US3635651A

US3635651A - Burner

Info

Publication number: US3635651A
Application number: US24221A
Authority: US
Inventors: Denis Henry Desty
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1969-04-28
Filing date: 1970-03-31
Publication date: 1972-01-18
Anticipated expiration: 1989-01-18
Also published as: DK140810C; DE2020408A1; BE749678A; SE357813B; CH507478A; DE2020408B2; NL164382B; GB1247406A; FR2040281A1; JPS5313822B1; NL7005804A; AT301107B; NO126882B; NL164382C; DE2020408C3; DK140810B

Abstract

A burner, preferably of rigid material such as metal foam which is pierced by airholes for the passage of combustion air and arranged to carry fuel across the combustion zone. Conveniently the burner has an evaporator in which the airholes are situated and a feeder for lifting fuel to the level of the evaporator. In the preferred configuration gravity assists fuel transfer. For example, the evaporator comprises two sloping portions joined together at their lowest portions and each joined to a feeder at its highest portion.

Description

Uite States Patent 15] mmwsi Desty 5] Jan. W, W72

[54] BURNER 2,229,717 1/1941 Brace et al ..43l/328 x [72] Inventor: Denismmy nestyweybridgefingland 2,879,837 3/1959 Downs ..431/328 [73] Assignee: The British Petroleum Company Limited, FOREIGN PATENTS OR APPLICATIONS Moor Lane, London, England 117,929 6/1930 Austria ..43 U326 22 Filed: Mar.3l, 1970 [21] Appl. No.: 24,221

[58] Field of Search ..43l/326, 328, 300, 298

Primary Examiner-Carroll B. Dority, Jr. A ttarney- Morgan, Finnegan, Durham 34 Pine [57] ABSTRACT A burner, preferably of rigid material such as metal foam which is pierced by airholes for the passage of combustion air and arranged to carry fuel across the combustion zone. Conveniently the burner has an evaporator in which the airholes are situated and a feeder for lifting fuel to the level of the evaporator. In the preferred configuration gravity assists fuel transfer. For example, the evaporator comprises two sloping [56] References Cited portions joined together at their lowest portions and each UNITED STATES PATENTS joined to a feeder at its highest portion, 2,227,899 1/1941 Grubb ..431/328 13 Claims, 8 Drawing Figures mimti m mmmmmmmtmmsi j i PATENTEnJAmzsrz 3.635.651

SHEET 1 0F 4 F/G.7. 11 14 r n mmmm'wuuummmmmmmgl I I I l PATENTED JAN] 81972 SHEET 3 BF 4 H6. 7. 72g 20g 27 209 22g C ooooo oc 000000 0 0 0 0 oooc 000 gofo c 74 7 v PATENTEUJAMIQH 3,635,651

sum UF 4 FIG.

BURNER This invention relates to burners and in particular to burners which operate on vaporizable liquid fuels such as kerosene.

It is known to burn liquid fuels for heating purposes in wick burners in which the wick conveys fuel from a reservoir situated below the combustion zone to the bottom of the combustion zone. Such burners give satisfactory combustion with silent blue flames but they have only been used for small portable heaters. This invention relates to a burner which is suitable for use in applications which require a higher output than a small portable heater and for which automatic control is desirable, e.g., domestic central heating.

The invention consists in a burner at least a portion of which is an evaporator adapted:

a. to hold liquid fuel by surface tension,

b. to evaporate liquid fuel held within the evaporator into a combustion zone adjacent to one surface of the evaporator by transfer of heat from the combustion zone, and

c. to permit flow of fuel held within the evaporator so as to replenish fuel lost by vaporization, the evaporator being pierced by airholes arranged so that, during the use of the evaporator as a component of a burner buming a vaporizable fuel, combustion takes place between air passing through the airholes and fuel evaporated from the evaporator.

According to a preferred embodiment of the invention the evaporator also has one or more downwardly depending, preferably vertical, feeders adapted to dip into a fuel bath whereby, during use in a burner, the feeders raise liquid fuel to the level of the evaporator.

(Note in this specification the words horizontal, vertical" and downward" are related to a burner which is in its optimum orientation. In fact the orientation is not critical, at least within the limits acceptable in domestic practice.)

The evaporator may be horizontal or it may comprise sloped portions, e.g., it may be dome shaped or dish shaped. Burners in which the evaporator comprises a portion downwardly sloped in the direction of fuel flow are advantageous because gravity assists the fuel transfer. Particularly suitable angles of slope are 20-70 to the horizontal, preferably 40-50 to the horizontal.

In a preferred configuration the evaporator comprises two sloping portions, preferably rectangular, joined together at their lowest regions.

In the case of burners which comprise feeders and evaporators with sloping portions the feeders should be joined to the evaporators at the highest portions.

It will usually be desirable to use the thinnest evaporator consistent with achieving the necessary fuel transfer since the greater the thickness the greater the heat capacity of the evaporator and the fuel contained therein. A low-heat capacity facilitates ignition since the smaller the heat capacity the more easily the fuel is raised to evaporation temperature. (Ease of ignition is particularly important in the case of a burner which is turned on and olf by an automatic control system.) A thin evaporator confers an additional advantage when the burner is turned out. Combustion continues until the supply of fuel contained in the evaporator is exhausted and this exhaustion is more satisfactorily achieved when the supply is small, i.e., when the evaporator is thin. Thus it is desirable that the evaporator is less than l mm. thick and for certain applications, e.g., domestic central heating, it is desirable that the evaporator be less than mm., and preferably less than 3 mm., thick.

In order to achieve uniform air distribution it is desirable that the airholes be uniformly distributed over the evaporator. The airholes are preferably circular in shape and preferably of an area of 0.1 -l cm. The number of airholes is such that they account for -50 percent, preferably -35 percent of the total area of the evaporator.

The invention includes both rigid and nonrigid burners. Nonrigid burners are made of flexible material, e.g., ceramic fibers, and therefore, since the burner would collapse under its own weight, it must be used in conjunction with a support member adapted to hold the burner in the required configuration. On the other hand rigid burners, e.g., wicks made of a porous metal such as a metal foam, are capable of supporting their own weight and therefore no support member is necessa ry.

Anisotropic burners are particularly suitable for most applications. Anisotropic bumers offer a lower resistance to liquid fuel flow in directions across the combustion zone than in the direction of airflow, e.g., the pore size is smaller measured in the direction of airflow than at right angles thereto. For embodiments which comprise a horizontal evaporator there is a lower resistance to liquid flow in a horizontal direction than in the vertical direction, e.g., the pore size is larger measured in horizontal directions than in the vertical] direction.

Anisotropic burners have the property of facilitating fuel flow in desired directions (i.e., across the combustion zone to achieve uniform fuel distribution) but hindering fuel flow in undesired directions i.e., fuel dripping out of the burner.)

The invention includes burners which incorporate burners according to the invention as defined above.

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

FIG. 1 is a vertical cross section of a burner having a foam metal evaporator,

FIG. 2 is a top view of the burner shown in FIG. ll,

FIG. 3 is a modification of the burner shown in FIG. ll having a flat horizontal evaporator and feeders,

FIG. 4 is a supported evaporator version of the burner shown in FlG. 3,

FIG. 5 is a modification of the burner shown in FIG. 3 having a separate feeder,

FIG. 6 is a vertical cross section through a burner having the preferred configuration,

FIG. 7 is a plan view of the burner shown in FIG. 6, and

FIG. 8 is a diagrammatic cross section of a domestic central heating boiling which includes a burner as shown in FIGS. 6 and 7.

The arrangement shown in FIGS. l and 2 comprises a burner I0 which has an evaporator 11 having the form of a horizontal circular disc of crushed metal foam. The evaporator I1 is pierced by airholes l4l arranged in a hexagonal pattern, i.e., each internal hole has six nearest neighbors whose centers form a regular hexagon.

The burner 10 was made from a cylindrical block of metal foam by crushing it parallel to its axis in the ratio 3:1 and the airholes M were cut at the same time. Originally the metal foam took the form of a three dimensional network of tubular nickel stands which defined dodekahedral pores of average diameter 0.3 mm. The compression flattened original pores so that they became thin in the vertical direction.

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

The circumference of the evaporator Illl engages with a fuel bath 13. During use the evaporator lll contains a reservoir of liquid fuel which is held in the pores by surface tension and conveyed horizontally by passage from pore to pore. The pores offer a high resistance to vertical flow so that no fuel drips from the under side of the burner.

While the burner is alight the heat of combustion draws air through the airholes M and the heat vaporizes fuel into the combustion zone, which is adjacent to the upper surface of the evaporator, so that the burner remains alight.

The heat output can be controlled by reducing the flow of fuel into the fuel bath 13 thereby reducing the rate of fuel supply to the combustion zone. As the fuel supply is reduced to zero the burner first goes out at the center because of fuel starvation. This creates a dead" area at the center which spreads (as the fuel supply reduces) until the whole burner is extinguished. Thus the burner is capable of good heat control.

To reignite the burner the fuel supply is restarted and a light is applied. This starts combustion and the burner operates as described above.

The burner shown in FIG. 3 comprises a foam metal evaporator 11 having a feeder 12. (The evaporator I1 is formed of crushed metal foam as described above and the feeder consists of an annular cylinder of uncrushed metal foam.)

The feeder l2 stands in a fuel bath l3 and it lifts fuel to the level of the evaporator 11 by means of wicking action. Transfer of the fuel across the burner and combustion then take place as described above. The fuel supply to the burner is conveniently controlled by varying the level of liquid in the fuel bath l3. Evaporation of fuel outside the combustion zone is hindered by the guard ring 19.

The burner shown in FIG. 4 is similar to that shown in FIG. 3 except that the evaporator is constructed from a fibrous ceramic material instead of a metal foam. Since the fibrous material is not self-supporting it is carried by a support member (generally indicated by the numeral 18) which comprises a horizontal base plate upon which the evaporator 11 rests. The base plate 15 has upwardly extending tubes 16 whose bores form the airholes 14. The support member 18 also comprises a circular support ring 17 which stands in the fuel bath l3 and holds the evaporator in the working position.

In the embodiment shown in FIG. 5 the evaporator 11 is formed of crushed metal foam (as in F IG. 3) but the feeder 12 is formed of ceramic fibers which contact the evaporator 11 and hence transfer the liquid fuel. The feeder 12 is held in contact with the evaporator 11 by the guard ring 19.

Laboratory tests were carried out on a burner as illustrated in FIG. 3 using kerosene as the fuel and during these tests it was noticed that the combustion zone had a circular cross section with combustion complete about 0.5 cm. above the top of the evaporator. The important dimensions of the burner are given in table l and table 2 gives experimental results.

(The smaller wicking height" means the vertical distance between the fuel level in the fuel bath l3 and the bottom of the evaporator 11 and the larger wicking height means the distance to the top of the evaporator.)

The burner shown in FIGS. 6 and 7 is made of the same metal foam described earlier. It comprises an evaporator 11 which is formed of two rectangular sheets a and 20b which are 3 mm. thick, slope at 45 to the horizontal and are joined at their lowest edges along the line 21. The sheet 20a is joined to a feeder 120 at its upper edge. Similarly the sheet 20b is joined to a feeder 12b. The

sheets

20a and 20b are pierced by circular airholes 14 arranged in a uniform hexagonal pattern (i.e., each has six nearest neighbors.) The diameter of the airholes 14 is 3 mm. and their total area represents 30 percent of the total area of the evaporator.

The burner was made by pressing and bending a flat sheet of the metal foam and the curvature between the feeders and the two sheets was chosen to avoid over straining the metal foam. During the pressing the foam in the evaporator was reduced in thickness in the ratio 3:1.

When the evaporator shown in FIGS. 6 and 7 is incorporated in a burner each of the

feeders

12a and 12b stands in a fuel bath to which liquid fuel is supplied. FIG. 8 shows a domestic central heating appliance of the hot water circulating type which incorporates such a burner.

The appliance shown in FIG. 8 comprises a combustion chamber 30 of rectangular cross section which has an air inlet 31 and a flue gas outlet 32 which are connected to a balanced flue.

A fuel bath I3 is situated on the longer sides of the rectangle and the

feeders

12a and 12b of an evaporator as shown in FIGS. 6 and 7 stand in the fuel bath 13. A blower 33 is situated below the burner and a heat exchanger 37 (for heating the circulating water) above.

Fuel is supplied to the bath 13 by the fuel line 38 under the control of the flow controller 34. The setting of the flow controller 34 is varied by a thermostat 3S situated in the outlet 36 of the heat exchanger 37. When the temperature of the water in the outlet line 36 falls below the setting of the thermostat 35 the flow controller increases the fuel supply; when the temperature rises too high the fuel supply is reduced or cut off so that the burner goes out.

(As a safety factor a level controller, not shown, is usually fitted. This overrides the flow controller should the fuel level tend to rise so high that there is danger of overflow.)

The burner comprised in the appliance shown in FIG. 8 operates substantially as described above. The

feeders

12a and 12b lift fuel by capillary action to the upper edges of the

sheets

20a and 20b and then gravity assists the transfer of the fuel (which is in the liquid phase) towards the line 21. As it passes through the evaporator fuel vaporizes into the combustion zone adjacent to the upper surface of the evaporator where it burns with air flowing through the airholes.

Automatic ignition is conveniently achieved by an electrical system (not shown in any drawing.) The system comprises a coil heater for heating a small area of the burner and an electrode for generating sparks in cooperation with the metal foam. To achieve ignition the ignition system is switched on and the fuel is restarted. When the liquid fuel reaches the heated area some vaporizes and the vapor is ignited by the sparks. This produces a flame which spreads over the whole area of the burner.

It is emphasized that in all the burners described in the examples the whole of the base of the combustion zone is adjacent to the upper surface of the evaporator. Thus the evaporator and the liquid fuel contained therein are heated and hence fuel is evaporated into the combustion zone. The evaporator holds a supply of liquid fuel which is able to move within the evaporator and thereby replenish the supply as fuel vaporizes into the combustion zone. Because the evaporator provides a good supply of fuel adjacent to the whole of the base of combustion it gives an evenly distributed supply of fuel vapor with the combustion zone.

The thermal conductivity of the metal foam mentioned in respect of FIGS. 1 and 2 has been found to give good heat transfer to the liquid fuel held within the evaporator. The pore size is good for retaining and transferring liquid fuels.

I claim:

I. An anisotropic burner which comprises an evaporator less than lO-mm. thick and having a body of porous material for holding liquid fuel in its pores by surface tension to provide a reservoir of liquid fuel contained in the evaporator from which fuel may be evaporated into a combustion zone adjacent to one surface of the evaporator by transfer of heat from the combustion zone, said body of porous material being pierced by airholes for conducting air through the evaporator to said combustion zone, said airholes being arranged so that, during use of the evaporator as a component of a burner burning a vaporizable fuel, combustion takes place in said combustion zone between air flowing through the airholes and fuel evaporated from said reservoir, and the pores of said porous material being smaller measured in the direction of the airflow than at right angles thereto so as to offer a lower resistance to liquid fuel flow in said reservoir in directions across the combustion zone than in the direction of airflow and facilitate flow of fuel in directions across the combustion zone to achieve uniform fuel distribution in replenishment of fuel lost by evaporation from said reservoir while hindering fuel flow against the direction of said airflow.

2. A burner according to claim 1, which also comprises a fuel bath and in which said evaporator comprises, at least one downwardly depending feeder adapted to dip in said fuel bath whereby, during use in a said burner, liquid fuel is raised to the level of the evaporator.

3. A burner according to claim 1, which also comprises a fuel bath and in which said evaporator comprises, at least one vertically downwardly depending feeder adapted to dip in said fuel bath whereby, during use in said burner, liquid fuel is raised to the level of the evaporator.

l. A burner according to claim 1, in which the evaporator comprises a portion downwardly sloped in the direction of fuel flow and at an angle of 20-70 to the horizontal.

5. A burner according to claim 4, in which the evaporator comprises two portions, each downwardly sloped in the direction of fuel flow, each at an angle of 20-70 to the vertical and joined together at their lowest regions.

6. A burner according to claim 5, in which each sloping portion is rectangular.

7. A burner according to claim 5, in which each downwardly sloped portion is at an angle of 40-50 to the horizontal.

8. A burner according to claim 7, in which each sloping portion is rectangular.

9. A burner according to claim 8, in which the evaporator is less than 5 mm. thick.

110. A burner according to claim 9, in which the evaporator is less than 3 mm. thick.

11. A burner according to claim 1, in which the evaporator is rigid.

12. A burner according to claim 11 in which the evaporator is made of porous metal.

13. A burner according to claim 12 in which the evaporator is made of metal foam.

' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,635,651 Dated January 18, 1972 Inventor (1;) Denis Henry Desty It is certified that error appears in the above-identified! patent and that said Letters Patent are hereby corrected as shown below:

Col. 5, Line 15, for "comprises at least comprises at one" read least one Col. 5, Line 20, for "comprises at least comprises at one" read least one Signed and sealed this 18th day of July 1 972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents i FORM PC4050 (069) USCOMM-DC 6O376-P69 Q ".5. GQVERNMENT PRINTING OFFICE I 959 0"366'334

Claims

1. An anisotropic burner which comprises an evaporator less than 10-mm. thick and having a body of porous material for holding liquid fuel in its pores by surface tension to provide a reservoir of liquid fuel contained in the evaporator from which fuel may be evaporated into a combustion zone adjacent to one surface of the evaporator by transfer of heat from the combustion zone, said body of porous material being pierced by airholes for conducting air through the evaporator to said combustion zone, said airholes being arranged so that, during use of the evaporator as a component of a burner burning a vaporizable fuel, combustion takes place in said combustion zone between air flowing through the airholes and fuel evaporated from said reservoir, and the pores of said porous material being smaller measured in the direction of the airflow than at right angles thereto so as to offer a lower resistance to liquid fuel flow in said reservoir in directions across the combustion zone than in the direction of airflow and facilitate flow of fuel in directions across the combustion zone to achieve uniform fuel distribution in replenishment of fuel lost by evaporation from said reservoir while hindering fuel flow against the direction of said airflow.

4. A burner according to claim 1, in which the evaporator comprises a portion downwardly sloped in the direction of fuel flow and at an angle of 20*-70* to the horizontal.

5. A burner according to claim 4, in which the evaporator comprises two portions, each downwardly sloped in the direction of fuel flow, each at an angle of 20*-70* to the vertical and joined together at their lowest regions.

6. A burner according to claim 5, in which each sloping portion is rectangular.

7. A burner according to claim 5, in which each downwardly sloped portion is at an angle of 40*-50* to the horizontal.

8. A burner according to claim 7, in which each sloping portion is rectangular.

10. A burner accorDing to claim 9, in which the evaporator is less than 3 mm. thick.

11. A burner according to claim 1, in which the evaporator is rigid.