US2245720A - Air heater - Google Patents

Description

June 1 7, 1941. Y J, H ROHRER 2,245,720

AIR HEATER Filed Jan. 1o. 1959v s sheets-sheet 1 June 17, 1941. 1 H, ROHRER u 2,245,720

l AIR HEATER' Filed Jan. 10.. 1959 3 Sheets-Sheet 2 Izdeno June 17, 1941. J, H ROHRER` ,2,245,720

' AIR HEATER Filed Jan. lO. 41959 3 Sheets-Sheet 3 o n o l D I 0 lll/,121,115,12-

Patented .une 17, 1.941:

STATES PATENT oEFlcE 15 Claims.

This invention relates to air heaters and more particularly to an air heater of the type in which air for supporting combustion in a steam generator or other combustion unit, is preheated by the exhaust or combustion gases from the steam generator or other combustion unit.

It is an object of the present invention to provide an air heater which will be eiective in the transfer of hea't from hot gases passing through one portion of the air heater to air passing through another portion thereof.

It is a further object of the present invention to provide an air heater in which the heat transfer surfaces are disposed for a high effective heat transfer.

It is a further object of the present invention to pro-vide an air heater which will not require an excessive space.

It is a further object of the present invention to provide an air heater which will be substantially self cleaning and in which access to the interior is made easy.

Other objects of the invention will appear from the annexed specification and claims.

The nature and characteristic features of the invention will be more readily understood from the following description, taken in connection with the accompanying drawings forming part hereof, in which:

Figure 1 is a front elevational View of a preferred embodiment of the air heater of the present invention, as seen from the gas inlet side of the air heater, portions of the outer casing being broken away to show the internal construction;

Fig. 2 is a rear elevational view of the air heater shown in Fig. 1, as seen from the gas outlet side of the air heater, portions of the outer casing being broken away to show the internal construction;

Fig. 3 is a vertical sectional view taken approximately on the line 3 3 of Fig. 2;

Fig. 4' is a horizontal sectional View taken approximately on the line li-f of Fig. 2;

Fig. 5 is a top plan view of the air heater, part of the top wall being broken away to show the internal construction of the air heater;

Fig. 6 is a Vfront elevational view of a modified form of the air heater of the present invention, portions of the outer casing being broken away to show the internal construction;

Fig. 7 is a vertical sectional View of the air heater shown in Fig. 6 taken approximately on the line 'l-l of Fig- 6;

Fig. S is a horizontal sectional view taken approximately on the line 8 8 of Fig'. 7;

Fig. 9 is a horizontal sectional View taken approximately on the line ill-9,01" Fig. 7; and

Fig. 10 is a horizontal sectional view taken approximately on the line IQ-l of Fig. '7.

,It will, of course, be understood that the `description and drawings herein contained are illustrative merely and that various modifications and changes may be made in the structure disclosed without departing from the spirit of the invention.

The air heater of the present invention is pref erably constructed from a plurality of sheets or plates` of boiler plate or sheet metal ci suitable strength and thickness, the edges, abutting portions and overlapping portions being secured by welding or in any other preferredmanner to provide suitable air tight and gas tight joints upon assembly. The air heater preferably includes an outer casing or housing composed of sheet metal plates, the exterior surface being provided with suitable insulation for retaining the heat therein.

The preferred l'orm of air heater is illustrated in Figs. 1 to 5, inclusive, and includes a horizontal bottom wall I0, lower side walls I I and I2 and a horizontally disposed lower or bottom tube sheet Ill which is preferably o1 heavier material than the adjoining walls. The bottom tube sheet i4 is spaced above the bottom wall I0 and extends across the air heater from one side wall I I Ito the other side wall I2. The bottom tube sheet I4 preferably has outwardly extending portions for supporting the air heater upon a suitable framework, the framework being indicated at I5.

A vertical partition wall I5 is provided and extends between the side walls II and I2 and from the bottom wall I) to the lower tube sheet Ill. The partition wallv It is united at its edges to the bottom wall I, the side walls II and I2 and the lower tube sheet I4 to divide the space to the front andrear thereof into a gas inlet chamber II and a gas outlet chamber I8, respectively. A door I9 maybe provided in the side wall Ii of the gas outlet chamber I8 for access to the interior of the gas outlet chamber i8 for cleaning.

The bottom tube sheet Id preferably has secured thereto at the front end thereof a forward extension plate 2B, which extends from the side Wall I I to the side wall I2 and to which the laterally extending wall portion ZI is secured for directing the gases upon their entry into the gas inlet chamber I1. The rear edge of the bottom tube sheet I0 has a wall portion 22 provided therebelow for guiding the gases from the gas outlet chamber I0.

A top tube sheet 2Q vis also :provided spaced from the bottom tube sheet i4 and banks of tubes as hereinafter referred to extend therebetween. Intermediate side walls 25 and 26 are provided, extending between the top tube sheet 2li and the bottom tube sheet I4, and in vertical alinement with the lower side walls II and I2.

A rear wall is provided consisting of a plurality of vertically extending plates 28 and 29 secured together, the rear wall exten-ding between the intermediate side walls and 25 to which the end plates 28 are secured, and also extending downwardly from the top tube sheet 24 to a location adjacent but spaced from the bottom tube sheet I4 to provide an air inlet opening 35.

The alternate plates 28 as will be seen particularly in Fig. 2 are extended below the plates 29 therebetween to guide, direct and distribute the air entering the air inlet opening 3.

A front wall preferably composed of a plurality of stepped plates 3I is also provided and is spaced forwardly from the forward edge of the upper tube sheet 24 and connected thereto by a horizontal wall portion 32. The lower end portion of the front wall may be inclined inwardly as at 33, as hereinafter more fully referred to. The front wall preferably has a door 34 thereon for access to the interior.

A top casing wall 35 is also provided, and preferably has a door 36 therein for access to the space therebelow and an upper front wall 31 and an upper rear wall 38 extend downwardly therefrom and are secured to the upper tube sheet 24. The upper rear wall 38 is preferably in vertical alinement with the rear wall 29. per side walls 40 are also provided, preferably in vertical alinement with the side walls 25 and 26, for closing the space above the upper tube sheet 24 and between the top casing wall 35, the upper front wall 31 and the upper rear wall 38 to provide a gas transfer chamber 42. An access door 39 is preferably provided in one of the upper side walls 40.

A plurality of spaced upper baille members 43 are provided, these bales 43 being secured to r the rear wall portions 29, extending horizontally forwardly therefrom, and being spaced below the upper tube sheet 24. A forward extension 44 of these baille members 43 is provided and welded thereto and extends forward approximately as far as the forward edge of the upper tube sheet 24, this extension 44 extending from the side wall 25 to the side wall 26 and being secured at its ends thereto.

A vertical partition wall 45 is provided and eX- tends from the junction of the upper baffle members 43 and the baille extension 44 to the .lower tube sheet I4, and from one side wall 25 to the other side wall 26, to which it is secured. The

partition wall 45 is in alinement with the partirf.

tion I6 separating the gas inlet chamber I1 from the gas outlet chamber I8.

Dividing walls 46 are also provided which ex tend from the upper baille portions 43 to a location spaced from the lower tube sheet I4 and also extend from the partition 45 to the rear wall portions 28 and 29. The dividing walls 43 separate from the rear portion of the air heater a plurality of alternately disposed compartments (see Figs. 2, 3 and 4) the lower ends of the cornpartments being closed by horizontal walls 41 which are spaced upwardly a short distance from the lower ends of the dividing walls 45.

Each of the spaces between these alternately disposed compartments has a bank 48 of vertically disposed straight tubes therein which extend from the upper tube sheet 24 to the lower tube sheet I4 and are secured to these tubesheets by welding or in any other suitable manner to provide gas tight joints. The tubes in the banks 48 are closely spaced to provide for a division into thin streams of the air passing over the exterior thereof.

The interiors of the banks 48 of tubes are thus in communication at the bottom thereof with the gas outlet chamber I8 and at the top thereof with the gas transfer chamber 42.

The spaces 50 which are provided between each bank of tubes 48 at the lower part thereof, above the lower tube sheet I4 and below the respective horizontal walls 41, facilitate the entry and distribution of the air which is directed into the air `heater so that the air passes among the passageways between the outer surfaces of the tubes in the banks 48 and divides so that it passes among the tubes on each side of each respective space 53. Access is thus provided for the air to be heated so that it may pass into the air heater as far as the partition wall 45 and adequate introduction and distribution of the air to the passageways around all the tubesin the banks 48 is provided.

In like manner the spaces 5I which are provided above the baille portions 43, below the upper tube sheet 24 and in front of the rear wall portions 28 and 29, permit of free exit of the air from around the banks of tubes 48, while at the same time, air may pass directly from the banks of tubes 48.

In front of the partition wall 45, a single solid bank of tubes 53 is provided, and the respective rows of tubes are staggered to permit the use of a large number of tubes therein and to provide a relatively narrow spacing between tubes so as to separate the air therearound into a large number of thin streams.

The front wall 3l, as will be seen particularly in Fig. 3, is spaced forwardly from the front row of tubes of the bank of tubes 53 and has a baille 54 extending rearwardly therefrom, and approximately to the middle of the bank of tubes 53. An air transfer space is thus provided at 55. The partition wall 45 has a baffle 56 extending forwardly therefrom below the baille 54 and, as will be seen from Fig. 3, an air transfer space 51 is provided in front of the bank of tubes 53. As hereinbefore indicated, the lower portion 33 of the front wall is inclined inwardly and at the lower end thereof a baille 58 extends rearwardly approximately to the middle of the bank of tubes 53. An air outlet 59 is provided below the baille 58 and above the tube sheet I4.

The door 34 on the front wall 3l preferably extends above and below the baille 54.

The modified form of air heater shown in Figs. 46 to 10, inclusive, includes side walls 25a and 26E, a rear wall 29a, front wall portions 3 Ia extending between the side walls 25EL and 26a, and bottom wall portions IIIa are provided. A gas -inlet chamber is provided at |15. A horizontally disposed lower or bottom tube sheet I4a is provided between the side walls 25a and 26a, the rear wall 29a and extends forwardly towards but is spaced iom the lower part of the front wall portions A lower intermediate tube sheet 24EL is provided spaced from the bottom tube sheet |49, and banks of tubes 53a, as hereinafter referred to,extend therebetween. An upper intermediatetube sheet 24b is also provided and is spaced from the lower intermediate tube sheet 24a to provide a gas transfer chamber 42EL between the tubesheets 24a and 24h, the side walls 25a and 26a, the rear wall 29a and an interior wall portion 31a.

An upper tube sheet I4b is provided spaced above the upper intermediate tube sheet 24h and banks of tubes 48a, as hereinafter referred to, extend between the upper tube' sheet I4b and the upper intermediate tube sheet 24h.

A top casing wall II)b is also provided and a gas outlet chamber I8a is formed in the space below the top casing wall Illb and the upper tube sheet Mb. f

Spaced upper barile members 4la are provided which extend from the rear wall 29a to vthe front wall 3|, and dividing walls 46a are also provided which extend from the upper baflie members llla to a location spaced from the upper intermediate tube sheet 24b and also extend from the rear wall 29a to the front wall Sla. The dividing walls 46a and the bafe members 47a separate the portion of the air heater in which they are disposed into a plurality of alternately disposed compartments, the lower ends of the compartments being closed by horizontal walls 43a which are spaced upwardly from the upper intermediate tube sheet 24h.

Each of the spaces between these alternately disposed compartments has a bank of vertically disposed tubes 43a therein which extend from' the upper tube sheet Mb to the upperinter- Vmediate tube sheet 24D,l and are connected to these tube sheets by welding or any other suitable gas tight joints. The tubes in the banks 48a and the spacing compartments therebetween are substantially the same as the banks 48 and their spacing compartments. The vspaces 58a provided between each bank of tubes 48a at the upper part thereof, below the upper tube sheet I4b and above each of the upper baffle members 41a facilitate the entry and distribution of air throughout the passageways between and around the tubes in the banks 48a.

In like manner, the spaces la provided below the wall 43a and above the upper intermediate tube sheet 24b permit of the free exit of the air from around the banks of tubes 148a in the manner heretofore indicated with respect to the banks of tubes 48.

A single solid bank of tubes 53a is provided between the lower intermediate tube sheet 24a and the lower tube sheet |42, this bank of tubes 53EL being similar to the bank of tubes 53.

Baies 54a, 56*L and 58a and air transfer spaces 55a and 5la are provided in a manner similar to the baflles 54, 55 and 58 and the air transfer spaces 55 and 5l illustrated in Fig, 3.

An air outlet chamber 5Sa is provided below the baille 58a and above the lower tube sheet Illa and, if desired, branchducts G may extend from the air outlet chamber 59a for distributing the air heated in the air heater.

This form of air heater may also have suitable doors 34EL in the front wall 3Ia thereof for access to the interior, and suitable doors |911, 39a in the side wall 26a for access to the gas inlet chamber Ila, the gas transfer chamber 42a and the gas outlet chamber 18a, respectively.

The operation of the air heater shown in Figs. 1 to 5, inclusive, will now be pointed out. Y

Hot gases, such asA products of combustion from a steam generator containing heat units available for utilization are introduced through the gas inlet chamber ll. The hot gases pass upwardly through the interior of the tubes 53 of the solid bank of tubes where a portion of the available heat is given up and then pass to the gas transfer chamber 42 in which the streams of hot gases from the tubes 53 are merged. The gases `then pass downwardly from the gas transfer chamber 42, being distributed among the tubes 48 in the spaced banks of tubes, and give up further portions of the available heat. The gases then pass into the gas outlet chamber I8 from which they are discharged in thef-customary manner, such, for example, as to a stack (not shown).

The air which is to be heated by the transfer of heat from the hot gases is passed in a counterflow relation to the hot gases circulated within the tubes. The air is introduced at the air inlet 30, and upon its introduction, some portions thereof pass directly into the passageways between the tubes 48 of the spaced banks of tubes and other portions thereof pass into the spaces 50 between the banks of tubes 48. The air passing into the spaces 50 is distributed by the spaces to the passageways surrounding the tubes 48 closer to the partition wall 45 and is thus rapidly and thoroughly distributed over all the banks of tubes 48. 'I'he air then passes upwardly in contact with the outer surfaces of the tubes 48 of the spaced banks of tubes, between the several spacing compartments, where it is heated by the heat transferred from the hot gases within the tubes 48. The air then passes from the banksof tubes 48 directly, andralso by means of the spaces 5I, to the portion of the bank of tubes 53 below the tube sheet 24 and above the baffle extension 44. The air is thoroughly distributed in the passageways between the tubes, is heated by the contact therewith and passes linto the upper air transfer chamber 55. The air passing into the air transfer chamber 55 is guided by the front wall 3l and. the baffle 54 and returned to contact with the exterior surfaces of the banks of tubes 53 passing around and below the baffle 54 and above thefbaiile 56. From this location the airvpasses into the air transfer chamber 5l, and then passes from the air transfer chamber 5l below the baie 56 and above the baffle 58 into contact with the outer surfaces of the tubes 53 of the solid bank of tubes. The air is guided by the inclined portion 33 of the front wall and passes around the baffle 58 and then thereunder to the air outlet 59 from which it is discharged for delivery tov the place of use.

The operation of the form of air heater shown in Figs. 6 to 10, inclusive, is substantially the same as that heretofore referred to in the description of the air heater illustrated in Figs. 1 to 5, and will be `clear from that description. Gas entering the gas inlet chamber l'la passes upwardly through the interiors of the tubes of the solid bank of tubes 53a, thence to the gas transfer chamber 42a and thence from the gas transfer chamber through the interiors of the tubes of the banks of tubes 4ta to the gas outlet chamber l8r. Air introduced at the air inlet 3l)a passes in counterow relationship to the hot gases around the exterior surface of the tubes 48a and is dis- Ytributed around the tubes of the banks of tubes 43a by means ofthe spaces 58gq above the walls 41a. The air, after traversing the exterior sur- 'face of the banks of tubes 48a passes by means of the air transfer chamber 55a into contact with the exterior surfaces of the tubes of the solid bank of tubes 53 and is guided in contact therewith by and between the baflies 54a, 56a and 58a and. the air transfer chamber 5la to the air outlet 59a.

It willbe noted that the air heater may be readily cleaned, all the tubes thereof being straight tubes and access to the interior thereof being readily obtained. At the same time, if access to the spaces surrounding the tubes -is desired, such access may be readily obtained.

I claim: Y

1. `rinair heater including a casinghaving a ga-'s inlet chamber, ra gas -outlet chamber, land a gas transfer chamber spaced from 'said gas inlet and gas outlet chambers, a solid. bank of tubes connecting said gas inlet chamber with said gas transfer chamber, an air outlet from the exterior space between and around said tubes, tubes connecting said gas transfer chamber with said outlet chamber, an air inlet to the exterior space between and around said tubes, means .extending lengthwise of said latter tubes and from said air inlet for dividing said tubes into spaced groups, and means extending 'transversely of certain of the tubes for dividing the spaces between said tubes extending to said gas inlet chamber into a plurality of passes.

2. An air heater including a casing having a gas inlet and a gas outlet, a gas transfer chamber spaced from said gas inlet and from said gas outlet,a tube sheet contiguous to said gas inlet, ajsolid bank 'of tubes connecting said tube sheet `with said gas transfer chamber, an air outlet from said bank, a plurality of spaced banks of `tubes connecting said gas transfer chamber with said gas outlet, an air inlet to said spaced banks of tubes, a member within the casing and extending longitudinally of the tubes of said banks vfor separating the rst named bank of tubes from said other banks of tubes.

3. An air heater including a casing having a gas inlet, a gas outlet, and an intermediate gas transfer chamber spaced from said gas inlet and said gas outlet, a solid bank of closelyspaced tubes connecting said gas inlet with said gas transfer chamber, an air outlet from the spaces surrounding thel tubes in said solid bank, a plurality of spaced banks of tubes connecting said gas transfer chamber with said gas outlet, an air inlet to the spaces surrounding said spaced banks of tubes, and baffles intermediate the ends of the tubes of said solid bank of tubes forv providing a -plurality of passes for circulating air in the spaces between the tubes of said solid bank.

4. An air heater including a casing having a gas inlet, a gas outlet, and an intermediate gas transfer chamber spaced from said gas inlet and said gas outlet, a solid bank of closelyr spaced tubes connecting said gas inlet with said gas .transfer chamber, an air outlet from the spaces surrounding the tubes of said solid bank, a plurality of spaced banks of tubes connecting said gas transfer chamber with said gas outlet, a vspacing compartment between said spaced banks, 'an air inlet Vto the spaces surrounding the tubes in'said spaced banks at one end of said compartment, and a plurality of baffles intermediate the ends of said solid bank of tubes for provid- .ing a plurality of passes for circulating air in the spaces between the tubes ofsaid solid bank in its passage to the air outlet.

. 5. An air heater including a casing having a gasA inlet, a gas outlet, and an intermediate gas transfer chamber spaced from said gas inlet and said gas outlet, a solid bank of closely spaced Vtubes connecting said gas inlet with said gas transfer chamber, an air outlet from the spaces vsurrounding the tubes of said solid bank, spaced banks of tubes connecting said gas transfer chamber with said gas outlet, an air inlet to the spaces surrounding thev tubes in said spaced banks, a plurality of baffles intermediate the ends of said solid bank of tubes for providing a plurality of passes for circulating air in the spaces between the tubes of said solid bank, and a plurality of air transfer chambers disposed between certain ofV said bailies. V

Y6. An air heater including a casing having a gas inlet and a gas outlet, a gas transferv chamber spaced from said gas inlet and said gas outlet, a tube sheet contiguous to said gasinlet and said gas outlet, a solid bank of tubes connecting said tube sheet with said gas transfer chamber, an air outlet from said solid bank of tubes, a plurality of spaced banks of tubes connecting said gas transfer chamber with said tube sheet, an air inlet to said spaced banks of tubes, a member within the casing and extending longitudinally of said tubes for separating the first named bank of tubes from said other banks of tubes, and .baffles for dividing the spaces around the tubes of said solid bank of tubes into a plurality of passes.

'7. An air heater including a casing having therein a gas inlet chamber, a gas outlet chamber and a gas'transfer chamber spaced rfrom said gas inlet and' gas outlet chamber, a solid bank of tubes connecting said gas inlet chamber with said gas transfer chamber, an air outlet from said solid bankof tubes, a ,plurality of spaced banks of tubes connecting said gas transfer chamber with said gasV outlet chamber, an air inlet to said spaced banks, a member within the casing for separating the first named bank of tubes from said other banks of tubes, and baiiles for dividing the spaces around the tubes of said solid bank of tubes into a plurality of passes.

8. An air heater including a casing having therein a gas inlet chamber, a gas ourtlet chamber and a gas transfer chamber spaced from said gas inlet chamber and said gas outlet chamber, a solid bank of closely spaced tubes connecting said gas inlet chamber'with said gas transfer chamber, a plurality of banks of tubes connecting said gas'transfer chamber with said gas outlet chamber, a member for separating the solid bank of tubes from said other banks of tubes, spacing members for separating the banks of tubes of the plurality of banksand providing dis--v tributing spaces above and below the spacing members in Comunication with adjoining pairs of banks of tubes of said plurality of banks.

9. An air heater including a casing having therein a gas inlet chamber, a gas outlet chamber and a gas transfer chamber spaced from said gas inlet chamber and-said gas outlet chamber, a solid bank of closely spaced tubes connecting said gas `inlet chamber with said gas transfer chamber, a plurality of banks of tubes connecting said gas transfer chamberY with said gas outlet chamber, a member within said casing for separating the solid bankA of tubes from said other banks of tubes, spacing members for separating the banks of tubes of the plurality of banks and providing distributing spaces above said spacing members in communication with adjoining pairs of banks of tubes of the plurality Aof banks and in communication with the solid bank of tubes.

10. An air heaterV including a casing composed of exterior walls, and having a gas inlet chamber, a gas outlet chamber, and a gas transfer chamber spaced from said gas inlet and said gas outlet chambers, a solid bank of tubes connecting said gas inlet chamber with said gas transfer chamber, an air outlet from said bank contiguous to said gas inlet, spaced banks of tubes connecting said gas transfer chamber with said gas out- Ylet chamber, an air inlet to said banks contiguous to said gas outlet, a partition within said casing Vfor separating the solid bank of tubes from said spaced banks of tubes, bafflesV for dividing the spaces between the tubes of said solid Ibank of tubes into a plurality of passes, certain of said baffles being connected to said partition.

11. An air heater including a casing composed of exterior walls, and having a gas inlet chamber, a gas outlet chamber, and a gas transfer chamber spaced from said gas inlet and said gas outlet chambers, .a solid bank of tubes connecting said gas inlet chamber with said gas transfer cham-v ber, an air outlet from said bank contiguous to said gas inlet chamber, spaced banks of tubes connecting said gas transfer chamber with said gas outlet chamber, an air inlet to said spaced banks contiguous to said gas outlet chamber, a partition within said casing for separating the solid bank of tubes from said spaced banks of tubes, baiiies for dividing the spaces between the tubes of said solid bank of tubes into a plurality of passes, certain of said baiiies being connected to an exterior wall of the casing.

12. An air heater including a casing composed of exterior walls, and having a gas inlet chamber, a gas outlet chamber, and a gas transfer chamber spaced from said gas inlet and said gas outlet chambers, a solid bank of tubes connecting said gas inlet chamber with said gas transfer chamber, an air outlet from said bank contiguous to said gas inlet chamber, spaced banks of tubes connecting said gas transfer chamber with said gas outlet chamber, an air inlet to said banks contiguous to said gas outlet chamber, a partition within said casing for separating the bank of tubes extending to said gas inlet chamber from rthe banks of tubes connected to said gas outlet chamber, bailies for dividing the spaces around the tubes of said solid bank of tubes into a plurality of passes, certain of said baiiies being connected to an exterior wall of the casing, said exterior wall being spaced from said rst mentioned bank of tubes to provide with the baiiles carried thereby a plurality of chambers for receiving air from around the tubes and returning the air to the tubes.

13. An air heater comprising a casing having a gas inlet chamber and a gas outlet chamber therein each extending substantially the entire distance between a pair of walls of the closure, a partition separating said gas outlet chamber from said gas inlet chamber, a gas transfer chamber spaced from said gas inlet and said gas outlet chambers, a solid bank of tubes connecting said gas inlet chamber with said gas transfer chamber, an air outlet from said solid bank of tubes, spaced banks of tubes connecting said gas transfer chamber with said gas outlet cham. ber, an air inlet to said spaced banks of tubes, a partition between the aforesaid pair of walls separating the bank of tubes extending from said gas inlet chamber from the banks of tubes extending to said gas outlet chamber, said partition having its termination at a location spaced from said gas transfer chamber, partition members connected to said partition at the termination thereof, walls extending along the margins of said spaced banks of tubes and providing with said partition members compartments between said spaced banks of tubes, said partition members controlling the flow of air from said spaced banks of tubes to said other bank of tubes, and baffles for dividing the spaces between the tubes of said solid bank of tubes into a plurality of passes.

14. An air heater comprising a casing having a plurality of opposite pairs of enclosing Walls a gas inlet chamber and a gas outlet chamber therein each extending substantially the entire distance between a pair of walls of the casing, a gas transfer chamber spaced from said gas inlet and said gas outlet chambers, a solid bank of tubes connecting said gas inlet chamber with said gas transfer chamber, an air outlet from said solid bank of tubes, spaced banks of tubes connecting said gas transfer chamber With said gas outlet chamber, an air inlet to said spaced banks of tubes, a partition separating the bank of tubes extending from said gas inlet chamber from the banks of tubes extending to said gas outlet chamber, said partition having its termination at a location spaced from said gas transfer chamber, partition members connected to said partition at the termination thereof and extending transversely with respect to the tubes of said solid bank and of said spaced banks, Walls extending along the margins of said spaced banks of tubes and providing with said partition members compartments between said spaced banks of tubes with passageways outside the upper and' lower ends of said compartments, certain of said passageways controlling the ow of air from said spaced banks of tubes to said solid bank of tubes, and b-aflies extending transversely with respect to saidsolid bank of tubes providing a tortuous air path therearound for the air, certain of said baffles being carried by said partition and others being carried by a wall of said casing.

15. An air heater comprising a casing having a gas inlet, a gas outlet, and a gas transfer chamber spaced from said gas inlet and said gas outlet, a solid bank of closely spaced tubes connecting said gas inlet with said gas transfer chamber, an air outlet from said solid bank of tubes, spaced banks of tubes connecting said gas transfer chamber with said gas outlet, an air inlet to said spaced banks of tubes, walls extending along the margins of said spaced banks of tubes and walls extending transversely in the spaces between said spaced banks of tubes and providing with said walls compartments between said spaced banks of tubes and passages outside the upper and lower ends of said compartments, baies extending transversely with respect to the tubes of said solid bank and intermediate the ends thereof for providing a plurality of passes for air circulated in the spaces around the tubes of said bank.

16. An air heater comprising a casing having a gas inlet, a gas outlet, and a gas transfer chamber spaced from said gas inlet and said gas outlet, a solid bank of closely spaced tubes connecting said gas inlet with said gas transfer chamber, an air outlet from lsaid solid' bank of tubes, spaced banks of tubes connecting said gas transfer chamber with said gas outlet, yanV air inlet to said spaced banks of tubes, walls extending along the margins of said spaced banks of tubes and walls extending transversely in the spaces between said spaced banks of tubes and providing with said walls compartments between said spaced banks of tubes and' passages outside the upper and lower ends of said compartments, and baffles extending transversely with respect to the tubes of said solid bank and intermediate the ends thereof for providing a plurality of passes for air circulated in the spaces around the tubes of said bank, said casing having a plurality of air transfer chambers for receiving the air from one pass between certain of said baiiies and' returning it to another pass.

JOSIAH H. ROHRER.

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