US3315647A

US3315647A - Marine steam generator having fluid cooled furnace

Info

Publication number: US3315647A
Application number: US467908A
Authority: US
Inventors: Leonard E Triggs
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1965-06-29
Filing date: 1965-06-29
Publication date: 1967-04-25
Anticipated expiration: 1984-04-25
Also published as: ES327906A1; NL6608286A; GB1135590A; NO117243B

Description

April 25, 1967 E. TRIGGS 3,315,547

MARINE STEAM GENERATOR HAVING FLUID COOLED FURNACE Filed June 29, 1965 SSheetS-Sheet l INVENTOR. LEONARD E. 7' H1665 ATT ORNE Y April 25, 1967 L. ETRIGGS 3,315,647

MARINE STEAM GENERATOR HAVING FLUID COOLED FURNACE Filed June 29, 1965 3 Sheets-Sheet 2 /NVE/VTOA7 LEONARD E 772/665 ey @Wi-M# A 7*'TOR/VE Y April 25, 1967 E. TRIGGS 3,315,647

MARINE STEAM GENERATOR HAVING FLUID COOLED FURNACE Filed June 29, 1965 v 5 Sheets-Sheet 5 42 42 Q I K@ 6a F/G 5 fab* INVENTOR Eo/M20 157e/66s BY QMWJMML? v /4 7` TORNE Y United States Patent O poration of Delaware Filed June 29, 1965, Ser. No. 467,908 Claims. (Cl. 122-478) This application is a continuation-in-part of U.S. patent application Ser. No. 448,570, tiled Apr. 8, 1965, now Patent No. 3,229,671.

The present invention relates to the construction and operation of marine-type vapor generating and superheating units. More particularly, the invention relates to a novel construction of a natural circulation marine vapor generator of the two-drum type wherein the furnace chamber is completely water cooled by tubes that additionally serve as the gas enclosure structure of the chamber. The invention further engenders a marine vapor generator and superheating unit wherein the furnace oor, superheater screen, and superheater roof are formed from a single circuit of vapor generating tubes arranged in welded panels.

v In vapor generators of the marine type, it is common to employ vapor generating tubes for fluid cooling the walls and roof of the furnace chamber; however, the furnace floor in such units is normally lined with downcomer tubes connecting the liquid drum with the riser circuit, such as at the lower header of the front or side walls. This practice requires the application of a layer of insulating material upon the floor tubes so as to prevent heat absorption therein which would be sufficient to effect the generation of vapor in these tubes and thereby impede the circulation of fluid within the system. The insulation most commonly employed consists of a layer of thermal insulating material, such as magnesite, covering the tubes, and a layer of furnace refractory applied to the top of this layer. The use of insulation and refractory material within a furnace, while being effective for the purpose intended, is undesirable since it adds materially to the cost of the unit in that, in addition to its initial cost, the effects of high temperature require frequent repair and replacement throughout the life of the unit. This repeated need for maintenance gives rise to undesirable shutdown of the unit which could be dangerous to a vessel at sea, as well as increasing the operating costs of the unit due to the labor and material costs required in replacing the spent materials.

The present invention provides a vapor generating unit wherein the presence of thermal insulation and refractory material is eliminated from the furnace chamber due to the fact that, in the instant unit, the floor of the furnace chamber as well as the walls and roof thereof are completely fluid cooled. This is accomplished by lining the oor and walls of the chamber with tubes in which vapor is generated whereby the velocity of the fluid flowing through the tubes is sufficiently great such that the flowing fluid absorbs the heat generated in the furnace thereby preventing burnout of the tubes.

The invention also provides for the construction of the walls that define the furnace enclosure in the instant unit of preformed, welded tubular panels. By constructing the unit in this manner, the wall-lining tubes are effective to serve as part of thegas-enclosing structure in the area of the furnace chamber thereby eliminating the need for heavy skin casing around the chamber and concomitantly reducing the cost of fabrication of the unit.

According to the invention the walls of the furnace chamber are formed by a plurality of panels each comprising a number of spaced, parallel tubes with the spaces therebetween being lled with a gas-tight metallic membrane. The floor of the chamber is formed by a uid circuit that comprises similar tubular panels extending between the lower front wall header at one end and the vapor and liquid drum at the other end. The fluid circuit panels are so constructed as to additionally permit the tubes thereof to define a screen protecting the superheater tubes against slag and the radiant effects of the llame generated in the furnace chamber. These panels also form a cover overlying the superheater chamber thereby protecting the furnace roof in this area against the high temperatures existing in the superheater chamber. Moreover, the invention engenders the arrangement of the panels that define the side walls of the unit such that some comprise extended tubes that are offset and overlie the top of the furnace chamber thus forming the roof thereof. By forming the roof in this manner, there is provided a convenient means for adequately distributing the liquid supply to all of the furnace-lining tubes such that none of the tubes receive an inadequate supply of liquid that would render the tubes subject to excessive thermal stresses.

Therefore it is an object of the present invention to provide a marine-type vapor generating and superheating unit characterized by a furnace chamber which is completely fluid cooled. v

A further object of the present invention is to provide a marine-type vapor generating and superheating unit wherein the need for the application of thermal insulating and furnace refractory material to the oor of the furnace chamber is eliminated.

Another object of the present invention is to provide a marine-type vapor generating and superheating unit wherein the furnace floor, superheater screen, and superheater roof are formed of a single tubular circuit comprising a plurality of parallelly arranged vapor generating tubes.

Still another object of the present invention is to provide a marine-type vapor generating and superheating unit wherein the furnace chamber is formed by welded tubular panels that serve as part of the gas-enclosing structure of the unit.

Various other objects and advantages will appear from the following description of the invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

The invention will be understood from the following description when considered in combination with the accompanying drawings forming a part thereof wherein:

FIGURE 1 is a side elevation of a marine-type steam generating and superheating unit embodying the present invention;

FIGURE 2 is a section taken along line 2 2 of FIG- URE 1;

FIGURE 3 is a plan view taken along line 3-3 of FIGURE 1;

FIGURE 4 is a section taken along line 4 4 of FIG- URE 1;

FIGURE 5 is a section taken along line 5-5 of FIG- URE 1; and

FIGURE 6 is a detailed view of a typical corner indicated as area 6 of FIGURE 1.

Referring now to the drawings, there is shown a marime-type vapor generating unit 10 comprising a furnace section 12, a boiler bank section 14, and a superheating section 16. The furnace section 12 comprises a pair of spaced side Walls 18, a front wall 20 and a roof 22 formed of closely spaced fluid-bearing tubular members arranged to form a chamber into which a plurality of fuel burners 24, mounted in the roof 22, is adapted to fire. The boiler bank section 14 is located in rearward alignment with the furnace section 12 and comprises an upper vapor and liquid drum 26, a lower liquid drum 28 and a plurality of substantially upright vapor generating tubes 30 connecting the two. The superheater section 16 occupies a position intermediate the furnace section 12 and the boiler :bank section 14. Within this section is positioned a vertical superheater 32 formed of a plurality of vertically oriented, laterally aligned, inverted U-shaped tubes 34 arranged in a bundle that extends across substantially the full Width of the unit and connected at their terminal ends by elongated headers 36 which are arranged to connect the superheater tubes in parallel. The furnace, superheater and

boiler bank sections

12, 14 and 16 are arranged for the straight through flow of combustion gases which exit the unit through a rear gas pass 17 that is adapted to house a vaporizable liquid preheater or economizer 15.

The disclosed vapor generator embodies a generally box-like construction so as to permit unimpeded cubical expansion during operation. The base of the unit includes horizontally disposed liquid drum 28 extending transversely of the sides of the vapor generator structure. In spaced parallel relation thereto is a lower front Wall header 40. Lower side wall headers 38 extend between and uidly connect header 40 with the liquid drum 28. As shown, header 40 is located at a lower elevation than the liquid drum 28 such that lower side wall headers 38 are inclinedat a slight angle with respect to the horizontal. The periphery of the upper portion of the unit is dened by the vapor and liquid drum 26 parallelly arranged with respect to the liquid drum 28, upper side wall headers 42 and upper -front wall header 44. The upperside wall headers 42 and front wall header are constructed of a continuous U-shaped header as shown in FIGURE 3. The vapor and liquid drum 26 4is supported in an elevated position above liquid drum 28 by external downcomer conduits 46 that extend between the two drums to supply the drum 28 with operating liquid and the tubes 3 0 which form the boiler bank. The forward ends of upper side wall headers 42 and the front wall header 44 are vertically supported by upright vertical buckstays 48 which extend between and connect with the

headers

42, 44 and lower front wall header 40. The headers 42 are downwardly inclined parallel with lower side wall headers 38 -with their rear ends terminating short of the vapor drum 26. Vapor relief conduits 58 unite the vapor drum 26 and the

headers

42 and 44 in lluid circulation. This arrangement presents a framing construction for the steam generator that is structurally sturdy and that will permit the pressure parts of the unit to thermally expand in an unimpeded manner. Augmenting the support of the structure are diagonal external downcomer conduits 52 which connect the water space in vapor and liquid drum 26 with lower front wall header 4t) and also serve as diagonal bracing members tying the lower front wall header 40 to the drum 26.

The radiantly heated walls, 18 and 2t), and roof 22 of the furnace section 12 are formed lby substantially vertically oriented parallel vapor generating tubes. Those tubes emanating from lower front wall header 40 are substantially straight tubes that connect directly to upper front wall header 44 while those which emanate from the lower side wall headers 38 do so in a manner which etfects formation of the roof 22 of the furnace section 12 as well as the side walls 18 thereof as described in greater detail hereinafter. The outer end rows 30 of the boiler bank tubes 30 contain tubes that are nned and covered by skin casing 31, while those tubes 30 that form the rear row of the boiler bank 14 are spaced to permit the ow of combustion gas to the gas pass 17.

In accordance with the present invention the

walls

18 and 20 that enclose the furnace chamber 12 and superheater chamber 16 are arranged as preformed, welded tubulous panels. Each panel comprises a row of parallel vapor generating tubes 19 that are fusion welded in sideby-side relation thereby presenting a gas-tight, metallic filler 21 between adjacent tubes as shown best in FIG- URE 6. The endmost tubes 19 in each panel are provided with an elongated fin 23 that permits welding, indcated as 25, of adjacent panels upon their erection. The panels 28a, 2Gb, 28e and 29d that form the front wall 20 comprise substantially straight tubes that extend between the lower and upper

front wall headers

40 and 44, being slightly offset at their ends for attachment to the headers. On the other hand, however, the side walls 18 and roof 22 are formed of alternating L-shaped and straight tubulous panels 18a, 181), 18C and 18d, that are arranged to close the furnace 12 and superheater chamber 16. In the disclosed arrangement the panels 18a and 18C comprise L-shaped tubes 19 that extend from the lower side wall header 38 of one wall to the top of the furnace chamber 12 where they are laterally offset, as shown in FIGURE 5, and extend across the top of the furnace to the upper header 42 on the opposite wall thereby serving to form the roof 22. The

panels

18b and 18d comprise substantially straight tubes that extend between

headers

38 and 42 in the same wall. The panels are erected with each side Wall 18 being formed by alternating L-shaped and

straight panels

18a, 18b, 18C and 18d. The Lashaped panels 18a and 18C that form one wall are opposed by

straight panels

18b and 18d in the opposite wall, thus the roof 22 is formed by the tubes of four panels, 18a and 18e of one side wall and 18a and 18C of the other side wall.

In order to provide for the mounting of burners 24 in theroof 22, some of the tubes, indicated as 27, are vertically and laterally offset, as shown in FIGURE 4, to provide space 29 within which the burners 24 can be mounted for directing the products of combustion that issue therefrom onto the furnace chamber 12.

There is also provided in accordance with the invention a generally Z-shaped fluid circuit 54 formed of

panels

54a, 54h, 54e and 54d including fusion welded vapor generating tubes 56 which connect at one end to lower front wall header 40 and at the other end to the vapor and liquid drum 26. The

tubular panels

54a, 54b, 54C and 54d are welded in side-by-side relation across the width of the furnace section 12 with the lower leg 58 forming the floor of the furnace, the upper leg 60 forming a roof over the superheater 32 and the generally upstanding connection portion `62 forming `a superheater screen. In forming these

panels

54a, 54b, 54C and 54d, fusion welding is applied to the tubes only along that portion that forms the lower leg 58 portion of the panels. Adjacent the end of the weld the tubes 56 are offset substantially normally out of the plane of the oor thereby forming the upstanding or screen portion 62 of the circuit. Alternate tubes in this portion .are both longitudinally and laterally oifset from the plane of the circuit so as to provide spaces 63 through which gases from the furnace are permitted to pass to the superheater section 16 and boiler bank section 14. The tubes 56 are generally normally offset again to overlie the top of the superheater 32 and are returned to substantially coplanar relation and connect with the vapor and liquid drum 26. The operation of the disclosed steam generating and superheating unit is as follows:

With the burners 24 ring into the furnace section 12 feedwater is admitted to the vapor drum 26 where it mixes with the downcomer fluid or water separated from kthe vapor deposited in the drum. This mixture flows downwardly to the liquid drum 28 through the external downcomer conduits 46 and through downcomer tubes located within the boiler bank section 14. A portion of the liquid mixture in drum 26 flows from the drum to the lower front wall header 40 through the diagonal downcomer conduits 52. From the liquid drum 28 the mixture flows into the lower side wall headers 38 Which supply the tubes forming the side walls 18 and root` 22 as well as the front wall header 49. Lower front -wall header 40 supplies the tubes forming the front Wall 20 and those forming the fluid circuit 54 with fluid. Heat transfer occurs within the vapor generating tubes and thereby fluid circulation is established from the water drum 28 and front Wall header 40 to the vapor drum 26. Due to the thermal siphonic action existing within the tubes fluid is caused to flow upwardly through the side walls 18, front wall 20 and the fluid circuit 54, the tendency to flow being enhanced by the upward inclination of the unit from front to rear. Fluid flow through these tubes serves to cool the furnace chamber 12. In addition, the lluid flowing through the tubes 56 yforming the fluid circuit 54 provides the superheater 32 with both a screen to protect the tubes thereof from the radiant effects of the furnace and a roof or cover to protect the roof lining elements against overheating.

Because it is important that the velocity of the fluid owing through the fluid circuit 54 be relatively great, it is necessary to insure an adequate liquid supply to these tubes. In the present arrangement such adequacy of supply is provided by the fact that the roof 22 is formed by tubes in the side 'walls 18 that are supplied by headers 38 and the tubes that form the front wall 20 emanating from the same supply header 40 as the tubes forming the lluid circuit 54 are limited in length, extending only the height of the front wall thereby limiting the amount of heating surface presented by the tubes and concomitantly the `amount of liquid necessary to be supplied to the tubes. This permits the greater portion of the liquid supplied to the header 40 to be passed through the fluid circuit tubes. Were the tubes that form the front wall 20 also employed to form the roof 22 as by offsetting them over the top of the furnace chamber 12 to connect directly to the drum 26 rather than to header 42 there would be provided such a great amount of heating surface by the tubes that the amount of liquid in header 40 available to service the tubes in circuit 54 would be reduced thereby presenting the danger of an inadequate supply of operating fluid and possible burnout. By limiting the amount of heating surface presented 4by the tubes in wall 20 this danger is removed.

By means of the present invention there is provided a marine-type steam generating and superheating -unit having a completely liuid-cooled furnace section that requires no insulation and refractory material in this section. By means of the arrangement of side wall tubes the roof along with the walls of the furnace section can be formed of welded, gas-tight panels that eliminate the n-eed for separate skin casing thereby further reducing the overall cost of the unit. Marine-type boilers constructed in accordance with the present invention are simple in design yet eliicient in operation and, because of the fact that the furnace section is completely lluid cooled the units are substantially less expensive to construct and to operate.

It will be apparent to those skilled in the art that many alterations or modications of the herein disclosed structure can be made without departing from the spirit of the invention. For this reason the invention is to be limited only by the scope of the appended claims.

What is claimed is:

1. A vapor generating and superheating unit `comprising tubular walls dening a gas enclosure including a furnace chamber7 a superheating chamber and a boiler bank; said boiler bank including a horizontally extending upper drum, a lower drum underlying said upper drum in parallel relation therewith and a bank of upwardly extending tubes connecting said drums; said furnace chamber including rectangularly disposed front and side walls; upper and lower header means delining the terminal ends of said walls; said walls comprising a plurality of panels each including a number of heat exchange tubes extending between said upper and lower header means, said tubes being disposed in spaced, parallel relation and having the spaces therebetween lilled by a gas-tight metallic membrane, said front wall panels comprising substantially straight tubes and said side wall panels including some panels having their upper ends laterally olfset to overlie the top of said furnace chamber and connecting `with the upper header means of the opposite side wall thereby forming the roof of said furnace chamber; burner means operatively associated with said furnace chamber; superheater means disposed in said superheating chamber intermediate said furnace chamber and said boiler bank; a lluid circuit disposed across the width of said unit including a number of panels of parallel, spaced heat exchange tubes extending between said lower header means and said upper drum; said fluid circuit tubes extending along the bottom of said furnace chamber to a point adjacent the lower end of said superheater means thereby forming the floor of said furnace chamber; gastight metallic membrane lilling the spaces between said tubes in the licor-forming l,portion of said fluid circuit; said lluid circuit tubes being upwardly offset from said point and extending to said upper drum in screening relation to said superheater means; a number of said tubes in said upwardly offset portion being longitudinally and laterally odset with respect to the plane of said portion to form openings therebetween, said tubes in said upwardly offset portion being devoid of metallic membrane to form spaces therebetween thereby permitting the flow of gas from said furnace chamber to the other operating sections of said unit.

2. The organization of claim 1 including means disposed externally of said gas enclosure for supplying operating liquid to said lower header means.

3. The organization of claim 1 including downcomer conduit means disposed externally of said gas enclosure connecting said lupper drum and said lower header means for supplying operating liquid to said header means.

4. A Vapor generating and superheating unit comprising tubular `walls delining a gas enclosure including a furnace chamber, a superheating chamber and a boiler bank; said boiler bank including a horizontally extending upper drum, a lower drum underlying said upper drum in parallel relation therewith and a bank of upwardly extending tubes connecting said drums; said furnace chamber including rectangularly disposed front and side walls, rectangularly disposed upper and lower headers defining the terminal ends of said front and side walls; said walls comprising a plurality of panels each including a number of heat exchange tubes extending between said headers, said tubes being disposed in spaced, parallel relation and having the spaces therebetween filled by a welded, gas-tight, metallic membrane; said front wall panels comprising substantially straight tubes extending between upper and lower front wall headers; said side wall panels including alternate panels having their upper ends laterally offset to overlie the top of said furnace chamber `and connecting with the upper header of the opposite side wall thereby forming the roof of said furnace chamber; side wall panels intermediate said alternate panels being substantially straight and extending between upper and lower headers of the respective side walls; burner means operatively associated with said furnace chamber; superheat means disposed in said superheating chamber intermediate said furnace chamber and said boiler bank; a uid circuit disposed across the width of said unit including a number of panels of parallel, spaced heat exchange tubes extending between said lower front wall header and said upper drum; said fluid circuit tubes extending along the bottom of said furnace chamber to a point adjacent the lower end of said superheater means thereby forming the floor of said furnace chamber; gastight metallic membrane filling the spaces between said tubes in the floor-forming portion of said fluid circuit; said fluid circuit tubes being upwardly olTset from said point and extending to said upper drum in screening relation t0 said superheater means; a number of said tubes in said upwardly offset portion being longitudinally and laterally offset with respect to the plane of said portion to form openings therebetween said tubes in said upwardly offset portion being devoid of metallic membrane to form spaces therebetween thereby permitting the ow of gas from said furnace chamber to the other operating sections of said unit.

5. The organization of claim 4 including means disposed externally of said gas enclosure for supplying operating liquid to said lower header means.

6. The organization of claim 4 including downcomer conduit means disposed externally of said gas enclosure connecting said upper drum and said lower header means for supplying operating liquid to said header means.

7. A vapor generator including a boiler bank operatively associated with a furnace chamber; said furnace chamber comprising rectangularly disposed tubular front and side `walls and a tubular roof; upper and lower header means connected in the uid system of said vapor generator defining the terminal ends of said walls; each of said walls comprising a plurality of preformed, tubular panels bonded together in side-by-side relation; said panels each comprising a plurality of tubes bonded in parallel relation with the ends thereof adapted to connect between said upper and lower header means; said side walls including rst, substantially vertically straight panels having the tubes therein extending between and connecting substantially vertically aligned upper and lower header means, and second, angularly offset panels having the tubes therein containing a laterally offset portion overlying the top of said furnace chamber to form the roof thereof with the ends of said tubes extending between and connecting diagonally opposed upper and lower header means.

Y' 8. The organization of claim 7 wherein said side walls each include said tirst and second panels in alternate disposition with the corresponding panels in opposite walls being in staggered relation whereby the laterally offset portions of said second panels are disposed in side-by-side relation along the top of said furnace chamber.

j9. The organization of claim 8 wherein said panels each comprise a plurality of tubes disposed in spaced, parallel relation; and the spaces formed between said tubes being filled by a metallic membrane bonded between said tubes.

10. A vapor generator organization including, in cornbination, a boiler bank and a furnace chamber having rectangularly disposed tubular front and side walls and a tubular roof in open gaseous communication with said boiler bank; said boiler bank comprising a horizontally elongated upper drum, a lower drum disposed parallel Q Q with, and spacedly below, said upper drum, and a bank of generally upstanding tubes extending between and connecting said drums; said furnace chamber comprising an upper front wall header spaced forwardly of and parallel with said upper drum, opposed upper side wall headers extending between and `connecting the ends of said upper front wall header with said upper drum, a lower front wall header spaced forwardly of said lower drum, and in substantial alignment with said upper front wall header, opposed lower side wall headers extending between and connecting the ends of said lower front wall header with said lower drum; each of said walls -comprising a plurality of preformed, tubular panels bonded in side-by-side relation, said panels each comprising a plurality of tubes bonded together in parallel relation with the ends thereof adapted to connect between said upper and lower headers; said front wall comprising substantially vertically straight panels having the tubes therein extending between and connecting said upper and lower front wall headers; and said side walls including rst, substantially vertically straight panels having the tubes therein extending between and connecting substantially vertically aligned upper and lower side wall headers and second, angularly offset panels having the tubes therein containing a lateral offset portion overlying the top of said furnace chamber to form the roof thereof with the ends of said tubes extending between and connecting diagonally opposed upper and lower headers; said side walls each including said rst and second panels in alternate disposition with the corresponding panels in said opposed walls being in staggered relation whereby the laterally offset portions of said second panels are adapted to be bonded together in side-by-side relation along the top of said furnace chamber.

References Cited by the Examiner UNITED STATES PATENTS 1,619,447 3/1927 Thomsen 122-329 X 1,815,086 7/1931 Wood 122-336 X 2,112,202 3/1938 Lucke 122-336 3,030,938 4/1962 Vorkauf 122-328 X 3,229,671 l/1966 Triggs 122-478 FOREIGN PATENTS 846,340 6/1939 France.

474,087 10/ 1937 Great Britain.

794,046 4/ 1958 Great Britain.

CHARLES I. MYHRE, Primary Examiner.

Claims

7. A VAPOR GENERATOR INCLUDING A BOILER BANK OPERATIVELY ASSOCIATED WITH A FURNACE CHAMBER; SAID FURNACE CHAMBER COMPRISING RECTANGULARLY DISPOSED TUBULAR FRONT AND SIDE WALLS AND A TUBULAR ROOF; UPPER AND LOWER HEADER MEANS CONNECTED IN THE FLUID SYSTEM OF SAID VAPOR GENERATOR DEFINING THE TERMINAL ENDS OF SAID WALLS; EACH OF SAID WALLS COMPRISING A PLURALITY OF PREFORMED, TUBULAR PANELS BONDED TOGETHER IN SIDE-BY-SIDE RELATION; SAID PANELS EACH COMPRISING A PLURALITY OF TUBES BONDED IN PARALLEL RELATION WITH THE ENDS THEREOF ADAPTED TO CONNECT BETWEEN SAID UPPER AND LOWER HEADER MEANS; SAID SIDE WALLS INCLUDING FIRST, SUBSTANTIALLY VERTICALLY STRAIGHT PANELS HAVING THE TUBES THEREIN EXTENDING BETWEEN AND CONNECTING SUBSTANTIALLY VERTICALLY ALIGNED UPPER AND LOWER HEADER MEANS, AND SECOND, ANGULARLY OFFSET PANELS HAVING THE TUBES THEREIN CONTAINING A LATERALLY OFFSET PORTION OVERLYING THE TOP OF SAID FURNACE CHAMBER TO FORM THE ROOF THEREOF WITH THE ENDS OF SAID TUBES EXTENDING BETWEEN AND CONNECTING DIAGONALLY OPPOSED UPPER AND LOWER HEADER MEANS.