the hot metal becomes smooth.

In the pilot-scale application of this process, the

paper web is supported by a fine metal wire and a

thick plastic wire and is fed between two metal bands

(Condebelt process). The top or bottom band is

heated by steam and the other band is cooled by

water. Two proposed Condebelt arrangements are

shown in Figure 35.25. Pilot-scale testing of this

process at Valmet–Tampella Research Center gave

promising results [73,74]. Drying rates close to

500 kg/m2 h are reported for low-grade paper in

these tests [73]. The advantages are claimed to be

much higher drying rates, improvement in sheet

properties (e.g., smoothness, enhanced strength pro-

perties, and no shrinkage), and the possibility of

reduced space requirements.

There are two commercial installations of Conde-

belt dryers on board machines. According to recent

reports [75,76], the minor operational issues have

been resolved with modified design and expected im-

provements in product properties are realized.

35.2.2.4.9 Impulse (High-Intensity) Drying

High-intensity contact drying denotes the drying

under sufficiently intensive heating conditions such

that, following a brief warm-up period, the moist

paper web operates at internal temperatures in excess

120 to 1808C (248 to 3568F) 2 to 10 bar (abs) (2 to 10 ata)

T = 20 to 908C (68 to 1948F)P = heating steam pressure

No air in the void spacesbetween the two steel platens

Steel platen

Steel platen

WebFine-mesh plastic wireCoarse plastic wire

of Valmet–Tampella.)

Air-removal unit

Possible subzone partitionsSteel band

Dry web out

Dry web out

Wet web in

Steam

Steam

Water

Steam

Suction

Sliding seals

Coarse wire

Doctor

Suction

Steel band

Fine wire(a)

(b)

Steam or hot gas

Hot gasHot steel band preheater

Air-removal unit

Wet web inBooster heater

Steel band

SuctionIn

Cooling water Out

Steel band

Band supports

Doctor

Suction

Coarse wire

Fine wire

Optional steel band precooler Water or aircooling

FIGURE 35.25 Condebelt dryer arrangements: (a) high z-pressure arrangement; (b) low z-pressure arrangement with pre-

and booster-heated steel band. (Courtesy of Valmet–Tampella.)

of the ambient boiling point [77]. Dryer surface tem-

perature might be elevated to 200 8C or higher and

sheet contact pressure in the range of 7 to 35 kPa or

higher. These can be compared with the typical dryer

surface temperatures, which range from 125 to 175 8Cand contact pressures that are less than 2 kPa. Drying

rates may be 10 to 20 times those obtained in conven-

tional drying [77].

Poirier and Sparkes [78] have successfully run a

two-roll impulse drying unit at PAPRICAN pilot

paper machine at speeds up to 800 m/min. Solids

contents of 60% have been achieved with newsprint

using this single-nip arrangement.


The main problems associated with this technique

are delamination of the sheet, brightness and opacity

losses, sidedness, and sticking of the sheet to the

surfaces. There is a great investment in experimental

and theoretical development work in order to gain a

better understanding of the mechanisms involved in

this highly promising process. Various theoretical

views and debate published in the last 5 y show that

a widely accepted mechanism has not been pro-

posed yet [61–64,79–83]. Drying rates and process

conditions for typical impulse drying process as com-

pared with press and Condebelt drying are shown in

Table 35.6.

TABLE 35.6Comparison of Press, Condebelt, and Impulse Drying

Press

Drying

Condebelt

Drying

Impulse

Drying

Temperature (8C) 100–250 120–180 150–500

Pressure (MPa) 0.1–0.4 0.02–0.5 1–5

Residence time (ms) 200–300 250–10,000 15–100

Drying rate (kg/m2 h) 25–120 100–400 500–8,000

Energy usage

(kJ/kg H2O) 2,500–3,500 2,200–3,000 550–1,400

Source: From Paulapuro, H., Developments in Wet Pressing, PIRA

Information Services, Leatherhead, Surrey, U.K., 1993.

35.2.2.4.10 Superheated Steam Drying

Loo and Mujumdar [84] made a transient analysis to

evaluate the technical feasibility of using superheated

steam as a drying medium replacing hot air. The main

attraction of the steam CIT (combined impingement

and through-drying) process they propose is the pos-

sibility of extremely high thermal efficiencies attain-

able through reuse of the exhaust steam by reheating,

compression, or use in other process-heating applica-

tions. Subsequently, Cui and Mujumdar proposed an

alternate configuration for steam drying of paper and

developed a simple mathematical model to estimate

the drying rate and energy consumption [85]. They

showed that although the drying rates for tissue prod-

ucts could be increased up to 25 to 30% compared

with those of a Yankee dryer, the net heat consump-

tion was extremely low. Their model was verified in a

static drying test apparatus operated in the constant-

rate period [86].

The effects of steam drying on paper quality need

to be examined closely. However, as steam drying of

pulp has been used successfully, it is not very likely

that it will have adverse effects on sheet properties.

Preliminary studies indicate that steam drying may

actually enhance the strength properties of paper.

In a recent review of this process, Mujumdar [87]

reported the drying rates of 100 to 200 kg/m2 h and

compared the limitations and advantages of both air

and steam drying. He concluded that the process

appears to be a viable concept due to its higher energy

efficiency, enhancement of certain quality indices for

at least some types of pulps, elimination of fire haz-

ard, and reduced space requirements. The technical

issues to be resolved are startup and shutdown, con-

densation of steam on web, air infiltration (sealing at

high speed), materials of construction due to corro-

sion and erosion, steam cleaning, recirculation, com-

pression, heating, and sheet-quality aspects.


35.2.2.4.11 Drying in the Presence

of an Electrostatic Field

Recently, Rounsley [88] reported a 5 to 18% increase

in the drying rates of paper and coatings in the pres-

ence of a nonuniform, static, electric field. This tech-

nique has been tested for both felted and unfelted

drum drying as well as air impingement and radiant

drying. If commercially successful, this concept has

the advantage of fast response and may be used for

moisture profile control.

In summary, it may be noted that most of the

paper drying carried out industrially is accomplished

by conventional multicylinder dryers. With the ad-

vent of more energy-efficient and economic dryers

based on novel concepts, however, it is likely that

new drying technology will find industrial acceptance

within a decade.

35.3 DRYING OF PULP

If pulp is produced for use in an integrated paper-

making machine, there is no need for drying. How-

ever, for market pulp, drying up to 10% moisture

(90% fiber, 10% water) is necessary.

The pulp web was dried exclusively by contact

with steam-heated cans until the mid-1950s. However,

air drying (air floater dryers) of pulp is predominant

today (e.g., 70% of the U.S. paper industry). Al-

though the application of flash drying in the industry

is relatively new, it has found an appreciable market

(e.g., 15% of the U.S. paper industry). As the steam-

dryer system was introduced to the industry only

recently, it is not yet an established drying technique

for pulp.

35.3.1 CONVENTIONAL PULP DRYING

In the conventional method, the pulp web is produced

on either a fourdrinier wire or a revolving cylinder in

a vat in which the level is kept constant by continuous

pulp supply. A modern fourdrinier system has a

closed head box working at constant level, wet suc-

tion boxes to allow the drainage on the wire, and hot

water or steam boxes for preheating the web. Pressing

is accomplished either with feltless press rolls or dou-

blefelted press rolls. After the press rolls, the dryer

section begins. Cylinder dryers or air floater dryers

can be used in this section. The described modern

fourdrinier system achieves high dryness ahead of

the drying section, which affects the economy of the

system and allows higher capacities and machine

speeds up to 100 t/d and 200 m/min, for example.

This unit may have a machine width up to 6.5 m.

The revolving cylinder system is less expensive, but

it has lower capacity and presents some problems

related to the quality of the sheet [89].

Although the conventional steam-heated cylinder

dryers are still predominant in the paper industry,

their share in pulp drying has diminished rapidly

after the application of air floater dryers. The dryers

are 1.2- to 1.5-m diameter case iron cylinders and the

steam systems and air systems are similar to the paper

dryer system described earlier.

The air floater dryers are also similar to those

described in the paper-drying section. Hot air is im-

pinged to the web from blow boxes above and below,

and the web floats supported by the airflow. The low

sheet tension ensures a greater ability to tolerate dis-

turbances or sheet defects, and the quality of pulp is

less affected. Pulp dryers also consist of stacked dry-

ing decks, as shown in Figure 35.16. Pulp dryers are

larger than paper dryers as pulp is air dried from the

press section to the cutter.

35.3.2 FLASH DRYING

Flash drying is a process in which wet pulp is intro-

duced into a stream of hot gases and its moisture is

vaporized.

Dewatering of wet pulp is accomplished by vari-

ous types of presses. After dewatering, moist pulp is

introduced into a hot gas stream. The pulp–hot gas

mixture passes through a number of flash-drying

towers (number depends on the design of the system,

but usually two double-towered systems with a cyc-

lone separator between them are used as shown in

Figure 35.26) and the dried pulp particles and the

moist gases are separated in cyclone separators.

EF

B

D

B

G

B

C

A

H

FIGURE 35.26 A flash-dryer unit for pulp (A, moist pulp;

B, air; C, oil; D, moist air; E, dried pulp; F, circulation fans;

G, air heater; H, steam-heated heat exchangers).


There are also single, rotating horizontal units avail-

able for pulp drying.

Flash dryers for pulp are in operation with cap-

acities of up to 500 BDMT/d (bone-dry metric tons).

One such plant dries pulp from approximately 60 to

12% water in a two-stage dryer. The lower moisture

bound is critical, as overheating below it can cause

thermal cross-linking that makes reconstitution of the

original fiber difficult. Pulp temperature is maintained

below 70 8C in both stages, with air inlet temperatures

of 400 and 170 8C, respectively. Exhaust for the sec-

ond stage is mixed with inlet air to the first stage.

Surface moisture is removed in the first stage (about

30% moisture), and the more delicate second-stage

drying is carried out at a lower temperature. Dried

pulp is cooled before baling. The first-stage air heater

may burn oil or natural gas; the second-stage dryer

uses steam-heated air.

Although there are still some questions about the

high gas temperatures involved in flash dryers and the

absence of wide market acceptance of the products,

flash drying is very promising because of its very low

operating and capital costs compared with conven-

tional pulp dryers [89].

35.3.3 STEAM DRYING

The steam drying of pulp is a very recent application

and not yet widely accepted. Pilot-plant and full-scale

tests have shown very attractive results.

The principle of the steam dryer is outlined in the

flow diagram shown in Figure 35.27. Wet pulp is fed

into the dryer by means of a plug feeder, then disin-

tegrated and fluffed in a steam atmosphere and blown

through the drying stages by means of fans. Each

stage is a shell and tube heat exchanger, with steam

of a higher temperature condensing outside the pulp-

transport pipes. The dried pulp and carrier steam are

then separated in a cyclone, and the pulp is fed out of

the pressurized system by a specially designed dis-

charge screw and blown with air to a cooling cyclone.

The generated steam from the pulp moisture is with-

drawn to keep the pressure constant and the rest is

reused as carrier steam.

The first commercial installation of a steam dryer

in Sweden for a chemithermomechanical pulp

(CTMP) line with a capacity of 150 t/day showed a

30% reduction in overall drying costs per ton of mar-

ket pulp compared with an equivalent-size flash dryer

[90].

Lower power consumption, very short drying

times, easy control, no risk for fire in steam atmos-

phere, and minor or no effect on pulp quality are

among the advantages claimed by the manufacturer.

A

F

G

I J

E

D

C

H

B

FIGURE 35.27 Flow diagram of a steam dryer for pulp (A, wet pulp; B, heating steam at 6 to 15 bar; C, dried pulp; D,

generated steam at 2 to 5 bar; E, condensate; F, plug feeder; G, fluffer; H, discharge screw; I, circulation fans; J, cooling-air

fan). (Courtesy of MoDo-Chemetics.)

35.4 CONCLUSION

This chapter summarized the current technology for

drying of paper and pulp. Recent developments and

trends are also indicated. The reader is referred to the

literature cited and other relevant sections or chapters

of this handbook for further details and additional

information.

ACKNOWLEDGMENT

The authors wish to thank Purnima Mujumdar for

her patient typing of this manuscript, without which

this work could not have been completed within the

required time frame.

NOMENCLATURE

A cross-sectional area

B basis weight of the sheet out of the dryer

section as dried (wet basis), g/m2

c concentration

C1, C2 constants

D diameter of the dryer cans

Dp nozzle-plate diameter ratio

D diffusivity

f open area

F1–2 overall interchange factor

h heat-transfer coefficient

H dimensionless jet-plate spacing

k thermal conductivity

L length across which DT is measured

M kilograms of water evaporated per kilogram

of paper dried (wet basis)

N number of dryer cans in contact with the

sheet

Nu average Nusselt number


P pressure

Pr Prandtl number

q heat flux

Qe condensate flow rate

Qs blow-through steam rate

R gas constant

Re Reynolds number

Sm speed of the machine, m/s

Sc Schmidt number

Sh average Sherwood number

t time

T temperature

u sheet velocity

w weight of water removed per unit area per

unit time, or drying rate

x coordinate axes

X sheet thickness

Xp nozzle-plate width ratio

Greek Symbols

a thermal diffusivity

b mass-transfer coefficient

s Stefan–Boltzmann constant, 5.67 � 10�8

W/m2 K4

l latent heat of vaporization

Subscripts

a air

cyl cylinder

cs cylinder to sheet

f felt

o overall

r radiant

s steam

sa sheet to air

sf sheet to felt

REFERENCES

1.

� 200

Luikov, A.V., Heat and Mass Transfer in Capillary-

Porous Bodies, Pergamon Press, London, 1966.

2.
Montgomery, A.E., Tappi J., 27(1):1, 1954.
3.
Kuhasalo, A., Niskanen, J., Paltakari, J., and Karlsson,
M., in Papermaking, Part 2, Drying, M. Karlsson, Ed.,


4.
Coveney, D.B. and Robb, G.A., in Pulp and Paper
Manufacture, Papermaking and Paperboard Making,

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5.
Kershaw, T.N., in Pulp and Paper Chemistry and Chem-
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New York, 1980.

6.
Nissan, A.H. and Kaye, W.G., Tappi J., 38(7):385,
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7.
Nissan, A.H., George, H.H., and Hansen, D., Tappi J.,
45(3):213, 1962.

8.
Kirk, L., in Advances in Drying, Vol. 3, A.S. Mujumdar,
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9.
Race, E., in Drying of Paper and Paperboard, G. Gavelin,
Ed., Lockwood, New York, 1972.

10.
Bell, D.O., Seyed-Yagoobi, J., and Fletcher, L.S., in
Advances in Drying, Vol. 5, A.S. Mujumdar, Ed., Hemi-

sphere, Washington, DC, 1992.

11.
Keey, R.B., Drying Principles and Practice, Pergamon
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12.
Nissan, A.H. and Hansen, D., AIChE J., 6(4):606,
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13.
TAPPI, Paper Machine Drying Rate, TIS-014.17, 1970.
14.
Perrault, R.D., Tappi J., 66(9):65, 1983.
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Simmons, T., in Drying of Paper and Paperboard,
G. Gavelin, Ed., Lockwood, New York, 1972.

16.
Walker, P.J., in 1977 Handbook: Practical Aspects of
Pressing and Drying, TAPPI, Atlanta, GA, 1977.

17.
Marshall, H.G., in Drying of Paper and Paperboard,

18.
Kottick, G.J., in Drying of Paper and Paperboard,

19.
Soininen, M.A., in Drying of Paper and Paperboard,

20.
Smook, G.A., in Drying of Paper and Paperboard,

21.
Palazzolo, S., Paper Trade J., 33(May 16–31), 1978.
22.
Johansson, B., Pulp Paper Can., 84(11):29, 1983.
23.
Osterberg, L. and Norinder, S.-O., in Drying of Paper
and Paperboard, G. Gavelin, Ed., Lockwood, New

York, 1972.

24.
Richter, G.A., Tappi J., 41(12):777, 1958.
25.
Nuttall, G.H., in Drying of Paper and Paperboard,

26.
Atkins, J.W., Rodencal, T.E., and Vickery, D.E., Tappi J.,
65(2):49, 1982.

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