When the paper sheet enters
the paper machine Dryer Section, it is about 50% water. It must be
dried to less than 10% water for a finished product. The most common
Dryer Sections include Steam Cylinders and Air Dyers. Infrared Dryers
are most commonly used on coating lines. The purpose of this web site
is to promote the increased use of Gas IR Heaters in the paper manufacturing
Dryer Section.
Drying occurs in Three
Phases or Zones. In the Heating Phase the sheet enters at about 100F
and is brought up to about 180F. Generally, about the first 5 cylinders
are considered the 'Heating Phase'. In the Constant Rate Zone, heat
is added at about the same rate that evaporation is removing the heat.
Most of the water is removed in the Constant Rate Zone. The Falling
Rate Zone is the most difficult as it must remove the last 10% or
so of moisture without causing problems related to un-even or over-drying.
Dryer
Types
Steam Cylinders/Cans/Drums
(Conduction)
Paper
Board Paper
Air Dryers
(Convection)
Tissue - Yankee Dryer
Market Pulp - Flakt Dryer
Infrared Dryers
(IR) (Radiation)
Coatings - Non-contact
Dryer
Steam
Cylinders
Steam cylinders are 4 -
5 feet in diameter and slightly longer than the width of the paper
sheet. A typical paper machine has 40 to over 100 steam cylinders,
depending on the line speed; the faster the line speed, the longer
the drying section. Typical machines are as long as a football field
and 3 to 5 stories tall. They require 1,275 to 1,575 BTUs steam input
per pound of water dried from the sheet.
Air Dryers
Air dryers are direct fired
or use steam-to-air heat exchangers to produce a hot air stream that
is forced over the surface of the paper. Hoods or 'caps' are used
to contain and direct the air flow. Air dryers tend to be used on
lighter weights of paper, such as tissues, and to supplement the drying
of steam cylinders.
A Yankee Dryer is a specific
kind of dryer used most commonly for Tissue and Toweling manufacturing
that combines a large steam cylinder and an air hood.
The Flakt Pulp Dryer is
also a special kind of air dryer that is used to dry "market
pulp" (Pulp that is made in one location and sold for final paper
manufacturing at another location). A Flakt Dryer is constructed in
decks that the pulp serpentines through as hot air is blasted through
it by a series of jets.
IR Dryers
IR
Dryers produce infrared radiation from a hot surface. Different wave
lengths are produced from different kinds of heaters and varies with
temperature. How receptive a material is to IR energy is a function
of its surface reflectance and the IR wavelength. The more receptive
the material, the higher the IR system efficiency.
Natural
Gas IR Heaters operating at about 1,800F (1,000C) produce an IR wave-length
between 2 and 3.5 microns. This is also the segment that both paper
and water have a high receptiveness. When the IR energy is absorbed
by the surface water, its temperature rapidly increases and it evaporates.
When the paper absorbs the IR, its temperature also increases and
drives out the moisture from within the sheet.
The
net effect is a very efficient system that uses over 50% of the available
energy to dry the sheet. In some applications the exhaust gas can
be recovered to pre-heat incoming air and further increase efficiency.
The
IR Advantage
Gas IR heaters
produce more drying, in a smaller space, more efficiently, than any
other drying system available.
Steam
Cylinders
IR
Dryers produce infrared radiation from a hot surface. Different wave
lengths are produced from different kinds of heaters and varies with
temperature. How receptive a material is to IR energy is a function
of its surface reflectance and the IR wavelength. The more receptive
the material, the higher the IR system efficiency.
Natural
Gas IR Heaters operating at about 1,800F (1,000C) produce an IR wave-length
between 2 and 3.5 microns. This is also the segment that both paper
and water have a high receptiveness. When the IR energy is absorbed
by the surface water, its temperature rapidly increases and it evaporates.
When the paper absorbs the IR, its temperature also increases and
drives out the moisture from within the sheet.
The
net effect is a very efficient system that uses over 50% of the available
energy to dry the sheet. In some applications the exhaust gas can
be recovered to pre-heat incoming air and further increase efficiency.