EP0002124B1

EP0002124B1 - Burner for burning a liquid fuel

Info

Publication number: EP0002124B1
Application number: EP78300617A
Authority: EP
Inventors: Jacques Bouvin
Original assignee: Esso SA
Current assignee: Esso SA
Priority date: 1977-11-11
Filing date: 1978-11-10
Publication date: 1980-11-12
Also published as: IT7851863A0; DE2860286D1; EP0002124A1

Description

The present invention relates to a burner for burning a liquid fuel, particularly (but not exclusively) a liquid fuel of the type used for domestic heating.
In one known burner of this type, as described in French patent specification No. 2257036 and in U.K. patent specification No. 1479686, fuel is heated and vapourized in an upwardly-extending tube, and the vapours are separated from the unvapourized liquid in a chamber above the tube, the liquid being recovered from the chamber in a downwardly extending tube which receives, in addition, a feed of cold liquid fuel, and which communicates at its bottom with the upwardly extending tube. The fuel vapour is sucked out of the chamber by entrainment in combustion air passing through an ejector which communicates the reduced pressure in its jet pump to fuel vapourizer thereby reducing the fuel boiling temperature. A drawback of this burner is that the air supplied to the ejector must be at a relatively high pressure to suck the fuel vapour effectively from the fuel vapourizer and to overcome the back pressure of the ejector. Accordingly, a relatively noisy air supply fan must be used. Another drawback of this type of burner is that it has been observed that the vapourization of the fuel does not proceed in a uniform manner and that accordingly, the size and appearance of the flame tends to vary.
In another burner of this type described in U.S. patent specification 2,123,884, air at a relatively high pressure is employed to generate a low pressure in an ejector so as to entrain vapourized fuel from a vapourizing chamber, the air and fuel vapour being mixed on passing through the divergent diffuser of the ejector where the kinetic energy is at least partly converted to pressure energy, and some of the mixture is recirculated to the vapourizing chamber from the downstream end of the diffuser, after combustion, in order to convert liquid fuel to fuel vapour. This burner also is noisy since a relatively high pressure blower must be employed to provide sufficient energy to entrain the fuel vapour, to overcome the back pressure of the ejector and also to furnish sufficient pressure energy to recirculate some of the air-fuel vapour mixture to the vapourizing chamber and to bubble the mixture through liquid fuel in the chamber. The liquid fuel in the chamber is exposed to hot recirculated combustion gases and/or oxygen-containing gases which tend to cause the fuel to degrade and form deposits in the chamber necessitating relatively frequent burner shutdowns for cleaning and maintenance. Moreover, the control of the flow of fuel vapour must be effected by a throttle valve and no means are provided for automatically securing a particular ratio of air to fuel.
Burners of the foregoing types are so constructed that the mixture of air and fuel vapour pass to the burner head through a relatively long downstream section of conduit, and there is, in consequence, an undesirably large amount of fuel vapour in the downstream conduit section which gives rise to fuel vapour condensation (especially during the start-up of the burner) and fouling on the walls of the downstream conduit section necessitating that the latter must be constructed and arranged in such a way that it can be removed for cleaning from time-to-time thus increasing the cost and degree of complication of the burner. In addition, at shut-down of the burner, there may tend to be an emission of fuel vapour to the atmosphere. The fuel vapour condensation which tends to take place during start-up tends to make the timing of the start-up of the burner more erratic than desirable.
The present invention provides a burner for burning a liquid fuel, comprising a burner head in which a mixture of air and vapour of the liquid fuel is burned in a substantially stable flame; air supply means operable to supply at least some of the air for forming the said mixture of air and fuel vapour; a conduit for conducting air from the said air supply means to the burner head; air swirl means for imparting a swirling or vortex flow to air in the conduit; a vapourizer which is operable independently of the presence of any flame at the burner head for converting liquid fuel to vapour of the liquid fuel out of contact with oxygen-containing gas or hot gases; and a fuel vapour duct for conducting fuel vapour from the vapourizer into the said conduit at or downstream of the location of the air swirl means.
In this patent specification, the term "air" is intended to comprehend any oxygen-containing gas.
In preferred embodiments of the invention, the length of conduit between the air swirl means and the burner head is as short as possible.
Because the fuel vapour is mixed with swirling air, problems due to condensation and fouling in the conduit at locations between the region at which fuel vapour enters the conduit and the burner head are substantially reduced or eliminated.
Preferably the fuel vapour duct is so disposed as to introduce fuel vapour into the conduit at a region where the static pressure of the fuel vapour is substantially equal to, or preferably slightly greater than, the static pressure of the air in that region. It is preferred that the static pressure in that region should be approximately equal to atmospheric pressure. The vapourizer preferably operates to generate fuel vapour at a pressure approximately equal to, or slightly greater than, atmospheric pressure.
Preferably, the vapourizer has no more than two open orifices therein, one being for the supply of liquid fuel to the vapourizer and the other being the entrance to the fuel vapour duct.
The vapourizer preferably comprises a substantially vertical riser conduit side-by-side with a substantially vertical downcomer conduit, the riser conduit and downcomer conduit communicating with each other at or near their bottoms, a disengaging chamber connecting the tops of the riser and downcomer conduits for the disengagement of fuel vapour from liquid fuel, and heating means disposed for heating liquid fuel in the riser conduit to its boiling temperature, said heating means preferably defining with the internal wall of the riser conduit a relatively narrow substantially vertical space for the upward circulation of a mixture of liquid and vapourized fuel generated by the action of the heating means on liquid fuel when in the vapourizer.
Liquid fuel is preferably supplied to the bottom region of the riser conduit from a constant-liquid-level regulating device.
In one type of embodiment of the invention, the emission of fuel vapour from the burner at shut-down may be substantially prevented by the provision of a suitable valve member which closes the fuel vapour duct when operated. The valve member may be operated by electromechanical means which are actuated on burner shut-down. In preferred embodiments of the invention, the air swirl means is substantially symmetrically disposed about the axis of the exit of the fuel vapour duct. This arrangement facilitates the provision and operation of the valve member.
In another type of embodiment, means may be provided for passing a relatively large volume of unheated liquid fuel at a relatively fast regulated rate into the vapourizer when the burner is being shut down in order to cool fuel in the vapourizer below the temperature range at which fuel vapour is produced in order to avoid or reduce the amount of fuel vapour emission at burner shut down. Means are preferably provided to enable liquid fuel above the level set by the constant level device to return to a liquid fuel reservoir when the burner is being started.
The invention is now further described with reference to embodiments thereof, given by way of non-limitative examples, and with reference to the accompanying diagrammatic drawings in which:

Figure 1 is an elevation of a vertical cross-section through a burner according to the invention;
Figure 2 is a perspective drawing showing, schematically, the principal parts of one type of air-swirl device which can be used in the burner of figure 1;
Figure 3 is an elevation of a vertical cross-section of the air-swirl device of figure 2;
Figure 4 is a plan of a horizontal section through the device of figures 2 and 3;
Figure 5 is a schematic elevation of a vertical cross-section through the top part of another burner according to the invention and;
Figure 6 is a plan of a horizontal cross-section of part of the equipment shown in Figure 5.

Referring first to figure 1, the burner is generally indicated by reference 10 and comprises a vapourizer 11, for converting liquid fuel into vapours of the liquid fuel, surrounded by insulation and supplied with unheated liquid fuel from a constant level tank (not shown) via pipe 12 and feed pipe 13. The constant level tank is of any type and maintains the liquid fuel level at the level L. A metal casing 14 surrounds the insulation so as to define a space 15 between the casing 14 and the insulation through which air for combustion of the fuel vapour can pass. At the bottom of the casing 14 is mounted an axial flow air fan 16 generating relatively low pressures to avoid noise. The fan is driven by an electric motor 17 and has a maximum pressure drop of about 40 mms H₂0 at zero flow and about 5 mms H₂0 at normal flow rate, with a power consumption of about 25W. The fan, when operating, passes air for combustion upwardly through the space 15. The top of the space 15 is bounded by a horizontal metal casing member 18 having a central hole 19 therethrough. Beneath the hole 19, and substantially coaxial therewith, is a conduit 20 which serves as the outlet for fuel vapour from the vapourizer 11. An air swirl device 21 is disposed in the top of the space 15 to impart to air passing out of the space 15 via the hole 19 a strong swirling or vortex motion so that the mixing with fuel vapour passing out of the top of conduit 20 and through hole 19 is thorough and a relatively uniform mixture of air and fuel vapour is produced above (i.e. downstream of) the hole 19.
The hole 19 is arranged to discharge swirling air from the space 15 and fuel vapour from the top of conduit 20 as a substantially uniform mixture into a burner head 23 having a suitable burner grid on (in the illustrated embodiment) the upper surface, so that during operation, the mixture of fuel vapour and air burns above the burner head 23 in an annular flame 24. A burner head and grid of the type used for burning gaseous fuel may be used.
Preferably, the burner 10 is so constructed, arranged and controlled that the amount of air passed to the burner head 23 is between 80% and 120% of the requirement for complete combustion of the fuel vapour.
The vapourizer 11 comprises an upwardly extending riser 27 terminating at its top at one side of a separating chamber 28 and a downcomer 26 which extends downwardly from the diametrically opposite side of the chamber 28, the bottom of the downcomer 26 curving smoothly downwards into communication with a bottom region of the riser 27. The riser is of circular cross-section, and a heating element 25 of circular cross-section extends upwardly in the riser 27 from the closed bottom thereof to near the top, and defines therewith an annular clearance of small radial width (e.g. 2.5 mms.). The heating element 25 is heated by an internal electrical heating resistance (not shown) supplied with electric power from terminals E and wires F. Liquid fuel is supplied to the vapourizer 11 from the pipe 12 which has a substantially vertical part terminating at the lower end of the feed pipe 13. The latter is upwardly sloped at a small angle to the horizontal to prevent the passage of fuel vapour into pipe 13 and also to prevent the accumulation of high boiling components of fuel therein. The upper end of the feed pipe is connected into the riser 27 at just above the level at which the downcomer 26 communicates with the riser 27.
For operation, liquid fuel, preferably boiling in the range 150°C to 400°C, is passed into the vapourizer 11 to the level L slightly below the bottom of the chamber 28 and preferably slightly below the top of the heating element 25, the level being regulated by the constant level tank. When electric power is supplied via the terminals E, the skin temperature of the heating element 25 rises and liquid fuel in the narrow annular space between the element 25 and the surrounding riser wall is heated. The lighter fractions of the liquid fuel eventually begin to vapourize and bubbles of vapour form in the liquid in the riser 27 thereby reducing the overall density of the liquid therein. The bubbles tend to rise in riser 27 and to promote an upward flow of liquid and vapourized fuel in riser 27. With the passage of a short time, the narrow annulus containing fuel in the riser 27, particularly towards the top end, tends to contain a foam of fuel vapour bubbles in liquid fuel and the fuel circulates upwardly into the chamber 28 where the fuel vapour separates from the liquid fuel. Unvapourized fuel circulates to the top of the downcomer 26, and cool fuel from the bottom of the downcomer circulates into the bottom of the riser 27. The fuel vapour rises in the chamber 28, initially giving up heat of vapourization and sensible heat to the chamber 28 and circulating to the downcomer 26, but eventually passing out of the vapourizer 11 via the conduit 20 for admixture with swirling air and combustion of the resulting uniform mixture of air and fuel vapour at the bumer head.
Because the fuel vapour and air are mixed under turbulent conditions, good mixing is achieved and the tendency for fuel to be deposited on the surfaces of the burner is reduced even when the surfaces are cool. Moreover, the hold-up of fuel vapour in the burner is relatively small, and problems associated with ignition and flame stabilization at burner start-up, and with fuel vapour emission to the atmosphere at burner shut-down, are substantially eliminated or relatively insignificant.
Reference is now made to figure 2 from which it will be seen that the air swirl device comprises two co-axial spaced-apart annular metal plates 31, 32 attached to each other by a plurality of angled flat blades 33 which extend from the outer to the inner edges of the plates 31, 32. The spaces between adjacent blades converge inwardly in a generally non-radial, non-tangential sense relative to the central orifices of the plates to define air pathways 34 to a cylindrical central zone inwardly of the blades 33. The central zone receives fuel vapour from the conduit 20 (figure 1) (e.g. via a plenum, not shown in figure 2) and air entering the central zone swirls therearound and mixes thoroughly with the fuel vapour to form a substantially uniform combustible mixture. The resulting mixture passes out of the top of the swirl device via a short duct 35.
From figure 3, it will be seen that the lower plate 32 is slightly dished in a downward direction, and fuel vapour passes into the central zone via a plenum chamber 37.
The arrangement of the blades is somewhat like that of a centrifugal impeller and in the central zone of the device, there is a relative depression of pressure along the axis thereof so that it is possible to adjust the pressure at the exit from conduit 20 to approximately atmospheric pressure.
With reference to figures 3 and 4, it will be seen that the separation of the plates 31, 32 at their periphery by the blades 33 is by a distance h, the diameter of the central zone around which the blades 33 are disposed is d, and the blades 33 are fixed at an angle alpha to radii from the centre of the central zone.
In a particular embodiment of a burner of the type shown in figure 1 having a fuel consumption of 1 kg./hr (output of about 10 kW) fitted with a Mecker-type combustion grid of area 25 cm² and having an air swirl device 21 of the type shown in figures 2, 3 and 4, the following characteristics of the air swirl device were found to give satisfactory operations:

alpha = 50°
h = 10mm
d = 22 mm

The foregoing data were obtained experimentally and are typical practical characteristics rather than optimum or unique values for satisfactory operation of the burner.
The fuel employed had the following approximate properties:

specific gravity 0.827 @ 10°C
viscosity 3.60 cS @ 20°C
distillation (ASTM)
10vol.% distilled 175°C
50 vol.% distilled at 260°C
90 vol.% distilled at 353°C

The prior problems of fuel vapour condensation and flame instability at start-up, fouling by fuel degradation products (e.g. coke) at locations upstream of the burner head necessitating burner shut-down for cleaning, and the discharge of fuel vapour to atmosphere at burner shut-down were found to be substantially eliminated.
Reference is now made to the embodiment of figure 5 and 6 wherein parts common to the embodiment of figure 1 are given the same reference numerals.
In figures 5 and 6, fuel vapour produced in the vapourizer (not shown) passes upwardly via conduit 20 into the central zone of the air swirl device 21 wherein it is mixed with swirling air entering from the air space 15 between the insulation and the outer casing 14. The resulting mixture of fuel vapour and air passes out of the hole 19 in the horizontal top casing member 18 into a volute 40 which directs the mixture to a linear combustion grid 41 of any known or suitable type used in burning gaseous fuels at the outlet 42 of the volute 40.
In addition to the benefits and advantages stated above, the symmetry of the principal parts of the burner about the central (vertical) axis enables the burner to be of simple and compact design and of relatively low cost.
It will be appreciated that a feature/or features shown or described in relation to one embodiment may be employed in another embodiment of the invention if technically feasible.

Claims

1. A burner for burning a liquid fuel comprising a burner head (23) at which a mixture of air and vapour of the liquid fuel is burned in a substantially stable flame (24); air supply means (16) operable to supply at least some of the air for forming the mixture of air and fuel vapour, a conduit (15) for conducting air from the air supply means (16) to the burner head (23), a vapourizer (11) which is operable independently of the presence of any flame at the burner head (23) for converting liquid fuel to fuel vapour without contact with hot gases and/or oxygen-containing gas, and a fuel vapour duct (35) for conducting fuel vapour from the vapourizer (11) into the said conduit (15), characterized in that the burner also comprises air swirl means (21) for imparting a swirling or vortex flow to air passing through the conduit (15) and in that the said fuel vapour duct (35) is disposed for conducting fuel vapour into the conduit (15) at the location of, or downstream of, the air swirl means (21).

2. A burner according to claim 1 characterized in that the fuel vapour duct (35) is disposed for introducing fuel vapour into the conduit (15) at a region of the conduit where the static pressure of the fuel vapour is substantially equal to, or slightly greater than, the static pressure of the air at that region, during operation.

3. A burner according to claim 2 characterized in that said region is selected so that, during operation of the burner, the static pressure of air in said region is approximately equal to air pressure.

4. A burner according to any one of claims 1 to 3 characterized in that the air swirl means (21) comprises a plurality of flat blades (33) in the duct (15), adjacent blades converging in a direction away from the wall of the duct (15) to define a plurality of convergent paths for some of the air, and each such convergent path being inclined, relative to a radius from the centre of the duct (15) non-radially and non-tangentially, and in the same sense as every other convergent path.

5. A burner according to any one of claims 1 to 4 characterized in that the upstream end of the burner head (23) is substantially immediately downstream of the exit from the air swirl means (21 ).

6. A burner according to any one of claims 1 to 5 characterized in that the vapourizer (11) has no more than two open orifices therein, one orifice being an inlet for the supply of liquid fuel into the vapourizer and the other (20) being the entrance to the fuel vapour duct (35) for the exit of fuel vapour from the vapourizer.

7. A burner according to any one of claims 1 to 6 characterized in that the vapourizer (11) comprises a substantially vertical riser conduit (27) side-by-side with a substantially vertical downcomer conduit (26), the riser conduit (27) and the downcomer conduit (26) communicating with each other at or near their bottom ends, a disengaging chamber (28) connecting the top ends of the riser and downcomer conduits (27, 26) for the disengagement of fuel vapour from liquid fuel, and heating means (25) disposed in the riser conduit (27) for heating liquid fuel in the riser conduit to its boiling temperature.

8. A burner according to claim 7 characterized in that a liquid fuel feed pipe (13) is located for supplying liquid fuel to the bottom region of the riser conduit (27) from a constant-liquid-level regulating device.

9. A burner according to any one of claims 1 to 8 characterized by comprising means operable to cause the evolution of fuel vapour out of the vapourizer (11) substantially to cease when the burner (10) is stopped.