US2029291A

US2029291A - Method and apparatus for heating fluids

Info

Publication number: US2029291A
Application number: US615358A
Authority: US
Inventors: Joseph G Alther
Original assignee: Universal Oil Products Co
Current assignee: Universal Oil Products Co
Priority date: 1932-06-04
Filing date: 1932-06-04
Publication date: 1936-02-04
Anticipated expiration: 1953-02-04

Description

Filed June 4, 1932 5 Shets-Sheet 1 aNvENToR JOSEPH G. ALTHER BY Z g ATTORN Filed June.4, 1932 5 Sheets-Sheet 2 lll VIL IIIL rllL rllL rllL rllL UHH HHHHHHHHHHHUUH lNVENTOR JOSEPH G. ALTHER Feb. 4, 1936. J. G. ALTI-1ER METHOD AND APPARATUS FOR HEATING FLUIDS Filed June 4, 1932 5 Sheets-Sheet 5 JOSEPH G ALTHER BY Q l ATTORNEW Feb. 4, 1936. J. G. ALTHER METHOD AND APPARATUS FOR HEATING FLUIDS 5 Shee'is-Sheet 4 Filed June 4, 1952 E E S.

Feb. 4, 1936; f J. G. ALTHER y 2,029,291

I METHOD AND APPARATUS FOR HEATING FLUIDS- Filed June 4, 1932 5 sheets-sheet 5 TEMPERATURE oF FLUID PROGRESS OF FLUID THROUGH HEATING COIL FIG; 8

IVENTOR x JOSEPH c. ALTHER ATTORNE barema Feb. t, ieee mail rica

UIDS

Joseph G. Alther, Chicago, Ill., assignor to Universal Oil Products Company, Chicago, Ill., a

corporation of South Dakota Application June 4, 1932, Serial No. 615,358

12 Claims.

This invention particularly relates to an improved method and means of heating fluids substantially by convection or fluid heat during their passage in a continuous stream through a conduit which comprise subjecting the maximum surface of the fluid conduit to high rates of heating in order to obtain a high average rate of heat input over the entire surface of the conduit.

While the invention may be applied to the heating of fluids generally, its preferred use is in the heating of hydrocarbon oils and it is especially useful in the cracking of hydrocarbon oils.

More. particularly the invention refers to an improved method and means for heating fluids wherein a heat carrying medium, e. g. hot combustion gases, are directed by positive means to opposite sides of the conduit through winch the fluid is passing, whereby they are uniformly distributed over the surface of the fluid conduit in such a manner as to effect an average rate-of heating around the entire periphery of the conduit approaching the maximum rate of heating and whereby direct control for any desired rate of heat input is obtained. Among other advan-` tages of the present invention may be mentioned (a)V the control of velocity, pressure drop and turbulence 'in the fluid conduit irrespective of the total quantity of oil undergoing treatment at one time (b) high average heat input over the entire surface of the fluid conduit and uniform application of heat to opposite sides of the conduit (c) flexibility of control of heat application to obtain any desired rate of heat input by direct control of the ring conditions and direct control of heat distribution to obtain any desired heating curve for the material undergoing treatment (d) high tube eiciency as compared with convection heating methods of conventional character. Maximum capacity for a fluid conduit of given size or, vice versa, minimum tube surface for a given capacity, (e) quick heating of the fluid to a high temperature in a minimum of time. As applied to the conversion of hydrocarbon oils, this feature reduces excessive cracking and gas formation as well as the formation and deposition of coke in the heating zone or y fluid conduit, thereby increasing the yield and Zone.

installation operating and maintenance costs and increases the safety factor. By the application of the inventionx a large quantity of oil may receive the same heat treatment asy a small quantity of oil with all of the resulting advantages.

In the heating of fluids substantially by convection, it is common practice to pass hot combustion gases over the fluid conduit, comprising 5 a plurality of tubes through which the uid flows in a continuous stream. When the tubes are arranged in rows parallel to the flow of combustion gases there is a tendency for the gases to channel through the space between the rows of l tubes, permitting a large volume of hot gases to pass through the tube bank without coming into intimate contact with the tubes and transmitting heat to the fluid. ,To overcome this channeling of the hot combustion products it has become l common practice to place the tubes of the fluid conduit in staggered formation so that the path of flow of the hot gases is continually changed in an attempt to distribute them more uniformly around the tubes and obtain more intimateand uniform contact between the combustion gases and the tube surfaces. l However, such methods do not secure positive uniform distribution of the gases about the tubes, different tubes in the same row perpendicular to the general direction Y of flow of the combustion gases often being i heated at different rates and the uniform distribution of hot gases around the circumference of each tube being the exception rather than the rule. 'I'he staggered arrangement of tubes 30 which breaks up the stream of gases, changing their uni-directional flow, offers increased resistance to the iiow of gases which, with a given draft condition, decreases the volume of gases which may pass through the. tube bank in a given time, thereby decreasing the velocity and heating efficiency of the gases which come into actual contact with the tube surfaces.

In the operation of the process of the present invention, the hot combustion gases, which fur- 40 nish heat substantially by convection to uid passing in a continuous stream through the tubes of a uid conduit, are directed against opposite sides of the tubes in such a manner that a substantially uniform distribution of gases and substantially uniform heating is obtained on opposite sides of each individual tube in the fluid conduit. This insures a high average heat input per unit area of tube surface for any given maximum rate of heating, which means high tube efllciency and maximum capacity for a fluid conduit of given size or, vice versa, a minimum of heating surface for a given capacity. i Also, by employing the features of the present invention, wide differences in heat intensity at different kpoints around the circumference of the same tube are avoided, thereby eliminating stresses, due to unequal expansion, and increasing the safety of a tube of given diameter and wall thickness or permitting the use of a thinner wall in tubes of the same diameter. This reduces the cost of.

installation. Also, as a result of increasing the average rate of heat input, the time of treatment may be reduced and overcracking minimized, which results in smaller gas production per barrel of oil being treated, greater gasoline'yield and higher octane number, as stated previously.

As a. further future of the invention, the fluid conduit may be divided into a plurality of tube banks each of which is independently heated from opposite sides. The various tube banks may be connected either in series or in parallel. When connected in series, this arrangement provides a method and means of obtaining a close control over-the intensity of heating in each section of the entire fluid conduit, independent of that in any other section so that the rate of heat input may be varied in different sections of the iiuid conduit to suit requirements or may be maintained substantially uniform in any or all of the various sections of the heating coil or fluid conduit. This makes it possible to employ any desired type of heating curve (i. e. any-desired rate of temperature rise in various sections of the furnace) and permits changing the heating curve at will by simply changing the firing conditions without any change in the furnace structure or in the ow through the heating coil.

With parallel flow through the various sections of the heating coil, the features of the present invention permit increasing the capacity of the furnace by the simple addition of unit sections comprising additional tube banks without changing any of the heating characteristics. For example, given a furnace with a fluid conduit comprising two banks of heating tubes, the tubes of each bank being connected in series and the two banks being connected in parallel, the capacity of the furnace may be doubled by the addition of two similar unit sections having two similar banks of tubes, without increasing the pressure drop through the total fluid conduit, which Vnow comprises the four tube banks connected in parallel, without changing the velocity of the iiuid flowing-through the heating coil and Without otherwise changing the heating conditions such as the intensity of heating in each unit section and each portion of the various unit sections of the furnace and without changing the type of heating curve.

A multiple-unit furnace of the type provided by the features of the present invention may also be utilized to special advantage, as applied to the conversion of hydrocarbon oils, when it is desirable to'subject two or more different Atypes of oil tov independently controlled heating conditions in the same cracking system. In this connection, any number of unit sections of the same furnace may be employed for any number` of different oils, one or more sections being utilized for each type of oil with independently controlled heating conditions in each unit section.

In the accompanying drawings,

Figure 1 is a sectional end elevation of the furnace and Figure 2 is a sectional side elevation. Figures 3 and 4 of the drawings illustratea somewhat different form of furnace structure also embodying the features of the present invention. Figure 3 is a sectional end elevation and Figure 4 is a sectional side view. Figures 5, 6 and 'I indicate three diiferent methods of connecting the various tube banks of the fluid conduit in eitherl of the different specific forms of furnace structure illustrated in Figures l, 2, 3 and 4. Figure 5 shows the tube banks connected in parallel while 5 Figures 6 and 7 indicate two diiferent methods of connecting the several tube banks for series flow of fluid therethrough. Figure 8 of the drawings illustrates several types of heating curves which may be obtained in accordance with the provisions of the` present invention in either of the specific types of furnace illustrated.

Referring to Figures 1 and 2 of the drawings, the main furnace structure comprises side walls I and I', end walls 2 and 2', a roof 3 and a floor 4. The fluid conduit comprises a multiplicity of

vertical tube banks

5, 6 and 1, each consisting of a number of horizontally disposed tubes 8, extending between end walls 2 and 2' of the furnace. The tubes of each of the vertical rows or banks are connected in series by means of suitable headers or return bends 9. Inlet and outlet ports I0 are provided-at opposite ends of each bank of tubes each of which may be utilized as eitherthe inlet or the outlet port so thatthe iiow of oil o'r other fluid to be heated may be either upward 'or downward through any individual bank of tubes, as desired.

, means of suitable burners, not shown in the v drawings, through firing ports located in the top. Hot combustion gases pass from the firing tunnels II, I2, I3, |4, I5 and I6 through openings I1 in the roof of the furnace into the respective distributing zones I8, I9, 20, 2|, 22 and 23, located on opposite sides of the

respective tube banks

5, 6 and 1.

The

tube banks

5, 6 and 1, respectively, are located in heating zones 25, 26 and 21. Heating 5 zone 25 is separated from distributing zones I8 and I9 by

perforated walls

28 and 29, respectively. Heating zone 26 is separatedfromA distributing zones 2|! and 2| by perforated walls 30 and 3|, respectively. Heating zone 21 is sepa- 50 rated from distributing zones 22v and 23 by perforated walls 32 and 33, respectively.' In this manner the entire furnace structure is divided into three unit sections, each of which comprises a bank of tubes within a heating zone with a firing tunnel and distributing zone on each side of the vertical center line through the tube bank, as viewed from the end of the furnace, as in Figure 1.

Equal quantities of hot combustion gases are directed to the opposite sides of the tubes of each bank through perforations or openings 34 in

walls

28, 29, 30, 3I, 32 and 33, passing downward over the tube surfaces in such a manner as to heat the opposite sides of each individual tube equally. f

Firing conditions may be so controlled that combustion is substantially completed in the firing tunnels II, I2, I3, I4, I5 and I6 or some combustion may be allowed in the distributing compartments I 8, I9, 20, 2|, 22 and 23. Particularly in the latter case, however, the

walls

28, 29, 30, 3| 32 and 33 are constructed of suitable material of low thermal conductivity, such as fused or molded alumina or silica, natural or sp'acing of openings 34, in

walls

28, 28, 30, 3|, 32

artificial mullite, etc., (i. e. material which will transmit only a minor amount of radiant heat from the distributing compartments to the tubes in the heating zone) in order that the tubes may be heated primariy by convection or fluid heat from the hot combustion gases which come into direct contact therewith.

The spent combustion gases having passed over the tubes of the uid conduit pass from the heating compartments 25, 26 and 21 through openings 31' in the floor 8 of the furnace into the

respective gas passageways

38, 39 and 46 from which they pass through a flue 43| to a stack, not shown in the drawings.

In the case here illustrated, the size and/or and 33 are so graduated from top to bottom that progressively increasing quantities of combustion gases pass throughthe openings into contact with the tubes in the heating zone as they pass downward through the furnace. This provides a means of heating the tubes in the lower portion of each bank or row'at a higher rate than the tubes in the upper portion of the same row. This condition may, of course, be varied so that greater heat intensity is concentrated around the upper portion, central portion or any desired portion of the tube bank or so that every tube in each or any individual row is heated equally by regulation of the. size and spacing of openings 34 to suit requirements. However, both perforated walls of each unit section of the furnace, located on opposite sides of any individual row or bank of tubes, preferably have openings of equivalent size and spacing so that opposite sides of the individual tubes of each tube bank are subjected to substantially equal heating. y

In accordance with the features of the present invention, the several tube banks may be heated at a substantially uniform rate or different rates of heating may be employed in any or every tube bank, the conditions of heating being independently controlled in each unit section of the furnace. The use of separating walls, such as indicated in the drawings by the

numbers

35 and 36, for the purpose of separating adjacent distributing zones I9 and 20 or 2| and 22 is optional but has been found advantageous in case it is desired to employ different heating conditions in adjacent tube bank-.

`Another AspecificV form of furnace structure somewhat different from that just described but embodying the features of the present invention, is illustrated in Figures 3 and 4 of the attached diagrammatic drawings.

The main furnace structure comprises side walls 5| and 5|', end walls 52 and 52', a roof 53 and a floor 54. The fluid conduit or heating coil comprises a plurality of

tube banks

55, 56 and 51,

each comprising a vertical row of tubes 58 horizontally disposed

betweenthe end walls

52 and 52. 'Ihe tubes of each of the vertical rows or banks are connected in series by means of suitable headers or return bends 59. Inlet and outlet ports 60 are provided at opposite ends of each bank of tubes, each of which may be utilized as either the inlet or the outlet port so that the ow of oil, or other uid to be heated, may be either upward or downward through any individual bank of tubes, as desired.

In this particular form of furnace structure the firing and combustion tunnels 6| 62, 63, 64, 65 and 66 are located within the main furnace structure, one firing and combustion zone being provided on each side of the

several tube banks

55, 56 and 51. lThe firing and combustion compartments are supplied with any desired form of fuel by means of suitable burners, not shown in thepdrawings, through firing ports 14 which are here shown in the roof of the furnace but may, when desired, be located in the end walls of the firing compartment or both the top and end walls.

Walls

64, 85, 86, 81, 88 and 89 separate the firing and combustion compartments 6|, 62, 63, 64, 65

Aand 66 from -the respective distributing

zones

68, 69, 16, 1I, 12 and 13 and openings 61 beneath each of these walls permit the pas:age of hot combustion gases from the firing and combustion compartments into the distributing compartments.

Perforated walls 1B and 19, located on opposite sides of the vertical row or bank of tubes 55, define heating zone 15 and separate it from the adjacent distributing

zones

68 and 69 refpectively. Similar

perforated walls

80 and 8| located on opposite sides of the vertical row or bank of tubes 56, define heating zone 16 and separate it from the adjacent distributing zones 16 and 1I, respectively. In the same manner, perforated

walls

82 and 83, which are located on opposite sides of the vertical row or bank of tubes 51, define heating zone 11 and separate it from the adjacent distributing zones 12 and 13, respectively.

A wall 95, the use of which is optional, serves to separate firing and

combustion zones

62 and 63 while a similar wall 96 may be provided for the purpose of separating combustion zones 64 and 65.

In this manner the furnace here illustrated is divided into three unit sections, each of which comprise: a vertical row or bank of vtubes within a heating zone with a distributing zone and a firing and combustion zone located on opposite sides of each bank of tubes, making each section of the furnace structure symmetrical on opposite sides of the vertical center line through the tube bank, as viewed from the end of the tubes as in Figure 3.

Equalized firing conditions are employed in both firing and combustion compartments of each unit section of the furnace and, due to the symmetry of the furnace structure on opposite sides of each tube bank, equal quantities of hot combustion gases are directed to opposite sides of the tubes in each bank through perforations or openings 90 in

walls

18, 19, 80, 8|, 82 and 83, said combustion g-ases passing downward over the tube surfaces in such a manner as to heat the opposite sides of each individual tube equally.

Firing conditions may be so controlled that combustion is substantially completed ther@-

ing tunnels

6I, 62, 63, 64, 65 and 66 or intercombustion may be allowed in the distributing

compartments

68, 69, 1D, 1I, 12 and 13. Particularly in the latter case,`however, the

walls

19, 19, 80, 8|, 62 and 83 are constructed of suitable material of low thermal conductivity, such as fused or molded alumina or silica, natural or artificial mullite, etc., l(i. e. material which will transmit only a minor `amount of radiant heat from the distributing compartments to the tubes in the heating zone) in order that the tubes may be heated primarily by convection or fluid heat from the hot combustion gases which come into direct contact therewith.

The spent combustion gases having passed over the tubes of the uid conduit pass from the heating compartments 15, 16. and 11 ythrough openings 9| in the roof 53 of the furnace into the

respective gas passageways

92, 93 and 94 from which they pass through a ilue 91 to a stack, not shown in the drawings.

In the case here illustrated, the size and/or spacing of openings 90 in the walls 18, 18, 80, 8l. 82 and 83 are so graduated from top to bottom that progressively decreasing quantities of combustion gases pass through the openings into contact with the tubes in the heating zone as they pass downward through the furnace. This provides a means of heating the tubes in the lower portion of each row or bank of ltubes at a higher rate than those in the upper portion of the same row. This condition may,'of course, be varied so that greater heat intensity is concentrated around the upper portion, central portion or any desired portion of the tube bank or so that every tube in the row is heated equally, regardless of its position in the tube bank, by regulation of the size and spacing-of openings 90 to suit requirements and/or by regulated firing through burner ports, not shown, located at any desired position in the end walls of the furnace. However, both perforated walls of each unit section of the furnace, preferably have openings of equivalent size and spacing so that the opposite sides ofthe individual tubes of each tube bank are subjected tol substantially equal heating.

Figures 5, 6 and '7 of the attached drawings illustrate diagremmatically the two general types of iiow through the heating coil which may be employed in either form of furnace structure above illustrated and described. Figure 5 illustrates parallel flow and Figure 6illustrates series ow through the heating coil. Figure 7 illustrates a somewhat modified form of series flow arrangement. It will be understood that many modications of the two general forms may be employed without departing from the scope of the invention. i

The numerical designations given to the-tube banks in Figures 5, 6 and 'l are the same as 'those in Figures 1 and 2, although the flows illustrated in Figures 5, 6 and apply equally well to the fluid conduit illustrated in Figures 3 and 4.

Figure 8 of the attached drawings illustrates ve general types of heating curves obtainable with the improved furnace of the present invention. Curve A illustrates a constantly` increasing rate of temperature rise through theentire heating coil; curve B illustrates a constantly decreasing rate of temperature rise; curve C illustrates a uniform rate of temperature rise; curve D illustrates a heating curve with a maximum rate of temperature risein the first stages of the heating coil and curve E illustrates a heating curve with the maximum rate of temperature rise in the latter stages of the coil.

Any of the types of heating curves illustrated in Figure 8 may be obtained with either of the furnace structures illustrated and above described and with either parallel or series flow of fluid through the heating coil. For example, when a constantly increasing rate of temperature rise, such as indicated by curve A in Figure 8, is desired with parallel flow through the heating coil, as illustrated in Figure 5, ring conditions are equalized in the several sections of the furnace so that equal heating is obtained in every bank of tubes and the rate of heating is progressively increased from top toy bottom of every vertical tube row. When this same type of heating curve is desired with series flow, as illustrated in Figure 6, progressively more severe heating conditions are employed from top to bottom of veach tube bank and progressively more severe heating conditions are employed in the unit sections of the furnace from left to right. With a iiow such as illustrated by Figure '1, the same type of heating curve may be obtained by employing progressively more severe firing conditions in the unit sections of the furnace from left to right and by alternating the rf ing conditions around adjacent tube banks, progressively increasing heat intensities being employed from top to bottom of the iirst tube bank, progressively increasing heat intensities being employed from bottom Ato top of the second tube bank and so on through any number of tube banks in the uid conduit. In a similar manner, by suitable regulation of the firing conditions, curves of the type illustrated by curves B, C, D and E in Figure 8 may be obtained with flows through the fluid conduit such as illustrated in vany of the Figures 5, 6 and 7 or in any modification of the flows illustrated.

` As a specific example of the increased emciency obtainable by use of the features of the present invention in .either of the furnace structures illustrated and heretofore described, we will consider two cases. In the rst case convection heating by conventional methods is employed wherein hot combustion gases are passed downward through a bank of tubes without any provision for directing them equally toward opposite sides of each tube. In this case the combustion gases entering the tube bank-may be at a temperature heat intensity by continuously supplying fresh increments of hot combustion gases to the cooled gases passing over the tube surfaces throughout the entire iiuid conduit. In this manner the combustion gases surrounding the tubes are maintained at a minimum average temperature of about 1600 F. In each case the oil enters they heating coil at a temperature of about 500 F. and is discharged at a temperature of about 950 F. The log mean temperature difference is approximately 540 F. in the iirst case and, assuming an average rate of heat absorption of 9 B. t. u. per sq. ft. of tube surface per 1 F. log mean temperature difference, the average rate of heating in the rst case is approximately 4900 B. t. u. per sq. ft. per hour. The log mean temperature difference in the second case is approximately 890 F. and the rate of heat absorption of tube surface per 1 F. log mean temperature difference may be increased to approximately 11 B. t. u. per sq. ft. of tube surface, or an increase of about 20 percent as compared with the rst case, due to more efficient use of the hot combustion gases by directing them equally to opposite sides of the tubes whereby higher average heat intensity is obtained around the entire Vcircumference of each tube. These conditions will give us a heating rate of approximately 9800 B. t.'u. per sq. ft. per hour or an increase of about 100 percent over that obtained by conventional heating under the conditions above given.

As a further example of the results obtainable of somewhat diiferent heating conditions which are substantially the same as those given in the second of the above examples except that the temperature of the combustion gases is allowed to decrease from approximately 1650 to about 1400" ditions wherein iiuid is passed inv series through a plu- F. as they pass over the heating coil, these congive a log mean temperature difference of about 790 F. and a heating rate of about 8700 B. t. u. per sq. ft. per hour which represents an increase of approximately 78 percent over the rate of heating obtained in the rst case.

It will be understood that the conditions given in each of the above examples may be obtained with either series or parallel ilow through the heating coil.

It is to be understood that by the term convection as used'in the above specication and the appended claims I mean heat carried by the gases, which may also be termed uid heat.

I claim as my invention:

1. In a process for heating uid of the type wherein theuid is passed through a. plurality of banks of conduits of restricted cross section connected'in series, each bank disposed within a separate heating zone, the improvement which comprises simultaneously subjecting opposite sides of each uid conduit to convection heat by contact with directed streams of hot products of combustion, whereby to independently control the heat input over the entire surface of each conduit, and independently controlling the rate of heat input in each bank.

2. In a process forl heating fluid of the type wherein the fluid is passed through a plurality of banks of conduits of restricted cross section connected in series, each bank disposed within a separate heating zone, the improvement which comprises uniformly heating opposite sides of each uid conduit by convection heat from directed streams of hot products of combustion, whereby to independently control the heat input over the entire surface of each conduit and independently controlling the rate of heat input ineach bank.

3. In a process for heating fluids of Athe type wherein fluid is passed through a plurality of banks of conduits of restricted cross section, each bank disposed within a separate heating zone and the several banks connected in series, the improvement which comprises uniformly heating opposite sides of each fluid conduit by convection from directed streams of hot products of combustion whereby to independently Acontrol the heat input over the entire surface of each conduit and independently controlling the rate'of heat input in each bank.

4. In a process for heating fluids of the type wherein fluid is passed in series through a plurality of banks of conduits of restricted cross section, each bank disposed within a separate heating zone, the improvement which comprises simultaneously subjecting opposite sides of each fluid conduit to heat by contact'with directed streams of hot products of combustion, whereby to independently control the heat input over the entire surface of each conduit, and independently controlling the rate of heat input in each bank.

5. In a process for heating fluids of the type wherein iiuidl is passed through a plurality of l banks of conduits of restricted cross section, each rality of banks of conduits of restricted cross section, each bank disposed within a separate heating zone, the improvement which comprises uniformly heating opposite sidesr of each uid conduit by heat from directed streams of hot products of combustion, whereby to independently control the heat input over theentire surface of each conduit, and independently controlling the rate of heat input in each bank.

'7. In a process for heating `uids of the type wherein iiuid is passed in seri'es through a plurality of banks of conduits of restricted cross section, each bank disposed within a separate heating zone, the improvement which comprises simultaneously directing separate streams of hot products of combustion over opposite sides of the fluid conduits in each bank and adding fresh increments of hot products of combustion to each separate stream at various Vpoints in its path of flow over the bank, whereby to independently control the heat input over the entire surface of different conduits in the same bank while independently controlling the rate of heat input in each bank.

8. In a process for heating iluids of thev type wherein iuid is passed in series through a plurality of banks of conduits of restricted cross section, each bank disposed within a separate heating zone, the improvement which comprises simultaneously directing separate streams of hot products of combustion, of equal heating value, over opposite sides of the fluid conduits in each bank and adding fresh increments of hot products of combustion to each separate stream, equally on opposite sides of each bank, at various points in its path of flow over the bank, whereby to equally heat opposite sides of each conduit and independently control the heat input over the entire surface of different conduits in the same bank while independently controlling -the rate of heat input in each bank.

9. A furnace for heating fluids which comprises, in combination, a plurality of convection heating zones, a plurality of tube banks connected in series and each comprising a plurality of connected tubular elements disposed within a common plane, centrally located in each heating zone, a combustion zone located on opposite sides of each heating zone, perforated walls separating each heating zone from the adjacent combustion zones, said perforated walls serving to shield the tubular elements from radiation from the combustion zones, and means for directing streams of hot combustion gases from the combustion zones to opposite sides of each tubular element of the tube banks.

10. In a method for heating fluids wherein a restricted stream of the uid is passed in series through a plurality of tube banks, each comprising a plurality of series connected, substantially horizontal tubular elements disposed in common vertical plane, the improvement which comprises uniformly heating the opposite sides of each tubular element from directed streams of hot combustion products of approximately equal heat intensity, and independently controlling the heating conditions in each tube bank. 11. In a method for heating fluids wherein the fluid is passed in a restricted stream in series through a plurality of tube banks, each comprising a plurality of serially connected, substantially horizontal tubes disposed in a common vertical plane in a separate heating zone, the improvement which comprises directing streams of hot combustion products of approximately equal heat intensity through each heatingzone on opposite sides o! the vertical plane of the tubes and in a direction substantially normal to the horizontal planes of the tubes, uniformly heating the opposite sides of each tube by direct transmission of heat from the combustion products, and independently controlling the heating conditions in the separate heating zones.

12. In a method for heating uids wherein a restricted stream of the uid is passed in series through a plurality of tube banks, each comcommon vertical plane, the improvement which' comprises uniformly heating the opposite sides of each tubular element by heat of approximately equal heat intensity including heat derived by passing streams of hot combustion gases generated in combustion zones disposed on opposite sides of each tube bank in heat transmitting relation thereto, and independently controi- 10 I ling the heating conditions in elatc'l'i tube bank.

i JOSEPH G. ALTHER.