Note: Descriptions are shown in the official language in which they were submitted.
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WO 95/10659 ~ ~ ~ ~ PCl'/US94/09566
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PATENT APPLICATION
IMPULSE DRYER ROLL WITH SHELL OF HIGH THERMAL DIFFUSIVITY
FIELD OF THE INVENTION
This invention relates to paper making machines and calenders.
More particularly, this invention relates to rolls used in paper making
machines which transfer heat to a paper web. Still more particularly, this
invention relates to a heated roll used in a paper making dryer or calender
which has improved heat transfer to the paper web.
BACKGROUND OF THE INVENTION
Paper manufacture is a capital intensive industry. A drive for
increased productivity has led to efforts to increase paper production by
increasing the width of the paper web which can be made on a paper
making machine. Another trend for increased productivity is to increase
the speed of the web through the machine. The width of the web can be
increased by making the machine and the paper handling rolls wider.
Increasing the speed at which the paper web is manufactured creates a
problem in the dryer section of the paper making machine. As web speed
increases, heat transfer to the dry paper web from each dryer cylinder
decreases. Thus, to deal with higher web speed, dryer sections of paper
making machines have had to be made longer.
One solution to the problem of longer dryer sections is the impulse
dryer. The impulse dryer employs a high temperature roll which is heated
to 232°C (450°F) or higher. The heated roll is used to form an
extended
nip between the roll and a shoe. The paper web is brought into contact
with the high temperature surface of the roll. The paper is backed by a
press felt which in turn overlies a blanket which rides a lubricating film
over
the shoe. By using such extended nip impulse dryers it has been possible to
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WO 95/10659 . ' ' ' ' ' ' ~ PCTlUS94/09566
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significantly decrease the length of the dryer section of a paper making
machine.
Further improvements in the impulse dryer require an increased heat
transfer between the roll and the paper web. Increasing the roll surface
temperature necessitates increasing the temperature of the entire roll. This
has deleterious effects on the roll strength. Increased roll temperature can
also complicate the use of internal crown support mechanisms which rely
on hydraulics. Further, high temperatures can cause searing of the press
felt, picking of the web surface fibers, excess roll head stresses, and
overheated bearings. In U.S. patent No. 4,324,613 to Wahren, it is
recognized that an impulse dryer roll would benefit from being made of a
material of relatively high thermal conductivity. Wahren also teaches the
desirability of a roll of low surface conductivity to achieve high surface
temperature. Wahren proposes a material selection criteria for at least the
surface layer of the roll which does not favor aluminum over steel or nickel.
Wahren discounts copper as not suitable for his roll.
U.S. Patent No. 4,631,794 to Riihinen, describes a roll for use in a
calender which is inductively heated. The roll uses a non-magnetic
insulating layer between an outer, ferromagnetic layer and the inner core of
the roll. Riihinen provides inductive heating of the roll in a calender and
allows access to the interior of the roll for crown control, but does not
disclose how to use increased thermal conductivity to improve heat
transfer.
U.S. Patent No. 4,738,752 to Busker et al suggests a press roll for
use in a heated, extended nip press which includes a first co-axial layer,
and a second co-axial layer extending about the first layer. The second
layer has a coefficient of thermal conductivity greater than the coefficient
A~,~ENDED SMELT
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WO 95/10659 . ' ' . ' ' ! F'CT/11594/09566
3/A
of thermal conductivity in the first layer. The first layer is a material
having
a low coefficient of thermal conductivity and the second layer is metallic.
It is further suggested that the first layer may be ceramic and the second
layer metallic.
Busker et al suggests that the second outer layer has a thickness in
the range of .0127-.127 cm (0.005 inches to 0.050 inches). Busker et al
does not disclose how to calculate the proper thickness of a metal outer
shell over an inner metal base shell. Nor does Busker teach the importance
of thermal diffusivity in the choice of material for and the thickness of the
outer shell.
What is needed is a roll for use in an impulse dryer or calender with
increased heat transfer capability.
SUMMARY OF THE INVENTION
The roll of this invention is composed of two parts. One is a metallic
base shell, which is constructed of conventional, spun-cast steel alloy. The
second part of the roll consists of a thin outer shell a few tenths of an inch
thick which is in intimate contact with the surface of the steel alloy base
shell.
The outer shell is constructed of a material of high thermal
diffusivity. Thermal diffusivity is proportional to the thermal conductivity,
and inversely proportional to the specific heat and density of a material.
Examples of materials of high thermal diffusivity are copper and aluminum.
One way to construct the roll of this invention is to flame spray a
layer of copper approximately two tenths of an inch thick on the surface of
a roll. The roll is employed in the extended hot press nip of an impulse
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WO 95/10659 ~ ~ ' ~ ' ' ' ' pCT/US94/09666
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dryer. The extended nip is formed by a shoe. The shoe has a smooth,
A~AENDED Sr~~ET ,
I
WO 95/10659 ~ ' ~ ' . t '.' r ~ NL T'/U594/09566
4/A
arcuate surface of slightly greater curvature than the surface of the roll of
this invention. An extended nip blanket is drawn over the surface of the
shoe at a speed equal to the surface° speed of the roll. A paper web
travels
adjacent to the roll surface, and a press felt is positioned between the
blanket and the web. The effect of the shoe is to hold the paper web
against the roll over a circumferential distance of perhaps ten inches on a
five foot diameter roll. The roll is rotated to produce a surface velocity on
the order of 914 meters/minute (3,000 feet per minute). The extended nip
blanket, press felt and web of paper all move at an identical speed past the
roll.
The roll is constructed of a heavy-walled metal cylinder. The
cylinder wall is typically several inches , in thickness. The roll is heated
by
induction heaters which heat the surface of the roll upstream of the
extended nip. When an analysis is performed on the heat transfer between
the roll and the web, it is observed that only the outermost portion of the
roll experiences a fluctuating temperature. If the surface of the roll is
heated to 232 ° Celsius (450° Fahrenheit) by induction heaters,
the entire
interior of the roll will reach an equilibrium temperature of 232°C
(450° F).
The surface of the roll that comes into contact with the web may
experience a temperature drop at the surface of 93°C or 149°C
(200°F or
300°F). However, this temperature fluctuation is only experienced by
the
top one- to three- tenths of an inch of the roll. Thus, the effective heat
transfer portion of the roll is only a thin, outer annulus. In a conventional
roll, the only way to increase heat transfer is to increase roll temperature,
increase nip length, or reduce web speed. Because all three of these
approaches have costs and disadvantages associated with them, the
present invention increases the heat transfer between a roll and the web by
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1N0 95/10659 ~ ' ' ' ' ' ' f'CT/US94/09566
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increasing the thermal diffusivity of the role by constructing it to include a
copper layer of a precisely calculated depth.
AMENDED SHEET
WO 95/10659 4 ~ PCT/US94/09566
It is an object of the present invention to provide an impulse dryer
roll with increased heat transfer to a paper web being dried.
It is another object of the present invention to provide a roll for use
in a calender which provides increased heat transfer to a paper web for a
given roll surface temperature.
It is a further object of the present invention to provide a roll for use
in the dryer section of a paper making machine which requires a shorter
section.
It is a still further object of the present invention to provide a roller
for use in an impulse dryer which reduces picking of the web surface
fibers.
It is a still further object of the present invention to provide a roller
for use in an impulse dryer that reduces roll head stress.
Further objects, features, and advantages of the invention will be
apparent from the following detailed description when taken in conjunction
with the accompanying drawings.
FIG. 1 is a cross-sectional elevational view, somewhat in schematic
form, of a wide area, or long nip or extended nip type, paper making
machine which employs the two-layer roll of this invention.
FIG. 2 is a cross-sectional elevational view, somewhat in schematic
form, of a wide area, or long nip or extended nip type paper making
WO 95110659 PCT/US94/09566
machine which employs a stretched, extended nip blanket, and utilizes the
roll of FIG. 1.
FIG. 3 is a graphical view of temperature versus time at various
depths from the surface of the roll of FIG. 1.
FIG. 4 is a cross-sectional elevational view, somewhat in schematic
form, of a calender which employs the two-layer roll of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring more particularly to FIGS. 1-4, wherein like numbers refer
to similar parts, an impulse dryer 20 is shown in FIG. 1. The impulse dryer
20 includes a press roll 22 which forms a nip 26 with a shoe 24. The shoe
24 is provided with a concave. surface facing the roll 22 and is mounted so
that it is urged towards the roll 22. The press nip 26 is formed between
the roll 22 and the shoe 24. A web of paper 28 passing through the nip
26 is subjected to a pressing pressure over an extended length of time. A
press felt 32 moves beneath the web 28 and a looped belt 30 passes
through the nip over the shoe 24 beneath the felt 32.
Oil is supplied between the shoe 24 and the belt 30. The oil causes
a hydrodynamic wedge of fluid to build up between the belt 30 and the
shoe 24. The fluid wedge transmits pressure to the web while at the same
time lubricating the movement of the web 28 through the nip 26. The
press felt 32 passes through the nip 26 while underlying the paper web 28
and riding on the belt 30. The paper web 28, the press felt 32, and the
belt 30, as well as the roll 22, are in engagement and so driven at the same
speed. Thus the paper web 28 does not experience significant sheer force
because there is no relative motion in the plane of the web 28 and the
press felt 32 and the surface 34 of the press roll 22. Thus the paper web
WO 95110659 PCT/US94/09566
7
28 is subject to principally compressive forces as it moves thrQUgh the
extended nip 26. The effect of this compressive force is to bring the web
into intimate contact with the surface 34 of the press roll 22.
The intimate engagement of the web 28 with the press roll surface
34 under pressure facilitates the rapid heat exchange between the surface
34 of the roll 22 and the web 28. The rapid heat transfer between the roll
22 and the web 28 produces a not completely understood drying
mechanism which is characteristic of the impulse dryer. The rapid heating
of the paper web vaporizes some of the water contained in the web. The
steam which has been produced from the water in the web is trapped
between the surface 34 of the roll 22 and the paper web 28. Its only route
of escape is through the paper web 28 into the press felt 32. The rapid
downward movement of the steam from the upper surface of the paper
web 28 downwardly into the press felt 32 has the effect of blowing water
contained in the web 28 into the felt 32. This process, impulse drying,
results in the rapid removal of water from the paper web 28.
The press roll 22 in the impulse dryer 20 has improved effectiveness
over a conventional impulse dryer because of the construction of the roll
22. The roll 22 has a metallic base shell 38 constructed of conventional
steel alloy. The base shell 38 is overlain by an outer shell 40 which is
constructed of a material of high thermal diffusivity such as copper. It
should be noted that the rolls as illustrated in the Figures are not precisely
to scale, and for illustrative purposes that relative thicknesses of the base
shell and the outer shell may be exaggerated. The outer shell 40 is
intimately joined to the outer surface 42 of the base shell 38. The metal
base shell 38 forms the structural support for the outer shell 40. The
increased thermal diffusivity of an outer copper layer can double the
effective heat transfer between the roll 22 and the paper web 28. A
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WO 95/10659 . ~ ' ~ ~ PCT/US94/09566
8/A
typical impulse dryer consists of a press nip in which the press roll reaches
a surface temperature in excess of 93°C to 149°C (200° to
300°F) and
preferably between 232°C and 288°C (450° and
550°F). A typical
impulse dryer may employ an extended nip press employing a shoe which
presses an impermeable blanket against an inductively heated roll. The
high surface temperature rapidly heats the wet web as it passes through
the nip and softens the paper fibers. This greatly enhances the removal of
water and the development of sheet strength properties.
The ability to increase the heat transfer by using a thin outer shell
40 of increased thermal diffusivity allows impulse dryers to be designed to
achieve selected advantageous effects. First, the surface temperature of
the roll 22 may be lowered and still provide the same level of heating. This
overcomes several problems associated with high shell temperatures. High
temperatures can cause searing of the press felt, picking of the web
surface fibers, excess roll head stresses and overheated bearings. Second,
more heat can be supplied to the paper to remove more moisture. Third,
the speed of the web 28 may be increased, while holding input and drying
effectiveness constant. In actual optimized designs employing the high
thermal diffusivity outer shell press roll 22, all three advantages of lower
press roll temperature, higher operating speed, and more complete drying of
the paper web 28 may be incorporated.
FIG. 1 is a cross-sectional view of an impulse dryer 20 employing the
improved press roll 22 and taken along the direction of travel of the paper
web 28. As will be appreciated by those versed in the art of paper making,
the cross-machine width of the paper web 28 will normally be between
one-hundred and four-hundred inches, with the components of the impulse
dryer such as the roll 22 being in general somewhat longer, as necessitated
by their particular function.
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The looped belt 30 and its support 44 are conventional and are
described more fully in U.S. Patent No. 4,673,461 (Roerig et al). The belt
30 is a continuous loop. It has a cross-machine width greater than the
press roll 22, so that the ends of the belt (not shown) may be sealed to
circular closures (not shown) which seal the ends of the belt, thus
containing the nip lubricating oil within the sealed belt 30. A stationary
beam 33 is contained within the belt 30. The beam adjustably supports
the shoe 24 by means of a hydraulic piston chamber 35 in which is
positioned a piston 37. The shoe is pivotally supported on a roller pin 39,
seated in a downward facing groove in the shoe 24 and an upward facing
groove in the piston 37. The piston is urged upward by fluid pressure
beneath the piston in, the chamber 35, which is in the form of an elongated
slot, slidably receiving the piston, and extending the full width of the
machine beneath the shoe 24. The belt 30 is guided by means of curved
side guides 41 and 43, upper guides 45 and 47, and a lower guide 49.
The guides are adjustably mounted to the beam 33 and serve to stabilize
the belt 30 during start-up. The guides also stabilize the belt 30 if any
fluttering or instability should occur during normal operation.
Once the belt 30 has reached operational speed, centrifugal force
will cause the belt 30 to assume a naturally circular shape, except where
traversing the nip 26 between the shoe 24 and the press roll 22. Thus, at
speed, the centrifugal forces will normally hold the belt 30 a short distance
off the guides, 41, 43, 45, 47, and 49.
The press felt 32 is supported on an infeed roll 46 and a press
outfeed roll 48. The infeed press felt roll 46 and outfeed roll 48 will
typically have a diameter of .61 meters (two feet), where the
corresponding diameter of the press roll is 1.52 meters (five feet). The
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rolls 46, 48, serve to bring the press felt into
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position to be fed through the nip 26 of the impulse dryer 20. The press
felt 32, after leaving the outfeed roller 48, is processed by a felt dryer
(not
shown), which removes water and excess moisture from the felt 32 before
it returns for reuse over the infeed roller 46.
The press roll 22 in FIG. 1 is shown employing a hydraulic crown
control mechanism 50 which has a non-rotating crown support beam 52.
The crown support beam has an oil supply port 54 which supplies oil to
piston cavities 56 which drive pistons 58 against the inner surface 60 of
the metallic base shell 38. Pistons 58 which are spaced along the central
beam or shaft 50 serve to apply a constant pressure between the press roll
22 and the shoe 24. In FIG. 1, an induction heater 62 is shown
schematically. It has coils 64 which are energized with high frequency
current. The induction heater 62 is conventional in nature. It employs
oscillating magnetic fields caused by the high frequency alternating current,
which create eddy currents in the surface 40 of the roll 22. The currents
induced produce resistance heating in the surface 40 thereby heating it to
the desired temperature.
From a heat transfer point of view, the shell 40 appears to be a
"semi-infinite plate." This is because of the very short time a given
segment of the roll is in the nip. With external induction heating of the
shell 40, the entire roll operates with a uniform temperature from the inside
60 to the outside 34, with the exception of a thin outer layer which
undergoes cyclical temperature fluctuations. The effective depth of this
outer layer can be estimated according to the following expression:
X= 4
'. ;
WO 95/10659 . . PCT/US94/09566
1 1 /A
Where:
X = the depth of the layer which experiences cyclical temperature
variations in ft.
S = the press roll surface speed in meters/hr (ft/hr).
L = the nip length in ft.
a - thermal diffusivity of the outer surface in square meters/hr
(ft2/hr).
This depth is estimated based on the assumption that the surface
experiences a step-change in temperature. In practice, the change will not
be this dramatic, so the above estimate will tend to be on the high side.
FIG. 3 is a graphical representation of the temperature fluctuations
with time of the surface of the roll and three points at successively greater
depths. In FIG. 3,.the vertical axis is temperature and the horizontal axis is
time. The uppermost curve 66 represents the temperature on the surface
of the roll versus time. Because the press roll 22 is approximately five feet
in diameter, it has a circumference of approximately 16 feet. The nip, 26, .
shown in FIG. 1, is ten inches long, with the result that the roll is exposed
to the paper web 28 for approximately five percent of the time as it
revolves around. The upper curve 66 has a repeated saw-tooth wave form
68 which represents the surface temperature over an entire rotation of the
drum 22.
- Taking as the starting point the precipitous drop in temperature 70,
which represents the contact of the surface 34 of the roll 22 with the
paper web 28, on the scale of FIG. 3 ~,rvhere the entire web contact time
represents five percent of a wave-length 68, the drop in temperature of the
surface is essentially instantaneous. The surface 34 of the roll 22 then
AA~ENDED SHEET
'. ' ;.,
WO 95/10659 ~ ' ' ' PCT/US94/09566
1 1 /B
maintains the temperature of the paper of the web 28 as it is heated by
contact with the roll.
AMEi~ED~ ~;~~ ,
CA 02173140 1998-09-10
WO 95/10659 T/U~~-...,9566
12/A
As a point on the surface exitsnip, it experiencesgradual
the a
increasein temperature 72 as heat from the interior the
flows of roll
towardsthe lower temperature surface.Finally, the roll surface
experiences at 74 a steep increase in temperature as it moves past the
induction heater 62. Thereafter, the temperature 'remains constant until it
again at 70 comes into contact. with the paper web 28.
Thermal diffusivity (a) is defined as:
k
CI - pc
Where:
a = thermal diffusivity.
k = thermal conductivity.
c = specific heat.
p = density.
The second graph 75 represents the temperature profile of an
arbitrary point 'some distance removed in depth from the surface 34 of the
roll 22. The wave length 76 of the second curve 75 is the same as the
wave length 70 of the surface temperature curve 66: However, the
change in temperature is less abrupt as the point represented by the curve
75 is somewhat removed from the surface.
A third curve 78 has a wave length 80 which is identical to the wave
lengths 76, 68, and is governed by the rotation rate of the roll 22. This
curve represents a point further removed from the roll than the curve 75
and shows how, at greater depths within the roll, the changes in
temperature caused by the cyclical nature of the surface 34 coming in
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WO 95/10659 ~ ' ~ , ~ ~ ~ r ' '~ 'PCT/US94109~66~
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contact with the roll and being heated by the induction heater 62 are
damped out.
The fourth curve 82 shows a constant temperature with time. It
represents the approximate depth at which the above equation predicts no
fluctuation in temperature. It is also the depth at which the thermal
A~JlEf~0E0 SHEET
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WO 95/10659 ' ~ ~ ~ ~ PCT/US94/0956C
13/A
diffusivity of the material is no longer important in governing heat flow
between the roll 22 and the paper web 28.
An alloy cast iron shell has a thermal diffusivity of only about .066
sq. meters/hr (0.6 to 0.7 ft2/hr), whereas the thermal diffusivity of
aluminum is 3.3 and copper is approximately 4.4.
TABLE 1
hermal DiffusivityNip Length Web Speed Effective
(Inches) (Ft./min.) Depth
(Inches)
0.7 10 3,000 0.086
2 10 3,000 0.146
3 10 3,000 0.179
4 10 3,000 0.206
0.7 1 3,000 0.027
2 1 ~ 3,000 0.046
3 1 3,000 0.060
4 1 3,000 0.065
0.7 10 6,000 0.061
2 10 6,000 0.103
3 10 6,000 0.126
4 10 6,000 0.146
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WO 95/10659 ~ ~ ' ' ' ~ " ' pCT/US94/0956~ 1
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Referring to Table 1, column 1 has various values of diffusivity; 0.7
corresponding roughly to the typical material from which press rolls are
constructed. A thermal diffusivity of 3, corresponding to aluminum, and a
thermal diffusivity of 4, corresponding to copper.
In Table 1, two common nip lengths in inches are listed, with 10
corresponding to a typical impulse dryer extended nip, and one inch
corresponding to a typical calender application as shown in FIG. 4. Two
web speeds in feet per minute are listed. Nine hundred fourteen meters per
minute (Three thousand feet per minute) is a typical speed for modern
paper making machines. Six thousand feet per minute is the approximate
upper range of paper speeds currently being discussed by the industry.
Table 1 indicates that the approximate maximum radial thickness of
the outer surface shell 40 need be only approximately two-tenths of an
inch thick with a combination of thermal diffusivity, nip length, and web
speed as shown. Taken in conjunction with FIG. 3, Table 1 also illustrates
the two layer construction of a press roll 22. '
The volumetric specific heat, i.e., the amount of thermal energy per
unit volume, is very roughly constant over a broad range of metals,
inasmuch as low density metals tend to have high specific heat and the
high density metals have a low specific heat but more material per unit
volume. Thus, the volume of material experiencing cyclical heating is a
good indicator of the total amount of heat being supplied by the roll 22 to
the paper web during its cyclical contact with the paper web 22.
FIG. 3 may be viewed with the understanding that the depth of the
points corresponding to curves 75, 78 and 82 double as the effective
depth, as shown in Table 1, doubles. Thus, the movement from an alloy
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WO 95/10659 ~ ~ ~ ' PCT/US94/0956fi~ ~ ,
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steel roll with an effective depth of .218 cm (eighty-six-thousandths of an
inch), to copper with an effective depth of .525 cm (207-thousandths of
an inch) means that over twice the volume of material is undergoing
cyclical temperature fluctuation. Thus, the heat flow during a given cycle
is roughly twice as high.
Another observation which can be gleaned from Table 1 is that as
the nip length decreases the effective depth decreases, so that in a
calender, as shown in FIG. 4, where the nip 76 is in the range of one-half
to one-and-a- half inches, very thin, high conductive outer shells 78 are
effective at dramatically increasing heat transfer.
Press rolls 22 are conventionally manufactured by centrifugal casting
of iron alloy against a chilled mold. Centrifugal casting against a cold
metal shell typically results in a roll with high surface hardness and
internal
ductility. The outer shell of higher thermal diffusivity material such as
copper or aluminum will preferably be sprayed onto the exterior surface 42
by spraying molten metal on the surface. Typical metal spray techniques
involve sand blasting the surface of the roll to develop a surface to which
the sprayed metal will adhere, heating a wire of the desired metal through
an electrical arc or plasma, and spraying the resulting metal on the surface
42 of the metal base shell 38. Typically, a metal spraying operation would
be performed while the base shell 38 is rotating, with successive layers of
sprayed metal applied until the requisite thickness is achieved. After this,
the roll is surface finished to provide the surface smoothness required of a
press roll in an impulse dryer or a calender roll in a calender.
One possible method of forming the exterior shell involves
centrifugally casting a thin outer layer first, perhaps by spraying molten
metal into the chilled centrifugal mold, then casting the steel base shell 42
J~IIEND ED 5~ '' ~'
WO 95/10659 ~ PCT/L1S94/09566
16
onto the outer shell 40. This method would require preventing an oxidized
coating from forming on the outer shell 40, so as to produce a molecular
bond between the outer shell 40 and the base shell 34.
Other possible methods include chemical or electrical deposition from
solution onto the surface 42 of the base shell 38. In chemical deposition, it
is important to avoid porosity in the deposited metal. Another possible
method, especially for very thin layers such as used in a calender, would be
vapor deposition.
FIG. 2 shows an alternative impulse dryer 120 which has a press roll
122, and forms and extended nip 126 between a shoe 124 and the surface
134 of the roll 122. The impulse dryer 120 employs an impervious
extended nip blanket 130. The shoe 124 is provided with a concave
surface facing the roll and is mounted so that it is urged towards the roll
122. The nip 126 subjects a web of paper 128 passing through the nip to
a pressing pressure over an extended length of time. The nip blanket 130
passes through the nip between the shoe 124 and the roll 122.
Oil is supplied between the shoe and the blanket 130. The oil
causes a hydrodynamic wedge of fluid to build up between the blanket 130
and the shoe 124. The fluid wedge transmits pressure to the web while at
the same time lubricating the blanket's movement through the nip 126. A
press felt 132 passes through the nip 126 while underlying the paper web
128 and riding on the blanket 130. The paper web 128, the press felt
132, and the blanket 130, as well as the roll 122, are driven at the same
speed by their inter-engagement through frictional force. Thus, the paper
web 128 does not experience significant sheer action because there is no
relative motion in the plane of the web 128 and the press felt 132 and the
surface 134 of the press roll 122. Thus, the paper web 128, is subject to
WO 95/10659 ~ ~ ~ PCT/US94I09566
17
principally compressive force as it moves through the extended nip 126.
The effect of this compressive force is to bring the web into intimate
contact with the surface 134 of the press roll 22.
In a way similar to the dryer 20, the press roll 122 in the impulse
dryer 120 has improved effectiveness over a conventional impulse dryer.
The roll 122 has a metallic base shell 138 constructed of conventional steel
alloy. The base shell 138 is overlain by an outer shell 140 which is
constructed of a material of high thermal diffusivity. The outer shell 140 is
intimately joined to the outer surface 142 of the base shell 38. The metal
base shell 138 forms the structural support for the outer shell 140.
The blanket 130 and its support 44 are conventional. The blanket
130 is a continuous loop. It has a cross-machine width greater than the
press roll 122, so that the edges of the belt (not shown) extend past the
edges of the paper web 128. A stationary beam 133 is beneath the belt
130 and the shoe 124. The beam adjustably supports the shoe 124 by
means of a hydraulic piston chamber 135 in which is positioned a piston
137. The piston 137 is urged upward by fluid pressure beneath the piston
137 in the chamber 135, which is in the form of an elongated slot, slidably
receiving the piston, extending the full width of the machine beneath the
shoe 124. The belt 130 is guided by means of upper guide rolls 141 and
143, and lower guide rolls 145 and 147, and a stretcher roll 149. The
stretcher roll 149 is adjustably mounted to vary tension in the blanket 130.
The press felt 132 is supported on an infeed roll 146 and a press
outfeed roll 148. The infeed press felt roll 146 and outfeed roll 148 will
typically have a diameter of two feet, where the corresponding diameter of
the press roll is five feet. The rolls 146, 148, serve to bring the press felt
132 into position to be fed through the nip 126 of the impulse dryer 120.
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The press felt 132, after leaving the outfeed roller 148, is processed by a
felt dryer (not shown), which removes water and excess moisture from the
felt 132 before it returns for reuse over the infeed roller 146.
The press roll 122 in FIG. 2 is not shown employing a hydraulic
crown control mechanism. Rather the roll 122 is formed with a diameter
which is greater in the center and which tapers axially. The disadvantage of
using a crowned roll 122 is that the pressure is only created for one level
of pressure between the roll 122 and the shoe 124. Thus it may be
desirable to employ an active crown if a range of drying pressures is
desired. In FIG. 2, an induction heater 162 is shown schematically. It has
coils 164 which are energized with high frequency current. The induction
heater 162 is conventional in nature. It employs oscillating magnetic fields
caused by the high frequency alternating current, which create eddy
currents in the surface 141 of the roll 122. The currents induced produce
resistance heating in the surface 134 to the desired temperature.
FIG. 4 illustrates the use of a roll 86 which has an outer shell 87 of
high thermal diffusivity in a calender 84. Calenders are used to smooth and
improve the surface finish of a paper web 82.
FIG. 4 is a schematic illustration of a machine stack, or "on
machine" calender 84. The calender comprises a calender stack,
constituted by calender rolls 86, 88, 90, and 92. Calenders may typically
have a hard nip between hard rollers or may have a soft nip wherein the
hard rollers 86, 90 are alternated with soft rollers 88, 92 composed of
elastic fiber, the construction being conventional.. A soft nip calender
typically has nips 94 of 1.27 cm to 3.81 cm (one-half to one-and-a-half
inches) in length. The calender roll 86 may be heated by infrared or
induction heating. When a calender roll having a metal base shell 96 and
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an outer shell 87 with high
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thermal diffusivity is used, improved heat transfer is effected between the
roll 86 and the paper web 98. This increases the effectiveness of the
calender 84.
As shown in Table 1, calenders, because of their shorter nip length,
require thinner shells of high thermal diffusivity, in the neighborhood of
.152 cm (60-thousandths of an inch). The relatively thin radial thickness of
the outer shell thus may facilitate the formation of the roll, being more
adaptable to chem-film deposition. The relatively thin film also means that
as non-metal, high thermal conductivity coatings such as diamond become
available, it may be possible to coat rolls with materials such as
vapor-deposited diamond.
It should be understood that as the web speed increases, the
required thickness of the outer higher thermal diffusivity shell decreases.
It should also be understood that wherein the equation provided
above for effective depth, as illustrated in Table 1, gives maximum
effective depth of heat transfer in the roll material, using depths less than
the maximum effective depth will still serve to increase the heat transfer
capability of a roll 22.
It should also be understood that the nip length, roll diameters, and
roll temperatures are illustrative, by way of example, and that other
temperature ranges, roll size, and nip lengths could advantageously employ
the roll 22 of this invention.
It should also be understood that where a crown support mechanism
is shown and illustrated, the roll could have a machined crown surface
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21"~3~.40
upon which is deposited or formed a thin outer shell of high thermal
diffusivity.
It should also be understood that wherein materials such as copper
and aluminum have thermal diffusivities of three or more, thermal
diffusivities in the range of one might be practical in some circumstances.
It is understood that the invention is not confined to the particular
construction and arrangement of parts herein illustrated and described, but
embraces such modified forms thereof as come within the scope of the
following claims.
