Note: Descriptions are shown in the official language in which they were submitted.
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Field of the Invention
This invention relates to web drying apparatus and is more particular-
ly concerned with a web guiding device for drying apparatus whereby a length-
wise moving web is floatingly supported while being guided around a turn.
Background of the Prior Art
In drying a moving web of paper or the like, it is often desirable
that the web be contactlessly supported during the drying operation, in order
to avoid damage to the web itself or to ink or coating on the web. One common
arrangement for contactless support of a w~b comprises upper and lower sets
of air bars extending along a straight substantially horizontal stretch of
the web. Air issuing from the lower set of air bars floatingly supports the
web, and air issuing from the upper set of air bars steadies the web to maintain
it substantially straight and at a substantially constant distance from the
air bars of both sets. The air blown from both sets of air bars is usually
heated to expedite web drying, and usually the air bar array is inside an
enclosure which is maintained at a slightly subatmospheric pressure by an
exhaust blower that draws off the vapors emanating from the web.
A drying arrangement that operates upon a straight horizontal stretch
of web has the obvious disadvantage of occupying a substantial amount of floor
space, and therefore much attention has been given to web drying apparatus in
which the stretch of web being dried is supported to have a plurality of
stretches that extend more or less vertically. Where the web to be dried can
be passed in contact with drying rolls, it is usually trained around a succession
of such rolls, arranged to define a markedly sinuous path for it. However a
freshly imprinted or freshly coated web of paper or plastic must be contactlessly
supported during drying, and for guidance of such a web along a path that
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occupies minimal floor area the drying apparatus should comprise a plurality
of contactless turning guides cooperating to define a desired path for the
web. It will be evident that contactless turning guides in a web drying appara-
tus should preferably accomplish some drying of the web in addition to
supporting and guiding it~
United States Patent No. 3,279,091 discloses a device for drying
a moving web while contactlessly supporting and guiding it around a tu m .
The device comprises a non-rotating cylindrical shell of less than circular
extent having in its outer surface a series of axially extending air outlet
slots that are spaced apart circumferentially. From each of these slots
heated pressure air flows radially outwardly *o impinge against the inner
surface of a web that is partially wrapped around the shell, to floatingly
support the web while drying it. The shell also has randomly distributed
air holes that provide an exhaust path for heated air. Flanges along the
edges of the web serve as air dams that prevent substantial escape of pressure
air from under the web except through the exhaust holes. The outlet nozzle
configuration is such as to require a very high air horsepower to ensure con-
tactless support of the web, as is apparent from the disclosure that the heated
air issues from the outlet slots "in the form of elongated jets having a
velocity of from 12,000-20,000 fpm and impinges on the bottom surface of the
moving web...". The apparatus has also been criticized (in United States Patent
No. 4,218,833) on the ground that"in order to maintain a workable air cushion
for the moving web, impossibly fine tension control is required since a balance
has to be continuously maintained between the radially outwardly acting forces
on the web arising from the air jets impinging thereon and the radially in-
waTdly acting forces arising as a result of the tension in the web...".
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Another form of apparatus for drying a moving web while contactless
supporting it and guiding it around a turn is disclosed by United States Patent
No. 4,218,833, wherein a plurality of nozzle pairsare arranged in a part-
circular array around which the moving web passes. Each nozzle is a Coanda
nozzle that discharges in one circumferential direction, while a closely
adjacent nozzle with which it is paired discharges in the opposite circum-
ferential direction, so that the two nozzles of each pair discharge circum-
ferentially divergent air streams. The nozzle pairs alternate around the
circumference of the shell with exhaust apertures in the shell surface. Into
these exhaust apertures flow the converging streams from circumferentially
adjacent nozzle pairs. The surface of the shell defines a continuous arc of
uniform radius broken only by the Coanda nozzle pairs and the exhaust aper-
tures. The flow of pressure air circumferentially around the cylindrical
shell surface, between that surface and the web, serves to float the web at
only a small distance from that surface, inasmuch as pressure against the
inner surface of the web is very small unless the web is very close to the
shell surface. ~s a result, the web tends to drag on the shell in some places,
particularly near the exhaust inlets, and the apparatus tends to pose problems
with respect to web flutter and web wrinkling. Furthermore, because of the
small pressure exerted against the web, the device cannot float a web that is
under substantial lengthwise tension.
In the past, various contactless turning guides for running webs have
been devised that were intended for web guidance without regard to any web
drying function and were aimed at minimizing the amount of air flow required
for adequate contactless support of a web as it moved around a curve. In
general, the most satisfactory turning guides were arranged to provide a
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cushion of pressure air between the web and an arcuate surface of the device
and to require a relatively small flow of pressure air for maintenance of
this cushion. In any such turning guide it is of course essential that
pressure against the radially inner surface of the web be uniform all across
the width of the web--particularly under its edge portions--and all along the
arc of its turn. Prior turning guides that were successful with respect to low
air flow and uniform pressure cushion are disclosed in United States Patents
Nos. 4,182,472, 4,197,972 and 4,288,015, all of which have a common assignee
with this application. Although these turning guides were very satisfactory
as web guiding devices, their otherwise desirable ability to operate with low
air flow rates made them unsatisfactory for use as web drying devices~ in~s-
much as a substantial flow of air in contact with the web is needed to effect
the rapid heat transfer necessary for fast web drying~
However) since adequate web drying can often be accomplished in the
straight stretches of web between turning guides, this disadvantage was of
lesser importance than the relatively high cost of such prior turning guides.
As is apparent from the drawings of the three last mentioned patents, the
turning guides that they disclose comprised extruded or cast parts. The
tooling needed to produce some of those parts had to be specialized for each
width and/or radius of turning guide and therefore had to be amortized over the
production of relatively few units. Needless to say, the need for a substan-
tially less expensive contactless turning guide was clearly apparent, but how
to achieve it was far from obvious.
Thus there has heretofore been an unfulfilled need for a substan-
tially less expensive device whereby a lengthwise moving web is contactlessly
supported and guided around a turn, and whereby a substantial amount of web
lZ405Q~
drying can also be accomplished. To be satisfactory, such a device must meet
rather stringent requirements. Insofar as it is intended to function as a
dryer, it should provide for sufficient flow of heated air in contact with the
web to achieve the necessary heat transfer; but it should not require an air
flow that is substantially greater than is adequate for drying, in order to
avoid waste of power. The device should in any case support the web at a
substantial distance outward from its surface, which distance should be at
least .125 in. (32 mm) and is preferably about .25 in. (63 mm). If the web
normally comes closer to any part of the surface of the device than .125 in.,
the web may at times drag against the deviceJ or other web handling problems
may arise. Notwithstanding this rather substantial distance between the web
and the surace around which it is guided, the web must move smoothly and
without flutter. In particular, there must be no more than insignificant edge
flutter thTough a wide range of web tensions and air supply pressures. Since
low cost is an important objective, production of the device should require a
minimum of specialized tooling, and desirably a given device should be capable
of accommodating webs of a substantial range of widths, to minimize both
production and inventory costs.
Summary of the Invention
The general object of this invention is to provide a low cost
device for contactlessly guiding a lengthwise moving web around a turn,
whereby the web is supported at a sufficient distance from the device to ensure
against its engagement thereagainst even though the web is under significant
lengthwise tension, and whereby the web is guided for substantially steady
flutter-free motion.
It is also an object of this invention to provide an inexpensive
web guiding device of the character described which achieves the objectives
just stated and which is also capable of producing a flow of air in contact
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66925-243
with the web that is high enough to ensure adequate heat transfer
for fast drying but is not uneconomically high.
The invention provides a device whereby a web that moves
lengthwise along a defined path is floatingly supported and guided
around a turn in said path, said device being characterized by:
A. a plurality of air bars, each being elongated and having (1)
opposite side walls, and (2) a front wall that is located between
said side walls and has opposite longitudlnally extending edge
portions which are spaced from said side walls to cooperate with
them in defining a pair of air outlet slots, each extending along
the length of the air bar, said edge portions of said front wall
being curved widthwise rearwardly so that each of said outlet slots
comprises a Coanda nozzle for directing pressurized air outwardly
from the interior of the air bar and laterally across said front
wall towards the other outlet slotj B. means defining a plenum
chamber whereby said air bars are supported in lengthwise parallel
relation to one another, extending across the width of said path,
with their front walls substantially lying on a convex arc of one
radius that substantially defines said turn, said plenum chamber
having its interior communicable with a source of pressure air and
communicated with the interior of each air bar at the rear thereof;
C. air dam plates having surfaces normal to the lengths of the
air bars, located adjacent to the ends of the air bars and extend-
ing edgewise across the spaces between them to define air dams
that restrain flow of pressure air outwardly from said spaces in
the directions lengthwise of the air bars; and D. means in the
space between each pair of adjacent air bars defining an outwardly
facing surface that extends laterally from one to the other of said
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66925-243
adjacent air bars and extends lengthwise from one to the other of
said air dam plates, each said surface lying substantially on a
second arc which is substantially concentric with said convex arc
and has a smaller radius.
The invention also provides a device whereby a web that
moves lengthwise along a defined path is floatingly supported and
guided around an arcuate turn in said path by pressure air acting
against the CGncaVe back surface of the web at said turn, said
device being characterized by: A. means defining a plenum chamber
communicable with a source of pressure air; B. means defining a
plurality of elongated substantially flat first front surfaces on
sald plenum chamber (1) extending lengthwise parallel to one
another across the width of said path; (2) lying substantially on
the arc of said turn, and (3) spaced laterally from one another at
intervals around said arc; C. means cooperating with each of said
first front surfaces to define a pair of elongated outlet slots in
communication with the interior of said plenum chamber, each com-
prising a Coanda nozzle extending along the opposite longitudinal
edges of said surface and arranged to direct obliquely outwardly
and laterally convergent streams of pressure air across said sur-
face; D. means defining a plurality of elongated substantially
flat second front surfaces on the device, each having its width
extending across an interval between a pair of said first front
surfacés, said second surfaces lying substantially on an arc con-
centric with the arc of said turn but of smaller radius; and E.
air dam means defining, at each end of each of said second front
surfaces an air dam surface which is substantially normal thereto
and projects forwardly therefrom, whereby air is restrained against
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~2~0S04 66925-243
flow away from the second front surface in directions lengthwise
of it.
The web guiding device can be constructed at low cost by
reason of its being substantially entirely of sheet metal construc-
tion and incorporating more or less convention and standard air
bar structures as nozzle elements. The device is capable of being
readily adjusted to accommodate webs of different widths and to
accommodate web tensions through the range of values from near
zero to about 2 lbs. per linear inch. It can accommodate small
variations in the angles at which the web approaches and leaves
the device, so that demands for high precision are not imposed upon
the locations of other turning and guiding devices that are arran-
ged directly ahead of the device and directly behind it for guiding
the web to and from the device.
It is preferred that the device comprise a plurality of
air bars of the general type disclosed in United States Patents
Nos. 3,964,656 and 4,197,971, each of which provides one of said
first flat front surfaces and the pair of Coanda nozzle outlets at
the opposite longitudinal edges thereof. The
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second flat front surfaces are then defined by rectangular filler plates, each
interposed between a pair of laterally adjacent air bars.
Brief Description of Drawings
In the accompanying drawings, which illustrate what is now regarded
as a preferred embodiment of the invention:
Figure 1 is a more or less diagrammatic view in elevation of one type
of web drying apparatus in which the device of this invention can be incor-
porated;
Figure 2 is a perspective view of a contactless web guiding device
embodying the principles of this invention;
Figure 3 is a view of the device in ve~tical section, on a plane
parallel to its end walls;
Figure 4 is a view of ~he device in front elevation, on a somewhat
smaller scale than Figure 3; and
Figure 5 is a fragmentary perspective view of a modified embodiment
of the device.
DetaiIéd Description of Preferred
Embodiments of the Invention
In a typical application, a dryer device 5 of this invention guides
a lengthwise moving web W around a curve that carries it through a substantial
change of direction as it comes up from a vertical dryer 6 and then moves down
to an inclined dryer 7. The arrangement of the dryers 6 and 7 is intended
to accommodate limitations in both horizontal and vertical room dimensions,
and the apparatus as a whose can achieve further compactness because the
web can undergo a substantial amount of drying at the device 5.
It will be observed that a rather long stretch 8 of unsupported and
B unguided stretch ~ extends vertically upwardly from the vertical dryer
6 to the device 5, and a similarly unsupported and
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unguided stretch 9 extends obliquely downwardly from the device 5 towards the
inclined dryer 7. Such an arrangement is feasible with the device 5 because
of its tolerance for small variations in the angles at which the web moves toward
and from it.
In general, the device 5 comprises a plenum chamber 10 and a plural-
ity of elongated air bars 12 which are mounted lengthwise parallel to one
another on the front of the plenum chamber to receive pressure air from it and
which have Coanda nozzles 29, described hereinafter, through which the pressure
air is discharged. The air bars ~2 are arranged laterally adjacent to one
another in an arc that substantially defines the turn or curve of the web
around the device 5, and the air issuing from their Coanda nozzles 29 provides
an air cushion by which the web is floatingly supported around the turn.
Pressure air is fed into the plenum chamber 10, at its rear, through
a suitable duct 14 connected with a pressure air source indicated at 15. It
will be understood that when web drying is to be effected at the device 5, the
pressure air source 15 will ordinarily comprise a heater.
The plenum chamber 10 is defined by imperforate end walls 17, a rear
wall 18 that is imperforate except for the inlet to which the pressure air duct
14 connects, and a front wall 19 that can comprise a single piece of sheet metal
bent generally into an arc extending from one to the other of the side edges of
the rear wall 18, to thus also define side walls 20. More specifically, the
front wall 19 is bent to a partial polygonal shape as seen from either end of
the plenum chamber, to have a plurality of flat, substantially identical rec-
tangular panels 22 that meet at obtuse angle corners and on each of which an air
bar 12 is mounted. Each flat panel 22 extends lengthwise between the end walls
17 and has a width substantially equal to the width of the air bar mounted there-
lZ4050~
on. Each panel 22 has numerous uniformly distributed perforations 23 through
which pressure air flows from the interior of the plenum chamber 10 into the
interior of its air bar 12. These perforations 23 can be formed directly in
the front wall 19, or else each panel of the front wall can have a large
aperture across which extends a screen or sheet 231 that defines the perfora-
tions. In any event, the passage of the pressure air through the perforations
23 brings about a uniform pressure distribution inside the air bar, and the
perforations thus serve as a flow straightener.
Each of the air bars 12 is generally similar to the air bars dis-
closed in United States Patents Nos. 3,964,656 and 4,197,971, to which reference
can be made for information about details of construction. Each air bar thus
comprises a pair of flat end walls 25, a pair of elongated side walls 27 that
are mirror images of one another, and a front wall 28 that cooperates with the
side walls 27 to define a pair of slot-like air discharge outlets 29, each
comprising a Coanda nozzle extending along the full length of the air bar. Where
B standard air bars are used, the end walls ~q of each air bar will lie flatwise
inwardly adjacent to the respective plenum chamber end walls 17, which project
edgewise beyond its front wall 19 as hereinafter explained; or the side walls
27 and front wall 28 of each air bar can extend all the way to the plenum
chamber end walls 17, which would then also serve as the air bar end walls. The
front walls 28 of each air bar comprises the central portion of a channel member
35 that has a hat-shaped cross-section.
Extending along the rear edge of each air bar side wall 27 is a
laterally inwardly projecting flange 30 that flatwise overlies the plenum
chamber panel 22 on which the air bar is mounted~ and said flange 30 is secured
to its underlying panel as by means of bolts 31. Each air bar side wall 27 is
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bent along its length, as at 33, to define an outwardly projecting ridge of V-
shaped cross-section that is near the front edge of the side wall but spaced
therefrom. A small laterally inturned lip 34 that extends all along the front
edge of each air bar side wall 27 defines the adjacent Coanda nozzle outlet 29
in cooperation with the hat-section channel member 35 that comprises the front
wall 28. Each of the lips 34 is rearwardly offset by a small distance from the
flat, forwardly facing surface of the air bar front wall 28. To cooperate with
the lips 34 in defining the Coanda outlets 29, the hat-section channel member
35 has opposite rearwardly projec~ing legs 36, each joined to the front wall
28 at a rounded corner 37 and from each of which a flange 38 projects laterally
outwardly to be received in the groove defined by the V-shaped bend 33 in the
adjacent air bar side wall. The outer edge of each flange 38 is spot welded
to the adjacent air bar side wall 27 at intervals along its length.
Holes 39 in each flange 38, at closely spaced intervals along it,
permit pressure air from the interior of the air bar to flow through the flange
38, thence through the space between the adjacent leg 36 of the channel member
and its adjacent air bar side wall 27, and out through the Coanda nozzle slot
29. The lip 34, in conjunction with the rounded surface 37, has the well known
function of causing the air stream issuing from the slot 29 to flow away from
the nozzle in an oblique direction, laterally inwardly across the air bar front
wall 28 as well as forwardly towards the web. Upon impinging the web, the
stream issuing from éach nozzle 29 divides, part of it flowing laterally inward-
ly under the web in converging relation to a stream component from the other
air outlet 29 of the air bar, and another part flowing laterally away from the
air bar.
In some cases it may be desirable to provide outlet openings 41 in
lZ4~)5~4
the air bar front wall 28, at regular intervals all along its length and midway
between the outlet slots 29, through which the air streams that converge across
the front wall 28 can flow into a tubular exhaust chamber 42 in the front
portion with the air bar, all as shown in Figure 2 and in accordance with
principles exp]ained in United States Patent No. 3,873,013. In such cases the
exhaust chamber 42 is defined by a plate 43 that bridges the rearwardly extend-
ing legs 36 of the channel member, in cooperation with those legs and the front
wall 28; and the end walls 25 of the air bar have openings through which the
tubular exhaust chamber 42 communicates with the atmosphere at both of its ends.The outlet openings 41, their associated exhaust chamber 42 and its venting
outlet will of course tend to increase the flow of air in contact with the web,
and they may be particularly desirable where the device 5 is relied upon to
effect significant web dryingO The holes 41 have been found to be unnecessary
where the device is intended to function solely or primarily as a turning
guide; and where the holes 41 are omitted (as in the embodiment shown in Figure
5) or are not utilized, the plate 43 can be omitted from each air bar or re-
placed by brace struts, and the end walls 25 of the air bar will completely
close its ends.
The flat front walls 28 of the air bars lie substantially on an arc
of constant radius that defines the path of the web as it moves around the
device 5. The angle between the front walls 28 of adjacent air bars is related
to this radius of web curvature, and should of course be uniform from air bar
to air bar around the assembly. Successful results have been achieved with
web tensions from about .5 PLI to 2.0 PLI when the angle between front walls 28
of adjacent air bars was in the range of about 20 to about 30, with a web turnradius of about 9 inches to about 12 inches. With these parameters the web did
not contact the device even with changes in the amount of wrap about the device
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5 that corresponded (Figure 1~ to swinging of the stretches 8 and 9 through
angles of up to approximately 10 about the device 5.
Because adjacen~ air bars are at an angle to one another, there are
gaps between them, but these are filled by plates 44, each of which bridges
across the space between a pair of adjacent air bars and extends along their
full length. Preferably each plate 44 rests on, and is tack-welded to, the
ridges (defined by bends 33) on the adjacent side walls 27 of the air bars
that it bridges. The several platcs 44 are thus disposed subst~ntially on an
arc which is of smaller radius than, but concentric with, the arc that sub-
stantially contains the~front walls 28 of the several air bars.
As mentioned above, a portion of the air stream issuing from each
outlet slot 29 turns laterally away from the air bar after impinging the web,
and thus flows into the space between the web and a plate 44. Each of the end
walls 17 of the plenum chamber has its front edge curved to the arc upon which
the front walls 28 of the air bars are substantially located so that the radially
outermost portions of the plenum end walls serve as fixed air dams 117 that
project forward from the filler plates 44 and restrain air flow lengthwise out-
wardly along them. For convenience in manufacture, the air dams 117 can com-
prise portions of a separate plate 17' that is secured to each plenum chamber
end wall 17, flatwise outwardly overlying it as best seen in Figure 5. Each
filler plate 44 thus cooperates with the fixed air dams 117 to maintain an air
cushion in the space in front of the plate, so that the web is maintained at a
uniform curvature all around the device.
If the gaps between air bars are left open by omission of the filler
plates 44, the web tends to have straight, flat stretches between adjacent air
bars, and thus tends to drag on the air bars, unless substantially higher air
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pressure values are used than are needed with the plates 44 installed. Thus
the plates 44 may be omitted in cases where a relatively high air flow in
contact with the web is desired for web drying and where the need for higher
air pressure is acceptable. On the other hand, moving the plates 44 forwardly
from the positions shown, so that they are more nearly at the same radius as
the front walls 28 of the air bars, tends to reduce the pressure needed to
maintain a smooth curvature of the web around the device and correspondingly
reduces the rate of flow of air in contact with the web.
To minimi~e flutter of edge portions of the web, there is an adjust-
able air dam 45 outwardly adjacent to each edge of the web, to restrict air
flow edgewise outwardly frcm behind the web. The two adjustable air dams 45
are shiftable towards and from one another to accommodate webs of different
widths. Each adjustable air dam 45 comprises a plate 46 that is curved to an
arc such that the plate can closely overlie the front walls 28 of all air bars
of the device. Extending along an inner edge of each plate 46, to be adjacent
and parallel to an edge of the web, is a radially outwardly projecting flange
47 that constitutes the air dam proper. The flange 47 projects radially out-
wardly beyond its plate 46 by a distance at least equal the normal maximum
spacing of the web from the front walls 28 of the air bars, typically about .25
in. to .75 in. The air dams have been found to be most effective when each is
about .25 in. to .75 in. from its adjacent edge of the web.
The adjustable air dams 45 prevent substantial waste of pressure
air when the width of a web being dried is substantially less than the length
of the air bars of the device, because the curved plates 46 of these air dams
overlie the then-unused end portions of the air bar outlet slots 29, so that
the air stream from each such slot will not be substantially more than suffi-
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12405~
cient to impinge the web all across its width. The curved plate 46 need not
make sealing engagement with the unused portions of the air bar outlet slots;
if it closely overlies the front walls of the air bars it will throttle outflow
from the unused portion of the slot to the point where any waste of pressure
air is negligible. It will be observed that each curved plate 46 also coop-
erates with its adjacent fixed air dam 117 to define a pocket that traps pres-
sure air in the space that is in front of the plate 44 but outward of each edge
of the web.
A device 5 of this invention was constructed in accordance with
Figure 5, and tests on it have established that it tends to maintain a web at
a desirable clearance distance from the front walls 28 of the air bars even
under appreciable changes in web tension. This web clearance stability is due
to a favorable relationship between clearance distance and the pressure of
the air cushion between the web and the front walls 28. Without any change in
the pressure air being fed to the air bars, a small decrease in web clearance
distance results in a prompt and relatively large increase in air cushion
pressure, and a small increase in clearance distance causes an immediate and
substantial decrease in cushion pressure. The device tends to maintain a
balance between radially outward air cushion force against the web and the
radially inward force component of web tension. If web tension increases,
upsetting the balance in the direction to decrease the web clearance distance,
the resulting prompt and rapid increase in air c~shion pressure under the web
ensureSthat the decrease in clearance distance will be small enough to keep the
web well away from contact with surfaces on the device 5. This corrective action
takes place automatically, without the need for any change in the pressure air
supply or any other operating variable. Therefore, the problem of "impossibly
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fine tension control ~which] is required..." ~Patent No. 4,21~,833) with some
prior turning guides is avoided with the device of this invention.
It will be apparent that the device 5 of this invention can be
partially enclosed in an exhaust hood or the like (not shown) by which vapor-
laden air that has passed in contact with the web can be drawn away. It will
also be apparent that a web could be completely dried by passing it successively
over two or more of the devices 5 of this invention, arranged to carry the web
through curves first in one direction and then in the other. Furthermore,
because the device 5 provides adequate supporting force against the concave
rear surface of the web, streams of drying air can be blown in any suitable
manner against the front surface of the web at the turn.
From the foregoing description taken with the accompanying drawings
it will be apparent that this invention provides a simple, inexpensive and very
efficient web drying device whereby a lengthwise moving web is floatingly sup-
ported while being guided around a turn. It can be seen that the device of
this invention accommodates webs of different widths3 under different length-
wise tensions, and ensures adequate heat transfer for fast drying of a web with
a flow of air that is not uneconomically high.
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