Doctor blade, drying or sealing assembly

The described apparatus has utility as a doctor on a paper machine, as a drying element in an aircap dryer and as a sealing element in such a dryer. It involves a foot portion having an outer surface generally conforming to the curvature of the cylinder surface on which it is used. It is resiliently supported in close juxtaposition to the cylinder surface so that a gap diminishing in thickness towards one longitudinal edge of the foot portion is provided. At least two rows of nozzles communicate through the foot portion in order to project pressurized air into the gap. Each nozzle is angled relative to three orthogonal planes, one of which is tangent to the cylinder surface at the point where the nozzle axis meets the surface, another of which is parallel to the longitudinal edge of the foot portion. The flow from the nozzles is such as to support the foot portion and to seal the support flow from any disorganized flows. The jetting air will tend to aid in doctoring a paper sheet from the cylinder surface or to dry a paper sheet passing thereunder or to seal an opening from the passage of air therethrough if the opening is defined by a cylindrical surface.

The present invention is particularly concerned with the utilization of 
pressurized fluids in the paper making industry and is particularly useful 
in drying and doctoring operations. 
As is well known paper making in the past was as much an art as it was a 
science. However, with the high speeds of present day paper making the 
production of good quality paper can only be enhanced if the paper maker 
himself is very talented. Computer controls and improved technology are 
moving to decrease the downtime of the paper machine and to make it more 
efficient and effective. The paper maker thus is able to devote more of 
his time to his "art." One of the problem areas which still has a good 
deal of downtime associated therewith is the creping operation which 
involves the use of a doctor blade on the Yankee roll in the dryer stage. 
Experience shows that there is considerable wear both to the Yankee shell 
and the doctor blade although the doctor blade has more wear associated 
therewith. Both the blade and the shell have to be reground periodically 
to bring them back into dimensional congruity. The doctor blade regrinds 
can be carried out with little machine disruption since they are replaced 
on the run every 20 to 120 minutes. But the Yankee regrinds are 
time-consuming, frequent and costly, and, of course, both the doctor blade 
and the Yankee have structural limits which determine the total amount of 
material that can be ground off. Since the Yankee roll is pressurized it 
is understood that as the shell thickness decreases so must the internal 
pressure and/or the loading. This greatly limits the usefulness of this 
very costly item, most Yankees now costing over one million dollars to 
replace. 
In many mills the bottleneck for increased speed potential is the dryer 
section. The dryers, of course, may include the Yankee mentioned above 
which is steam heated to vaporized moisture from the paper web thereon. In 
some instances dryer performance is aided by the provision of a peripheral 
hood which utilizes nozzles and high velocity air jets. These high 
velocity hot air or gas hoods or caps are placed over standard steam 
dryers to improve the rate of uniformity of drying. Because there is a 
boundary layer of moisture upon the web which impedes heat flow to and 
moisture flow away from the sheet, high expenditures of energy are 
required in order to obtain the hot gas velocities in the region of 10,000 
to 20,000 feet per minute that are needed to decrease the boundary-layer 
effect. The temperatures of the heated air used in the range of, say 
600.degree. F. to 800.degree. F. An attendant problem with hot air caps 
is, of course, the removal of the vast volumes of high temperature 
saturated air. The balance between air flowing to the sheet and back 
pressure involved in removing the mositure and air has always been a 
design compromise. 
It has been known in the past to use doctor blades supported on a film of 
air. Minton in his Canadian Pat. No. 223,760 issued Apr. 5, 1922 described 
and claimed such a device but it never enjoyed any commercial success. In 
the Minton doctor a relatively narrow structure was provided with a 
passage ending in a port near the end of the toe portion. Air escaped at 
the port in the form of a jet adjacent a cylinder and the jet would 
allegedly strip the web from the cylinder. A portion of the fluid under 
pressure would back up to form a cushion between the toe and the cylinder 
to preclude contact of the doctor with the cylinder. It is noted in 
particular that the passage is in a plane perpendicular to the cylinder 
axis and hence the air jetting from the port will be unstable and would 
tend to back up on either side of the port and would not be organized. 
Thus the action of the doctor and of the creped paper resulting from 
operation thereof would not be consistent and predictable. In addition, no 
cleaning of the cylinder surfaces either to the front or back of the toe 
portion would be provided. 
The present invention is intended to overcome the problems of the prior art 
with respect to doctoring and drying. The structure of the present 
invention is such that it can be used as an air-supported doctor, as an 
element in a high-velocity hot air dryer and as a sealing element in an 
air cap type dryer. The present invention utilizes air exitting from a 
curved surface positioned along the length of a cylinder to support the 
surface away from the cylinder. The jetting air is angled obliquely 
relative to the cylinder axis so that its flow is organized and 
predictable. In addition a portion of the jetting air is used to 
effectively seal the remainder to the jetting air from escaping in an 
unwanted and hence inefficient flow pattern. 
In its broadest aspects the present invention contemplates apparatus for 
use in conjunction with a cylinder surface rotatable about an axis. The 
apparatus comprises a foot portion having an outer surface which 
substantially conforms to the curvature of the cylinder surface and means 
for resiliently supporting the apparatus in close juxtaposition to the 
cylinder surface whereby a gap of thickness diminishing progressively 
towards one longitudinal edge of the foot portion is formed between the 
outer surface and the cylinder surface. At least two rows of nozzles 
communicate through the foot portion to exit therefrom at the outer 
surface. A source of pressurized fluid is connectable to the nozzles, each 
of the nozzles being directed obliquely to three orthogonal planes, one of 
which is parallel to the one edge and another of which is tangent to the 
outer surface at the juncture of the nozzle axis with the outer surface. 
Thus, fluid which issues from the nozzles in the row farthest from the one 
edge will coact with the fluid issuing from adjacent nozzles in the same 
and adjoining rows to act as a seal to preclude fluid from the remaining 
nozzles from passing thereby.

FIGS. 1 and 2 are intended to depict the basic configuration for the 
present invention, without any particular application such as doctor, 
dryer, or seal in mind. In some instances in the Figures distances and 
separation have been exaggerated for effect. 
FIG. 1 shows a cylinder 10 of radius R for rotation in the direction of the 
arrow A. In close juxtaposition to the cylinder surface 12 is the 
apparatus 14 of the present invention. It is provided with a foot portion 
16 having an outer surface 18. Resilient and adjustable means 20, 
connected to a solid support structure which, under load, maintains the 
juxtaposition referred to above so that the gap diminishes progressively 
towards one edge, such as 22, of the foot portion 16. At least two rows 
24,26 of nozzles 28 communicate through the foot portion 16 to exit at the 
outer surface 18 thereof. A course (not shown) of pressurized fluid, such 
as air, is connectable with the nozzles 28 through hose 30 and plenum 32, 
the latter formed within housing 34 sealed to foot portion 16. As is seen 
in FIG. 1 and particularly in FIG. 2 each nozzle is directed obliquely to 
three orthogonal planes P-1, P-2, P-3. Plane P-1 is parallel to the 
tangent at the cylinder surface at the juncture of the nozzle axis with 
the cylinder surface. Plane P-2 is parallel to the edge 22 and, of course, 
plane P-3 is perpendicular to the planes P-1 and P-2. As seen in FIG. 2 
the centerline of a nozzle 28 makes an oblique angle .alpha. with plane 
P-1, an oblique angle .delta. with plane P-2 and an oblique angle .beta. 
with plane P-3. 
Referring to FIG. 3, air exitting from the nozzles in row 24 as flow 34 
will thus issue in such a manner that it tends to be directed towards edge 
22 but at an angle oblique thereto. The overall accumulated flow thus has 
components parallel to the edge 22 as well as perpendicular thereto. The 
air exitting from the nozzles in row 26 as flow 36 is initially directed 
in the same general direction as that issuing from nozzles 24. However, 
the air from nozzles 26 will encounter the air flow from the nozzles in 
row 24 and this air flow from the nozzles in row 26 will tend to have a 
greater flow component parallel to the edge 22. In effect this air is 
turned more towards a parallel flow direction and it acts as a "skirt" or 
seal along the back portion of the foot 16. This is very advantageous 
since the sealing air ensures that there is no backward escape of air from 
the nozzles, even when the cylinder is rotating toward edge 22 and this 
sealing air also prevents the intrusion of dust and debris into the small 
gap formed between the surface 18 and the surface 12. Needless to say, the 
air and the flow thereof operating in the gap g maintains the gap since 
the structure and its operation is analogous to the structure and 
operation of an air bearing or a skirtless air cushion. 
The flow pattern and the properties thereof as described above can be put 
to practical use in the paper industry. The structure described above can 
be used in the paper drying stage to impinge high velocity air at the wet 
web from very close proximity; the sealing effect can be used to prevent 
detrimental escape of the drying air from dryer hoods; and the very close 
juxtaposition of the foot portion to the cylinder surface permits the 
structure to be used as a doctor for the removal of paper from a rotating 
cylinder, especially for the creping of the web from a Yankee roll. This 
is particularly useful in the production of high bulk tissue sheets which 
are formed and dried to high degrees of dryness with minimum mechanical 
work being exerted on the paper web. Such work is detrimental to softness 
and strength of the web but it has been found that differential creping 
action is of benefit to softness and strength properties. 
These various applications of the present invention will be depicted in 
FIGS. 4 to 8 and described hereinafter. It is noted that most of these 
Figures show the application of the present invention in essentially 
schematic or sectional form. It is understood that the configuration is 
three-dimensional and, for example, that the individual apparatus of the 
present invention would probably extend along the full length of any 
cylinder shown although sections of short length can be employed to remove 
trim from the edges of the dryer cylinder without wear problems. 
FIGS. 4 and 4a show a very basic concept for use as a doctor for a Yankee, 
or other, cylinder 10 rotating in the direction of the arrow. The 
apparatus of the present invention is resiliently mounted on a rotatable 
shaft 38 which is biased as by a torsion spring, air bag or piston (not 
shown) to bring the foot portion 16 into close juxtaposition with the 
cylinder surface 12. A gap g is formed between the surface 12 and the 
complementary curved outer surface 18 of the foot portion, the gap g 
diminishing progressively in thickness towards the edge 22 of the foot 
portion 16. The apparatus is provided with a plenum 32 which is 
continuously supplied with a pressurized fluid, usually compressed air at 
various pressures, say 10 to 40 p.s.i. It is understood that doctoring 
applications require higher loads and extremely close juxtaposition to the 
web cylinder. The air is permitted to escape from the plenum through 
nozzles 28 in a pair of parallel rows 24 and 26, the nozzles being 
oriented as depicted in FIGS. 1 and 2 and having a flow pattern as 
depicted in FIG. 3. The issuing air forms a thin wedge of fluid between 
surfaces 12 and 18 as shown in FIG. 4a, the thickness of the web 
diminishing in the direction of projection, that is, towards edge 22. The 
nozzles are designed so that the air issued therefrom at or near 
supersonic speeds and exits from the gap with sufficient velocity and 
force to break the adhesion of the web of paper 40 to the cylinder surface 
12. The action of the air-support doctor is such as to impart a creping 
configuration to the web or to assist, in various degrees, mechanical 
action of the leading edge of the blade, as shown in FIGS. 4 and 4a. It is 
important to note that the air wedge formed between the surfaces 12 and 18 
is sufficient to prevent contact between the doctor and the cylinder 
surface and the air exitting from the front edge of the foot portion 
assists in varying the degree of contact between the paper and the doctor. 
Since there may be somewhat random intrusions of pressurized fluid into 
the region bounded by the paper, the cylinder surface 12 and the edge 22 
the resultant creping action can be considered to be differential along 
the length of the edge 22 and, as pointed out above, this is a desirable 
effect. Because of the orientation of the nozzles all of the air is 
effectively used for support and creping interaction as it all exits in a 
preferential direction. This flow can be effective in cleaning doctoring 
surfaces of any fibre or lint buildup. Doctoring is achieved over the full 
length of the doctor and is readily controlled through variations in air 
pressure and doctor loading. Because there is no contact between the 
doctor and the cylinder there will be no wear and no chance of damage to 
the sheet of paper by worn, chipped or damaged equipment. 
It is understood that the resultant radially outward forces generated by 
the support wedge are counterbalanced by the nozzle unit and support 
weight together with resilient mounting and loading systems of the 
apparatus on shaft 38 whereby the gap g is properly maintained at 
equilibrium. 
FIGS. 5 and 5a show a somewhat more sophisticated version of the doctor 
apparatus of FIGS. 4 and 4a. As with the previous embodiment the cylinder 
10 rotates in the direction of the arrow at its usual speed of roughly 
4000 feet per minute, carrying paper web or sheet 40 on its outer curved 
surface 12. Foot portion 16 is resiliently biased towards cylinder surface 
12 by resilient means 20, similar to that shown in FIG. 1, and L-shaped 
arm 42 is affixed to shaft 38 adjacent and parallel to cylinder 10. A 
plurality of spaced spring fingers 44 extend from the end of one leg 42a 
of arm 42 and are removably attached to housing 34 of the doctor 
apparatus. Fingers 44 are spaced along the length of the arm 42 and the 
doctor apparatus so as to effectively bias the doctor therealong toward 
the cylinder. Adjusting means 46, shown as a nut passing through finger 44 
to be received in a threaded hole 48 in leg 42b of arm 44 permits the load 
on finger 44 and hence the load on the doctor apparatus to be adjusted. In 
this manner the gap g can be adjusted so that it remains constant or it 
can be varied as required to account for discrepancies in the surface 
smoothness or position of cylinder 10. 
The foot portion 16 is provided with parallel rows 24 and 26 of nozzles 28 
which are oriented and operate as previously described. In addition, 
however, the surface 18 is stepped, as at 50, in order to accomodate a 
blade 52 which is removably pivoted to the foot portion as at 54 and which 
extends outwardly towards and possibly beyond edge 22 along the length of 
the doctor. A third row of nozzles 56 communicates through foot portion 16 
from plenum 32 in order to supply pressurized air to the space between 
blade 52 and the outer surface of the stepped portion 50 of surface 18 to 
thereby provide further resilience to the blade support system. 
In operation the diminishing gap g is maintained by the air issuing from 
the nozzles in rows 24 and 26 and most of that air exits from the front 
edge 58 of blade 52 to break the adhesion of the paper 40 from surface 12 
and to assist in giving it its creped effect (see FIG. 5a). The air from 
nozzles 56 exits adjacent edge 22 from between stepped portion 50 and 
blade 52 and serves to provide a resilient support to the blade as well as 
to create a source of pressurized air to deflect the creped paper away 
from the doctor. Thus the chances of difficulties arising from 
interference by the doctor support with the creped paper is reduced and 
the creped paper is in a better position to be fed to subsequent 
operations. 
There are many variaties of mounting means for the doctor apparatus, 
incorporating load adjustment, air feed pressures, etc. In fact it is 
possible by using an air bag in series with the plenum and the air supply 
to render the loading automatically appropriate to the pressure conditions 
present at the gap g. These variations in the mounting means and other 
variations in the doctor apparatus itself will be readily apparent to 
someone skilled in the art who is faced with a custom installation of the 
apparatus for a specific cylinder. Needless to say, all mountings provide 
a means of quickly releasing built up paperjams or clearing of the drier 
or feeding of paper by pivoting away from the drier for maximum clearance. 
Opening of the hood of an air cap dryer for major cleaning of paper 
build-ups will be by various lifting systems or automatic opening 
arrangements now in use. 
FIG. 6 shows a dryer situation in which the present invention can be 
utilized to achieve high efficiency drying of paper. A standard press 
system, depicted by reference number 60 feeds a sheet of partially dried 
paper 50 to the inlet 62 of a hot gas cap dryer. The standard gas cap 
system is depicted by reference number 64 and need not be considered in 
full herein. It suffices to say that in the configuration of FIG. 6 a 
Yankee or similar dryer cylinder 10 having an outer surface 12 is 
supported for rotation in the direction of the arrow and it is covered 
over at least 50 percent of its surface by the cap 66. Exterior to cap 66 
is a high pressure burner 68 fed by compressor 69 with air extracted from 
the interior of the cap. The burner 68 provides high pressure hot gases of 
sufficiently low relative humidity for use in the dryer devices 72, four 
of which are shown. A conduit 70 leads from the burner to each dryer 
device 72, the free end of each conduit comprising a resilient mounting 
means 74 analogous to resilient means 20 of FIGS. 1 and 5. In fact, it 
might be found that an intermediate position of the resilient mounting 
means may permit the air exiting from the nozzles in devices 72 to assist 
in threading the sheet over the cylinder surface. 
Dryer devices 72 are completely analogous to the apparatus 14 shown in FIG. 
1 and need not be described in detail. It suffices to say that high 
velocity hot air issues from the nozzles in each foot section to float the 
foot portion on a wedge of fluid and to impinge on the paper sheet as it 
travels past. The jetting air cracks the boundary layer of moisture on the 
sheet due to the juxtaposition of the dryers from the sheet, in the range 
of, say 1/8 to 1/4 of an inch, and the high velocity air. Tests have shown 
with other impingement devices that 35 to 40 percent drying efficiency 
increases can be realized with jets moved from 1 inch to 1/2 inch from 
dryer or web surfaces. Thus a more efficient drying system is achieved 
while at the same time there is a reduction in the power required over 
existing systems. The gap between the nozzles in existing air dryers and 
the sheet must be be fixed at some compromise clearance for air removal 
balance with turbulent and unorganized flows. This gap is extremely large 
in relation to the gap achievable with the present invention and this goes 
a long way to reducing the power required to achieve the desired degree of 
drying. In the present invention an optimum proximity can be achieved by 
automatic balance of the incoming nozzle hot gases impinging on the sheet 
with the required exhaust area required to release the hot air and 
vaporize moisture from the sheet. The organized flow of the angled jets 
will provide the best flow of hot gases with optimum removal conditions. 
Also, since the dryer cylinder is reduced in diameter due to repeated 
grinding the present invention automatically adjusts to the best new 
clearance position. It is also worthwhile noting that with the relatively 
high velocities of the nozzle jets as compared to the dryer surface speed, 
the nozzles need not necessarily be directed in one direction or the other 
with regard to the dryer rotation or web direction. 
In the embodiment shown in FIG. 6 a doctor apparatus 76 is shown exterior 
of the dryer cap 66 to remove the dried paper from the cylinder. The 
doctor apparatus 76 could take the form shown in FIG. 4 or FIG. 5 or it 
could be custom designed to the particular operation. Other similar 
doctors could be used subsequent to doctor 76 to clean the cylinder 
surface of extraneous fibres or debris. 
FIG. 7 shows a second dryer embodiment which utilizes the present invention 
in all of its modes of application, namely drying, sealing and doctoring. 
The dryer of FIG. 7 has an insulated vacuum hood 78 extending around more 
than 50 percent of the periphery of Yankee cylinder 10. At each end of the 
hood 78 a seal device 80 is resiliently mounted so as to direct 
pressurized hot gases or air against the paper sheet 40 at the entrance to 
the hood and at the exit of the hood thereby providing a dynamic 
non-wearing seal for the interior of the hood. Each seal device 80 is 
analogous to the basic apparatus 14 in that it rides on a wedge of 
pressurized air and it need not be further described since there is no 
difference in the air flow. As with the embodiment of FIG. 6 the present 
embodiment is provided with an air supported doctor device 76 which 
removes the dried sheet 40, from the cylinder 10. 
Within the vacuum hood 78 is at least one, and preferably a plurality of, 
dryer devices 82 resiliently mounted so that a narrow gap diminishing in 
the direction of fluid projection is established between each foot portion 
and the paper sheet 40. No further discussion of the structure or mounting 
of devices 82 is required in view of previous discussions. Each device 82 
is connected to a manifold 84 within hood 78 which acts as a source of 
relatively hot and dry high pressure air. The inlet of the manifold is 
sealed to the hood as at 86 and the outlet of the hood is sealed to 
conduit 88. Because of the sealing provided by seals 80 the only airflow 
within the hood 78 can be from manifold 84, through the nozzles in dryer 
devices 82 and hence through outlet conduit 88. Since the outlet conduit 
is connected to a vacuum/compressor pump 90 the interior of hood 78 can be 
maintained at a partial vacuum and hence the rate of evaporation from the 
paper sheet 40 can be considerably increased. Also, since the present 
apparatus works under partial vacuum conditions the temperatures of the 
hot air can be reduced from the temperatures required in an atmospheric 
system such as that shown in FIG. 6. In order to provide an essentially 
closed air system, showers 92 spray cooling water into the conduit 88 to 
initially cool the air in and hence remove moisture by condensation while 
providing the vacuum pump with sealing water and aid in the compression of 
the air. The compressed moist air from pump 90, heated somewhat by 
compression passes through separator 94 to remove the water droplets and 
then passes through heat exchanger 96 to heat and thus relatively dry it 
even further. Thus there will be very little air loss in the system and 
efficiencies will increase. It should be noted that gases heavier than air 
could be utilized to enchance drying by increased gas densities or 
improved properties. 
FIG. 8 shows a third dryer configuration typical of many speciality or 
paper board dryer cylinder configurations except that in the present 
embodiment these dryers are enclosed to provide a system which is 
recirculatory, thereby resulting in enormous savings in energy. As with 
the previous embodiment the present embodiment uses low temperature air 
combined with a vacuum condition to achieve high dryer efficiencies. The 
paper sheet 40 is fed from the paper machine press section 60 around an 
input cylinder 98 at the entrance to the dryer. A pair of air supported 
seal devices, similar to devices 80 already described for the embodiment 
of FIG. 6, are provided. The dryer has a vacuum hood 102, interrupted only 
by the paper input at cylinder 98, the paper output at cylinder 104 
(sealed by air-supported seal devices 100 as well) and the air outlet 106. 
The air outlet 106 is connected to conduit 108 which has cooling water 
spray 110 associated therewith and in turn is connected to 
vacuum/compressor pump 112. The moist air is compressed and fed to a 
separator 114 where the moisture (water) is removed. The compressed air is 
then fed to manifold 116 from which it is distributed to the seal devices 
100 and the dryer devices 118 (distribution being schematically shown by 
the arrows leaving manifold 116). 
As is shown in FIG. 8 the dryer devices are distributed about the portions 
of rolls 120 which support the paper sheet 40. As the sheet passes around 
a roll 120 it is subjected to high velocity impingement of air issuing 
from the dryer devices 118. With the air from the dryer devices being 
exhausted through vacuum/compressor pump 112 a partial vacuum is obtained 
within the dryer hood and evaporation of moisture from the paper sheet is 
enhanced. The fluid exhausted is cooled, dried, recompressed and recycled 
to the drier devices via manifold 116. Thus there is very little loss of 
fluid and the energy input is extremely small in comparison to existing 
dryers. After exitting the dryer hood around the exit cylinder 104 the 
sheet is fed to the reel section 122 where it is wound for subsequent 
finishing. 
As can be seen from the examples of particular applications described 
herein the apparatus of the present invention is extremely versatile. It 
can be used to enchance the efficiency of regular dryers; it can be used 
to seal the inlet and exit ends of a vacuum dryer efficiently; it can be 
used to doctor a paper sheet from a rotating cylinder; it can be used in 
industries other than the paper industry for similar applications; and, in 
fact, it can perform multiple duty as, for example, a dryer and a seal 
device. This last function is performed of course by the two devices 100 
which face the paper sheet, one at the inlet roll 98, the other at the 
outlet roll 104 (FIG. 8). The present invention therefore, while appearing 
to be relatively straight forward in basic structure, is capable of 
alleviating many problems associated with the paper industry, espcially at 
the dryer end of the paper machine and it does provide an additional new 
field of utilizing fluids heavier than air to provide energy savings 
unheard of at the present time where systems are limited to air.