Note: Descriptions are shown in the official language in which they were submitted.
~%3~3Z~
The present Invention relates to a felt condltlonlng
system having particular application to papermaklng machinery In
which traveling felts absorb water from a paper or board sheet
being formed by Lowe machine. In order to assure efficient
machine operation It Is necessary to debater the felt and remove
other materials picked up by the tell from the paper web such as
loose fibers, clays, etc.
In the press section of a Papermaklng machine top and
bottom endless press felts are used to remove water from a paper
or board sheet being formed. For proper functioning of the end-
less felts It Is necessary to remove all water absorbed by the
felt In each revolution otherwise the felt becomes supersatu-
rated. It Is particularly Important to remove absorbed water
from the felt before It reaches the press nip so that the felt Is
properly conditioned, I.e., water has been removed to enable the
felt to absorb the maximum quantity of water from the paper
sheet. In conventional practice It Is common
~L~3~2~L~
to see a paper Rhine operating with a wet nip, i.e., a back flow
of water to the incoming side of the press nip - a clear indication
that the felt is supersaturated. A wet nip occurs because the felt
conditioning suction boxes are rot removing the quantity of water
taken up by the felt for each felt cycle. A supersaturated felt travel-
lying at 3000 ppm encounters high hydraulic forces at the press nip
causing removal of fines from the paper sheet and requiring reduction
in nip pressure to avoid hydraulic forces which would destroy the
sheet. Of course, with reduced nip pressure less water is removed
from the sheet.
Accordingly, conventional techniques for conditioning
felts on operating paper machines have inherent limitations so that
press felts are not properly dried.
In felt conditioning with suction boxes a saturated felt
passes over a vacuum opening or slot extending across the machine
beneath the felt. At machine speeds of 3000 ppm any point in the
felt has a dwell time of 1.6 milliseconds over a 1 inch vacuum slot.
As machine speed increases the dwell time grows shorter limiting the
volume of water that can be drawn by vacuum through the slot. Moreover,
removal of water from a rivaling felt into a suction box requires
the force of air drawn through the felt to deflect each droplet of
water moving with the felt at machine speed. As machine speed increases
greater air force is required to remove water from the felt. To overcome
these l~nntations and to achieve increased water removal at greater
machine speeds con may use more than one suction slot, however, the
cost for this improvement is reduced felt life.
In practice, suction boxes are applied to the paper sheet -
side of the felt because the dirt to be removed is located toward
that side of the felt. The suction boxes then wear the nap of the
felt and diminish the ability of the felt to absorb water. Suction
I
boxes are also applied to a horizontal run of the top felt after the
paper side of the felt has passed over an outside roll which presses
the dirt into the felt before reaching the suction box.
Another technique for felt conditioning is the honeycomb
roll described in U. I. Patent No. 4,116,762 to Gardener. According
to Gardener the felt passes over a rotating honeycomb roll while Canada
toning air Yes through the pheromones structure of the rotating
roll and through the felt. Since the honeycomb roll rotates, the
conditioning air is supplied to a stationary plenum within the roll
in an axial direction from both ends of the roll. Supplying air through
the roll in an axial direction is not feasible because extremely high
air velocities are required in order to move the necessary volume
of air through the felt for conditioning. High velocity air loses
pressure as it moves through the axial supply tubes with resultant
loss of air temperature and volume and diminished ability for condition-
in the felt. The diameter of the honeycomb roll cannot be increased
to achieve greater conditioning air volume with lower air velocities
because the minimum pressure of conditioning air is inversely proper-
tonal to the radius of curvature of the felt passing over the roll
at a given felt tension. As a result any increase in honeycomb roll
diameter requires lower conditioning air pressures to avoid lifting
the felt away from the honeycomb roll surface.
Felt manufacturers recommend a minimum flow of conditioning
air for the honeycomb roll of 6 cubic feet per minute per square inch
of felt or approximately 100 aim per inch of felt width. For a 300-inch
wide felt 30,000 aim of conditioning air is required at velocities
approaching 25,000 ppm. As the conditioning air expands through a
honeycomb roll under these conditions its temperature drops to the
point of freezing the water carried by the felt. In addition, water
viscosity increases as temperature decreases inhibiting its removal
from the felt.
I
A further limitation of the honeycomb roll inheres in
the nature of the honeycomb roll itself. As the moving felt engages
the surface of the honeycomb roll, a pocket of ambient air is trapped
in the cells defined by the honeycomb structure between the felt and
the pressurized plenum within the roll. Felt conditioning air in
the interior plenum chamber of the honeycomb roll therefore must first
compress the trapped ambient air before passing through the felt.
In addition, the trapped ambient air will lower the temperature of
hot conditioning air. As a result of this limitation, time is lost
and the effectiveness of the conditioning air is diminished. It is
not likely that these air pockets can be eliminated since the honeycomb
structure requires a given depth of lattice work to achieve roll strength
sufficient to support the felt under tension. In addition with the
current industry trend to wider machines the honeycomb structure must
have greater radial dimensions to meet strength requirements. Accord
tingly, the honeycomb roll is limited in utility for purposes of left
conditioning by passing pressurized air through the felt and has not
been commercially used in the paper making industry.
Another felt conditioning device is disclosed in U. S.
Patent by 3,347,740 to Goumeniouk. This device utilizes either a
rotating or a stationary tube member for supplying air under pressure
to fill the voids created in a traveling felt as it expels water
under the influence of centrifugal force. In order to generate suffix
client centrifugal force for water removal, a very small diameter tube
or roll is required. Accordingly, for reasons elaborated above, felt
conditioning by use of centrifugal force and by moving air through
the felt are physically incompatible techniques and cannot be used
together with advantage.
The present invention is directed to a felt condo-
toning system in which air under pressure is delivered to a
felt for removal of water and trapped substances such as paper
fibers, clay, and the like accumulated in the felt in the
course of removing water from a paper or board web being
formed.
According to the present invention there is provided
an apparatus for conditioning a paper machine felt having a
paper side and a back side comprising a stationary air supply
plenum chamber defined by upstanding front, rear, and end
walls and extending substantially the entire width of the felt
in the cross machine direction, said plenum chamfer having an
open end lying in confronting relationship with the back side
of the felt, a plurality of rib members mounted across the
open end between the front and rear walls of the plenum for
engaging the back side of the felt being conditioned said rib
members each lying along a given radius of curvature with
respect to the open end of the plenum, said rib members being
spaced from each other in the cross machine direction to
define a plurality of air passages extending from the front
wall to the rear wall of the plenum and having the same radius
of curvature as the ribs so that the open end of the plenum
chamber communicates with the back side of said felt thereby
permitting air flow from plenum through the felt with minimum
obscuration by the ribs, means for sealing the peripheral
edges of the open end with the felt to prevent escape of air
from the plenum along the surface of the felt, and means for
supplying conditioning air at a pressure of between 3 and 15
inches of Mercury to the plenum chamber to flow there through
in a direction substantially radially of the felt and for
passing through the felt at a flow rate of from 7 to 25 aim
per square inch of plenum opening to remove dirt and water
aye
from the felt.
Thus according to the invention, a stationary elf
supply plenum chamber is located at the back side of the felt
for delivering conditioning air to the felt. Thy air outlet
from the chamber is pitted with a plurality of support ribs
for engaging and spreading the back side of the traveling
felt as conditioning air flows in a radial direction through
the felt. Preferably, hot air from a convenient source such
as the final dryer section of the machine is compressed and
delivered to the air plenum chamber as pressurized condition-
in air. The interior of the plenum is fitted with vanes for
directing conditioning air radially toward the felt. In the
system there is only minor loss of air temperature and there
is negligible pressure differential before heated and pressure
iced air passes through the felt for removing water. The hot
air reduces water viscosity which facilitates water removal
from the felt.
In a preferred form of the invention the felt sup-
porting ribs may be arranged in a herringbone pattern", i.e.,
at an acute angle to the machine direction in order to spread
the felt as it is being conditioned. According to the invent
lion the arcuate supporting ribs have a relatively small ray
dips of curvature and therefore are able to take advantage of
centrifugal force as an aid in water removal it being under-
stood that centrifugal force only aids in removing saturation
water from the felt thereafter being of negligible value.
Suitably, the rib members have a curved surface in the cross
machine direction for engaging minimum surface area of the
felt back side. Preferably, the front terminal portion of
each rib member is displaced in the cross machine direction
from its rear terminal portion a distance equal to twice the
cross sectional diameter of the rib.
Jo
.9
As felts continue in operation, they accumulate
dirt which reduces felt permeability and it is therefore
necessary to reduce the volume of air delivered to the felt to
avoid increasing air pressure which would lift the felt away
from the supporting ribs of the air supply plenum. According
to the present invention, the volume of conditioning air pass-
in through the felt is adjusted by monitoring plenum air
pressure and felt tension.
In another aspect thereof the present invention pro-
vises an apparatus for conditioning a traveling endless felt having a paper side and a back side and being installed in the
press section of a paper making machine for absorbing water at
its paper side from a paper web as the felt web travel through
a press nip comprising a plenum chamber defined by upstanding
wall members joined in an air tight manner, means for supply-
in air under pressure to the plenum chamber said plenum champ
bier having an open end extending substantially the entire
width of the felt and lying in spaced relationship with the
back side of the felt, a plurality of ribs extending across
the open end of the plenum chamber along a common radius of
curvature and engaging the back side of the felt, said ribs
being directed to spread the felt to open its interstices to
the water removal action of conditioning air, said rib members
being spaced from each other in the cross machine direction to
define a plurality of air passages extending across the open
end of the plenum and having the same radius of curvature as
the ribs so that the open end of the plenum chamber commune-
gates with the back side of said felt thereby permitting air
flow from plenum to felt with minimum obscuration by the ribs,
means for supplying pressurized conditioning air to the plenum
chamber, means for directing the conditioning air in a radial
direction through the plenum chamber and through the felt to
is
- 7
~2382~.~
remove water and girt therefrom so that the felt returns to
the press nip in a dry condition and means for sealing the
periphery of the open end of -the plenum with the felt to pro-
vent escape of air from the plenum along -the back side surface
of the felt. Suitably, said air supplying means further pro-
vises a maximum of 3" water pressure drop in the conditioning
air as it enters and moves across the plenum. Desirably, the
apparatus further includes a load cell cooperating with a felt
roll to monitor felt tension and a movable stretch roll for
changing felt tension, and means cooperating with the Toad
cell for mowing the stretch roll in order to maintain constant
felt tension. Preferably, an apparatus which further includes
means for reducing the volume of air flowing through the felt
at constant pressure as felt permeability decreases. Suit-
ably, an apparatus in which the plenum chamber has an egg
shaped cross-section characterized by a small radius of
curvature at the open end and a large radius of curvature at
the rear end. Desirably, an apparatus in which the plenum
chamber has an egg-shaped cross-section characterized by a
small radius of curvature of between 2 and 5 inches at the
open end and a large radius of curvature of between 6 and 14
inches at the rear end. Preferably, an apparatus in which the
plenum chamber has an egg-shaped cross-section characterized
by a small radius of curvature of 3 to 3-1/2 inches at the
open end, and a large radius of curvature of between 6 and 14
inches at the rear end.
More preferably, an apparatus in which the rear end
of the plenum is provided with an elongated opening and a
tapered duct for furnishing air there through to the plenum.
Suitably, an apparatus in which the opening is between 3 to 3-
1/2 inches measured along the curvature of the plenum
between points a and b. Desirably, an apparatus in which the
Jo
- pa -
3 I
opening is between 3 and 12 inches measured along the radius
of curvature.
The present invention also provides a method for
conditioning paper making felts comprising the steps of supply-
in pressurized air to a plenum chamber having an open end
defining a curved surface having a radius of curvature of
between 2 and 5 inches for guiding a felt to be conditioned,
and delivering from the plenum chamber of sufficient volume
air from between 40 and 120F., between 3 to 10 ysi gauge and
lo at a flow rate of between 7 to 25 aim per square inch of felt
passing over said open end. Suitably, a method including the
step of maintaining the tension in the felt at greater than
the product of the plenum air pressure in pounds per linear
inch times the radius of curvature of the open end.
The present invention again provides a method of
conditioning a paper making felt having a back side and a sheet
side comprising the steps of moving the felt through a given
radius of curvature defined by the open end of an air plenum
chamber, supplying pressurized air at a range of 3 to 15
inches of Mercury to said plenum chamber at a flow rate of 7
to 25 aim per square inch of plenum open end, directing said
air from the plenum chamber through the back side of the felt
at said flow rate to condition the felt, and sealing the felt
to the marginal edges of the open end of the plenum to prevent
escape of air from the plenum along the surface of the felt.
The invention thus provides a felt conditioning
system for a paper machine which removes the water absorbed by
the felt each operating cycle so that the machine operates
with a dry nip at the press rolls and with higher nip
pressure.
The invention also provides a felt conditioning
system which engages the back side of the felt and does not
Jo
- 7b
I 9
wear the pauperized nap of the felt.
The invention again provides a felt conditioning
system which effectively provides a sufficient volume of
heated air for removing water and dirt from the felt.
The invention further provides a felt conditioning
system which spreads the felt in a cross machine direction to
promote removal of water and dirt.
The invention again provides means for adjusting the
volumetric flow of conditioning air at constant pressure in
order to maintain substantially constant felt tension.-
A preferred embodiment of the invention has been
chosen for illustrating and describing its principles and is
shown in the accompanying drawing in which:
Figure 1 is a schematic view of a press section of a
paper making machine in which the felt conditioning system of
the invention is installed.
Figure 2 is a detailed schematic view of a felt con-
ditioning system of the invention installed in the press sea-
lion of a paper making machine.
Figure 3 is a front elevation of a felt conditioning
air plenum chamber according to the invention.
Figure 4 is a section view of the plenum taken along
line 4-4 of Figure 3.
Figure 5 is a fragmentary top plan view of the eon-
ton section of the plenum illustrating the felt support ribs.
Figure 6 is a schematic view illustrating the means
for maintaining substantially constant tension in the machine
felt and substantially constant air pressure in the condition-
in air plenum chamber.
- 7c -
FIGURE 7 is a fra~nentary perspective view of a modified
Penn according to the invention.
FIGURE 8 is a section view of the plenum of FIGURE 7.
FIGURE 9 is a side elevation Al view of a modified plenum
according to the invention.
FIGURE 10 is a section view taken along line 10-10 of
FIGURE 9.
Referring now to the drawing and in particular to FIGURE
], I have illustrated the press section 10 of a paper making machine
including an unsupported board sheet web W passing through the nip
of cooperating press rolls 14, 16 along with endless felts 18, 20
which remove water and a residue of fibers, clay, etc. from the board
sheet. Each felt is supported over a plurality of felt rolls 22,
and guiding rolls 24 and passes a felt conditioning station 26 having
the felt conditioning system 28 of the present invention. A save
all collection pan 30 collects and drains water and dirt removed from
the felt at each felt conditioning station. It is to be understood
that only one felt conditioning system is needed for each press felt.
'the felt conditioning stations shown in FIGURE 1 are typical however
they may be located at any accessible point of travel. A shower 29
fur flooding thy felt is located upstream of each felt conditioning
station.
Referring now to FIGURES 2 to 5 the felt conditioning
system according to the invention comprises a plenum chamber 32 in
the form of a box-like structure with top 34, front 36, rear 38, and
end 40, 42 walls joined in cry suitable air tight manner. An air
supply header 44 is preferably located in one of the end walls as
82~g
shown in FIGURES 2 and 3. Air directing vanes 46 are positioned within
the Penn between the front 36 and rear 38 walls for the purpose
of directing the conditioning air in a radial direction toward and
through the felt. If desired an air supply header may be located
in each end wall of the Penn chamber and in this case air directing
vanes cooperate with each header to divert conditioning air radially
toward the felt.
As shown in FIGURES 3 - 5, the felt c~ndltioning plenum
includes an open end 48 defined by a plurality of ribs 50 extending
along a predetermined radius of curvature from the front wall 36 to
the rear wall 38 of the plenum. The ribs are preferably fabricated
of steel rods having a circular cross section to achieve minimal Eric-
tonal contact with the felt and to minimize the area of felt obscured
by the ribs during the felt conditioning operation. Each rib is secured
at its front and rear terminal portions 52 and 53 to front and rear
plenum walls. A metal shield 54 covers the front and rear terminal
portions of the ribs 50 to prevent abrasion of the felt. Spaced stiffen-
in bars 56 support and maintain desired spacing between the adjacent
ribs.
In order to aid spreading of the felt during the conditioning
operation, the support ribs are oriented away from the machine center
line at an acute angle in the machine direction. Therefore as the
felt moves over the angled support ribs in the direction indicated
by the arrow in FIGURES 4 and 5, the felt spreads in the cross machine
direction to open its interstices to allow more efficient water removal
by the conditioning air. In order to provide uniform elf flow to
all sections of the felt, I prefer orienting the support ribs so that
the rear terminal portion 53 of each rib is displaced in the cross
machine direction from its forward terminal portion 52 a distance
approximately twice its cross sectional diameter.
3~L9
n~ls preferred relationship is shown best in FIGURE 5
where arrow A represents the machine direction and where the front
terminal portion 52 of rib 50 is displaced two diameters Ed in the
cross machine direction from its terminal portion 53. Iris spacing
and orientation of the ribs is essential to attaining the uniform
open area of the felt in the cross machine direction.
For felt conditioning, a press felt laden with water and
dirt received from the board web and from felt saturating showers
is trained over the open end of the conditioning plenum. As described,
the support rods being divergent in the direction of felt travel spread
the felt in the cross machine direction opening its interstices to
the purging action of the conditioning air. Heated air preferably
taken from the final dryer section of the machine is compressed and
introduced through air inlet 44 into the plenum chamber 28 thereafter
passing radially through the felt for removing water and dirt as shown
by arrows in FIGURES 3 and 4.
For ease of fabrication the supporting ribs forming the
open end of the air plenum chamber may be formed of a stainless steel
plate rolled to the desired radius of curvature with the supporting
ribs formed by cutting slots in the rolled plate. The ribs formed
in this manner have their lateral edges machined so that each rib
has a curved surface in engagement with the traveling felt. In this
form of the invention the ribs are also oriented in a divergent manner
with the forward terminal portion of the rib spaced twice its effective
cross sectional diameter from its rear terminal portion in the cross
machine direction.
It should be pointed out that the outer edges of the plenum
open end are provided with sealing strips 58, 60 whisk engage the
I, .
-- 10 --
32~L~3
lateral edges of the felt to prevent lateral escape of air from the
plenum.
In FIGURES 7 and 8, I illustrate a modified form of plenum
chamber 80 with side walls 82, 84 having a generally erg shaped cross
section characterized by an open end 86 having a small radius of curve-
lure r and an enclosed rear section 88 having a large radius of curvature
R. my this plenum chamber construction the felt F as it moves over
the open end conforms to the small radius r so that the felt tension
T is kept at a Mattel value for a given air pressure. Wherefore,
the full advantages of the invention are achieved by directing the
felt over as small a radius as possible with lull flow of air at a
given pressure through the felt without the necessity of increasing
felt tension. To provide an air seal I prefer to begin felt contact
with the plenum chamber a small distance, say 2 inches, before point
a and end felt contact a similar distance past point b in FIGURES
7 and 8.
In practice an egg shaped plenum 80 may have an open
end 86 defined by a small radius of curvature r of between 2 and 5
and preferably 3 to I inches with an opening of 3 to 12 and preferably
3 to I inches along the curvature between points a and b. The
rear section 88 of the plenum chamber has a larger radius of curvature
R of between 6 and 14 inches to provide a plenum of sufficient volume
to accommodate the value of air required for purging the felt. Air
flow may enter the plenum through a suitable end opening as in the
enbodinent of FIGURE 3. The outer surfaces of side walls are curved
for rigidity. The open end of the egg shaped plenum chamber is fitted
with a plurality of ribs 50 in the same arrangement as FIGURE 5.
The plenum sidewalls 82, 84 extend the full width of the machine as
with FIGURE 3. With a plenum chamber in these ranges of dimensions
and having an air pressure of between 3 to 10 prig, preferably 3 to
7 PRIG and a temperature between 40 and 12C~F. I achieve an air flow
through the felt of 7 to 25 aim per square inch of air opening at
open end of plenum chamber. This air flow range is sufficient to
purge water from felts of 20 to 120 inches (water gauge) permeability.
Additionally, this air flow range and felt purging is achieved regardless
of machine speed, a major advantage of the present invention.
In FIGURES 9 and 10 I illustrate a further modification
of the present invention comprising an egg shaped plenum 80 of FIGURES
7 and 8 with a tapered air supply duct 90 furnishing purging air through
an opening 92 extending the full length of the large end of the plenum.
The mEKimUm pressure of conditioning air is a function
of felt tension and radius of curvature of the conditioning zone.
With a given radius of curvature, it is necessary to maintain felt
tension at a known value so that conditioning air has sufficient pressure
for effective cleaning of the felt. For proper operation, the tension
in the felt is greater than the product of the plenum air pressure
in pounds per linear inch times the radius of curvature (inches) of
the plenum open end. As a new felt is being used it tends to stretch
or creep and it is necessary to take up the slack to maintain constant
felt tension. Accordingly I provide an Emery load cell 62 (FIG. 6)
or a strain gauge at a felt roll 22 journal to detect any change in
felt tension. The load cell cooperates with a viably stretch roll
64 through an actuating diaphragm 66 to restore desired felt tension.
As shown in FIGURE 6, load cell 62 detects felt tension and signals
a differential pot 68 which compares the signal to a reference value
for felt tension. If the felt tension is below a desired value the
differential pot will actuate an air valve 70 admitting compressed
air to the diaphragm 66 which moves slid ably mcunt0d stretch roll
64 to restore the tension of felt 20 to the desired value. A bleed
- 12 -
3L2 3~3~L~3
valve 72 allows for reducing diaphragm pressure should it be necessary
to reduce felt tension in an operating emergency.
A press felt normally accumulates embedded dirt in the
course of its useful life which cannot be removed resulting in decreased
permeability of the felt to conditioning air. Accordingly, as a felt
ages the pressure of a given volume of conditioning elf through the
felt increases tending to lift the felt off the supporting ribs so
that conditioning air vents at the edges of the felt without passing
through it. This being the case it is necessary to provide means
for maintaining the same conditioning air pressure and for reducing
the volume of air flow through the felt as it ages. As shown in FIGURE
6, a pressure transducer 74 in air plenum 28 detects variations in
air pressure in the air supply plenum chamber. The pressure transducer
signal is compared by the differential pot 68 to a standard value
for plenum air pressure. If the signal exceeds a predetermined inane-
mint, the differential pot will open or close a damper valve 76 in
the plenum air supply system 78 to change the volume of air entering
the air supply plenum at constant pressure. In this manner there
it no air pressure build up in the plenum chamber as the felt loses
permeability. It should be observed that permeability of new felts
varies and the foregoing system may be adjusted for desired values
of felt tension and plenum air pressure.
In operation, the felt conditioning system according to
the invention is applied to each felt used in the press section of
a paper making machine. Each felt emerges from the press nip laden
with water absorbed from the paper sheet and carrying dirt picked
up from the sheet. As the felt approaches the felt conditioning station
is is flooded with a shower to prepare it for purging. The felt then
passes over the air purging plenum opening through a predetermined
radius of curvature with the back side of the felt engaging diverging
.. " .
I 9
ribs which spread the felt and open it to purging action of the condition-
in air removing water and dirt. Air pressure (gauge) in the plenum
chamber may be in the range from 3 to 15 inches of Mercury and preferably
is 7 to 8 inches of Mercury. Air under pressure and at elevated tempera-
lure flows through the plenum chamber in a radial direction and through
the felt to condition it. Water removal is aided be centrifugal force
developed in the felt as it traverses the conditioning station at
high velocity. A felt conditioning system having a four inch radius
of curvature at the conditioning zone provides considerable operating
advantages over a conventional suction box having a one inch wide
suction slot. The felt conditioning system provides a ten-fold increase
in felt dwell time in the conditioning zone permitting Moe more effective
purging of the felt. The system also eliminates the need for expensive
vacuum pump and the approximately 100,000 gallons of seal water recolored
by a vacuum pump in a suction box system. Tension in the felt is
maintained at a constant value by means a load cell cooperating with
a diaphragm operated tension roll which adjusts for creep occurring
in the felt through continuous use. Mbveover, to adjust for gradual
loss of permeability as the felt ages I provide a pressure monitoring
system to sense build up of air pressure in the conditioning air plenum
chamber with decreasing felt permeability. As this occurs, the volume
of air flow into the plum chamber is decreased. In this manner
I achieve maximum conditioning air pressure for a constant felt tension.
From the foregoing description it will be understood the
present invention provides a new and improved system for supplying
conditioning air through a paper making felt for purging a felt so
that the felt arrives at the press nip in a dry condition.
- - 14 -
