US3408990A

US3408990A - Boiler

Info

Publication number: US3408990A
Application number: US595906A
Authority: US
Inventors: John C Cleaver; Gustav A Rehm; Fred G Wiegratz; Walter J Baron
Original assignee: Aqua Chem Inc
Current assignee: Aqua Chem Inc
Priority date: 1966-11-21
Filing date: 1966-11-21
Publication date: 1968-11-05
Anticipated expiration: 1985-11-05

Description

NOV. 5, 1968 J c, CLEAVER ET AL 3,408,990

BOILER 5 Sheets-Sheet 2 Filed Nov. 21, 1966 A 3 5320 555 mwwum 4 44 .C. :1 ,A/ 4 77 C E: Z

Nov. 5, 1968 Filed NOV. 21, 1966 J. C. CLEAVER ET ILER 3 Sheets-Sheet 5 V 5 a a l 5 I United States Patent BOILER John C. Cleaver, River Hills, Gustav A. Rehm, Milwaukee, Fred G. Wiegratz, New Berlin, and Walter J. Baron, Hubertus, Wis., assignors to Aqua-Chem, Inc., a

corporation of Wisconsin Filed Nov. 21, 1966, Ser. No. 595,906 12 Claims. (Cl. 122-328) ABSTRACT OF THE DISCLOSURE The invention relates to a boiler construction wherein the boiler is provided with longitudinally extending drums at its lower portion and connected by a plurality of tubes in panel form to a drum at the upper portion of the boiler. With the particular arrangement of the tubes and drum, superior circulation of water can be achieved and reduction in the amount of refractory material normally found in boiler constructions can be accomplished.

This invention relates in general to boilers and more particularly to boilers for use in generating steam.

In the past, it has been known to connect riser and downcomer structure between a water drum and a steam drum within a boiler housing having burner means therein. Heated water moves upwardly in the risers into the steam drum because of its lower density, and relatively cooler water, because of its greater density, moves downwardly in the downcomers into the water drum for recirculation through the risers. While such arrangements have functioned generally satisfactorily to produce a desired amount of steam, troublesome problems have been encountered in the manufacture of the units, their operation, and their servicing. Heretofore, in order to prevent the dissipation of the heat generated by the burner means within the boiler housing, it has been conventional to provide complicated insulating means, which is not only diflicult and time consuming to install, but which is also extremely expensive and very diflicult to replace. More particularly, boilers of known construction have usually included refractory brick around at least the lower portions of the risers and downcomers to effectively create a sealed heating chamber in which the water in the risers is heated, and the water in the downcomers is isolated from the heating zone. In addition to the usual refractory material, it has been conventional to provide heavily insulated casing side walls, which are difficult to install and replace. It has also been conventional in prior art boilers of the type described above to weld each of the risers and downcomers to the water and steam drums, and the multiple openings necessitated thereby has weakened the drums to the point that heavy walled structures have been necessary. Additionally, with prior art boiler designs, to get the desired steam or hot water output, large heating surface areas have been required, which have resulted in extremely bulky boiler units. Still further, typical prior art boiler designs have been so cluttered internally, that there is no room within the boiler housing to service the unit. Accordingly, the purpose of the present invention is to provide a novel boiler arrangement which obviates the problems and disadvantages of prior art boiler designs.

An object of the invention is to provide a unique boiler construction wherein the risers are arranged to substantially eliminate the necessity of providing refractory brick around any portion of the riser structures.

Another object of the invention is to arrange the risers in a boiler so as to create an elfective water wall at the outer portions of the boiler to reduce the need for refractory materials, and to enable a light weight insula- 3,408,990 Patented Nov. 5, 1968 tion to be provided on the side walls of the boiler housing.

A further object of the invention is to provide a unique boiler design wherein less than all of the riser panels are connected to the water and steam drums, so that a reduced number of spaced openings may be provided in these drums whereby generally light walled structures may be provided.

Still another object of the invention is to provide a unique boiler construction wherein only alternate riser panels are connected directly to water drum means at the lower portion of the boiler housing while adjacent riser panels are connected to one another so that water will flow upwardly in each of the riser panels, although each of them is not directly connected to the water drum means.

A still further object of the invention is to provide a novel boiler arrangement, wherein a pair of spaced water drums are provided at the lower portion of the boiler housing, with a row of riser panels being connected to each of the spaced drums.

Another object of the invention is to provide a boiler as set forth in the preceding paragraph, wherein the rows of riser panels are spaced from one another, particularly at the lower portion of the boiler housing, to provide a clearance space for servicing of the boiler structure.

Still another object of the invention is to provide a boiler structure wherein only alternate riser panels are connected to the water and steam drum means, with novel tube means being provided for connecting adjacent riser panels to one another and to the drum means.

Yet another object of the invention is to provide a compact boiler structure, wherein the desired heat trans fer can take place with a heating surface area that occupies a minimum amount of space. A related object is to provide a boiler structure wherein maximum heat absorption is achieved by a single vertical pass of products of combustion through the riser structure.

These and other objects of the invention will hereinafter become more fully apparent from the following description, taken in connection with the annexed drawings, wherein:

FIG. 1 is a perspective cut-away view of a preferred embodiment of the invention, with the housing removed, and with certain other portions of the structure broken away for clarity of illustration;

FIG. 2 is a fragmentary view of one end of the boiler of the present invention taken generally along line 2-2 of FIG. 3, and with certain parts broken away for clarity of illustration;

FIG. 3 is a view taken generally along line 33 of FIG. 4;

FIG. 4 is a cross sectional view taken generally along line 44 of FIG. 2, with certain portions broken away for clarity of illustration;

FIGS. 5 and 6 are fragmentary sectional views, similar to FIG. 3, and showing modifications of the invention wherein different size steam drums are provided; and

FIG. 7 is a cross sectional view, similar to FIG. 3, and illustrating a further embodiment of the invention.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail an embodiment of the invention together with modifications thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated. The scope of the invention will be pointed out in the appended claims.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, a boiler is illustrated in its entirety by reference numeral (FIG. 3) and includes a housing 11 having a base 12 and

upright side walls

13 and 14.

Housing side walls

13 and 14 have insulating means 15 on the inner surfaces thereof, and the lower portions of

side walls

13 and 14 are preferably seated within

confining flanges

12a and 12b at opposite sides of the base 12. Housing 11 further includes a top wall 16 having an opening 17 therein that communicates with a steam line 18.

End walls

19 and 20 complete the housing structure, and

walls

16, 19 and 20 are preferably insulated as shown.

First drum means is provided at the lower portion of the housing 11, and includes a pair of spaced, longitudinally extending

drums

21 and 22 that are supported on the housing base 12.

Drums

21 and 22 are disposed in a common horizontal plane, and extend parallel to

housing side walls

13 and 14.

Drums

21 and 22 each include a plurality of longitudinally spaced openings 23 (FIG. 1), and in the illustrated embodiment, openings 23 are provided in the upper portion of the drums so as to be vertically disposed. Each opening 23 in tube 21 is positioned generally in the same plane as an opening 23 in tube 22, and the planes of the pairs of openings 23 are positioned generally perpendicularly to

side walls

13 and 14 and base 12.

Second drum means is provided at the upper portion of the housing 11, and is defined by a single longitudinally extending, horizontally disposed drum 24 whose axis is disposed in the vertical median plane between

drums

21 and 22. Drum 24 is substantially larger in diameter than

drums

21 and 22, as is evident from FIG. 3. A plurality of longitudinally spaced openings 25 (FIG. 1) are provided at opposite sides of drum 24, and in the illustrated embodiment, openings 25 are positioned approximately above a horizontal plane through the axis of drum 24.

A plurality of riser panels are connected between the aforedescribed first drum means and second drum means, and more particularly, a first row 26 of riser panels is connected between lower drum 21 and one side of upper drum 24, while a second row 27 of riser panels is connected between lower drum 22 and the opposite side of upper drum 24.

Rows

26 and 27 are identical structurally, except of opposite hand, so that common reference numerals have been utilized to designate corresponding parts of the riser panels of each row, with the subscript 0 added to the riser panel elements of row 27. Row 26 includes alternating riser panels 28 and alternating riser panels 29 each positioned between a pair of panels 28, while row 27 includes alternating riser panels 28a and alternating riser panels 29a each positioned between a pair of panels 28a.

As is seen in FIG. 3, each riser panel 28 includes a generally horizontally disposed header portion 30, with a central stem portion 31 extending downwardly therefrom, and being received within an opening 23 in drum 21, it being understood that stem 31 is secured to drum 21 as by welding or the like. Spaced upwardly extending

stems

32 and 33 are provided on the upper side of header 30, and vertically disposed

tubes

34 and 35 are secured to stems 32 and 33, as by welding or the like. A further vertically disposed tube 36 includes a curved lower portion 37 that is secured to the inner most end of header 30, while a still further vertically disposed tube 39 is secured to the outermost end of header 30 by a right angle reducing elbow 38. Stem 31 and tubes 3436 are of the same diameter, while tube 39 is of a larger diameter. In an exemplary embodiment of the invention, stem 31 and tubes 34-36 are 2 in diameter, while tube 39 is 2 /2 in diameter.

Riser panels

29 and 29a are generally similar to

riser panels

28 and 28a, except that they are not connected directly to the

drums

21 and 22. It will be readily appreciated that since only alternate panels are connected to the

drums

21 and 22, the openings 23 therein are relatively widely spaced, and light walled tubing can be used for

drums

21 and 22. The structure of

riser panels

29 and 29a will be best understood from a consideration of FIG. 3, where riser panel 29a is shown in detail. Said riser panel includes a horizontally disposed header 40a at the lower end thereof which is positioned in a plane spaced vertically above the header portion 30a of an adjacent riser panel 28a. Tube means in the form of an upwardly inclined connecting stem 31a connects the central portion of header 40a with the rnidportion of the header 30a on an adjacent riser panel 28a. Header 40a includes spaced, vertically disposed

stems

42a and 43a to which the ends of vertically extending

tubes

44a and 45a are secured, as by welding. The rounded end portion 470 of a further tube 46a is connected to the innermost end of header 40a, while a right angle reducing elbow 48a connects the outermost end of header 40a to a still further vertically extending tube 4911. Stem 41a and tubes 44a-46a are preferably of the same diameter, while tube 49a is of larger diameter. In an exemplary embodiment, stem 41a and tubes 44a-46a are 2" in diameter, while tube 49a is 2 /2" in diameter.

The tubes of the riser panels are spaced equally across the boiler housing, and the tubes are aligned in vertical planes parallel to the

side walls

13 and 14 of the boiler housing. Thus, clearance means is defined between each of the vertically extending riser panel tubes, and burner means for heating the water within the riser panels is disposed at the lower portion of the clearance openings. Said burner means includes a plurality of longitudinally extending burner pipes 50 that are rectangular in the illustrated embodiment, and each burner pipe is positioned between aligned pairs of riser panel tubes. Ribbon type burner elements 51 extend longitudinally of pipes 50 in the upper wall thereof, it being understood that a suitable gas-air mixture flows through each of the pipes 50, while the flame issuing from burner elements 51 extends upwardly a substantial distance therefrom. In an exemplary embodiment of the invention, the burner pipes 50 are 3%" by 8", and the flame issuing from burner elements 51 extends approximately 40" above the burner elements 51. A pilot ribbon burner 52 extends generally transversely across burner tubes 50, and receives a suitable combustible mixture through inlet pipe 53. Suitable electrical ignition means 54 may be provided adjacent one end of pilot burner 52, and suitable flame sensor means 55 may be provided adjacent the opposite end of pilot burner 52. Air for combustion is drawn in through air ducts 56 and 57 (FIG. 3). Burner tubes 50 are supported by bracket means 58 (FIG. 2) secured to the boiler base, while the pilot structure is mounted on bracket means 59 (FIG. 2) connected to bracket means 58.

Means are provided for increasing the heat exchange areas between the riser panel tubes and the products of combustion issuing from the aforedescribed burner assembly, and in the embodiment of FIGS. 1-4 said means include a plurality of fins 60 secured to each of the tubes of the riser panels. Each of fins 60 is secured adjacent its inner end to the respective riser panel tube as by welding; and notches 61, formed as by saw cutting, are preferably provided in each of fins 60. Fins 60 are preferably welded throughout their length, and notches 61 preferably extend entirely through the fins to the riser panel tube. As can be best seen in FIG. 4, in the illustrated embodiment, four fiins 60 are secured to the opposite sides of each of the riser panel tubes, except for the outermost tubes, which have four fins 60 secured to their innermost surfaces. Fins 60 preferably extend downwardly into the flame area created by the burner assembly, and the lower end 62 of the fins are preferably tapered. In the exemplary embodiment, wherein the flame issuing from burner elements 51 extends approximately 40" above the burner elements, the fins 60 terminate 30" above the upper portion of the burner elements, so that at least the lower 10" of the fin 60 are disposed within the flame area. Tapered portions 62 are 12" in length in the exemplary embodiment.

The embodiment illustrated in FIG. 7 is the same structurally as that illustrated in FIGS. 1-4, except that a different form of means for expanding the heat exchange area is disclosed. In the embodiment of FIG. 7, instead of fins, vertical pipes 63 are connected between adjacent tubes in each riser panel and extend para'llel thereto. Each pipe 63 has an inwardly bent lower inlet end 64 and an outwardly bent upper outlet end 65; and taking a riser panel 28 for example, the inlet end 64 of a first pipe 63 is connected to tube 36 and the outlet end 65 thereof is connected to tube the inlet end 64 of a second pipe 63 is connected to pipe 35 and the outlet end thereof is connected to tube 34; and the inlet end 64 of a third pipe 63 is connected to tube 34 and the outlet end 65 thereof is connected to tube 39. The connection of pipes 63 for the other riser panels is identical to the arrangement described above for panel 28.

The means by which the riser panels are connected to the upper steam drum 24 will be best understood from a consideration of FIGS. 2 and 3. As is shown in FIG. 3, the upper ends of riser panel tubes 34-36 are bent outwardly as shown at -72, and make right angle connections at vertically spaced positions with outermost tube 39. In a similar manner, the uppermost ends of tubes 44a46a of riser panel 29a are bent outwardly as shown at a- 82a and make right angle connections at vertically spaced positions with outermost tube 49a. It will be understood, of course, that the connections of

riser panels

28a and 29 are identical to those described above. The number of openings 25 in steam drum 24 is one-half the number of riser panels, so that tube means connect the upper ends of the outermost tubes of adjacent riser panels to one another and to the steam drum. By making this type of connection, the steam drum is not materially weakened, so that a relatively thin walled steam drum may be provided. As is evident from FIG. 3, the upper ends of

outermost tubes

39 and 49 of

adjacent riser panels

28 and 29 are bent inwardly as shown at 73 and 74, respectively,

where they make a right angle connection with a downwardly inclined connecting tube 75 that is secured to the side wall of steam drum 24 and which projects inwardly thereof through an opening 25. In a like manner, the upper ends of the

outermost tubes

39a and 49a in

adjacent riser panels

28a and 29a are bent inwardly as shown at 83a and 84a in FIG. 3, and make a right angle connection with a downwardly inclined connecting tube 70a that is secured to the steam drum 24 at an opposite side from tube 75, and which projects inwardly of drum 24 through an opening 25 in the side wall thereof.

Generally vertically disposed steam separation baffles 76 are provided within steam drum 24 in alignment with openings 25, and baffles 76 are spaced above the normal water level L in drum 24. A steam outlet connection 27 is provided in the upper portion of steam drum 24 and communicates with the steam main 18. In the exemplary embodiment of the invention, steam drum 24 is 24" in diameter, and connecting

tubes

75 and 75a are disposed at an angle of 30 with respect to a horizontal plane through the axis of the steam drum. Connecting

tubes

75 and 75a are preferably larger in diameter than the outermost tubes of the riser panels, and in the exemplary embodiment of the invention wherein the outermost tubes are 2 /2" in diameter, the connecting tubes are 3" in diameter.

A slightly modified means of connecting the riser panels to the steam drum is illustrated in FIG. 5, and this arrangement is utilized in an exemplary embodiment wherein the steam drum is larger in diameter than that illustrated in the embodiment of FIGS. 1-4. In the embodiment of FIG. 5, primed reference numerals have been used to designate elements corresponding to those of the embodiment of FIGS. 1-4. To accommodate the larger steam drum 24, the side walls of the boiler housing are enlarged as shown at 90, and the inner surface of enlarged wall portion includes an outwardly inclined surface portion 91, and a generally vertically disposed surface portion 92. Tubes 34-36' of riser panel 28' have outwardly bent upper ends 70'72 that make right angle connections at vertically spaced positions with outer riser panel tube 39. Outermost riser panel tube 39 includes an outwardly inclined portion 93 adjacent inclined wall surface 91, and the upper end 94 of outermost riser panel tube 39' is generally vertically disposed and makes a right angle connection with a horizontal portion 96 of a connecting tube 95. The upper end of outermost tube 49 in riser panel 29' makes a right angle connection with the horizontal portion 96 of connecting tube 95, and the downwardly inclined inner end 97 of connecting tube is secured to steam drum 24 and projects inwardly thereof through an opening 25 in the drum side wall. In the embodiment of FIG. 5, the steam drum 24 is 36" in diameter, and connecting tube portion 97 is disposed at an angle of 30 with respect to a horizontal plane through the axis of steam drum 24'.

In FIG. 6, a still further mode of connecting the riser panels to the steam drum is illustrated, and double primed reference numerals have been used to indicate elements corresponding to those of the embodiment of FIGS. 14. In the embodiment of FIG. 6, the steam drum 24" is larger in diameter than the steam drum illustrated in the embodiment of FIGS. 1-4, but smaller in diameter than the steam drum illustrated in FIG. 5. The upper ends of the side walls of the boiler housing are slightly enlarged as shown at 100, and the inner surface of the boiler side Walls adjacent enlargements 100 are inclined as shown at 101 and 102. The upper end of the outermost tube 49a" in riser panel 29a" is inclined inwardly as shown at 103, and makes a right angle connection with a downwardly inclined connecting tube 104, which is secured to steam drum 24 and the inner end of which extends inwardly of the steam drum through an opening 25" in the side wall thereof. The upper portion of the outermost'tube 39a" in the riser panel adjacent to riser panel 29a" in cludes a first outwardly inclined portion 105 adjacent inclined wall surface 101, and a second inwardly inclined portion 106 adjacent wall surface 102 that makes a right angle connection with connecting tube 104. In the exemplary embodiment of FIG. 6, a steam drum 24" is 30" in diameter, and connecting tube 104 is disposed at an angle of 30 with respect to a horizontal plane through the axis of the steam drum.

As can be best seen by comparing FIGS. 1 and 4, the

outer tubes

39 and 49, and 39a and 49a are positioned in tangency with one another throughout a substantial portion of their height. In the exemplary embodiment of the invention, wherein the overall height of the boiler is 12 10", and the

headers

30 and 30a are spaced approximately 12" from the base 12; the outermost riser panel tubes are tangent throughout a length of approximately 9' 11". It will be readily appreciated that the water rising upwardly in the outermost riser panel tubes elfectively creates water walls at the sides of theboiler which produce a substantial cooling effect. The cooling effect is particularly etfective at the lower portion of the risei panels, where the water therein is at its coolest temperature; and it has been found that with the aforedescribed arrangement, it is not necessary to provide refractory brick around the lower portion of the riser panels. As is evident from FIG. 4 although the outermost riser pane] tubes contact one another, the intermediate riser panel tubes are spaced from one another, so that the products of combustion from the burner assembly can pass freely upwardly through the riser panels in effective heat exchange relationship therewith. Suitable refractory material 110 (FIG. 2) is provided at the ends of the riser panel rows to insulate downcomers 111 and 111a that are connected between the steam drum 24 and

water drums

21 and 22 at opposite ends thereof.

The advantages flowing from the structure of the present invention are believed evident from the above description. It will be noted from FIG. 3 that the relative wide spacing between the small

diameter water drums

21 and 22 cooperates with the

curved portions

37, 37a, 47 and 47a of the innermost riser panel tubes to define a clearance space in which a Serviceman M can crawl to service the lower portions of the riser panels and the

drums

21 and 22. Thus, it is believed clear that each of the objects have been fully achieved.

We claim:

1. A boiler comprising: a housing; first drum means at the lower portion of said housing including a pair of horizontally spaced drums extending longitudinally of said housing; second drum means at the upper portion of said housing; and means connecting said lower drum means and said upper drum means including a first row of panels extending between one of said spaced drums and said second drum means, and a second row of panels extending between the other of said spaced drums and said second drum means; each of said panels having a plurality of generally vertically extending tubes, the outermost tubes of adjacent panels being positioned in engagement with one another for a major portion of their length, whereby wall means is defined at opposite sides of said boiler, the arrangement being such that only alternate panels in said rows of panels are connected to one of said spaced drums and including tube means connecting the panels that are connected to one of the spaced drums to an adjacent panel that is not connected to one of the spaced drums.

2. A boiler as defined in claim 1 wherein each panel includes a generally horizontally disposed header at the lower end thereof, with the tubes of each panel extending upwardly from their respective header generally perpendicularly with respect thereto.

3. A boiler as defined in claim 2 in which the headers of the panels that are connected to one of said spaced drums are positioned at a level lower than the headers of the panels that are not connected to one of said spaced drums, and wherein said tube means connecting adjacent panels is inclined upwardly from a lower header to an adjacent header.

4. A boiler as defined in claim 3 wherein the panels of said first and second rows are aligned generally in the same plane.

5. A boiler comprising: a housing; first drum means at the lower portion of said housing including a pair of horizontally spaced drums extending longitudinally of said housing; second drum means at the upper portion of said housing; and means connecting said lower drum means and said upper drum means including a first row of panels extending between one of said spaced drums and said second drum means, and a second row of panels extending between the other of said spaced drums and said second drum means; each of said panels having a plurality of generally vertically extending tubes, the outermost tubes of adjacent panels being positioned in engagement with one another for a major portion of their length, whereby wall means is defined at opposite sides of said boiler, the innermost tubes in said first row of panels being spaced from the innermost tubes in said second row of panels, whereby a work space is defined at the lower center of said housing between the spaced drums and the spaced lower portions of said innermost tubes; wherein the tubes of each panel are spaced from one another, with each tube of each row of panels being longitudinally aligned with a tube of the other panels in the row to define longitudinal passages between the panel tubes, and wherein burner means are positioned in said passages.

6. A boiler as defined in claim 5 wherein heat transfer means are associated with said panel tubes abovesaid burner means.

7. A boiler a's defined in claim 6 wherein said heat transfer means is defined by fin structure extending outwardly of said tubes.

8. A boiler as defined in claim 6 wherein said heat transfer means is defined by further tubes, each having a lower inlet end connected to a panel tube and an upper outlet end connected to an adjacent panel tube.

9. A boiler comprising: a housing; first drum means at the lower portion of said housing including a pair of horizontally spaced drums extending longitudinally of said housing; second drum means at the upper portion of said housing, said second drum means being defined by a single longitudinally extending drum spa'ced centrally above the spaced drums of said first drum means, and wherein the tube means connects the outermost tubes of said adjacent panels to said single drum; means connecting said lower drum means and said upper drum means including a first row of panels extending between one of said spaced drums and said second drum means, and a second row of panels extending between the other of said spaced drums and said second drum means; each of said panels having a plurality of generally vertically extending tubes, the out ennost tubes of adjacent panels being positioned in engagement with one another for a major portion of their length, whereby wall means is defined at opposite sides of said boiler, the arrangement being such that only alternate panels in said rows of panels are connected to one of said spaced drums and including single tube means connecting a pair of adjacent panels to said second drum means; and tube means connecting the panels that are connected to one of the spaced drums to an adjacent panel that is not connected to one of the spaced drums, said tube means being defined by a header pipe having at least a portion inclined downwardly toward said single drum.

10. A boiler as set forth in claim 9 in which the outermost tubes of each panel are larger in diameter than the remaining tubes of each panel, and wherein said header pipes are larger in diameter than said outermost tubes.

11. A boiler as set forth in claim 9 wherein bafile means is provided within said single drum positioned in general alignment with the connection to said header tubes.

12. A boiler comprising: a housing; first and second horizontally spaced, longitudinally extending drums at the lower portion of said housing, each of said first and second drums having a plurality of openings therein; a third longitudinally extending drum at the upper portion of said housing, said third drum having first and second sets of plural openings therein; first and second rows of panels connected between said first and third, and second and third drums, respectively, the number of panels in each row being greater than the number of openings in said drums; said alternate panels in each row being connected respectively to said first and second drums; means connecting certain of said panels adjacent their upper and lower ends, whereby material circulating from said first and second drums to said third drum may pass through each of said panels; and common tube means connecting adjacent panels to said third drum.

References Cited UNITED STATES PATENTS KENNETH W. SPRAGUE, Primary Examiner.