US3870459A

US3870459A - Burner for use with fluid fuels

Info

Publication number: US3870459A
Application number: US415074A
Authority: US
Inventors: Denis Henry Desty; Barry Herbert Francis Whyman; David Montagu Whitehead
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1968-11-06
Filing date: 1973-11-12
Publication date: 1975-03-11
Anticipated expiration: 1992-03-11

Abstract

A matrix burner contains metal sponge between the air tubes and adjacent to the combustion zone. For gaseous fuels the metal sponge produces a high outlet resistance to give even fuel distribution. For liquid fuels the metal sponge gives a wicking action and it assists heat transfer to vaporise the fuel into the combustion zone.

Description

United States Patent Desty et a1. 1 1 Mar. 11, 1975 [54] BURNER FOR USE WITH FLUID FUELS 1,830,826 11/1931 Cox 431/328 2,227,899 1/1941 [751 Inventors: Henry D8511 Weybndge, 3,173,470 3/1965 Wright 431/328 Herbert Ffancls Whymall 3,266,893 8/1966 Duddy 75/222 Teddmgton; David Montagu 3,367,149 2/1968 Manske.... Whitehead, Camberley, all of 3,408,180 10/ 1968 Winkler 264/44 E l d p FOREIGN PATENTS OR APPLICATIONS 1731 Assgneei 1 Petmleum Cmnpany 102,676 12/1937 Australia 431/323 14111111111 London England 783,170 4/1935 France 431/3211 [22] Filed: Nov. 12, 1973 Primary Examiner-Carroll B. Dority, Jr. [211 App! 415074 Attorney, Agent, or Firm-Morgan, Finnegan, Durham Related U.S. Application Data & Pine [63] Continuation of Ser, No. 866,481, Oct. 15, 1969,

abflndoned- [57 ABSTRACT 52 11.5. cu. 431/328 A matrix mains metal SPmge [51 Int. c1. F23d 13/12 air tubes and adjacent CombusflO Zone- [58] Field 011 Search 431/7, 10, 326, 328, 329, For gaseous fuels the metal sponge produces a high 431/170 Outlet resistance to give even fuel distribution. For liquid fuels the metal sponge gives a wicking [56] References Clted action and it assists heat transfer to vaporise the fuel UNITED STATES PATENTS into the combustion zone.

286,914 1(1/1883 Dimock et a1. 431/328 369,411 9/1887 Schreiner 431/323 10 Clams 7 Drawmg F'gmes BURNER FOR USE WITH FLUID FUELS This is a continuation of application Ser. No. 866,481, filed Oct. 15, 1969 and now abandoned.

This invention relates to a burner for use with fluid fuels, and more particularly to a burner suitable for use with vaporisable liquid fuels and gaseous fuels.

It would be convenient for the manufacturers of the appliances which incorporate burners to have available a type of burner which enables a wide range of fuels to be burnt in burners of similar construction, size and with similar heat outputs.

According to the invention a burner for fluid fuels comprises a plurality of combustion air tubes which pass through a fuel chamber which is divided into a fuel inlet zone adapted to distribute fuel across the burner and a fuel outlet zone which is packed with a metal sponge through which, during the use of the burner, fuel flows into the combustion zone where it burns with air which flows into the combustion zone via the combustion air tubes.

In the case of a burner operating on a gaseous fuel the metal sponge provides a relatively high resistance to the flow of fuel and the combination of a low resistance inlet zone followed by a high resistance sponge encourages an even fuel distribution. The possession of a high resistance implies that the pore size of the metal sponge is sufficiently small, for example a pore size of 0.l 0.5 mm average diameter is particularly suitable.

In the case ofa burner operating on a vaporisable liquid fuel such as kerosene the metal sponge transfers fuel by capilliary action. However the metal from which the sponge is made is a good conductor of heat and therefore the sponge has sufficient conductivity to assist vaporisation into the combustion zone by transfering heat to the liquid fuel.

In a preferred embodiment of the invention the low resistance inlet zone is packed with a metal sponge whose resistance to fluid flow is less than that of the metal sponge in the fluid outlet zone so that the whole of the fuel chamber contains packing. With this arrangement the walls of the fuel chamber (including the walls of the combustion air tubes) do not need to be mechanically self supporting and they preferably take the form of impervious coatings applied to the appropriate walls of the sponge.

Metal sponges are available in which the metal is arranged as a three dimensional network of strands between which a liquid or gaseous fuel can pass and these sponges are suitable for use in burners according to the invention.

The strands define small volumes which are the pores of the sponge. Sponges with a pore size of 0.5 2 mm average diameter are particularly suitable for use in the fuel inlet zone (in the case of burners in which this zone is packed).

In some sponges the strands are hollow, i.e., the sponge is a network of tubular strands and these tubes connect with one-another throughout the sponge so that there are two possible flow systems, i.e., through the pores and through the bores of the tubes. The sponges are conveniently prepared by:

l. Forming a plastic sponge of the appropriate pore size 2. Metalizing the plastic sponge, and

3. Heating the metalized plastic sponge to remove the plastic material.

It is possible to mould the original plastic foam to the required shape and dimensions and it is also possible to mould layers of foam of different pore sizes. In other words the formation of the metal foam can be used to prefabricate packing elements of the required shape and of the right dimensions.

It is emphasized that the range of pore sizes suitable for high resistance (gas) burners overlaps the range of pore sizes suitable for wicking (liquid fuel) burners. This implies that it is possible to make a single burner which, without modification, will give satisfactory operation with a wide range of liquid and gaseous fuels, e.g., it will give satisfactory operation with kerosine, natural gas and towns gas. When it is desired to take advantage of this flexibility it will usually be necessary to make considerable adjustments to the fuel supply system or even, when both liquid and gaseous fuels are to be used, to provide two independent fuel supply systerns.

Cylindrical combustion air tubes are particularly suitable for use in all the burners described above. Most suitably the combustion air tubes are arranged with their axes parallel to one another.

The flow of fuel from a burner as described above tends to be aligned with the flow of air from the combustion air tubes. This gives satisfactory combustion but where very high fuel flow rates are required better combustion may be achieved if the fuel flow is deflected into the combustion air flowing out of the combustion air tubes. Where this is required the burner may comprise a baffle positioned over the fuel outlet zone.

The invention also includes a gaseous fuel burner as described above which incorporates one or more pilot tubes which terminate in the fuel outlet zone, the pilot tubes being so sized that, during use they supply enough fuel to provide a pilot flame for re-ignition. Preferably the pilot tube, or each pilot tube when there is more than one, terminates near the boundary between the fuel outlet zone and the fuel inlet zone.

(In optimising the performance of the burners described above the cross-sectional area and density of packing of the combustion air tubes are particularly im portant. The cross-sectional area of the combustion air tubes affects their resistance to air flow and reducing this area clearly increases the resistance. The density of packing defines the length of the difusion paths and short paths assist the mixing of the fuel and oxygen; the cross-sectional area is also relevant in that it limits the number of tubes which can be packed into a given area. We have found that optimum dimensions are usually achieved when the bore of each combustion air tube has a cross sectional area of 0.01 1.0 cm where it opens into the combustion zone and the bores of the tubes account for at least 25%, particularly at least 50%, of the surface of the fuel outlet zone adjacent to the combustion zone).

The invention includes combustion appliances which incorporate one or more burners as described above, e.g., a burner mounted in a combustion chamber adapted to be connected to a flue.

As an example of such a combustion device we quote a device intended for heating a fluid, e.g., a central heating boiler, which also comprises a heat exchanger positioned so as to receive hot gases when the combustion applicance is alight.

As a second example we quote a radiant heat combustion device which also comprises a ceramic element positioned in its combustion zone so that, when the device is alight, combustion heats the element. This combination is particularly suitable for furnace construction, i.e., the radiant elements form sides and/or the bottom and/or the top of the hot box. (It is clearly desirable to surround, in three dimensions, the hot box" with radiant elements but it is necessary to provide an opening for flue gas and a door. If desired the door could take the form of movable burner/ceramic element combinations.)

The invention will now be described, by way of example, with reference to the diagrammatic drawings accompanying the provisional specification in which:

FIG. 1 is a perspective view of a burner according to the invention,

FIGS. 2 and 3 are horizontal cross sections through a burner according to the invention, FIG. 4 is a vertical cross-section through the burner shown in FIGS. 2 and 3,

FIG. 5 shows a detail of the edge construction,

FIG. 6 is a top view of a burner made according to an alternative construction, and

FIG. 7 is a vertical cross section through the burner shown in FIG. 5.

FIG. 1 gives an impression of the appearance of a burner according to the invention. The burner comprises a rectangular box 10 through which pass a large number of combustion air tubes 11. A metal sponge I2 is contained in the interstitial space between the combustion air tube 11 and, when the burner is alight, fuel passes out of the top of the metal sponge 12 into the combustion zone where where it burns with combustion air issuing from the combustion air tubes 11. Fuel is supplied by the fuel inlet tube 13 and there may be a header 1.9 to facilitate even distribution of the fuel.

The metal sponge 12 took the form of a three dimensional network of tubular nickel strands which define pores of average diameter 0.3 mm.

(Irregular dodekahedra can pack to fill space and the sponge can be regarded as an example of such a packing. From this point of view the metal strands form the edges of the dodekahedra.)

FIGS. 2 4 show in greater detail an embodiment in which the fuel chamber is formed ofa mechanically self supporting sheet metal case. FIG. 2 shows how the interstitial space between the combustion air tubes 11 is filled with a metal sponge 12 in the upper half of the burner. FIG. 3 shows that the lower half of the burner has the same construction except that the metal sponge 12 is omitted. This is also illustrated in FIG. 4 which shows that the combustion air tubes 11 are soldered in a fluid tight manner into holes in a base plate 20.

The operation of the burner illustrated in FIGS. 2 4 on liquid and gaseous fuels will now be described.

A gaseous fuel, after entering via the inlet tube 13, passes into the inlet zone 14 which has a low resistance to gas flow. In order to escape the fuel has to pass through the metal sponge 12 which offers a high resistance to flow. The high escape resistance encourages the fuel to distribute itself evenly throughout the inlet zone 14 and this encourages even flow distribution throughout the metal sponge 12 so that there is even distribution into the combustion zone. The header space 19 assists in the achievement of an even fuel distribution. As the fuel issues from the top of the metal sponge l2 combustion air issues from the combustion air tubes 11 and combustion takes place by a silent diffusion mechanism which gives blue flames extending less than 5 mm from the top of the burner.

Low resistance channels through the metal sponge l2, e.g., between the metal sponge l2 and the walls of the combustion air tubes or between the metal sponge l2 and the sides of the burner, tend to produce an uneven flow of fuel into the combustion zone and therefore optimum performance will be achieved by avoiding these low resistance passages.

In some cases a burner may display a tendency to burn fuel rich as its outer periphery. This tendency usually arises because of the missing combustion air tubes which extend beyond the periphery. The tendency may be corrected by means of a metal plate which blocks off part of the metal sponge 12 between the outermost combustion air tubes or by extending upwardly and inwardly the sides of the rectangular box 10 so that they inwardly deflect the peripheral gas flow (these extensions are not shown in any drawing). Alternatively extra combustion air tubes may be provided as shown in FIG. 5. It will be noticed that the extra tubes 21 have a smaller diameter than the tubes 11 in order that they fit into the reduced interstitial space available within the boundary of the burner.

A liquid fuel also enters via the fuel inlet tube 13 and in this case it floods the inlet zone 14. During use it is necessary to maintain the liquid level high enough to submerge the base of the metal sponge 12. Once the base of the sponge is immersed capillary action conveys the fuel throughout the metal sponge and the heat of combustion vaporises the fuel into the combustion zone. The thermal conductivity of the metal is high enough for the sponge to assist in this vaporisation. It will be noted that the liquid fuel actually enters the combustion zone in the vapour phase and therefore combustion takes place as described for a gaseous fuel burner.

It will be appparent that a higher liquid level, i.e., a greater proportion of metal sponge l2 immersed below the liquid surface, implies a higher rate of fuel supply to the combustion zone and hence a greater heat output. This can be exploited by two different control mechanisms. In the first mechanism the fuel supply incorporates an adjustable level controller which can be set for high and low levels to give high and low thermal outputs. In the second mechanism the fuel supply incorporates an adjustable flow controller. If the flow rate is too high for the thermal output the liquid level will rise and hence increase the thermal output; if the liquid level is too low it will fall and hence decrease the thermal output. Thus the thermal output will adjust itself to the flow rate.

Any of the constructional details mentioned above in connection with gas burners will give satisfactory operation in the case of liquid fuel burners. However it is not necessary to ensure the absence of low resistance channels through the metal sponge 12 since these have no adverse effect upon a wicking mechanism. This gives an extra possibility for controlling the tendency to burn fuel rich at the periphery, that is the metal sponge 12 does not completely fill all the interstitial space between the outermost combustion air tubes.

Certain metal sponges give satisfactory high resistance for gaseous fuels and satisfactory wicking for liquid fuels. A gaseous fuel burner constructed from such a metal sponge will also burn a liquid fuel.

FIGS. 6 and 7 illustrate an alternative method of constructing burners according to the invention. There are two special features of this alternative construction:

a. The inlet zone 14 is also packed with a metal sponge which has an average pore diameter of 1 mm to give a low flow resistance. Thus the entire fuel chamber is packed with metal sponge.

b. In FIGS. 2 4 the walls of the fuel chamber and the combustion air tubes ll are mechanically self supporting. In the modified construction the walls take the form of coatings deposited on and supported by the metal sponges.

The nature of packing of the fuel space is most clearly seen in FIG. 7 which is a vertical cross section. This figure shows that the upper part of the fuel space contains a relatively small pore size metal sponge 12a and the lower part contains a relatively coarse pore size metal sponge 12b. The base of the metal sponge 12b is sealed by a fluid proof base coating 16, the walls of the combustion air tubes ll are sealed by means of tube coatings l5 and the sides of the burner are sealed by means of side coatings 17.

The top face of the burner, i.e., the one adjacent to the combustion zone, is illustrated in FIG. 6. This shows the arrangement of the metal sponge 12a, the tube coatings l5 and the side coatings 17. This figure also shows a top coating 18 which extends between the outermost combustion air tubes ll in order to correct the tendency to burn fuel rich.

The operation of the packed fuel inlet zone of FIGS. 6 and 7 is exactly the same as the operation of the unpacked fuel inlet zone of FlGS. 2 4. In other words the differences of construction illustrated in FIGS. 2 4 as compared with FlGS. 6 and 7 have no effect upon the mode of operation of the burner.

The coatings can be applied to the burners illustrated in FIGS. 6 and 7 by dipping suitably shaped metal foam in ceramic. e.g., slips, and firing to give fluid proof coatings supported on the metal foam. The coating may be applied to a sandwich" of coarse sponge, fine sponge, coarse sponge which is dipped into ceramic so that all surfaces are coated. After firing the sandwich is cut through the middle to give two burners.

We claim:

1. A diffusion flame burner for fluid fuels which comprises a fuel chamber having a fluid-tight bottom por tion defining an air inlet zone, a top portion defining a combustion zone, and a fluid-tight side portion between said bottom portion and said top portion, said fuel chamber being open to the atmosphere only at its said top portion and being divided into upper and lower fuel zones, metal sponge having a pore size of 0.1 to 0.5 mm average diameter packed in said upper fuel zone for controlling the flow of fuel therethrough towards the combustion zone, said metal sponge being exposed to the atmosphere and unobstructed at its upper surface, said lower fuel zone constituting a fuel inlet zone adapted to be connected to a fuel supply and said upper packed fuel zone constituting a fuel outlet zone adjacent to and communicating with said combustion zone, and a plurality of combustion air tubes passing through said fuel chamber from said bottom portion to said top portion for conducting primary combustion air through said chamber from said air inlet zone to substantially the entire area of the surface of said fuel outlet zone adjacent to and communicating with said combustion zone in confined streams out of contact with fuel in said chamber, whereby, during the use of the burner primary combustion air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet zone into the combustion zone thereabove, said fuel and said primary combustion air mixing only in said combustion zone by low velocity diffusion, and the resulting fuel-air mixture burning as a diffusion flame in said combustion zone, the bore of each of said combustion air tubes being 0.0l1.0 cm where it opens into the combustion zone and the bores of the tubes accounting for at least 25 percent of the surface of said fuel outlet space adjacent to the combustion zone.

2. A burner according to claim 1, in which the bores of the combustion air tubes account for at least 50% of the surface area of the fuel outlet zone adjacent to the combustion zone.

3. A burner according to claim 1, in which the combustion air tubes are all cylindrical.

4. A burner according to claim 3, in which the axes of the cylinders are parallel.

5. A burner according to claim 1, in which the fuel inlet zone is packed with a metal sponge whose resistance to fluid flow is less than that of the metal sponge in the fluid outlet zone.

6. A burner according to claim 5, in which the metal sponge in the fuel inlet zone has a pore size of 0.5 2 mm average diameter.

7. A burner according to claim 5, in which the walls of the fuel chamber and the walls of the combustion air tubes take the form of an impervious coating applied to the appropriate walls of the sponge.

8. A burner according to claim 5, in which the combustion air tubes are all cylindrical.

9. A burner according to claim 8, in which the axes of the cylinders are parallel.

10. A burner according to claim 5, in which the bores of the combustion air tubes account for at least 50% of the surface area of the fuel outlet zone adjacent to the combustion zone.

UNITED STATES PATENT OFFICE (JERHHCATE 0F 0RRETMN PATENT NO. 3, 870 459 DATED I March ll, 1975 INVENTOWS) Denis Henry Desty, Barry Herbert Francis Whyman,

David Montagu Whitehead It is certified that error appears in the ab0verdentified patent and that said Letters Patent are hereby corrected as shown below On the first page of the patent, please insert the following:

- [30] Foreign Application Priority Data November 6, 1968 Great Britain .52543/68 figned and gealcd this f f Day of August1975 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting ff (ummr'sximu'r uflalcnrs and Trademarks

Claims

1. A diffusion flame burner for fluid fuels which comprises a fuel chamber having a fluid-tight bottom portion defining an air inlet zone, a top portion defining a combustion zone, and a fluid-tight side portion between said bottom portion and said top portion, said fuel chamber being open to the atmosphere only at its said top portion and being divided into upper and lower fuel zones, metal sponge having a pore size of 0.1 to 0.5 mm average diameter packed in said upper fuel zone for controlling the flow of fuel therethrough towards the combustion zone, said metal sponge being exposed to the atmosphere and unobstructed at its upper surface, said lower fuel zone constituting a fuel inlet zone adapted to be connected to a fuel supply and said upper packed fuel zone constituting a fuel outlet zone adjacent to and communicating with said combustion zone, and a plurality of combustion air tubes passing through said fuel chamber from said bottom portion to said top portion for conducting primary combustion air through said chamber from said air inlet zone to substantially the entire area of the surface of said fuel outlet zone adjacent to and communicating with said combustion zone in confined streams out of contact with fuel in said chamber, whereby, during the use of the burner primary combustion air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet zone into the combustion zone thereabove, said fuel and said primary combustion air mixing only in said combustion zone by low velocity diffusion, and the resulting fuel-air mixture burning as a diffusion flame in said combustion zone, the bore of each of said combustion air tubes being 0.01-1.0 cm2 where it opens into the combustion zone and the bores of the tubes accounting for at least 25 percent of the surface of said fuel outlet space adjacent to the combustion zone.

1. A diffusion flame burner for fluid fuels which comprises a fuel chamber having a fluid-tight bottom portion defining an air inlet zone, a top portion defining a combustion zone, and a fluid-tight side portion between said bottom portion and said top portion, said fuel chamber being open to the atmosphere only at its said top portion and being divided into upper and lower fuel zones, metal sponge having a pore size of 0.1 to 0.5 mm average diameter packed in said upper fuel zone for controlling the flow of fuel therethrough towards the combustion zone, said metal sponge being exposed to the atmosphere and unobstructed at its upper surface, said lower fuel zone constituting a fuel inlet zone adapted to be connected to a fuel supply and said upper packed fuel zone constituting a fuel outlet zone adjacent to and communicating with said combustion zone, and a plurality of combustion air tubes passing through said fuel chamber from said bottom portion to said top portion for conducting primary combustion air through said chamber from said air inlet zone to substantially the entire area of the surface of said fuel outlet zone adjacent to and communicating with said combustion zone in confined streams out of contact with fuel in said chamber, whereby, during the use of the burner primary combustion air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet zone into the combustion zone thereabove, said fuel and said primary combustion air mixing only in said combustion zone by low velocity diffusion, and the resulting fuel-air mixture burning as a diffusion flame in said combustion zone, the bore of each of said combustion air tubes being 0.011.0 cm2 where it opens into the combustion zone and the bores of the tubes accounting for at least 25 percent of the surface of said fuel outlet space adjacent to the combustion zone.

6. A burner according to claim 5, in which the metal sponge in the fuel inlet zone has a pore size of 0.5 - 2 mm average diameter.