transfer of water evaporating from the 
web and is valid up to a dry content of 
about 75 %.  
(Formula 1)
β – mass transfer coeff.[m/s]
m
v
 – evaporation rate [kg/s]
A
contac
 – paper surface area [m²]
p
o
 – total pressure [Pa]
p
v
 – vapour partial pressure of air [Pa]
p
vp
 – vapour partial   pressure of the paper 
web [Pa]
T
Paper
 – temperature of paper web [K]
R
v
 – gas constant of vapour [J/kgK]
When the dry content is above about 
75 % we have to consider diffusion 
effects because the moisture of the 
web has to permeate through dry paper 
areas to reach the web surface to be 
evaporated. The mass transfer at high 
dry content is denoted β*, it is given in 
Formula 2 [2]. Substituting β with β* 
leads to the modified Stefan equation 
(Formula 3) [4] for high dry content.
(Formula 2)
µ – diffusion resistance number [-]
D – diffusion coefficient [m²/s]
s – distance from water level in web to 
web surface [m] 
(Formula 3)
According to equations 1 and 3 the 
evaporation rate is strongly depending 
on the constant parameters such as R
v
, 
the paper surface area A
contact
, the total 
pressure p
o
 and the variable parameters β, 
p
v
 and p
vp
. The mass transfer coefficient 
β depends on different diffusion, mass 
transfer and paper surface parameters 
and cannot be influenced easily. The two 
remaining parameters give the major 
possibilities to influence the drying rate in 
a paper machine.
 1.) Decreasing vapour partial pressure 
of the air (p
v
)
 Vapour partial pressure of the air 
can be decreased by increasing the 
temperature or the dry content of the 
supply air in the drying hood or by 
improving supply air amount using 
blow boxes which are bringing fresh 
air to the paper surface.
 2.) Increasing vapour partial pressure 
of the paper web (p
vp
)
 can be influenced by changing 
the web temperature. Increasing 
the drying rate can be achieved 
by increasing the temperature of 
the cylinder surface. Furthermore 
web temperature can be raised by 
increasing the draw of the drying 
fabrics which presses the paper web 
to the cylinder surface leading to a 
better heat transmission to the paper. 
In both cases the steam pressure 
has to be increased so that more 
temperature difference between 
cylinder surface and paper web is 
achieved and thus the heat flux to 
the paper web will be increased.
The impact of these possibilities for 
influencing the drying rate will be further 
discussed in the case studies (see chapter 
2.1). To optimize a drying section one has 
to know the actual status of the machine. 
Therefore a mass and heat balance of the 
existing drying group including steam and 
condensate system is necessary. Based 
on the accurate heat and mass balance 
the physical drying simulation model is 
developed. The model is validated by 
M. Schneeberger
1, 
P. Leuk
1, 
U. Hirn
1, 
P. Fisera
2, 
W. Bauer
1
RAZISKAVE IN RAZVOJ
30
Papir za notranjost revije PAPIR je prispevala papirnica Vipap Videm Krško d.d., VIPRINT 80 g/m
2
IZVLEČEK
Naraščajoče cene energije so štiri avstrijske papirnice spodbudile, da so pristopile k skupnemu projektu o prihranku energije v 
sušilni skupini papirnega stroja. Razvili so računalniški model za prenos toplote in snovi na štirih različnih papirnih strojih. 
Pri modeliranju pretoka energije in vodne pare, ki izhaja iz papirnega traku v sušilni skupini, ter v sistemu rekuperacije toplote 
papirnih strojev so upoštevali fizikalne enačbe za prenos toplote in snovi. Modeli simulirajo različne nastavitve sušilne skupine 
in rekuperacije toplote na papirnem stroju. Uporabljajo se za optimiziranje nastavitev, ki se pozneje preverjajo s poskusi na 
strojih. Bistveni del računalniškega modela predstavlja grafični vmesnik, enak kot slika, ki jo ima papirničar, ko nadzira delovanje 
papirnega stroja. Tako je ta model simulacije papirničarjem preprost za uporabo ter primeren za poskuse in vajo.
Možnosti prihranka v sušilni skupini so obravnavane na osnovi modela in osnovnih znanj o poteku sušenja. Predstavljeni so 
trije primeri študije. Prvi je yankee sušilnik z vpihovalnikom vročega zraka, druga dva pa primera papirnih strojev z običajnima 
sušilnima skupinama. Obravnavani primeri so zahtevali nizke investicijske stroške in omogočili prihranke energije do 5 % oziroma 
4.000 MWh/leto.
Ključne besede: papirni stroj, sušilna skupina, prihranek energije, računalniški model za prenos toplote in snovi.
ABSTRACT
Due to the increasing prices of energy, four Austrian paper companies started a joint project to save energy in the drying section. 
A computer simulation model of the heat transfer and mass transfer in four different paper machines has been developed. This 
model employs the physical relations for heat- and mass transfer to model the flow of energy and vaporized water from the web 
through the drying section and the heat recovery system of the paper machines. The models are simulating different settings in 
the paper machine drying section and heat recovery system. Simulations are used to define optimum machine settings which 
are then subsequently tested during machine trials. One of the key features of computer models is a graphical user interface 
identical to the operator screens used to control the specific paper machine. Thus, the simulation model is easy to use by mill 
staff and can also be applied for operator training.
Opportunities for saving energy in the drying section are discussed based on the model and some drying fundamentals. Three 
project case studies are presented, one study from a Yankee dryer with impingement hood and two studies from paper machines 
with conventional drying sections. The case studies presented require low investment costs and provide energy savings for up to 
5 % or 4.000 MW
th
 per year.
Keywords: paper machine, drying section, energy saving, computer simulation model, heat transfer, mass transfer.
1 INTRODUCTION
The drying process is a highly cost 
intensive part in paper production as up 
to 75 % of the overall thermal energy are 
applied there. Due to the fact that energy 
is getting more and more expensive a 
project has been started by the Institute 
for Pulp, Paper and Fibre Technology, 
Graz University of Technology, to 
optimise the drying section in terms of 
energy consumption. 
As a basis to understand the 
mathematical model some drying 
fundamentals are discussed. Also the 
energy consumption necessary for 
evaporating the water out of the web 
in dependence of the dry content of the 
paper has to be determined. For each 
paper machine a physical simulation 
model of the drying group is developed. 
The models include the drying cylinders, 
IR and impingement dryers, heat 
recovery, condensate and steam system. 
It has been primarily developed by the 
company “Consulting Fisera” and by 
the project members of Graz University 
of Technology. 
2 FUNDAMENTALS OF PAPER 
DRYING AND SIMULATION 
MODEL
The whole drying process can be split in 
several periods as shown in Figure 1 [1].
The heat up phase (A-B) is primarily used 
to increase the web temperature and 
only some energy is used for evaporating 
water out of the web. The first drying 
phase is identified by a more or less 
constant drying rate (B-C). Reaching a 
critical dry content (C), all water on the 
fiber surfaces has been evaporated and 
to further increase the dry content water 
has to diffuse through the dry paper 
web before evaporation. In the graph we 
see a decrease of the drying rate – this 
is the second drying phase (C-D). The 
third drying phase (D-E) starts at above 
80 % dry content. The drying rate is 
decreasing rapidly in this third phase 
due hydrogen bonding and capillary 
effects, which are binding the water to 
the fibre surface. Additional evaporation 
energy is necessary to overcome these 
effects and this additional energy is 
Papir za notranjost revije PAPIR je prispevala papirnica Vipap Videm Krško d.d., VIPRINT 80 g/m
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RAZISKAVE IN RAZVOJ
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Figure 1: Moisture content changes as a function of paper moisture during paper drying. The x-axes shows the 
equilibrium moisture content X [kg/kg], y-axes shows the drying rate [kg/kgs]
Slika 1: Hitrosti sušenja v odvisnosti od ravnotežne vsebnosti vlage 
Figure 2: Heat of vaporization of unbleached Kraft pulp at 80 °C as a function of equilibrium moisture content 
(EMC) [3]. The heat of sorption is the difference between the latent heat of water (2308 kJ/kg) and the 
vaporization energy. 
Slika 2: Toplota izhlapevanja nebeljene sulfatne celuloze pri 80 °C kot funkcija ravnotežne vsebnosti vlage (EMC). 
Sorpcijska toplota je razlika med vsebnostjo navidezne toplote vode in energije izhlapevanja.
called sorption enthalpy [2], see Figure 
2. Water has a latent heat of 2.308,05 
kJ/kg at 80 °C [5], this is equivalent 
to the heat of vaporization up to an 
equilibrium moisture content (EMC) of 
about EMC=0.3. At higher dry content 
the vaporization energy is increased 
by the heat of sorption, at e.g. 97 % 
dry content (EMC=0.03) the necessary 
heat for evaporation is about 2.900 
kJ/kg instead of 2.300 kJ/kg implying 
that at this dry content we have a heat 
of sorption of 600 kJ/kg water. The 
integrated heat of sorption (shaded area) 
is up to 3 % compared to the latent heat 
of water, meaning that drying paper up 
to a dry content of 97 % the total heat of 
vaporization is about 3 % higher than the 
latent heat of water.
In order to understand the drying process 
the Stefan equation [4] (Formula 1) is 
discussed which describes the mass 
OPPORTUNITIES FOR ENERGY SAVINGS     
IN THE DRYING SECTION
MOŽNOSTI PRIHRANKA ENERGIJE V SUŠILNI SKUPINI

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Papir za notranjost revije PAPIR je prispevala papirnica Vipap Videm Krško d.d., VIPRINT 80 g/m
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Papir za notranjost revije PAPIR je prispevala papirnica Vipap Videm Krško d.d., VIPRINT 80 g/m
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RAZISKAVE IN RAZVOJ
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online DCS measurements and manual 
control measurements of temperature, air 
moisture and air flow at various positions 
in the drying section and the heat 
recovery system. The model is optimised 
and validated until a maximum deviation 
between model and DCS values of lower 
than 5 % is achieved.
The simulation model includes following 
parameters
 All paper machine parameters like 
machine speed, grammage, machine 
width and surrounding conditions of 
air and paper web.
 Drying equipment like steam cylinder, 
impingement dryers, IR-dryers
 Heat recovery system with heat 
exchanger, fans and air streams 
 The steam and condensate system for 
each drying group 
6 REFERENCES
[1] Siebenhofer M. (2011). Thermische 
Verfahrenstechnik I VO - Teil 1. Trocknung. 
Lecture Script of the Institut für chemische 
Verfahrenstechnik und Umwelttechnik, Graz 
University of Technology.
[2] Krischer, O., & Kast, w. (1992). 
Trocknungstechnik, Erster Band (3. Auflage 
Ausg., Bd. 1). New York: Springer Verlag.
[3] Leuk, P . (2012). Methoden zur Bestimmung 
der spezifischen Trocknungsenergie von 
Faserstoffen. Master Thesis. Graz University of 
Technology.
[4] Karlsson, M. (2009). Papermaking, Part 2, 
Drying. Helsinki: Association Paperi ja Puu Oy.
[5] Gnielinski, & al., e. (2006). VDI-Wärmeatlas 
(10. Ausg.). (V. D. Ingenieure, & V. G. (GVC), 
Hrsg.) Berlin Heidelberg: Springer.
1 
University of Technology Graz, Institute for 
Pulp, Paper and Fibre Technology, 8010 Graz 
2 
Consulting Fisera, 8072 Gössendorf bei Graz
Contact autor: 
michael.schneeberger@tugraz.at
 All relevant paper parameters in 
terms of drying (fibre mix, filler 
content, dry content before and after 
drying section)
 All the necessary equations for the 
heat and mass transfer during the 
drying process in the drying hood 
and heat recovery system.
 The images from the DCS user 
interface in order to facilitate 
application of the simulation model 
by operators as the program looks 
like the DCS system.
 Several paper properties at each 
cylinder like evaporation rate 
on cylinder, evaporation rate to 
next cylinder, paper and cylinder 
temperature, heat transfer coefficient.
 The prices for different energy 
sources (gas, steam, oil …) can be 
integrated in the simulation in order 
to simulate varying energy prices.
The main advantages for using such a 
simulation model are:
 Testing different machine settings 
for energy optimisation without any 
risk for troubles in production. The 
settings found in the simulation can 
then be tested in machine trials.
 Additional equipment can be 
added to the model fairly easy, 
like additional heat exchangers, 
additional impingement hoods et 
cetera. So the model can be used 
to calculate different scenarios for 
machine rebuilds and investments for 
new equipment.
 Application of the model as a training 
tool for operators.
 Increased cost awareness of 
operators. True costs for energy (gas, 
fuel, steam) can be integrated in the 
model and thus true cost results are 
obtained directly from the simulation.
3 APPLICATION CASE 
STUDIES
Three project case studies to save energy 
are discussed in this chapter 
3.1 Influence of supply air 
temperature on a yankee 
machine
As discussed in chapter 3 there are two 
possibilities to influence the evaporation 
rate of a paper web. In term of a Yankee 
machine we can increase the absorbed 
heat of the paper web by increasing 
the steam pressure or by increasing 
the temperature of the air in the 
impingement hood. The example below 
in Figure 3 shows these two possibilities 
by the simulation model: 
 Case A, the impingement 
temperature is 175 °C, cylinder steam 
pressure is 1.7 bar gauge
 Case B, the impingement 
temperature is 150 °C, cylinder steam 
pressure is 2.5 gar gauge
In both cases the heat absorption of the 
web and the drying performance are the 
same. The heat flux from the Yankee 
cylinder in case B however turned 
out to be more energy efficient than 
increased convection via impingement 
drying. The simulation model shows an 
energy advantage of about 1.8 % for 
case B. Another disadvantage of the 
impingement hood is that the supply 
air for the impingement dryer has to be 
heated by a gas burner and the energy 
prices for gas is 2,5 as high as for steam 
in the considered paper mill which adds 
another 3.5 % of cost difference. The 
combined effect leads to reduced energy 
costs of 5 % only due to increasing the 
steam pressure in the Yankee cylinder 
and lowering the temperature of the 
supply air in the impingement hood.
3.2 Possibilities to use exhaust 
air of infrared dryer
In some machines the exhaust air of an 
infrared dryer is not used in the heat 
recovery system. The amount of the 
exhaust air, in our case was close to 
10.000 m
3
/h with a temperature of 
165 °C and a moisture content of
 0.01 kg/kg. This corresponds to an 
energy amount of close to 
4.000 MW
th
/year compared to fresh 
air. This heated air with low moisture 
content can be used in air hoods. 
This will lead to direct energy savings 
of 4.000 MW
th
/year. Therefore the 
investment is just some piping and 
valves for redirecting the air stream, 
payback time for this investment is far 
below one year.
3.3 Optimisation of leakage air 
and recirculated air
In another case study the mass balance 
of exhaust and supply air turned out to 
be incorrect during our measurements. 
The amount of leakage air was more 
than 40 % of total air volume. The 
balance was somewhat complicated 
because exhaust air from the drying 
section was partly recirculated. With the 
simulation model it was demonstrated 
that the recirculation stream negatively 
influenced the energy demand for 
heating up the supply air. Also too much 
supply air was injected to the hood in 
order to avoid condensation. It turned 
out that the supply air stream had 
remained unchanged for several years 
although considerable modifications of 
the paper machine had been undertaken 
during that time.
The recirculation stream has now 
been closed and the supply air flow 
has been adapted to a value of 20 % 
leakage air, which turned out to be 
optimal according to modelling results. 
This lead to energy savings due to 
reduction of steam consumption for air 
heating and reduction of electric energy 
consumption from turning down the 
supply air fans. The energy savings were 
about 4.000 MW per year (thermal and 
electric combined), without any major 
investment in machine infrastructure.
4 CONCLUSIONS AND 
OUTLOOK
Based on an exact mass- and heat 
balance different approaches for energy 
optimization can be worked out and 
subsequently quantified using the 
developed simulation model. Evaluation 
of different settings in the model is 
easy because the simulation software 
has the same graphical user interface 
as the computer screens for controlling 
the paper machine. The results from 
the simulations are directly displayed 
on the screen and additionally the 
financial results are shown. A variety of 
optimization approaches and scenarios 
can be analyzed within a short time and 
the most promising machine settings 
found in the simulations can be verified in 
machine trials. 
Project work so far identified two general 
aspects of energy savings. The first one 
is rather straightforward: it is beneficial 
to use the heat of the exhaust air for 
heating up supply air or process water. 
While this idea seems rather obvious, 
it is not fully implemented in all paper 
machines, especially not in older ones. 
Many paper machines run with high 
supply air temperatures. As shown in 
case study 4. 1 it is more cost effective to 
decrease the temperature in the supply 
air to lower energy costs. This effect is 
also confirmed by [4]. 
Application of the simulation model in 
four paper machines revealed so far an 
energy optimisation potential of 1 % to 
5 % of total drying energy consumption. 
Payback time for the optimization was 
calculated to be between 0 months 
and two years, depending on the paper 
machine. Currently the optimization 
measures obtained from simulations 
are verified in machine trials at the 
participating companies. Results so far 
are in line with the predicted savings 
from simulations. 
5 ACKNOWLEDGEMENTS
We would like to thank Hamburger 
Containerboard Pitten, Lenzing 
Papier, Mondi Packaging Frantschach, 
SCA Laakirchen and the Austrian 
Industrial Research Foundation FFG 
Figure 3: Example for Yankee machine. Case A: T= 175 °C, P=1.7 bar; 
Case B T= 150 °C, P=2,5 bar 
Slika 3: Yankee stroj, primer A in B
for supporting this project (Project 
‘Trocknungsoptimierung mittels Simulation 
der Stoff- und Wärmeübertragung’ (FFG 
project number 3265427))