US2993479A - Fluid heaters - Google Patents

Description

J. THURLEY FLUID HEATERS July 25, 1961 3 Sheets-Sheet 1 Filed Aug. 11, 1958 J. THURLEY July 25, 1961 FLUID HEATERS 3 Sheets-Sheet 2 Filed Aug. 11, 1958 QQJ F/GB.

July 25, 1961 J. THURLEY 2,993,479

FLUID HEATERS Filed Aug. 11, 1958 a SheetsSheet s F/ G. 4. F/ G. 5.

Q7 72 U3 4 .37 U2 k p I N 1 3 I 3 0 w 9 0 0W0 0 0 9 00000 000 0 0 0 0 a 0 0 0 0 0 0 0 0 0 00 0000000v 0 0 0 0 0 0 s 0000000000v 9 P. Ir 3%57. 2 7 3@/5 7 I 7 I a q L@ 0000000000 0000000 00 W V 0 0 a 00000000 8 m 000000 0 P v 0 00000000 00000 000m0 9 7 00000000 0 9 h 0 3 H 2 @1 0:32 @B? United States Patent 2,993,479 FLUID HEATERS John Thurley, Westminster, London, England, assignor to Gibbons Heaters Limited, London, England, a

British Company Filed Aug. 11, 1958, Ser. No. 754,466 Claims priority, application Great Britain May 14, 1958 8 Claims. (Cl. 122-23) This invention concerns improvements in or relating to fluid heaters in which the fluid to be heated is conveyed through tubes in a furnace and heat is transferred to this fluid by radiation and/or convection from combustion gases or flames in the furnace. More particularly, the invention relates to heaters for heating temperature sensitive stocks, such as hydrocarbons.

In fluid heaters used heretofore, aluminous highly emissive flame has been considered the most satisfactory means for heating the furnace due to the ability of such a flame to radiate most heat for a given furnace temperature. In the methods used for heating by radiation, the tubes, which usually extend either horizontally or vertically, of the furnace, are arranged in sets lining up posite Walls of the latter and a plurality of burners are arranged at each end of the furnace to direct their flames between the sets of tubes. These tubes are arranged so as not to come into direct contact with the hot combustion gases or flames and a high flame emissivity is relied upon in order that as much heat as possible should be transferred to the fluid being heated before the combustion gases leave the furnace, e.g. to pass to a convection bank.

Such heaters suffer from the disadvantage that heat absorption is not uniform around the periphery or along the length of each tube and also varies from one tube to another depending on the position of each tube relatively to the burners. In refinery heaters Where the fluid to be heated is a temperature sensitive stock liable to coke when overheated, the side of the tube which is nearer to the burner flames becomes heated to a greater extent than that side remote from the burner flames and consequently coking of the stock inside the tube is greater at the side facing the burner flames, the resultant build up of carbon at this side resulting in high metal skin temperatures of the tube and ultimate failure of the latter. Further, in light hydrocarbon pyrolysis and catalytic reforming, high heat absorption rates and high temperatures are required at a high temperature level. Such high heat absorption rates and the uneven tube temperatures produced cause scaling of the outer surface of the tubes and ultimate failure of the latter.

Many attempts have been made to solve the problem of how to ensure even heat absorption around and along the tubes, but none of these attempts has proved entirely satisfactory.

In one heater, burner flames are made to impinge and flow down refractory walls located on each side of a double row of staggered tubes. In such a heater more even heating of the tubes is achieved and a higher average rate of heat absorption is permitted than in the case of conventional heaters. However, in order that the said refractory walls may form a radiant heat source they must be subjected to severe flame impingement and as a result become damaged in use, especially when oitfired burners are used.

In a second heater proposed in an endeavour to overcome the disadvantage of uneven heating of the tubes, a stream of waste combustion gases is blown at high speed between the burner flames and the tubes being heated in order to form abarrier preventing flames licking the tubes.- The velocity of these gases may be varied to control the height of the said barrier and thus the amount of heat absorbed by the tubes. This heater, besides not being perfectly effective in use, is more elaborate than the conventional form of heater and requires a fan for the recirculation of the waste gases.

In a third heater, which comprises a cylindrical furnace in which vertical tubes are arranged around the inner periphery of the furnace, a highly radiating luminous flame burner is arranged at the lower part of the furnace and, in order to compensate for the loss of flame radiation at the upper part of the furnace, a cone is arranged in such upper part, such cone increasing the velocity of the flue gases, increasing the convective heat transfer and also acting as a source of secondary radiation. This arrangement gives a constant heat flux along the whole length of each tube, but does not produce a constant heat flux around the tube periphery since the sides of the tubes remote from the centre of the furnace absorb less heat than those sides facing the centre of the furnace.

In a further heater designed in an endeavour to ensure equal absorption around the tubes, 9. special form of tile is located over the upper tubes so as to ensure similar gas flow over these tubes and also to compensate the high radiant heat absorption from the flames on the lower sides of the tubes by convective heat transfer to the upper sides of the tubes. This form of heater does not in fact provide sufficient compensation and uneven heat absorption persists.

Further attempts to equalize the heating of the tubes have been made by placing on each side of a single row or a double row of staggered tubes radiant panels, which may be the furnace walls, or radiant cups heated by flameless combustion multi-burner installations. These attempts have been to some extent successful, but the heaters incorporating radiant panels or cups are extremely expensive.

In all of the heaters discussed above, an attempt has been made to achieve maximum emissivity, either by using high emissivity flame burners or, if flameless burners are used, by using the burners to heat refractory panels or cups in order to create a highly emissive heat source.

It is an object of this invention to provide a new or improved fluid heater, particularly a refinery heater for hydrocarbons, in which even, or substantially even heating, of the tubes is achieved without the need for the use of extremely expensive equipment.

This object is achieved by using, for the or each burner, a short flame high intensity combustion burner capable of delivering a high velocity stream of combustion gases with a low emissivity and directing the or each stream of combustion gases into a furnace space, through which pass tubes carrying the fluid to be heated, from a position adjacent a wall of the furnace space towards an opposite wall thereof.

We have found that, contrary to expectations, the low emissivity radiation produced by a short flame high intensity combustion burner produces perfectly satisfactory heating of the tubes without radiating panels or cups being required and that the high velocity of the combustion gases of such burner can be used to cause automatic circulation of the combustion gases throughout the furnace without the need for using circulating fans and even, or substantially even, heating of the tubes is achieved.

In addition, the following further advantages are given by a heater according to this invention:

The heater may be constructed to be free from air infiltration, the combustion gases issuing at a high velocity from high intensity combustion burners occupy the whole of the furnace space and prevent air infiltration, which would otherwise reduce the furnace temperature, reduce the percentage of the important radiating gases CO and H O in the combustion gases and reduce the mean beam length of the radiating mass of combustion gases.

The heater may be made more compact since in high intensity combustion burners nearly complete combustion takes place inside a comparatively small refractory lined combustion chamber in the burner itself at combustion rates of up to 10,000,000 B.t.u./hr. for every cubic foot of combustion space and the furnace does not therefore require to act as a combustion space and the tubes carrying the fluid do not have to be spaced remote from the burners to avoid flame impingement.

The excellent mixing of fuel and air and the low heat loss during combustion in high intensity combustion burners enables substantially complete combustion of the fuel to be achieved without smoking, whilst using substantially no excess air above that required for burning of the fuel. This produces a higher flame temperature and a The tubes 7 are, as shown, arranged in parallel rows.

In one end wall 3 of the furnace 1 are mounted a plurality of short flame high intensity combustion bumers 8, each capable of producing a high velocity jet of combustion gases with a low emissivity. The burners 8 a are arranged spaced one above the other in a vertical row i 4 which is centrally located with respect to the rows of higher CO and H 0 content in the combustion gases of the tubes 7, the arrangement being such that the jets of combustion gases from the burners travel horizontally between the rows of tubes 7 and impinge on the end wall 3 of the furnace 1 opposite to that end wall 3 in i which the burners are mounted.

high temperature superheaters which arise due to low velocity combustion gas streams in which the gases are fusion temperature ashes including sodium sulphate.

Moreover, the operation of the. burners without excess air and the elimination of infiltrated air to the, furnace prevents the oxidation and scaling, and thereby prolongs the life, of intermediate tube supports andof the tubes in high temperature service.

The jet of combustion gases issuing from a high in:

tensity combustion burner is directional and thus the ori:

entations of a plurality of burners can be arranged to provide the optimum combustion gas circulation pattern in the furnace.

-High intensity combustion burners may be used which utilise refinery fuel gas or residual fuel oil which is now tending to replace refinery fuel gas as a refinery heating fuel. When using refinery fuel gas, low emissivity. comlow emissivity combustion gases at low velocity may be I produced with premix gas burners using a fuel gas pressure above 25 p.s.i.g. at the limiting orifice in, order to aspirate the primary air. 7 I v The furnace may be designed to be long and narrow, thereby giving optimum radiant absorption characteris tics and enabling the use of longer tubeswhich reduce the capital cost of the heater and reduce the process pressure drop.

In order that this invention may more readily be understood, reference will now be made by way of example to the accompanying drawings in which: FIGURE 1 is a diagrammatic perspective view of a heater according to this invention, part of this "heater being broken away to show the interior of the furnace; 7

FIGURES 4 to 7 are diagrammatic horizontal sections through further heaters according to this invention.

Referring to FIGURES l and 2, the heater there-illus trated comprises a furnace 1 of box type construction and including a furnace space 2 defined by walls 3.: The walls 3 are formed of a plurality of interconnected panels and are carried by a supporting frame of girders. 4 mounted on base supports 5. At its upper part thesaid {I have found that, using high velocity jets of combustion gases of low emissivity as described above, eificient circulation of these gases occurs, as indicated by the arrows in FIGURE 2, and perfectly satisfactory even heating of the tubes 7 is obtained.

Circulation of the combustion gases in the furnace space 3 may also be achieved if, instead of using burners which produce high velocity combustion gas streams in the form of jets, burners are used which produce high in addition swirled within the streams. Swirling of the gases produces a region of low pressure in the centre of the furnace space and thereby cooler combustion gases at the rear and sides of the tubes are drawn towards the furnace centre and this circulation of gases gives a more even furnace temperature with a more even heat flux around the periphery and along the length of each tube than is obtained with conventional heater designs.

In a modification illustrated in FIGURE 3, each burner 8 is fired into a muflle 9 which comprises a short cylindrical tube spaced from the furnace wall 3 in which the burner is mounted and surrounding the discharge end of the burner so as to be coaxial therewith. Due to the aspiration effect of this muffle 9, combustion gases in the furnace space 3 are drawn through the muflle and are entrained with the jet of combustion gases issuing from the burner so that increased circulation of the combustion gases is achieved. When a muffle is used, the combustion gases drawn through the mufile temper the high burner flame temperature before the gases leave the muffle. The circulation of combustion gases in the furnace not only creates even furnace conditions by lowering the temperature at the centre of the furnace and raising the furnace temperatures remote from the burners, but also increases heat absorption by convective as well as radiant heating. Although the furnace of the heater of FIGURE 1 has been shown as of box type construction, it will be appreciated that this furnace could be of circular or any other suitable form. Further, although the tubes 7 have been shown as being vertically arranged and the burners as firing horizontally, it will be appreciated that the tubes could be arranged horizontally or in any other desired orientation and the burners could be arranged to fire vertically, e.g. the heater could be floor-fired, or in any other desired orientation. Thus, FIGURE 2 could equally wellrepresent a heater in which the tubes are horizontally arranged and the burners are mounted in the floor of the furnace to fire vertically upwardly.

According to a further feature of the invention, the combustion gases may be removed from the furnace at the end opposite that from which the burner streams are directed and recirculated through the furnace by being brought externally of the latter and reintroduced into the furnace so as to be entrained with the coma bustion gases issuing from the burners. Advantageously, such external recirculation is achieved by firing. the burners, which are located externally of the furnace, into a muflle mounted in, or forming part of, the furnace wall.

FIGURES 4 to 7 show diagrammatically heaters in which the combustion gases are externally recirculated. These heaters are shown as being floor-fired and have extensions 10 at their upper parts which lead to flue stacks or waste heat recovery sections. Due to their compactness, several of these radiant heaters could be combined and the flue gases led off to a common process convection bank or waste heat recovery unit or stack. The burners 8 in each of these heaters are located externally of the furnace 1 and the mutfles 9 are mounted in the floor or lower wall 3 of the furnace. From apertures 11 in the extensions 10 conduit means, indicated schematically by the lines 12, lead to the muflles 9 so that combustion gases in the furnace space 2 are automatically drawn from the upper part of the furnace space, brought externally of the latter to the muflies 9 and entrained with the combustion gases issuing from the burners.

The high velocity streams of the burners used in heaters according to this invention enable external recirculation of the combustion gases to be effected without circulating fans being required.

Conveniently a damper 13 is provided in the external flow path of the recirculating combustion gases so as to afford control of the furnace atmosphere, i.e. control of the ratio of convection heat absorption to radiant heat absorption.

FIGURES 4 to 7 show heaters having various arrangements of tubes 7. The heater of FIGURE 4 has two horizontally spaced vertical rows of tubes, whilst the heaters of FIGURES 5 and 6 have respectively three and five such rows. The heater of FIGURE 7 has nine rows, the tubes of each row being vertically staggered relatively to the adjacent rows.

Those tubes 7 which are nearest in the line of the combustion gases issuing from the burners may be omitted as in the heater of FIGURE 6 or, alternatively, may, as in the heater of FIGURES, be shielded from direct impingement of the high velocity gas streams by baffles 14.

The spacings of the tubes 7 may be small or large depending on whether one requires minimum furnace size or maximum coil heat absorption. In a convenient form of the heater of FIGURE 1, the height of the furnace space 2 may be 40 feet and the rows of tubes 7 spaced apart by a distance of 3 feet.

Further, the heater according to this invention may be used to heat any gaseous or liquid medium and the invention may thus be applied to a steam boiler. It is, however, as previously indicated, particularly applicable to refinery heaters.

Examples of high intensity combustion burners which utilise residual fuel oil and produce a high velocity jet of combustion gases and which are suitable for heaters according to this invention are shown and described in US. Patents Nos. 2,625,795, 2,632,300 and 2,701,608, whilst examples of high intensity combustion burners which produce a high velocity combustion gas stream in which the gas is caused to swirl within the stream are shown and described in US. Patents Nos. 1,560,076, 1,560,078 and 2,698,050.

I claim:

1. A heater of the class described comprising: Wall structure defining a furnace space; a plurality of tubes passing through said furnace space and connected to be supplied with fluid to be heated, said tubes being spaced from said wall structure and from each other to allow the free flow of gas around the tubes; a plurality of short flame, high intensity combustion burner means having combustion chambers in which substantially complete fuel combustion takes place, said combustion chambers sivity to cause such jets to impinge on a part of said wall structure remote from the burner means orifices andto cause circulation of such gases around the tubes in the furnace space, said jets issuing from said burner means orifices being substantially the sole means trans ferring heat to said tubes.

2. A heater of the class described comprising: wall structure defining a furnace space; a plurality of tubes passing through said furnace space and connected to be supplied with fluid to be heated, said tubes being spaced from said wall structure and. from each other to allow the free flow of gas around the tubes; a plurality of short flame, high intensity combustion burner means having combustion chambers in which substantially complete fuel combustion takes place, said combustion chambers being substantially entirely shielded from said furnace space andv positioned adjacent one surface of said wall structure defining said furnace space, said burner means each having a delivery orifice at said one surface of said wall structure for delivering to said furnace space a high velocity jet of combustion gases having a low emissivity to cause such jets to impinge on a part of said wall structure remote from the burner means orifices and to cause circulation of such gases around the tubes in the furnace space, said jets issuing from said burner means orifices being substantially the sole means transferring heat to said tubes; and an axially short cylindrical muffle mounted adjacent each burner means and positioned relatively to said wall structure so that the combustion gas jet from each burner means orifice passes coaxially through its respective muffle before passing across the burner space and draws therethrough combustion gases circulating in the furnace space thereby to increase the circulation of these gases.

3. A heater of the class described comprising: wall structure defining a furnace space; a plurality of tubes passing through said furnace space and connected to be supplied with fluid to be heated, said tubes being spaced from said wall structure and from each other to allow the free flow of gas around the tubes; a plurality of short flame, high intensity combustion burner means having combustion chambers in which substantially complete fuel combustion takes place, said combustion chambers being substantially entirely shielded from said furnace space and positioned adjacent one surface of said wall structure defining said furnace space, said burner means each having a delivery orifice at said one surface of said wall structure for delivering to said furnace space a high velocity jet of combustion gases having a low emissivity to cause such jets to impinge on a part of said wall structure remote from the burner means orifices and to cause circulation of such gases around the tubes in the furnace space, said jets issuing from said burner means orifices being substantially the sole means transferring heat to said tubes; and an axially short cylindrical muffle mounted adjacent each burner means and positioned relatively to said wall structure so that the combustion gas jet from each burner means orifice passes coaxially through its respective mufile before passing across the burner space and draws therethrough combustion gases circulating in the furnace space thereby to increase the circulation of these gases and conduit means leading from a region of the furnace space remote from the burner means to said muflles so that combustion gases in the furnace space are withdrawn therefrom and entrain with combustion gas jets passing through said muflies.

4. A heater according to claim 3, wherein damper 7 means are provided in said conduit means ror controlling the atmosphere of the furnace space.

5. A heater according to claim 3, wherein baffles are positioned the furnace space for shielding the tubes closest to the burner means orifices from direct impingement of the combustion gas jets.

6. A fluid heater of the class described comprising: walls defining a furnace space; a plurality of tubes passing through said furnace space and connected to be supplied with the fluid to be heated, said tubes being arranged parallel to one another in a plurality of spaced rows, the outer rows of tubes being spaced from opposed 'walls of the furnace space and the tubes in each row being spaced from each other to allow the free flow of gas around the tubes; a plurality of short flame, high intensity combustion burner means, each burner means having a combustion section in which substantially complete combustion of fuel takes place and which delivers from a delivery orifice thereof a high velocity stream of combustion gases having a low emissivity, said combustion space being substantially entirely shielded from said furnace space, means mounting the burner means adjacent said furnace space; delivery orifices of the combustion sections of said burner means communicating through one of said walls with said furnace space so as to direct the combustion gases from said burner means into the furnace space and between the said rows of tubes, whereby said jets issuing from said burner means orifices constitute substantially the sole means transferring heat to said tubes, said orifices being closely adjacent said one wall and nearer to the latter than any of said tubes; and a plurality of axially short tubular mufiies, one associated with each of the burner means, each mufiie being mounted within the furnace space and adjacent, but slightly spaced from/the said one wall so as to surround the delivery orifice of its associated burner means coaxially of the combustion gas stream to issue therefrom.

7. In a fluid heater of the class described: walls defining -a chamber; a row of spaced apart parallel tubes for conducting fluid to be heated through said chamber, each tube extending in direction between two opposed walls of the furnace chamber and said row extending between a firing wall and a back wall; means for heating said tubes substantially uniformly throughout the respective exterior surface areas of each of them comprising burner means for'introducing into said chamber heated, low emissivity gases at high velocity from a location between said row of tubes and one of said opposed wall portions, said gases entering said chamber substantially at the interior surface of said firing wall in direction towards said back wall; whereby said gases are circulated between said row of tubes and said opposed walls and between all of said tubes to transfer heat thereto, said gases thus constituting substantially the sole means transferring heat to said tubes.

8. In a fluid heater of the class described: walls defining a chamber; a plurality of rows of spaced apart parallel tubes for conducting fluid to be heated through said chamber, each tube extending in direction between two opposed walls of the furnace chamber and said rows extending between a firing wall and a back wall; means for heating said tubes substantially uniformly throughout the respective exterior surface areas of each of them comprising burner means for introducing into said chamber heated, low emissivity gases at high velocity from locations between said rows of tubes and one of said opposed wall portions, said gases entering said chamber substantially at the interior surface of said firing wall in direction towards said back wall, whereby said gases are circulated between said rows of tubes and said opposed walls and between all of said tubes to transfer heat thereto, said gases thus constituting substantially the sole means transferring heat to said tubes.

References Cited in the file of this patent UNITED STATES PATENTS

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