US3495556A

US3495556A - Heat exchanger of the tube bundle type

Info

Publication number: US3495556A
Application number: US742374A
Authority: US
Inventors: Andrew B Steever; Walfred W Jukkola
Original assignee: Dorr Oliver Inc
Current assignee: Dorr Oliver Inc
Priority date: 1968-07-03
Filing date: 1968-07-03
Publication date: 1970-02-17
Anticipated expiration: 1987-02-17

Description

Feb. 17, 1970 A. a. s'rEl-:vER ETAL 3,495,556

HEAT EXCHANGER 0F THE TUBE BUNDLE TYPE Filed July 3, 1968 7 Sheets- Sheet A1 Pff f /ff/ r Feb. 17, 1970 A. B. STI-:EVER EIAl.v 35495556 HEAT EXQHANGE'R 0F THE TUBE BUNDLE TYPE 7 Sheets-Sheet 2 Filed July 3. 1968 mh/ I mvmqg/ EK 4 ...nu L 6 WSJO@ .W. d B Wm m ER RF @New Aw w FIG.

ATTORNEY.

Feb. 17, 1970s' A. B. STL-:EVER ETAL 3,495,555

i HEAT EXGHNGER 0F THE TUBE BUNDLE TYPE Filed July s, 1968 7 sheets-snee; s

s*2 INVENToRs. 55 ANDREW B. sTEEvER WALFRED w. JUK OLA ATTORNEY.

Feb. 17, 1970 A. B.s1'|-:EvER"ETAl-' '35495555' HEAT ExcHANGEE oF THE TuEE BUNDLE TYPE Filed .July s. 1968 7 sheets-sheet 4 INVENTORS. ANDREW B. STEEVER WALFRED W.JUK OLA ATTORNEY.

' Feb. 17, 1970 Ara; sTEEvl-:RITAi-f 3,495,555

n HEAT EXCHANGER 0F THE TUBE BUNDLE TYPE Filed July 3, 1968 l 7 Sheets-Sheet 5 FIG Il INVENTORS. ANDREW B. STEEVER WALFRED W. JU OLA BYOZMM Mn ATTORNEY.

Feb. 17, 1970 A. asTEEvER ET AL Filed July 3, 1968 HEAT EXCHANGER OF` THE TUBE BUNDLE TYPE 7 Sheets-Sheet 6 INVENTORS. ANDREW B. STEEVER WALFRED W. JUKKOLA ATTORNEY.

Feb. 17,1970 A. B. s'rEEvER ETAL 3,495,556

' HEAT ExcHANGER oF Tan TUBE BUNDLE TYPE Filed Jllly 3. 1968 INVENTORS. ANDREW B. STEEVER ATTORNEY.

BY WALFRED W. JUKKOLA United States Patent 3,495,556 HEAT EXCHANGER 0F THE TUBE BUNDLE TYPE Andrew B. Steever, Old Greenwich, and Walfred W. Jukkola, Westport, Conn., assignors to Dort-Oliver Incorporated, Stamford, Conn., a corporation of Delaware Filed July 3, 1968, Ser. No. 742,374 Int. Cl. F23g 5/00; F231 15/00 U.S. Cl. 110-8 19 Claims ABSTRACT 0F THE DISCLOSURE A heat exchanger or combustion air preheater unit of the tube bundle type wherein heat is transmitted through the tube walls from one uid or gas to another, wherein the loop-shaped tube elements constitute an L-shaped bundle in an L-shaped casing, the L-shape providing thermal stress compensation for the tube elements.

are joined to a single tube plate so that the air inlet ends and the air outlet ends are arranged at respective opposite sides of a median plane. A header connected to the tube plate, divided along said median plane, provides the inlet and outlet connections for the air. Both the cold air inlet and the hot air outlet are thus located at the same end of the shell. v

The invention is concerned With the problem of differentials of heat expansion of the tube portions on opposite sides of the median planes, due to temperature differentials between the incoming combustion gases of say 1500 F. and the incoming combustion air which may have only ambient temperature. l

With a resulting air exit temperature of say 400 F. to 500 F., the tube legs on the air outlet side of the median plane will be considerably hotter than the tube legs on the inlet side of that plane. The lresulting heat expansion differentials and stresses in the loop-shaped tube elements may result in damage and eventual failure of the terminal joints of the tubes. Also, lateral distortions or deflections may occur of the tube bundle with concurrent strains not only on the tube, but also on the shell and the header, and their connections.

This invention provides an improved shell and tube bundle type of heat exchanger wherein the above indicated thermal stresses are eliminated or compensated for at least to a degree where they are not objectionable and can be readily absorbed.

More in particular, this invention aims to eliminate destructive thermal stresses on the pipe joints of the hot legs or exit ends of the tubes, since these joints in the tube plate are much more vulnerable at the higher temperatures than the joints at the inlet ends of the cold legs of the tube.

To attain the foregoing objectives the invention provides a heat exchange unit wherein the arrangement and shape of the tube elements and of the surrounding shell are such that the thermal expansion or elongation of the hot and cold legs of the tube elements will occur substantially independent of, and unrestricted by, each other.

Patented Feb. 17, 1970 lCe Yet, lateral bending stresses upon the hot legs are precluded for the protection of the pipe joints at the hot air exit ends of the tubes where the hot combustion gases enter the shell preferably for the sake of general thermal eliiciency of the unit.

Therefore, according to the invention, the shell of the heat exchanger is of generally L-shaped configuration comprising for example a horizontal leg and a Vertical depending leg. This shell encloses a correspondingly L- shaped bundle of loop-shaped tube elements. The inlet ends and outlet ends of these tube elements terminate in a tube plate which closes the free end of the horizontal leg of the shell. A header connected to the tube plate provides the inlet and outlet connections for the tubes.

Thus, a representative tube element has an L-shaped cold leg and an L-shaped hot leg substantially parallel thereto, and a communicating cross-connection or U- bend portion between the depending lower ends of the parallel L-shaped legs.

For the purposes of this invention, the horizontal part of the hot leg is supported within the shell in such a manner as to permit thermal expansion thereof only in the longitudinal direction, while allowing for free expansion of the remaining part of the tube element.

The great length of the tube element horizontally as well as vertically relative to the tube diameter will readily absorb the expansion or elongation dilferentials between the parallel hot and cold legs of the tube element. While this may involve slight deflections of the unsupported tube portions, the stress effects upon the entire tube and the tube connections are negligible in view of the floating 'condition of the vertically depending portion of the tube elements.

In this Way, the vertical depending loop portion of the L-shaped tube element, in effect provides an expansion joint for temperature dierentials between the hot and cold horizontal portions of the tube element. The deflectable horizontal cold leg portion, in turn, provides an expansion joint for the vertical depending loop portion of the tube element.

According to one feature, the L-shaped shell or casing has a lateral L-shaped wall that is removable so as to provide for the mounting or inspection of the L-shaped tube assembly including the tube sheet connected to the hot and cold ends of the loop-shaped elements.

Another feature lies in the provision of a casing which comprises a removable corner portion constituting the juncture of the horizontal and the vertical portion of the L-shaped shell, allowing for inspection. Preferably, each leg of this L-shape has an intermediate shell portion ange-connected to the corner portion, as well as Hangeconnected to respective end portions.

In a preferred embodiment, the header has a horizontal partition defining an upper compartment which communicates with the hot legs, and a lower compartment communicating with the cold legs of the loop-shaped tube elements. This makes it possible to provide a tube bundle which comprises a plurality of assemblies of loopshaped tube elements nested into one another, and to have these assemblies extend in planes parallel to one another.

Other features and advantages will hereinafter appear.

As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the metes and bounds of the claims, or of forms that are their functional as well as conjointly cooperative equivalents, are therefore intended to be embraced by those claims.

FIG. l is a vertical semi-diagrammatic view of a combustion system wherein the air preheater unit is positioned in the manner of an inverted L.

FIG. 2 is an enlarged and more detailed side view of the air preheater unit of FIG. 1, showing one embodiment of the L-shaped bundle of tube elements in a casing having a removable wall portion.

FIG. 3 is a plan view of the air preheater unit of FIG. 2, with top wall portions of the casing broken away.

FIG. 4 is an exploded view of the preheater unit of FIG. 3, illustrating a manner of assembly of the unit.

FIG. 5 is a fragmentary view of the corner portion of the preheater unit, illustrating the removal of the corner portion of the casing.

FIG. 6 is a detail cross-sectional view taken on line 6 6 in FIG. 5, showing the arrangement of sub-assemblies of tube elements in the tube bundle, and spacer means for the sub-assemblies.

FIG. 7 is a detail cross-sectional view taken on line 7 7 in FIG. 5, showing support and guide means for the tube elements.

FIG. 8 is a fragmentary view of the depending terminal loop portion of the tube elements, including spacer means therefor.

FIG. 9 is a diagrammatic illustration of the manner in which thermal expansion differentials are compensated for in the loop-shaped tube elements of the preheater unit of this invention.

FIG. l0 is a side elevation of a modified form of the tube bundle of FIGS. 1 to 8.

FIG. 11 is a top view taken on line 11-11 of FIG. 10.

FIG. l2 is an end view taken on line 12-12 of FIG. 10.

FIG. 13 shows some of the tube elements of FIGS. 10 to 12 in perspective view with thermal expansion thereof indicated in dot-and-dash.

FIGS. 14, l5, 16 show other arrangements and modifications of the air preheater of FIGS. 1 to `8.

FIGS. 17 and 18 show a modified tube header for the air preheater of FIGS. 1 to 8.

FIGS. 19 and 20 show other forms of the air preheater.

By way of example (see FIG. l), the heat exchange unit of this invention is herein shown to be employed as a preheater for the combustion air in a combustion system which may comprise a vertical combustion unit 11, a blower 12 supplying the combustion air, and a scrubber 13 for purging the combustion gases of entrained solids such as y ash or the like.

The combustion unit in this example is of a well known type that operates on the principle of fluidization. That is to say, a constriction plate 14 divides the unit into a combustion chamber 15 above, and a windbox 16 below. Material to be burned, for instance dewatered sewage sludge or the like is supplied at 17 into a fluidized bed B of sand. After a start-up period with the burner R providing hot uidizing gas, the combustion of the material in the red hot sand bed may become substantially selfsustaining, with the combustion air from blower 12 providing the bulk of the fluidizing gas. A small amount of supplemental fuel may be added depending upon the degree of moisture in the material to be burned.

Due to the wind pressure from blower 12 maintained in the system, the combustion gases from neck 18 of the combustion unit are forced through the horizontal leg 19 of the air preheater, and then down through the depending leg 20 and then via duct 21 through the scrubber 13 whence the purged or scrubbed gases may escape through stack 22.

The combustion gases in passing through the L-shaped shell or casing of the air preheater surround the l-shaped bundle of loop-shaped tube elements indicated at 23, thus heating the combusion air forced through these tube elements by the blower 12.

That is to say, the blower through rising duct 24 forces air into the lower chamber 25 of a header 26, and through the loop-shaped tube elements into the upper 4 chamber 27 of the header, and then through a duct 28 downwardly into the windbox 16.

The loop-shaped tube elements comprise a pair of L- shaped legs 29 and 30 herein termed the cold leg and the ho-t leg respectively, extending in a common vertical plane. As indicated at 31 the hot leg 30 is supported in such a manner as to allow for its horizontal axially directed thermal elongation, while preventing deflection or angular deformation. At the same time, the cold leg 29 as Well as the Shanks 32 and 32a of the depending loop portion 33, are left free to expand, as well as to deflect to the extent necessary to accommodate the relative thermal elongations of both the horizontal and the vertical depending portions of the tube element. Any potential thermal stresses in the tubes thus become selfcompensating, inasmuch as the depending loop portion absorbs elongation differentials of the horizontal hot and col leg portions, while bending stresses or deflection of the hot leg is avoided, thereby protecting a potentially vulnerable connection of the hot leg with the tube plate P, all as will be furthermore explained.

Referring to FIGS. 2 to 8 there follows a detailed description of one form of the heat exchange unit or air preheater of this invention:

The air preheater unit arranged in the manner of an inverted L comprises an L-shaped bundle 34 of tube elements enclosed by a correspondingly L-shaped casing 35 provided with refractory lining K backed by a suitable layer of insulating material I. The casing comprises a horizontal section H and a vertical depending section V, both connected to a corner section C as by flange connections 35 and 36. The horizontal section H in turn comprises sub-sections H-l and H-Z having a flange connection 37 with one another, while the vertical section V comprises sub-sections V-l and V-2 with flange connection 38 between them.

The casing is of rectangular cross-sectional conguration, and thus may also be defined as comprising a pair of L-shaped

vertical side walls

39 and 40 interconnected by a horizontal rectangular outer wall 41, a horizontal inner rectangular wall 42, a vertical rectangular outer wall 43, and an inner vertical wall 44, and further an inner corner wall portion 45. The L-shaped side wall 40 is detachable from the L-shaped body 46 of the casing, in order that the L-shaped tube bundle may be inserted sideways into casing as illustrated in the exploded view of FIG. 3. A tube plate T closes the open end of the horizontal section of the casing by way of flange and bolt connection F.

The horizontal section of the casing has a downwardly directed hot gas inlet neck 47 connected to the top end of the combustion chamber, and a lateral spent gas outlet neck 48 at the lower end of the vertical depending section connected to the scrubber 13.

The vertical depending part of the tube bundle terminates a distance d above the bottom 49 of the casing, so as to provide a dust collecting pocket 50 accessible through a manhole 51.

The L-shaped tube bundle connected to the tube plate T comprises a plurality of assemblies 52 of loop-shaped

tube elements

53, 54, 55 nested one into the other in a common vertical plane.

Each tube element comprises a pair of L-shaped legs communicating with one another at the lower depending end, and having an inlet end 57 for the incoming cold air and an outlet end 58 for the heated air.

Connected to the outer face of the tube plate is a box or header 59 having a horizontal partition 60 defining a lower chamber -61 communicating through tube plate T With the cold air inlet ends of the tube elements, and an upper chamber 62 communicating with the hot air outlet ends of the tube elements. The lower chamber 61 through connection 63 receives cold air from the blower, while heated air passes from the upper chamber 62 through a connection 64 to the windbox 16 of the combustion chamber.

While both the cold air inlet ends and the hot air outlet ends of the tube elements are located in the zone of entry of the hot combustion gases into the casing, the upper hot air outlet legs of the tube elements will assume substantially higher temperatures than the lower cold air intake legs, thus rendering them what is herein termed the "hot leg and the cold leg respectively.

Since the tube joints of the hot legs are more vulnerable than those of the cold legs, provision is made to support or maintain the horizontal portions of the hot legs in such a manner as to prevent lateral deflection thereof, while leaving them unrestricted as to thermal elongation. Accordingly, there are provided horizontal transverse support bars `65 one for each bank of horizontal end Portions ofthe hot legs. As seenin FIG. 7, the ends of each bar are supported upon small brackets `66 or the like provided upon the inner faces of the vertical side walls of the casing.

Proper spacing of the vertical assemblies of tube elements is insured by the provision of inverted U-shaped spacer elements 67 applied to the tubes as indicated in FIG. 8. One set S-1 (see FIGS 2 and 4) of such spacer elements is located Within the corner portion of the casing, another set S-2 is applied to the depending terminal portion of the tube elements (see FIG. 6).

Prom FIG. 5 it will be seen that the body portion 68' of the corner section of the casing is removable by separating respective flange connection, while leaving the corner wall portion 45 in place. While thus exposing the corner portion of the tube bundle and providing access to the support bars 65, the removal of casing corner part 68 also allows for inspection and cleaning of the horizontal as well as the vertical portions of the tube bundle.

However, for periodical cleaning of the tube elements, soot blowers 68a may be provided, for instance in the form of horizontal transversely extending blower tubes or the like.

Transverse spacer bars 69 are provided between banks of vertical tube portions (see FIG. 6), in order to allow the depending portion of the tube bundle to be tcnsioned against the adjacent inner vertical Wall 44 of the casing when the horizontal tube portions are in a cold or contracted state. Thus, when -upon heating in the operation of the unit, the horizontal tube portions expand, the depending portion of the tube bundle will become untensioned an-d assume a desired median position in the depending section of the casing.

FIG. 9 diagrammatically illustrates the manner in which thermal stress compensation for the loop-shaped tube elements is achieved by this invention. The cold or inoperative condition of the tube element 70 according to the invention is indicated in full lines, while the dot-and dash outline indicates thermal elongation changes of the element when in operation.

Incoming cold air from chamber 71 of header 72 passes through the L-shaped cold leg 73 of the tube element concurrent with the ow of the combustion gases through the L-shaped casing 74 from inlet 75 to outlet 76 thereof. The air then passes through bend 77 into the L-shaped hot leg 78 countercurrent to the how direction of the combustion gases, into chamber 79 of the header for delivery to the combustion unit.

The horizontal exit portion P-1 of the hot leg 78 is supported and guided at 80 in such a manner as to prevent any lateral deection, While allowing for axially directed thermal elongation as indicated by the length differential t-1. Such guidance avoids any objectionable bending stress on the tube plate joint 81 at the hot end of the tube element.

Concurrently there is some thermal elongation t-2 of the horizontal portion P42 of the cold leg 73, of a lesser extent than elongation t41 of horizontal hot leg portion P-1. This dilerential, however, is absorbed by the considerable length of the vertical depending loop portion P-S of the tube element, which loop portion then in effect acts an expansion joint.

The vertical depending loop portion, in turn, is subject to thermal elongation as indicated by the length differential t-3. That is to say, the vertical shank Q-l of the loop elongates downwardly, while shank Q-2 elongates upwardly. Any resultant differential in the elongation of these two vertical Shanks, however, is absorbed by the considerable length of the horizontal cold leg portion P-Z, as indicated by a deilection t-4 thereof. While the amount of this deflection depends upon operating temperature conditions, it will impose no objectionable stress -npon the tube plate joint 82 which is subject to much lower temperatures than hot tube joint 81.

The modified form of the tube bundle in FIGS. 10 to 13 is such as to accommodate a vertical partition 83 in tube header 8'4, in differentiation from the horizontal par tition 60 in the tube header of FIG. 2. Accordingly, a representative tube element 84a (see FIG. 13) comprises a pair of L-shaped

legs

85 and 86 extending in vertical planes parallel to one another, conncted to a tube plate 87a at opposite sides of partition 83, and a transverse intermediate tube portion 87 interconnecting the lower ends of the L-shaped legs. Supporting and guide means for the horizontal portion of the L-shaped legs are indicated at 88. The dot-and-dash lines in FIG. 13 indicate potential thermal expansion of the tube element.

' The arrangement of the preheater in FIG. 14 differs from FIG. 2 only in that the vertical leg of the unit is pointing upwardly.

In FIG. 15 the L-shaped air preheater unit is connected to an outlet albow 89 of the combustion chamber, so that horizontal part of the FIG. 2 unit is now vertically arranged pointing upwardly from the top end of the combustion chamber.

In FIG. 16 the air preheater unit is of generally Z-shaped configuration, but otherwise similar in arrangement and function to FIG. 2.

In FIGS. 14 to 16 the tube bundle is indicated by a single tube element in dotted lines.

FIGS. 17 and 18, the tube header box 59 of the air preheater unit in FIG. 2 has been replaced by individual

tubular headers

90 and 91 providing the air inlet and air outlet chambers for the tube elements 93 which have extensions 94 extending outwardly beyond the tube plate 95 to which the tube elements are connected.

In the embodiment of FIG. 19, the casing of the air preheater unit has a U-shaped intermediate section 96, and end

sections

97 and 98 extending from the ends of the U-shape in opposite direction and in a common vertical plane with the intermediate section. Within this casing from end to end thereof and conforming to the shape of the casing extends a tube bundle here indicated only by a single tube element 100 in dotted lines.

The upper ends of the tube elements are connected to 'a tube plate 101 closing the upper end of the casing, and

having in turn connected thereto a header :box or air outlet chamber 102 communicating with the upper ends of the tubes.

The lower ends of the tube elements are connected to a tube plate 103 closing the lower end of the casing, and having in turn connected thereto a header box or air out- .let chamber 104 communicating with the lower ends of the tubes. The upper end of the casing has a lateral hot gas inlet neck 104. The lower end of the casing has a lateral spent gas outlet neck 105.

Support means 106 are shown at the upper end of the unit, while lower end is guided or secured at 107 against lateral displacement. An adjustable guy rod or suspension member 108 interconnects the end portions of the U-shaped section of the unit, as a weight transmitting element. Adjustment means for this element may comprise a plain turnbuckle 109 or a turnbuckle device 110 including a compression spring 111.

The embodiment of the air preheater unit in FIG. 20 is similar in principle to the one in FIG. 19 just described, dilering therefrom only substantially in respect to the fact that the depending U-shaped intermediate section 112 has horizontal

parallel end sections

113 and 114 pointing in the same direction.

Both embodiments, FIGS. 19 and 20, provide the opportunity for true countercurrent operation, in that hot gas inlet and hot air outlet may be located at one end, and spent gas outlet and cool air inlet may be located at the other end of the unit.

Support and guide means for the tubes are indicated in FIG. 14 at 115, in FIG. 15 at 116 and 117, in FIG. 16 at 118 and 119, in FIG. 19 at 120, and in FIG. 20 at 121.

Following is an example of thermal expansion (see FIG. 9) of one of the tube elements in operation.

(A) Approximate dimensions of tube element:

1. A heat exchange unit which unit comprises a plurality of loop-shaped tube elements each of which comprises a pair of L-shaped legs substantially parallel to one another and communicating with one another at one end, and having an open inlet end and a open outlet end, said tube elements being assembled so as to constitute an L-shaped bundle of such tube elements with all the open ends thereof located adjacent one another,

a tube plate connected to all said open ends of the tube elements with said open inlet ends occupying one portion of the plate, and said open outlet ends occupying the complementary portion of said plate,

header means comprising an inlet chamber cornmunicating with said inlet openings and having an inlet for the incoming cold uid, and an outlet chamber communicating with said outlet openings, and having an outlet for the heated fluid,

a casing surrounding said bundle of tube elements, substantially conforming to the L-shape thereof, with one end of the casing closed by said plate, and having an inlet for the incoming hot fluid at one end, and an outlet for the spent hot uid at the other end, and

support means in the casing for securing the outlet end portions of the tube elements against lateral deflection, while allowing for their longitudinal thermal elongation.

2. The heat exchange unit according to claim 1, wherein both legs of the L-shaped casing are of rectangular cross-sectional configuration, and wherein one L-shaped side wall of the casing is detachable.

3. The heat exchange unit according to claim 1, wherein both legs of the L-shaped casing are of rectangular configuration, wherein one L-shaped side wall of the casing is detachable, and wherein said bundle of tube elements together with the tube -plate is detachable from the casing.

4. The heat exchange unit according to claim 1, Wherein the casing has L-shaped side walls and rectangular walls interconnecting the side walls, wherein said header means has a partition extending parallel to the adjacent interconnecting walls, and wherein said tube bundle comprises a plurality of assemblies of said tube elements, each assembly in turn comprising a plurality of said tube elements nested into one another in a plane parallel to the L-shaped side walls of the casing.

5. The heat exchange unit according to claim 1, wherein the corner portion of said casing is detachable, and wherein said support means are located adjacent to said corner portion.

6. The heat exchange unit according to claim 1, where- 1n the casing is provided internally with a layer of heat insulation.

7. The heat exchange unit according to claim 1, wherein the casing is provided internally with a layer of refractory material.

8. The heat exchange unit according to claim 1, wherein said casing has L-shaped side walls and rectangular transverse walls interconnecting the side walls, wherein said header means has a partition extending parallel to said side walls, and wherein said tube bundle comprises a plurality of assemblies of said tube elements, each assembly comprising a plurality of tube elements nested into one another, so that the inlet and outlet end portions of the tube elements of each such assembly are located in respective planes parallel to the adjacent inner and outer interconnecting rectangular transverse wall portions of the casing, and the loop-shaped end portions of each such assembly are located in respective planes parallel to the adjacent inner and outer interconnecting rectangular transverse wall portions of the casing.

9. A combustion system which comprises a combustion unit with combustion gas outlet at the top, an L-shaped gas conduit having a horizontal leg and a depending leg, said horizontal leg having a lateral connection with said gas outlet, said depending leg having a spent gas discharge at the lower end, a tube plate closing the `free end of said horizontal leg,

a loop-shaped tube element comprising a pair of L- shaped legs substantially parallel to one another and communicating with one another at one end, and having an open inlet end and an open outlet end, and constituting an L-shaped loop located within and coextensive with the L-shape of said gas conduit said open inlet and outlet ends of the loop being connected to said tube plate,

header means comprising an inlet chamber communicating with said inlet end of the loop and provided with an inlet for cool air, and an outlet chamber communicating with said outlet end of the loop, and provided with an outlet for heated air, and

support means in said gas conduit for securing the outlet end portion of said tube against lateral dellection, while allowing for its longitudinal thermal elongation.

10. The combustion system according to claim 9, with the addition of a -gas scrubber connected to said lower end of the gas conduit so that dust particles in the gas are swept through said depending leg of the gas conduit downwardly into said scrubber.

11. The heat exchange unit according to claim 1, wherein said header means comprise a casing connected to the tube plate, enclosing all said open ends, and having a partition dening said inletand outlet chambers.

12. A combustion system which comprises a combustion unit with combustion gas outlet at the top, an L- shaped gas conduit having at one end of the L-shape a lateral connection with said gas outlet, with a spent gas discharge outlet at the other end, and having a tube plate closing said one end, a loop-shaped tube element cornprising a pair of L-shaped legs substantially parallel to one another and communicating with one another at one 6nd, and having an Open inlet end and an open outlet end, and constituting an L-shaped loop located within and coextensive with the L-shape of said gas conduit, said open inlet and outlet ends of the loop being connected to said tube plate,

header means comprising an inlet chamber communicating with said inlet end of the loop and provided with an inlet for cool air, and an outlet chamber comunicating with said outlet end of the loop, and provided with an outlet for heated air, and

support means in the gas conduit for securing the outlet end portion of said tube element against lateral deflection, while allowing for its longitudinal thermal elongation.

13. A heat exchange unit which comprises an openended tube element having an intermediate loop-shaped portion of substantially U-shaped formation, and straight end portions extending laterally from the respective ends of the U-shape and substantially in the plane of said U- shape,

a casing surrounding said tube element substantially coextensive therewith, and having a tube plate closing each end and connected to respective ends of the tube element, said casing providing a gas conduit surrounding said tube element, and having a hot gas inlet adjacent to one end, and a spent gas outlet adjacent to the other end,

header means comprising an inlet chamber communicating with one end of said tube element, and having an inlet for cool air, and an outlet chamber communicating with the other end of said tube element, and having an outlet for the heated air, and

support means in said casing for securing the outlet end portion of said tube element against lateral deflection, while allowing for its longitudinal expansion to be absorbed by said intermediate loop-shaped portion of the tube element.

14. The heat exchange unit according to claim 13, wherein said end portions of the tube element extend from said U-shaped intermediate portion in directions opposite to one another,

15. The heat exchange unit according to claim 13, wherein said end portions of the tube element extend codirectionally from said U-shaped portion.

16. The heat exchange unit according to claim 13, wherein said gas inlet and outlet are arranged so as to provide countercurrent ow of the air through said tube element and of the gas through said casing, relative to one another.

17. The heat exchange unit according to claim 13, with the addition of a tie element interconnecting the outer end portions of the Shanks of the U-shaped formation.

18. The heat exchange unit according to claim 13,

10 wherein said tie element is longitudinaly resiliently yieldable.

19. A heat exchange unit for preheating combustion air by combustion gases, which comprises a plurality of loop-shaped tube elements each of which comprises a pair of L-shaped legs substantially parallel to one another in a single plane and communicating with one another at one end, and having an open inlet end and an open outlet end, said tube elements extending in parallel planes and laterally in substantial registry with one another, and constituting an L-shaped bundle of such tube elements with all the open ends thereof located adjacent to one another,

tube plate means connected to all said open ends of the tube elements with said open inlet ends occupying' one portion of the plate, and said open outlet ends occupying the complementary portion of said plate, header means comprising an inlet chamber communicating with said inlet openings and having an inlet for incoming cold air, and an outlet chamber communicating with said outlet openings, and having an outlet for the heated air,

a casing structure enclosing said bundle of tube elements substantially conforming to the L-shape of said bundle of tube elements, said casing structure having a closed end adjacent to said communicating ends of the tube elements, and having its opposite end closed by said tube plate, said casing structure having a lateral gas inlet adjacent to the outlet ends of said tube elements, and having a lateral spent gas outlet located adjacent to the inlet ends of said tube elements, said casing structure being so constructed and arranged relative to said enclosed bundle of tube elements that the combustion gas passes from said lateral gas inlet to said lateral gas outlet of the conduit along a path along and counter current to the path of air passing through said tube elements from the air inlet chamber to the air outlet chamber of said header means.

References Cited UNITED STATES PATENTS 3,366,080 1/1968 Albertson 110-8 3,319,586 5/1967 Albertson et al. 1 10-8 3,079,992 3/1963 Otten et al 165-158 XR 2,779,573 l/1957 Kuroda 165--82 2,416,273 2/1947 Wallis et al 165--158 XR KENNETH W. SPRAGUE, Primary Examiner U.S. Cl. X.R. -56; 165-82