US3388479A - Pocket ventilator for web drying equipment - Google Patents

Description

June 18, 1968 T. A. GARDNER POCKET VENTILATOR FOR WEB DRYING EQUIPMENT 2 Sheets-Sheet 1 Filed April 20, 1967 .fillllllllllllLl \NUENT'OK TI /am s H. G aze/v5? AM, Mm

ATTORNE$ T. A. GARDNER POCKET VENTILATOR FOR WEB DRYING EQUIPMENT June 18, 1968 2 Sheets-Sheet 2 Filed April 20, 1967 EDGE 0F Dame, "A.

7Hamns H. neowse INUENTOK y, A'r'rozuivs United States Patent "ice 3,388,479 POCKET VENTILATOR FOR WEB DRYING EQUIPMENT Thomas A. Gardner, 513 Clark St., Neenah, Wis. 54956 Continuation-impart of abandoned application Ser. No.

486,760, Sept. 13, 1965. This application Apr. 20, 1967, Ser. No. 632,315

15 -Claims. (Cl. 34-15) ABSTRACT 0F THE DISCLOSURE Drying gas is supplied to the space between dryer cylinders and a felt roll by a manifold substantially equal in length to the width of the web to be dried. Pressure adequate to force the gas through the felt or such web and into the pocket is developed by directing the discharge from the manifold at an acute angle to the felt or web at or near the points of tangency to the respective cylindens, all .parts of the manifold being spaced from the felt or web with sufficient clearance to avoid contact in the event of irregularities in the web to be dried, the pressure maintaining seal being effected by requiring some of the gas to reverse its direction between the manifold and the dryer cylinder-s.

Background of the invention This application is a continuation-in-part of my application Ser. No. 486,760, and filed Sept. 13, 1965, and now abandoned.

It has not heretofore been possible to dry webs from edge to edge with anything approaching uniformity. The problem has become particularly serious in recent years due to increase in width of paper machines and the operation thereof at increased speeds. Large amounts of paper have been rejected by consumers because of uncontrolled variation in quality due to lack of uniformity in drying. Variations in drying between the center and the edges of the web have become so acute that some buyers specify that portion of the web that they will accept. Excess dehydration such as commonly occurs near the margins of the web results in loss in absorbency, loss of brightness, loss of strength, and loss of elasticity. Underdrying may cause blackening of the wetter portions of the web when these are calendered. Worst of all in many industries is the tendency of the web to lose its flatness as its moisture equilibrium varies due to subsequent eX- posure to ambient atmosphere.

Variation in drying has been traced to variations in humidity of the ambient air to which the web is exposed in traversing the dryer. All previous dryers have lacked the capacity to provide dependably uniform humidity profiles across the web. Attempts to control drying in accordance with observations of humidity profile in a few locations have failed, partly because the continuous determination of humidity profile of the ambient air is impractical and no way has been found to correlate humidity at selected stations with final moisture content of the web. Moreover, it is basically poor practice (because of shrinkage) to dry the paper in a non-uniform manner even if the final moisture .profile is uniform across the machine.

The present invention seeks to maintain the profile of humidity substantially uniform at all points throughout the dryer, whereby the quality of theweb during and after drying is uniformly superior. Additionally, the relative humidity can be controlled with accuracy over a wide range and it responds rapidly to control, thereby providing improved means for controlling the final moisture content. The cost is comparable to the least expensive 3,388,479 Patented June 18, 1968 system previously used and approximates one-third of the cost of some popular systems. Because of substantial increase in drying capacity, major savings are effected by reducing the over-all length of the dryer and in elimination of axuiliary equipment, as well as in operating costs.

Summary of invention The web to be dried passes alternately about drying cylinders in upper and lower tiers. The felt, if any, which holds the web to the cylinders of a given tier passes about a guide pulley, leaving the web exposed in its travel from tangency with the cylinder of one tier to tangency with a cylinder of the other tier and thence from the last mentioned cylinder back to the next cylinder of the first tier. The space between the runs of the web between the cylinders of the respective tiers is called the pocket.

The invention contemplates forcing drying air into the pocket by establishing a static pressure of drying gas between consecutive cylinders of one tier and the opposing cylinder of the other tier and above the felt guide pulley, if any.

It is .preferred to confine my pocket ventilators to the intermediate part of the dryer section. Ordinarily, they are not useful in the first ten percent of the web travel in the dryer section, since relatively little evaporation occurs from the web as it is being heated up to drying temperatures. Similarly, the last five to twenty percent show little gain, depending on the type of paper being dried, because evaporation rates have fallen so low in this range. I have found a very substantial improvement is effected by using the pocket dryers on only three of a possible thirty units in a given installation. Hower, my pocket ventilators may be used for a purpose other than uniform high rate drying in the last part of the dryer section. It is sometimes desired to condition paper in a very humid atmosphere after it is substantially dry. Because of the characteristic uniformity of my ventilating method, I can immerse the entire web in a uniform atmosphere at high humidity. The high humidity can be obtained by recirculating humid air from the dryers an'd/ or by injecting steam or moisture into the air supplied to my pocket ventilator or into the pressured space above the felt roll.

The ambient air around the drying web may not always be dehydrated. Near the sides of the machine in particular the humidity is commonly higher than in previous practice. The primary purpose of my device is niform humidity across the machine.

According to my invention, all parts of the ventilation apparatus are stationary. The apparatus includes an inlet plenum into which I introduce air. In practice this may be at 3000-5000 f.p.m. From the inlet plenum the air passes through a baffled distributor passage leading to a distributor plenum. In the distributor plenum, the velocity of the air transversely of the machine is substantially eliminated and the air is discharged obliquely toward the runs of the felt web at the points at which these leave the paper to pass about the felt roll. Some of the air turns upon itself to move reversely up along the back of the cylinder and the remainder is used for pocket ventilation.

The momentum of the transversely elongated jets directed obliquely downwardly against the felts or at least substantially tangentially t0 the respective rolls has the effect of providing a seal which permits buildup of static pressure in the Zone in which the manifold is located. Since this seal is achieved despite a very substantial clearance between the dryer rolls and the manifold, any irregularities of the felt or the paper will Pass freely through this clearance without being impeded by any structure required for sealing purposes.

Assuming a felt is used, the ventilating air is sealed within the pressurized space between the distributor plenum and the felt roll, being confined therein, in part, by end closures which are loosely spaced from the felt. There is ample air remaining under pressure to flow through the moderately permeable felt for ventilating the pocket. Because of the large exposed area of felt, no more than .3 inch water gauge pressure will normally be required. If there is no felt the operation will be essentially similar and may be considered as equivalent to a felt of zero resistance.

Flow of external ambient air into and through the pockets transversely of the machine is completely eliminated when my invention is used. This is important because ambient air is regarded as a contaminant because of its uncontrolled humidity and variable and unpredictable flow.

End seals may be completely omitted on dryer sections having no felt.

It is a significant and primary feature of the invention that the margins of the metal parts can be very substantially spaced from the paper and from the felt, if used, and also there is clearance of the end seals without impairing the ventilating action. Adequate clcarance is essential. Web breakage and wads of loose paper are a common occurrence in paper machines. Clearance of the end seals may be relatively less because these are flexible.

While many features of the invention are usable regardless of the presence or absence of felts, I regard the role of the dryer felt in connection with this invention as being of critical importance. Most webs encountered in the drying of paper are not only thin but extremely weak because of the weight of water contained and because the interfiber bonding that gives paper its strength is not fully developed until the paper is dry. It is therefore critically important to prevent air blasts or jet streams such as issue rom a ventilating device from striking the comparatively long and unsupported draw of weak paper passing from one dryer to the next and forming the side walls of the pockets. In this respect most of the previous ventilating devices have failed because they have attempted to blow air directly through felts selected for their high permeability (300 to 800), and the resulting disturbance of the web has forced operators to reduce the flow to the point that little effect was obtained.

The use of felts having a permeability range of from 50 to 250 is required with my invention when common lightweight webs such as newsprint, book, writings, etc. are being dried. The permeability must be low enough to almost completely screen out the air jets flowing from my nozzles but high enough to allow very low velocity pass age of air into the pockets when l pressurize the space outside the pockets. Typically the face velocity normal to and through the felts into the pockets is under 100 feet per minute, an almost imr..easurably low velocity, when my apparatus is used. The important characteristic of my invention is that it exerts pressure on the entire area of the felt from the point where it leaves one dryer cylinder, passing around the felt roll and forming one side of the pocket, to the point where it makes contact with the falowing dryer cylinder. Thus a very large area of felt is exposed to pressure, and in spite of the very low face ve locity, a relatively large amount of air is introduced into the pocket, and this with no danger of disturbing the web. As an example of this function, I might employ a felt having a permeability of 150 c.f.m. per square foot at one-half inch water gauge pressure and nozzle velocity of 5000 f.p.m., having a stagnation pressure of one and onehalf inches W.G., to develop a sealed in static pressure upon the felt of 0.2 inch W.G. This static pressure will cause a flow through the felt of about 70 f.p.m. at all exposed points, and yet the felt screens the 5000 f.p.m. jet down to a velocity (local) through the felt of only about 350 f.p.m. In contrast, conventional blowing devices typically produce local velocities through the felt (high permeability) exceeding 1000 f.p.m.

The air used to ventilate the pockets may have relatively high humidity (for example, 0.25 of water vapor per pound of dry air) and will still be effective. The air should, in any event, be heated to a level of 180 F. or thereabouts to prevent formation of visible fog in the pockets. Excessive heat is undesirable because it is wasted, adding little or nothing to drying capacity. It also shortens felt life. Since the air may be quite humid, economies in heat can be had by recirculating at least part of the air through the equipment. The humidity of the air exhausted from the machine may be raised appreciably and its quantity reduced accordingly.

Supply of air to each unit is desirably controlled by a damper for uniformity of distribution and reduction of flow in the case of oversupply. In practice, it is found desirable to have the lowest exit velocity from the pocket exceed about f.p.m. Typically, the average exit velocity from both ends of the pocket is in excess of 200 f.p.m. This tends to avoid the bad effects of natural drafts of ambient air. Greater velocity may be used to achieve greater reduction of humidity with resulting greater drying capacity. It is estimated that exit velocities in excess of 500 f.p.m. are perfectly safe but they are believed to be unnecessary. The flow of air into the pocket per foot of machine width normally falls in the range of 80 to c.f.m. per foot of width, but may sometimes exceed these limits.

Sealing air that flows over the backs of the drying cylinders is not wasted. The flow of sealing air is advantageous in that it purges the atmosphere back of the felts, or back of the paper in the absence of felts. The net result is to unify to a degree never before possible the entire atmosphere in which the drying machine operates. Overdrying of any part of the web is substantially elimimated with the exception of the inch or two which is normally trimmed from the edge in any event.

The important function of my invention is that I am able to produce the desired pressure on the aforementioned large area of felt without mounting equipment dangerously near the moving felt. By utilizing part of the flow from my nozzles as aerodynamic sealing I am able to develop and maintain the desired pressure while at the same time maintaining a clearance of two inches or more from the felt. While two inches has proved adequate in prolonged test, it is inadequate for other installation and the clearance may be increased to any reasonable amount without losing the aerodynamic seal.

This aerodynamic seal is very stable in spite of inevitable variation in nozzle width. The uniformity is at least partially attributable to the large dimensions of the static pressure zone at the entrance to the pocket. The pressure in this zone produces exit velocity from the pocket such that ambient air from the sides of the machine is completely excluded.

It is important that changes in drying rate can be made in a matter of seconds. Therefore, the final moisture content of the paper of the web responds rapidly to control. This is much more satisfactory than conventional attempts to control moisture by varying steam pressure in the drying cylinder, since a very substantial amount of time elapses after steam pressure is changed before the effect is perceptible in the paper. As much as 2000 feet of paper may pass through the dryer before the control operating on steam pressure is able to restore paper moisture to the desired level.

It is also possible to produce any desired variation in drying effect as between different parts of the dryer.

Brief description of the drawings In the drawings:

FIG. 1 is a fragmentary view in side elevation of a typical dryer having upper and lower drying rolls and felt rolls and illustrating the application of the invention to the pockets formed by the rolls.

FIG. 2 is a detail view taken in cross section on the line 2-2 of FIG. 1.

openings may be provided as exemplified by the ports 480 in FIG. 6. The ranges are broad but the invention is applicable to a wide variety of paper machines and the most desirable arrangement may depend in part upon the porosity of the felts or, in some cases, the complete absence of felts.

Felts having permeabilities in a range of 50 to 250 are usable but the preferred range is from 100 to 200. Felt permeability is expressed in cubic feet per minute. It represents the amount of dry air at 70 F. which will pass through one square foot of felt under pressure of 0.5 inch water column. Assuming that the felt used has porosity deemed excessive, the designer may, at his option, restrict air flow to allow relatively smaller percentages of the net air supply to enter the pocket through the felts. The location of the felt roll also has significance. If the drying rolls are farther apart so that the felt runs converge toward the felt roll, or if the felt roll is located far down in the pocket, in either case there will be a variation in the amount of felt area which is exposed to the air which is being used for ventilation.

I have derived the mathematical treatment necessary for designing and operating the apparatus. On the basis of momentum change of the jets from the slotted nozzle, the reaction pressure imposed on the space above the felt can be expressed as:

Q =Total volume of flow from both nozzles. Qf POI'tiOH of flow entering pressure space. Q =Portions of flow to backs of cylinders. V-=Velocity of flow.

=Density of gas.

:Angle of impingement with respect to felt. g=Acceleration rate of gravity. Y=Clearance between nozzles and felt. b=Length of the slots.

If the angle is small, e.g., 20, the formula may with little error be reduced to:

P Qr( 4gYb The resistance to flow through the known flow areas of felt and through the end seals can also be determined, and the pressure required to drive flow through both exits from the pressure space is:

C=The resistance factor. n=Determinable exponent between 1 and 2.

Since the pressures expressed in the above equations both represent the pressure in the pressurized space, the equations may be combined to eliminate the pressure factor. Furthermore, because Q,+Q =Q,, the equations may be solved to determine Q,, Q, and P, or F for any specified design configuration. Thus it is possible to determine how much air will enter the pocket.

For reasons explained above, the invention has been demonstrated to be of considerable value, particularly for the paper industry. It provides a method and means for obtaining uniform moisture profile. It will eliminate overdrying of the sides of the web and accompanying loss of quality. By allowing the sides of the web to retain moisture which is uniformly equivalent to that in the intermediate part of the web, there will be considerable increase in the value of the web since it is not only of superior quality but it weighs more.

In FIG. 7 I have illustrated humidity profiles secured by test of a dryer having equipment embodying the present invention as disclosed in FIGS. 1 to 5. The solid line 60 shows the uniformity resulting when the pocket ventilator is in operation. The dotted line 62 shows the effect when the supply of air to the pocket ventilators was cut off, leaving the pockets subject to a natural draft across the paper machine from back to front. The dotted line 64 shows the result when the natural draft was reduced approximately 50 percent by obstructing its flow at the back side of the machine.

Through the use of the invention, the drying capacity of a paper machine will be markedly increased. Operating problems such as calender marking will be reduced. The life of the dryer felt will be increased. The adverse random effects of ambient air movements have been demonstrated to be substantially eliminated. The steam consumption required for drying is reduced. Yet all of these advantages are available at low capital cost and low operating cost.

I claim:

l. A method of ventilating a pocket of a dryer having cylinders over which a web is trained, such method consisting in directing elongated jets of dehydrating gas at opposite acute angles approximately tangentially against portions of such a web supported on cylinders at opposite sides of the pocket and for substantially the entire width thereof, and confining a substantial quantity of the gas under static pressure to require it to flow across the web and in opposite directions toward the ends of the pocket.

2. A method of ventilating a pocket of a dryer having at least first and third and an offset intermediate second cylinders over which a web is trained having one portion engaged tangentially with the first and second cylinders and another portion engaged tangentially with the second and third cylinders to form an open ended pocket to be ventilated, said method including pressurization of all of the said portions of said web between the said cylinders with which such portions are tangentially engaged, such pressurization being effected by the step of supplying air under pressure to an elongated space extending across all portions of the web and overlying the pocket, and the step of sealing said space sufficiently to require a substantial volume of such air to move into the pocket for escape from the ends thereof.

3. A method of ventilating a pocket of a dryer having at least first and third and an offset intermediate second cylinders over which a web is trained to form an open ended pocket to be ventilated, said method including the step of supplying air under pressure to an elongated space extending across all portions of the web and overlying the pocket, and the step of sealing said space sufficiently to require a substantial volume of such air to move into the pocket for escape from the ends thereof, the sealing step including the jetting of components of the air so supplied oppositely toward the first and third cylinders and using for the sealing of said space the back pressure produced by change of direction of such of the air so jetted as it escapes from said space about the first and third cylinders.

4. Ventilating apparatus for a pocket between drying cylinders having a web trained thereover, said apparatus comprising a distributor plenum spaced between two cylinders, means for supplying a dehydrating gas thereto under pressure, said plenum including discharge orifice means directed across the space between said plenum and the respective cylinders toward both said cylinders and extending transversely for the width of the web and longitudinally of the respective cylinders, the direction of discharge of dehydrating gas from said orifice means being toward the pocket and at an acute angle to the web trained over the cylinders for the delivery of such gas toward the respective cylinders and toward a portion of said web adjacent the cylinders to be supported thereby, whereby to establish a head of static pressure notwithstanding the space between the plenum and the said cylinders for forcing a substantial component of such gas to flow into the pocket and from the pocket at the ends of said cylinders.

5. In a dryer comprising three cylinders over which is FIG. 3 is a detail view of the distributor spacer in plan as viewed from the line 33 of FIG. 2.

FIG. 4 is an enlarged fragmentary detail view taken in section on the line 44 of FIG. 2.

FIG. 5 is an enlarged detail view taken in section on the line 5-5 of FIG. 2.

FIG. 6 is a fragmentary view in perspective showing a modified embodiment.

FIG. 7 is a diagram comparing humidity profiles in a dryer pocket functioning with and without the equip ment of the present invention.

Description of the preferred embodiment The conventional dryer organization shown in FIG. 1 as a basis for exemplifying the invention includes a bank of upper drying rolls 10 and a bank of lower drying rolls 12. Over these is trained the web 14 of paper or the like which is to be dried. In most paper machine dryer sections, at least a part of a conventional dryer organization includes a conventional felt web 16 which holds the paper web 14 to the upper rolls 10. Between the upper rolls 1%), the felt web is trained about the felt rolls 18. Similarly, a felt web 20 holds the paper web 14 to the lower drying rolls 12 and between such drying rolls is trained over the felt rolls 22.

The pockets which are venilated in accordance with the present invention lie between each given drying roll of one bank and the staggered pair of drying rolls of the other bank. By definition, in paper machine dryer practice, a pocket is the space bounded by a steam cylinder on one side and by the paper web flowing to and from this cylinder on the two sides. If a felt is used, the felt completes the pocket as it is passed onto, around and off of a felt roll on the side oppoiste the steam cylinder. The pocket is open at both ends and may be open between the runs of the paper web if no felt is used. For example, a single pocket 30 (FIG. 1) is closed at its bottom by the roll 12' and at its sides by runs and 15' of the web 14 going to and from the drying roll 12'. The felt roller 18 and the runs of felt passing thereover depend into the pocket 30 and tend to close communication between the pocket and the air above it. However, assuming that a reasonably porous felt is used, it is posisble to ventilate the pocket through the felt as hereinafter disclosed.

Similar pockets are formed beneath each of the upper rolls 10 about which the web to be dried has been trained. It will be understood that for the vetnilation of these pockets, the apparatus will be inverted from the position in which it is used to ventilate the pocket 30.

Extending transversely across the dryer between each pair of drying rolls is an inlet plenum 34. Subject to the control of appropriate dampers 36, the several inlet plenurns are supplied with air from a manifold 38. Since the ventilating air leaves the manifold at high velocity and high pressure, most of it would tend to go through to the opposite side of the machine with high ventilation at one side and little at the other unless its flow were rectified.

Accordingly, the air is caused to flow from the inlet plenum 34 downwardly to a distributor plenum 40 via a narrow tube 42 which depends from plenum 34 and is subdivided by a distributor spacer 44 shown in detail in FIG. 3. This effectively kills all of the transverse velocity of the air streaarn and delivers the air downwardly against the closed bottom of the plenum For uniformity of delivery, this tube is sometimes tapered. Although there is considerable turbulence, the air has little remaining lateral velocity, but ample pressure for discharge of thin jets through the delivery slots or ports 56 and 48.

The flow of air from delivery ports 46 and 43 is illustrated diagrammatically in FIG. 4. The jets are directed at a sharp angle downwardly as indicated at 50. In practice, I have used an angle of about to the tangent line of the felt as it leaves one of the rollers 10 and returns to the other. In some parts of the dryer, felts are omitted. In such cases the planes in which the ventilating air is discharged from the nozzles may be approximately the same planes in which the air would be delivered if the felt runs were present.

Some of the air of each jet 50 turns abruptly to flow upwardly about the dryin roll and along the felt as indicated at 52. The velocity of the air of the jet is such that considerable energy is required to change its direction at this acute angle. Accordingly, the air which thus changes direction tends to function as a seal to require the rest of the air to accumulate under pressure in the space 54 which is in the bight of the felt 16 and between the felt roll 18 and the distributor plenum 40. The pressurized air in this space readily passes through the porous felt 16 into the pocket which is to be ventilated.

PEG. 6 suggests two different ways which are independently usable for modification of embodiments previously disclosed. The first modification suggested by FIG. 6 is the provision of auxiliary ports 480 for directing jets upwardly to provide supplemental air over the backs of the cylinders. A separately usable feature shown in FIG. 6 is the omission of felts so that jets 50 are directed from ports 48 obliquely against web 14.

The ends of the pressurized space 54 are at least partially closed if felts are used. For reasons hereinafter explained, it is not necessary to close this space if felts are not used. Assuming that felts are used, I prefer to employ flexible closure flaps 56 (FIG. 2) which may be supported from the distributor plenum and should be spaced at all points from the felt web and the felt roll. In practice, I employ about inch of space between the margins of the closure flaps 56 and all moving webs. All metal parts are spaced from the web by two inches or thereabouts. (Under some circumstances, it may be necessary to use several inches of metal part clearance.) The object is to avoid interference with the felt or with the web which is being dried. in all web dryers there are occasional breakages of the web, and more frequently there are wads of paper or the like on the web. Considerable damage may be done if these become entangled with any fixed object. A very substantial clearance may exist without materially decreasing the effectiveness of the drying operation because the quantities and pressures of air in the pressurized space 54 are adequate to assure that as much air will pass through the felts as can be handled by the porosity thereof. The physical location of the closure flaps 56 axially of the respective rolls will preferably be well within the margins of the felt web and preferably somewhat within the margins of the paper web (FIG. 2). The jets from slots 46 and 48 will desirably be substantially continuous for the width of the web or at least from seal to seal.

The pockets 30 are wide open at their ends for the discharge of the ventilating air supplied thereto in the manner described. The short arrows 58 in FIG. 4 diagrammatically illustrate the flow of ventilating air through the felt into the pocket.

It will be understood that particulars of dimensions and pressures are merely by way of exemplification and not by way of limitation. A wide variety of changes can be made by the designer. As used by me, the slots 46 and 48 extend at A1 inch opening for the full width of the distributor plenum 40 the full distance between end seals transversely of the machine. With such slots, I have used air velocities of approximately 7000 f.p.m. However, experiments indicate that slot cross sections of .10 inch up to at least one inch are practicable, with velocities ranging from 2000 f.p.m. to 12,000 f.p.m.

A designer also may wish to alter the angle of the jets which issue from the slots 46, 48. I have found the angle 20 to be approximately correct but if more air were to be desired over the backs of the cylinders, an les up to could be used. If additional air over the backs of the first and third cylinders is desired, supplemental trained a web to be dried, the first and third cylinders being proximate and the second cylinder being oflset, whereby a pocketis formed between the web and the second cylinder, a pocket ventilator comprising nozzle means elongated parallel to the axes of the cylinders and disposed between the first and third cylinders in a position offset from said pocket, said nozzle means comprising slots directed at actute angles toward said web adjacent lines of tangency of said web to the first and third cylinders, said nozzle means being spaced from the first and third cylinders with sufficient clearance to pass irregularities in said web, such portion of the flow from said nozzle means as escapes through the space between the nozzle means and the first and third cylinders being required to reverse direction whereby to establish a seal in each such space to the extent of the inertia of the air flow changing direction, remaining flow from said nozzles establishing static pressure in the pocket for forcing air toward the second cylinder to be discharged from the ends of the pocket.

6. A pocket ventilator according to claim in which partial closure means substantially closes the ends of the space between the nozzle means and the first and third cylinders whereby to direct into the pocket the air supplied by the nozzle means, and means for supplying air to the nozzle means in sufiicient volume in excess of all leakage from said space to force air to the pocket adequate for ventilation thereof.

7. A ventilator according to claim 5 in which the means for delivering air includes means for rendering air flow from the nozzles substantially uniform throughout the width of the web.

8. A ventilator according to claim 5 in which the air delivering means includes a distributor plenum between the first and third cylinders and on which said nozzles are mounted, the said plenum partially filling the space between the first and third cylinders over which the web is trained, and being spaced from said first and third cylinders leaving clearances through which a part of the air discharged from said nozzles can escape about the first and third cylinders, the dryer section including a felt roll and a felt of limited porosity trained over the felt roll and the first and third cylinders, the air supplied from said nozzles establishing static pressure between the plenum and the felt passing about the first and third cylinders and the felt roll, such pressure constraining a substantial component of the air to pass through the felt into the pocket.

9. In a paper machine cylinder dryer section having a web of paper trained over first and third cylinders and an intervening second cylinder at a different level, thereby defining a pocket, a pocket ventilator comprising a plenum extending substantially continuously between the first and third cylinders for the width of the web and having air discharge slots substan ially continuous for the width of the web and positioned to direct air oppositely toward said first and third cylinders, and against portions of the web which are adjacent portions thereof that are supported on said cylinders, means for supplying air in substantial volume and under pressure to the said slots and constraining it to flow wih substantial uniformity throughout the length thereof from one side to the other of said web, means including the slots for establishing pneumatic seals between the plenum and the first and third cylinders whereby only a part of the said air can escape about the first and third cylinders, means for confining in a pressurized space between the plenum and the second cylinder and communicating with said pocket a component of the air adequately supplied in such volume from said nozzles, the pocket being open at both ends for the discharge of said component of air.

10. A dryer section according to claim 9 in which a felt roll intervenes between the plenum and the second cylinder, and a felt of limited porosity is trained over said roll and the first and third cylinders and has portions intervening between said pressurized space and said pocket and through which said air component passes for pocket ventilation.

11. A dryer section according to claim 10 in which the permeability of the felt is in a range of 50 to 250.

12. A dryer section according to claim 11 in which the permeability of the felt is in a range of to 200.

13. In a dryer, the combination with two rows of drying cylinders, the cylinders being staggered in the respective rows and the cylinders in each row being closely proximate to each other and adapted to have a web to be dried trained alternately about the cylinders of said rows and defining pockets, each of which is bounded by one cylinder of one row and two cylinders of another row and the intervening web trained thereover, of means comprising a porous felt trained about said cylinders for holding the web thereto, a felt roll in each of said pockets about which the felt is trained, the felt extending tangentially from and to the cylinders of a given row and having an intermediate portion trained about the respective felt roll at each such pocket, and ventilating means for individual pockets and comprising in each such pocket a distributor plenum, means for delivering a dehydrating gas under pressure to said plenum, the said plenum having elongated slots which are substantially continuous transversely of the pocket for the full width of the said web, said slots being directed oppositely for the delivery of jets of gas against the felt and the cylinders over which runs of the felt are trained, the plenum having spaces between it and the last mentioned cylinders sufficient to pass any irregularities in the felt and the web to be dried and the direction of delivery of said jets being toward the pocket and at an acute angle to the felt runs adjacent the lines of tangency between the felt and the last mentioned cylinders whereby escaping air is required to turn upon itself, the inertia of such air tending to establish static pressure of a substantial component portion of the gas of said jets on the felt between said cylinders, a portion of said gas serving to provide sealing resistance to flow between the plenum and the adjacent cylinders in a direction away from the felt roll, another substantial component of the gas being forced thereby to flow through the felt between the felt roll and the cylinders over which the felt is trained, such gas ventilating the pocket and being constrained to escape in substantially equal amounts from opposite ends of the pocket.

14. A dryer for a felt-supported web and comprising first, second and third cylinders over which is trained a web to be dried, the first and third cylinders being proximate with narrow intervening space in one tier and the second cylinder being offset in a different tier, whereby a pocket is formed between the moving portions of the web and second cylinder, the said web portions constituting the sides of the pocket, a porous felt trained over the first and third cylinders and having a bight extending into the pocket, a felt roll in said bight and about which the felt passes, and a pocket ventilator comprising means for developing a static head of pressure of drying gas in the bight of the belt, said head being sufficient to force the gas through the felt into the pocket for the drying of the moving web portions at the sides of the pocket, the said means for establishing a static head of pressurized gas including a plenum extending longitudinally of the bight between the first and third cylinders, said plenum including nozzle means for discharging drying gas into said bight for substantially the length of the bight between said first and third cylinders, the plenum further comprising means extending across the space between the first and third cylinders for restricting escape of gas from said bight along the surface of the felt trained over said cylinders, means extending from said last means and free of contact with the felt for restricting flow from the ends of the bight, the head of pressure thus established in the bight being sufiicient to force drying gas through the porous felt into the pocket for the drying of the web portions constituting the sides of the pocket, the ends of the pockets beyond said felt roll being open for the escape of used drying gas.

15. A dryer for a moving web, said dryer comprising first, second and third cylinders over which is trained a web to be dried, first and third cylinders being proximate with narrow intervening space in one tier and the second cylinder being offset in a different tier, whereby a pocket open at its ends is formed between the portions of the moving web and the second cylinder, the web constituting the sides of the pocket, means for developing a static head of pressure of drying gas in the pocket, said means including a plenum in the said space between the first and third cylinders and having nozzle means extending longitudinally of the pocket for directing drying gas into the pocket for substantially the full width of the web, said plenum comprising means extending across part of the space between said cylinders for so restricting said space as to permit substantial static head to be developed in said pocket, and means extending from said plenum across the portions of the ends of the pocket which are nearest to said plenum for restricting endwise flow of gas from the pocket to require such endwise flow to occur through portions of the pocket which are remote from the plenum, thereby to require a major portion of such gas to contact the surfaces of those portions of the moving web which constitute the sides of the pocket.

References Cited UNITED STATES PATENTS 1,199,394 9/1916 Liebeck 34-11] 1,298,655 4/1919 Briner 34-114 1,660,640 2/1928 Asten 34-111 X 2,199,233 4/1940 Williams 34-157 X 3,302,304 2/1967 Goldenberg et. al 34-159 FOREIGN PATENTS 598,111 2/1948 Great Britain.

FREDERICK L. MATTESON, JR., Primary Examiner. A. D. HERRMANN, Assistant Examiner.

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