US2742895A - Gas heating furnace with tubular heat exchange means - Google Patents

Description

3 Sheets-Sheet l J. O. NAUCLER I'AL `April 24, 1956 GAS HEATING FURNACE WITH TUBULAR HEAT EXCHANGE MEANS Filed May 3l, 1950 April 24, 1956 J. o, NAUCLER ErAL 2,742,895

GAS HEATING FURNACE WITH TUBULAR HEAT EXCHANGE MEANS Filed May 5l, 1950 3 Sheets-Sheet 2 mm, Mm? ffm TOP/VEYS' J. O. NAUCLER ET AL April 24, 1955 GAS HEATING FURNACE WITH TUBULAR HEAT EXCHANGE MEANS 3 Sheets-Sheet 3 Filed May 5l 1950 l l l l i f l f f f l f l f C emar/'ei mi j y Per c/oz an fno' United States Patent GAS HEATING FURNACE WITH TUBULAR HEAT EXCHANGE MEANS Application May 31, 1950, Serial No. 165,202 In Sweden November 28, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires November 23, 1961 22 Claims. (Cl. 12b-1M) The present invention relates to furnaces for heating owing gases mainly by radiation from gaseous heat sources of relatively high temperature, and more particularly to radiation gas heaters of the kind in which the gases to be heated are passed through vertical tubes disposed in a ring in a vertical refractory shaft and forming a central vertical passage for the heating gas.

Gas heaters of this general character are known in the art, and similar structures are widely used for steam boilers and also employed in the art of cracking oils. However, in spite of the various precautions that have been resorted to for protecting the heating surfaces against deterioration, none of the devices heretofore known has been capable of using heating gases of very high temperature and at the same time attaining a high degree of heat transfer eiliciency. Y

Itis therefore a primary object of this invention to provide a tubular gas heater that will offer a very high efciency per unit of heat transmitting surface when using heating gas of a high temperature and make it possible to utilize the tube material to the utmost of its capacity.

It is also an object to provide a structure that will tolerate practically any attainable temperature of the heating gas and wherein practically the only factor on which the heat transfer is dependent is the quantity of heat supplied.

Another important object of the invention is to provide a gas heater structure that will be easy, practical and economical to build and so designed that substantially the entire tube system may be manufactured as a unit at any distance from the place where the gas heater is to be erected.

These and other valuable objects that will be evident as the description of the invention proceeds will be obtained primarily by arranging the tubes in spaced relationship vertically in a single ring Abetween upper and lower head boxes to form a tube-cage which is handleable as a unit, placing thiscage within the heat transmission space in the shaft so as to freely depend from a support for the upper head box, and covering the shaft by a refractory head-unit containing a vertical passage for the heating gases, the gas heater thus being composed of three separate units: the shaft, the tube-cage and the head-unit.

For a better understanding thereof, the invention will now be described more in detail with reference to the embodiments illustrated in the accompanying drawing, although it will be understood that the invention is not limited to these disclosures'which are illustrative only.

Inthe drawing, wherein like reference characters indicatev like parts throughout the several views:

Fig. 1 is a vertical section of one form of radiation gas heater embodying the invention.

Fig. 2 is a horizontal section taken along the line 2 2 in Fig. l.

Fig. 3 is a fragmentary elevation, partially in section, of an expansion device alternative to that shown in Fig. 1. Fig. 4 is a vertical section of a second form of gas heater including diagrammatic showings automatically controlling the supembodying the invention, of alternative means for ply of the heating gas.

As will be seen from Fig. l, the heat exchanger consists of three main parts: a shaft formed by the heat resisting wall 7, a tube-cage consisting of the tubes 6 and the upper and lower head boxes 10 and 11, and the head-unit 12 constituting a cover for the shaft and containing the heating gas inlet passage 2 which is coaxial with and communicates directly with the central unobstructed portion of the shaft. The shaft wall 7 is built up from different layers, such as, in order from within, refractory bricks, chamotte grains, rock wool and outermost a casing of iron plate.

The shaft has an airtight bottom 25 and rests upon a support S. Through the bottom projects a vertical ilue 9 which is coaxial with the shaft and provides a passage for the heating gas leading to the outlet conduit 5.

Suspended in the shaft is the tube-cage consisting of an annular upper head box 10, a ring of tubes 6 secured to and opening into said box 10, and an annular lower head box 11 secured to the lower ends of tubes 6 and into which the latter all open. In the form shown, the upper head box 10 is substantially triangular in cross section with a at bottom, while the lower head box 11 is rectangular in cross section. In order to obtain the most efficient heat transfer by radiation from and through a large bulk of gas, the length of the tubes 6 should be less than three times the diameter of the ring in which they are arranged. The tube-cage forms a removable unit which is supported in the shaft by means of a conical plate 26 having its lower edge resting on the T-iron 27 which is in turn supported by the iron plate casing of the shaft wall 7. The plate 26 is an integraly extension of, or may be fixed to, the outer inclined side of head box 10. Alternatively, the bottom may rest directly on the upper end of the brick layer of the wall 7 which forms a shelf 29 extending slightly above the rest of the shaft. The tubes shall, however, suspended freely from the upper head box. It is also within the scope of the invention to subdivide the tube-cage into vertical sections, a construction which is particularly advantageous in Veryflarge heaters.

By providing the upper head box 10 with a flat bottom the tubes can be attached to said bottom near its inner edge and a large portion of the box can overlie the shaft wall 7 or shelf 29, either to rest thereon or merely to be shielded or protected thereby from radiation. The other or inclined sides of the upper head box are also relatively flat which facilitates manufacture of the box, particularly when its diameter is as large as about 2 meters, for example. Eyes 28 may be welded or otherwise fixed-to the head box 10 so that the entire tube-cage can be lifted out of the shaft or lowered therein as a unit. The weight of the tube-cage ensures a perfectly airtight engagement be'i tween the supporting parts 26 and 27.

The shaft is limited at the top by the head-unit 12 coninclined wall of the upper head box 10 and is preferably held in gastight relation therewith solely by its own weight, although special means may be employed for the latter purpose, if desired. In the form shown, the head-unit 12 is provided with connections 3 a manner known to the art (cf, Patent No. 95,240). After substantially complete combustion, the gases reach the radiation space 1 at the upper radiation zone will be moved to a lower level in the shaft.

It will be apparent to those skilled in the art that, if desired, the gas inlet passage in the head-unit may be connected to any suitable source of hot gas instead of being combined with aburner as indicated in Pig. l.

It is also obvious that the head-unit may be supported by the wall 7 directly, or by the intermediation `of means other .than the upper head box 1G, without departing from the invention. On the other hand, it is important that the head-unit be a separate structure which is removable as a unit, but at the same time supported by the wall 7 so that it may follow the thermal movements thereof and thereby facilitate maintenance of .a gastight connection between the head and the shaft. The .removability of thc head enables ready access to the interior of the shaft and facilitatesl insertion and removal of the tube-cage.

Since, in a heater wherein the heating gas is introduced at the top, the strongest radiation emanates from the portion of the shaft marked 1, situated immediately below the gas inlet 2, and from adjacent surfaces of the headunit, the upper portions of the tubes 6 are exposed to Vthe greatest strain. Therefore it is particularly important that the upper portions of the tube system be properly shaped and arranged and, as the case may be, carefully protected. ln the apparatus shown in Fig. l, the desired result is attained partly by suitably shaping the head-unit so that it is provided with projecting portions 17 which screen off the upper head box 10, and partly by arranging a considerable portion of the box 10 above the wall 7 so as to be screened thereby.

The tubes 6 are arranged in a single ring along the cylindrical shaft wall 7 with suicient distance between the tubes and the wall (e. g., 1 to 4 decimeters), as well as between adjacent tubes (preferably 1/2 to 4 tube diameters, e. g., l to 2 tube diameters), to allow the portions of the shaft wall beyond the tubes to be subjected to radiation from the heat source in the center of the shaft and thereby attain such a high temperature that they, in turn, can effectively emit radiation to the rear or outer surfaces of the tubes and thereby ensure a more uniform heating of the latter.

As shown, the tube-cage tapers downwardly and the lower head box 11 is of smaller diameter than the upper box 10, a construction which facilitates insertion of the cage into the shaft and its vertical movement therein or removal therefrom, and also permits the tubes to expose a greater surface to the hot gases entering at the top. ln order to obtain the desired tapered arrangement, the distance between the tubes and the shaft wall may be, for instance, 1 to 2 decimeters at the top and 1.5 to 4 decimeters at the bottom in a furnace having a height of 4 to 6 meters.

The tube-cage comprising the upper and lower head boxes 10 and 11 and tubes 6 is preferably welded together, an operation which can be carried out in any suitable work shop regardless of its distance from the place where the gas heater is to be erected. In case atempering furnace large enough to house a structure of this size should not be available, the tempering may take place carefully after the cage has been mounted in the shaft. A welded tube-cage is more heat resisting and safer than a structure embodying tube ttings of other kinds, and can consequently be used at higher temperatures than the latter.

In the preferred embodiment, the tubes are :straight throughout their entire length and are attached tov the bottom of the upper head box and to the roof of the lower head box, thus facilitating an even distribution of the gas and forming a structure of minimum heat sensitivity. Such an arrangement also makes it possible to utilize practically the entire length of the tubes for Vheat transmission purposes. If desired, the tubes may be provided with insertions of the character indicated at 18 (Fig. 2) in order to speedup the transmission of heat from the tube walls 'to Vthe gas owing therein.

The gas to be heated may be introduced at the top into box 10 and discharged at the bottom from box 11 or, vice versa, introduced into the lower box' 11 and removed from the upper box 10. Due to the fact that the tubes must support not only their own weight but also that of the lower head box 11 and the discharge pipe or pipes 14, and since the strength of tubes decreases as the temperature rises, the gas to be heated is preferably introduced at the top, since in such case the tubes will be effectively cooled at the end of greatest strain. This is particularly important when, as in the preferred embodiment, the heating gas is also introduced at the top of the shaft and the radiation consequently is strongest there. Parallel flow of the heated and the heating gases coutributes to attainment of an even temperaature along the tubes, so that the tube material can be eciency utilized.

Accordingly, in the structure of Fig. 1 the gas to be heated enters the upper head box 10 through pipe 13 and is evenly distributed to the tubes 6 through which it flows downwardly to the lower head box 11. The heated gas collected in the lower head box 11 is discharged therefrom through pipe 14 which passes through the bottom 25 of the shaft and is connected to an expansion device 16 lead'- ing to the discharge main 15. The apparatus is preferably provided with a plurality of such discharge pipes 14, although the number is limited to, for instance, three so that the number of openings in the bottom 25 may not become too great. By using a plurality of pipes 14 Symmetrcally disposed, the cross section of the head box can be reduced, the gas distribution improved and distortion due to heat movements of the tubes avoided. When .several discharge pipes 14 are employed, they may be connected to an outer header which, in turn, may be connected to the discharge main 15 in any conventional manner.

The expansion device 16 consists of a sleeve 31 of extensible bellows-like construction rigidly secured at its ends to flanges 32 and 33 formed on the vertical portion of discharge main 15 and on pipe 14, respectively, inside of which sleeve the lower end of expansion pipe 24 moves telescopically. Packing material 34, such as as bestos, is placed around the joint between the device 16 and the pipe 14. Alternatively, the expansion device may be shaped as a kind of a liquid lock or liquid trap wherein the liquid consists of a medium which is uid, but substantially non-volatile, at working temperatures, e. g. lead, lead alloys, Babbitt metal or the like. Such an embodiment is illustrated in Fig. 3 in which the pipe 21 (having in some large plants a diameter of more than 0.5 meter) extends from the lower head box -11 and submerges in the liquid which is held in the annular con tainer or pocket 23 attached to the upepr portion of the outlet pipe 22.

It will thus be seen that the ilow of gas in the tube system is divided into four separate zones: the distribution zone 10, the radiation zone 6, the collecting zone 1I and the discharge zone 14; if desired, a fifth or secondary collecting zone may be added. Due tothe fact that the rst three zones, the main zones, communicate directly with one another and stand in rectilinear relationship to one another, this zone arrangement contributes greatiy to attainment of the objects of the invention, as by making it ypossible to use the above described unitary tubecage structure and consequently to manufacture a substantial part of the heater as a unit in a factory. Furthermore, a very quick, one-pass transport of the gases through the heater tubes is thereby attainable, a result which is :sought by the present invention partly in order to effect a sufficient cooling of the tubes to make hem capable of exracting the great quantities of Vheat desired, and partly in order to obtain as even a distribution of the gases as possible so that the gas will have the same ini-tial state in all tubes, which is also important.

Being dependent in the shaft, the tube-cage can expand freely, `even though equipped at the bottom 'with the large head box and the discharge pipes attached thereto. When the gas to be heated is introduced at the top and thus effectively cools and the upper parts of the tubes, the expansion will be concentrated at the bottom, in which even it is particularly important that the tubecage be suspended from its upper end and free to expand at its lower end.

Centrally of the shaft bottom 25, Ythe outlet ilue 9 for the heating gas projects into the shaft chamber 20 to a level above the lower head box 11. By this arrange ment, the flowing stream of heating gas will not directly contact with the head box, although the gas can nevertheless radiate to the tubes along their entire length down to the head box, and no part of the valuable tube lengh will be lost for the heat transfer, the tube material instead being utilized to the utmost.

The heat transmission preferably should take place only through the tubes and not to any material extent through the head boxes. To this end, in addition to screening the upper head box by the projecting portions 17 of the head-unit and by the wallA 7, the lower head box 11 may be protected from the radiation by ceramic or other refractory bricks 30 which are'light and loosely placed on the head box and on shelves on the wall 7 and the outlet llue 9, respectively, so that they do not interfere with the heat responsive movements of the tube-cage.

As the heat transmission surface of the gas heater is the product of tube diameter, tube length and number of tubes, and as the tube diameter and the number of tubes cannot be increased above a certain maximum, a demand for greater heat transmission surface must be met with an increase -in length of the tubes. But due to the fact that the tubes of the present form of heater are heavily strained by the load of the lower head box in addition to the load of the tubes themselves, a relatively low limit is also set for the length of the tubes (i. e., less than three times the diameter of the tube-cage). Consequently, in order to apply the present invention to an installation requiring more heat transmission surface than can be furnished by a tube-cage of the maximum permissible length, without the necessity for employing two separate gas heaters, the structure illustrated in Fig. 4 has been provided, wherein two tube-cages are arranged one above the other in the same heater shaft.

In the heater of Fig. 4, the wall 7 maybe built up, in principle, in the same way as that described with reference to Fig. l, and the upper tube-cage 41 is suspended from an upper shelf 42 adjacent the top of the shaft in substantially the same way as the tube-cage of Fig. l.

In this embodiment, the upper head box 10 which hangs on the plate 40 is rectangular in cross section and has a gas inlet pipe 47 which extends horizontally through the upper end of the wall 7. Since the space below the upper tube-cage is occupied by the lower tube-cage 43,

the discharge pipe 44 from the lower head box 11 of the upper cage (which in this instance is circular in cross section) passes horizontally through avertically elongated opening 45 in the wall 7, said opening being high enough to permit the pipe 44 to move vertically as a consequence of the thermal movements of the tubes 6. The portion of the opening 45 around the pipe 44 may be sealed against the escape of gas in any convenient manner known to the art. The head-unit l2 rests on a U-bar 39 carried by the plates 40 which rest on ring 38 lying on wall 7.

The lower tube-cage 43 is suspended from a shelf 46 formed in the wall 7 at the waist portion of the heater shaft, the upper head box 10 of said cage resting on said shelf and having a pipe 47 connected thereto which extends outwardly through an opening 48 .in the wall. Since the box 10 has practically no vertical movement relative to the wall, the opening 48 need be very little larger than the pipe 47. The tubes 6 and the lower head box 11 of the lower tube-cage are freely suspended from the upper head box 10 as described above. In the event that insuficient space is available for accommodating gas supply or discharge connections below the gas heater, the connecting pipe 44 of the lower head box 11 may-be passed through an opening 50 in the wall 7, as shown, similarly to the corresponding element of the upper cage 41. This connection can, of course, be passed vertically through the bottom of the shaft similarly to the construction of Fig. l, if space is available.

By the illustrated arrangement of connecting pipes 44, 44', 47 and 47 extending through the wall 7, the gas to be heated can be passed through the heater in any one of a plurality of alternative paths, inasmuch as anyone ofthe connecting pipes can be made the inlet and either of the connecting pipes of the other tube-cage can be made the outlet, the remaining pipes then being interconnected. For example, as indicated in Fig. 4, pipe 47 connected to the upper head box 10 of the lower tubecage 43 may be the inlet for the gas to be heated and pipe 44 connected to thelower head box 11 of the upper cage 41 may be the outlet, the pipes 44 and 47 then being interconnected as shown to lead the gas discharging from the lower box ofthe lower cage to the upper box of the upper cage. In case it is desired that the lower head box 11 of the upper cage 4l be permanently connected to the upper box 10 of the lower cage 43 so that the gas may flow straight through the two units, these boxes are preferably interconnected inside the shaft so as to eliminate the openings 45 and 48 through the shaft wall.

A shielding structure 51 is provided within the waist portionof the heater shaft in order to protect the head boxes l1 and 10 of the upper and lower tube-cages, respectively. The shield consists of a carrier :'52, as of sheet iron, which is provided at the bottom with an inwardly projecting flange 53 adapted to carry the refractory brick structure 57 and at about the middle with an outwardly projecting bracket S4 adapted to rest on the upper head box 10 of the lower cage. By the latter arrangement, no bracket need be interposed between the lower head box 11 of the upper cage and the upper head box 10 of the lower cage, the height of the gas heater thus being reduced by the height of such a bracket. The refractory brick portion 57 of the shield is shaped to protect the carrier 52 from radiation and impingernent of gas, a recess 55 being provided at the lower edge thereof to `house the flange 53.

Light refractory bricks 3i) rest partly on the lower head box 11 and partly on the shield 51 and a shelf 56 on the wall 7, respectively. The box 11 is thereby protected without impeding its vertical movement, while the inner ring of bricks 30 also protects the upper portion of carrier 52.

By the double tube-cage arrangement described, several advantages are gained. Apart from the more or less obvious saving in floor area and material for building the shaft as well as in piping for transport of the gas, the provision of a second tube set below the first one in the same shaft therewith, rather than laterally thereof in a separate shaft, offers a greater cooling surface to receive radiation from the zone i from which the strongest radiation emanates, in that a part of the radiation from said zone passes through the central passage of the shield 51 and reaches the tubes of the lower set. in the structure illustrated in Fig. l, a great portion of the radiation entering the outlet flue 9 is reflected or reradiated to the tube-cage and particularly to its upper portions near the hot zone 1, which portions of the tubes are already heavily strained due to direct radiation from said zone. Such reradiation is obviated, or at least minimized, in a double cage structure like that of Fig. 4. Another advantage resides in the fact that the shield 51 serves as a recollector for the vertical stream of heating gas so as to impede this gas from impinging directly on the tubes, which would otherwise'tendto occur if the tubes were very long. The double cage arrangement is also favorable because it `enables passage of the gas to be heated through Vthe heater Aarbitrarily in a variety of diiferent combinations of parallel and countercurrent flow. Usually the best mode of heating is, as shown in Fig. 4, to introduce the heating gas at the top of the shaft and to introduce the gas to be heated into the upper head box of the lower cagei43; after passing the gas beinghcated through the lower tube-cage and extracting it from the lower head box l1, it is conveyed to the upper head box 10 of the upper cage 41 and, after passing through this cage, is discharged from the lower head box 11 thereof. However, it is often advantageous to introduce the heated gas into the lower head box of the lower cage, so as to pass it countercurrcntly to the heating gas in this cage. A still further advantage of the subdivision of the tubes into two cages is that, being not so heavily strained as the upper one, the lower tube-cage can be made of a cheaper material than the upper one.

,inasmuch as gases have a very poor cooling effect, head boxes of the character herein disclosed are apt to be badly affected by the heat to which they are subjected, particularly at the places where the tubes are welded to the boxes` According to the present invention, however, the head boxes may be effectively protected against overheating by providing choking or baille plates 61 inside the boxes over and near the mouths 62 of the tubes 6. The plates 61 are secured to the walls of each head box by holders 63 attached to parts of the walls which are not exposed to direct radiation from the heat source or, generally, are not subjected to too much heat. The plates are so shaped and positioned near the walls of the boxes ,as to form narrow passages 64 through which all the gas being heated is forced to flow. The quantity and velocity of the gas passing a given area of the box wall surface are thereby greatly increased, a result which will in turn many times multiply the cooling effect of the gas. Although the choking plates 61 are shown only in the upper and lower head boxes of the upper tube-cage 41 of Fig. 4, it will be understood that similar plates may be embodied in the head boxes of the lower cage 43. and also in either or both of the head boxes of Fig. l.

The choking plate arrangement also improves the even distribution of the gas to the various tubes so as to assist in rendering equal the heat transmission status of all tubes, which is one of the main objects of this invention.

lt is common in heat exchangers to control the supply of heating gas, or gaseous fuel and combustion air, in accordance with variations in the temperature of the heated gas. This may be carried out automatically by providing impulse means in the exit line of the heated gas connected to means for controlling the supply of heating medium, whether in the form of heating gas or as one or both components of combustion. However, in the present gas heater, where such high temperatures and such great quantities of heating gas are employed as will strain the tubes to the utmost of their capacity, a control based on the temperature of the heated gas is usually not sufficient. Accordingly, another feature of the invention is the provision of an impulse member which is directly responsive to the tube surface temperature and is so connected to the control means for the heating medium as to prevent the tubes from being overheated.

As is diagrammatically indicated in Fig. 4, the impulse member is preferably a bimetallic therrnoelement '71 of any known design. the junction of the two metals being applied to the surface of one of the tubes of the upper cage 41 adjacent the upper end thereof and the electrical bimetals being passed outwardly through a channel 73 in the wall 7. The thermoelemcnt 71 and the leads 72 are preferably disposed on the rear or outer side of the tube so as to be protected as much as possible from the radiation heat. The leads 72 connect the element 71 with a relay 74 which transforms, `in any conventional way, the electric current produced by the heat in the bimetallic joint into mechanical power for regulating the lll control `means iof-theheating medium, here illustrated by the damper 75.

.Instead-0f, .orin .addition to, the thermoclement 71, a light sensitive-photoelectric cell 77 may be directed to ward `one of the `tubes 6 through an opening 78 in the wall 7 and the electric current produced thereby may be passed lthrough leads 79 to the relay 74. As the light emission of the V.tube is `proportionate to the temperature thereof, such a photoelectric `cell will be directly responsive tothe temperature of .the tube.

There is thus provided by the present invention an improved gas heater ofthe radiation type which is more efficient as a heat exchanger, is capable of using higher heating gas temperatures, and is more practical and economical to construct than similar devices previously known tothe art. Although two specifically different forms of heater embodying the invention have been described and illustrated in the accompanying drawing, it will be obvious -that ,the invention is not limited to the exact structures shown, but that various changes, which will now suggest themselves to those skilled in the art, may be made in the form, details of construction and arrangement of the parts without departing from the inventive concept. Reference is therefore to be had to the appended claims for a definition of the limits of the invention.

This application is a continuation-impart of Serial No. 514,474, filed December lr6, 1943, now abandoned.

Alternatively or in addition to controlling the heating medium the impulse members 71 and 77 may with advantage be connected by vmeans of leads 81 and 82, respectively, to another relay 83 actuating a damper B4 in the inletconduit for the gas to be heated.

What is claimed is:

l. A radiation gas heater comprising a wall forming a vertical shaft `and having an inner lining `of refractory material, a .head-unit covering the upper end of said shaft and forming a heating gas passage coaxial with and opening vertically into the central portion of said shaft, means closing `the bottom end of said shaft and forming a second heating gas passage coaxial with and opening vertically into said central portion `of the shaft, an annular upper head box adjacent the upper end of said shaft, an annular lower head box within said shaft, a set of substantially vertical tubes arranged in a single ring about the axis of said Shaftat a spacing between adjacent tubes of at least one half the tube diameter and such that radiation from the heating gas may pass between said tubes to the refractory lining of said shaft wall for reradiation thereby to the adjacent surfaces of said tubes, said tubes being substantially straight throughout their entire length and connected in parallel to the bottom side of said `upper head box and to the upper side of said lower head box and so positioned within the shaft that they are subject to radiation from the heating gas substantially throughout their entire length, said head boxes and said 4tubes forming an integral `tube-cage having a central unobstructed space throughout its entire length coaxial with said heating gas passages, gas conduits connected to said head boxes for supplying gas to be heated to one ofthe head boxes and discharging heated gas from the other head box, and means associated with said shaft wall for supporting said upper head box, said tubes and said lower 'head ibox being suspended in the shaft from said upper head box and freely movable in a vertical direction inbresponse to thermal expansion and contraction of said tu es.

2. A gas heater-.as defined in claim l wherein the heating gas inlet vto .the Vunobstructed space and the inlet to the tube-.cage of the gas to be heated are located at the same Vend of the tube-cage, 4so 4that `the heating gas and the lgas to be heated flow Tin parallel through the heater, and the .length ofthe tubes is less than three times the diameter 4of the Aring'in which they are arranged.

3. A gas heater as defined Vin claim l wherein the 9 wall of said shaft is so constructed as to form a shelf over which a substantial portion of the bottom of the upper head box of the tube-cage projects.

4. A gas heater as defined in claim 1 wherein said supporting means for the upper head box are so yconstructed and arranged as to form an airtight engagement between said upper head box and said shaft wall.

5. A gas heater as defined in claim 1 wherein said tube-cage is so constructed and arranged as to be insertable in and removable from said shaft as a unit.

6. A gas heater as defined in claim 1 wherein the diameters of said ring of tubes and said lower head box are less than the inside diameter of that portion of said shaft wall which surroundes said tubes and lower head box.

7. A gas heater as defined in claim 5 wherein the diameter of said lower head box is less than that of said upper head box and the tubes converge toward their lower ends so as to form a downwardly tapering tube-cage.

8. A gas heater as defined in claim 5 wherein the means for discharging the heated gas comprises at least one conduit connected to thebottom of'said lower head box and passing vertically through said bottom closure means.

9. A gas heater as defined in claim 1 wherein the means for supplying gas to be heated comprises an inlet conduit connected to said upper head box. y

10. A gas heater as defined in claim l including an inlet conduit for heating gas connected to the heating gas passage of said head-unit.

11. A gas heater as defined in claim 1 wherein said head-unit includes a burner for combustible fluid connected to said heating gas passage and a combustion chamber between the inlet of said burner and the mouth of said heating gas passage, said burner being so constructed and arranged as to provide substantially complete combustion within said combustion chamber.

12. A gas heater as defined in claim lincluding an inlet conduit for gas to be heated connected to said upper head box and an inlet conduit for the heating gas connected to the gas passage of said head-unit.

13. A gas heater as delined in claim 1 wherein said head-unit is removableas a unit from the shaft and is supported at least indirectly on said shaft wall.

14. A gas heater as defined in claim 13 wherein said head-unit is supported directly by said upper head box.

15. A gas heater as defined in claim 14 wherein said head-unit rests by gravity on said upper head box and is in airtight engagement therewith.

16. A gas heater as defined in claim 1 wherein a substantial portion of the bottom of said upper head box projects over and s shielded from radiation by a portion of said shaft wall.

17. A gas heater as defined in claim 16 wherein said upper head box is provided with a at bottom and said tubes are connected to the inner peripheral portion of said bottom.

18. A gas heater as defined in claim l including a second tube-cage suspended in the shaft below said first named tube-cage, said second tube-cage comprising an upper head box, a lower head box and a set of substanover the mouths of said tubes and the adjacent portions of the walls of `said head box at a relatively small distance from said walls to form a narrow passage between said plate and said walls leading to said tube mouths.

22. A gas heater as defined in claim 1 wherein at least one of said head boxes is so positioned as to expose a portion of its walls to said heating chamber andhas a plate therein extending over the mouths of said tubes and along said exposed portion of the walls of said box at a relatively small distance from said walls so as to form a narrow passage between said plate and said walls leading to said tube mouths.

References Cited in the file of this patent UNITED STATES PATENTS 171,844 Payne Jan. 4, 1876 Y 613,332 Temple Nov. 1, 1898 677,798 f Riblet July 2, 1901 1,599,613, Fahrenwald Sept. 14, 1926 1,662,142 Braun Mar. 13, 1928 1,671,686 Smoot May 29, 1928 1,717,334 DeFlorez June 11, 1929 2,119,817 Keller June 7, 1938 2,201,616 LaMont May 21, 1940 2,213,324 Niemitz Sept. 3,. 1940 2,276,527 'Ihrockmorton Mar. 17, 1942 2,320,911 Cooper June 1, 1943 2,409,801 Ruegg Oct. 22, 1946 2,421,387 Lysholm June 3, 1947 2,524,637 Ruegg Oct. 3, 1950 2,544,600 Keller Mar. 6, 1951 2,555,552 Litell June 5, 1951

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