[0001] The present inventien relates to a burner for burninga liquid fuel. particularly
(but net exeloxively) a liquid fucl of the typc used for domestie heating.
[0002] In one knewn buner of this type, as described in French patant 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.
[0003] 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 prerure blower. nast be cmployed to provide sufficient enerny
t:o entiain the fuel vapour, to overcome the back pressure of the ejector and also
to furnish sufficient piessure energy to recirulate some of the air-fuel vapour mixture
tc the vapourizing chamber and to bubble the mixture through liquid fuel in the chmber.
The liquid fuel in the chamber is exposed to hot recirculated combustion gases and/or
oxyegen-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.
[0004] 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.
[0005] The presont invention provides a burner for burning a liquid fucl, compriging a burner
herd in whieh a mixture of air (as hereinafter defined) 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 vapours 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.
[0006] In this patent specification, the term "air" is intended to comprehend any oxygen-containing
gas.
[0007] In preferred embodiments of the invention, the length of conduit between the air
swirl means and the burner head is as short as possible.
[0008] 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.
[0009] 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.
[0010] preferably, the vapoueizer bas no more than two open orifiees therein, one being
for the maply of liquid fuel to the vapourizer and the other being the entrance to
the fuel vapeur duct.
[0011] The vapoufaer preferably comprises a substantially vertical riser conduit side-byside
with a substantially vertical downcomer conduit, the riser conduit and downcomer conduit
communicating with each other at or near their bottoms, a disengaging chamber connesting
the tops of the risar 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 hearing means preferably defining with the
internal wall of the riser conduit a relatively narrow substantially vertical space
for the upward cireulation of a mixture of liquid and vapourized fuel generated by
the action of the heating means on liquid fuel when in the vapourizer.
[0012] Liquid fuel is preferably supplied to the bottom region of the riser conduit from
a constant-liquid-level regulating device.
[0013] In one type cf 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 electro-mechanical 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.
[0014] 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 comstent level device to returen
to a liquid fuel reserveir when the burner is being started.
[0015] The is now further deseribed with refarence to cmbodiments thereof, given by way
of non-limitative examples, and with reference to the accompanying diagremmatic drawings
in which:
Figure 1 is an elcvation 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.
[0016] 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 H
2O at zero flow and about 5 mms H20 at normal flow rete, with a power comgption cf
abont 25W. The fan, when eperating. passes air for combustion upwardly through the
spase 15. The tep of the space 15 is bounded by a horizontal metal casing member 18
having a centrral 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 vertex 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.
[0017] 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.
[0018] 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.
[0019] 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. 2.5mms.). The heating
element 25 is heated by an internal electrical heating resistance (not shem) supplied
with electric power from terminals E and wires F. Eiquid fuel is supplied to the vapourizer
11 from the pipe 12 which has a substantally vertical part terminating at the lower
end of the feed pipe 13. The latter is upwatdly 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 componentes of fuel therein. The upper end of the feed pipe is connected
into the riser 27 at just above the level at which the downeomer 26 communicates with
the riser 17.
[0020] 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 clement 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 burner head.
[0021] Because the fuel vapour and air arc mixed under turhulent conditions good mixing
is achieved and the tendeney for fuel to be deposited on the surfaces of the burner
is reduced even when the surfaces are cool. Moreovrer the hod-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 claiminated or relatively insignificant.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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 teh blades 33 are fixed
at sn eugl, alpha to radii frow the spntre of the ecniral zone.
[0026] In a particular cmbodiment of a burner of the type shown in figure having a foel
of l kg./hr (output of about 10 kW) fitted with Mecker-type combustion grid of aren
25 cm
2 and having an air swirl device 21 of the type shown in figures 2, 3 and 4, the following
charaeteristies of the air swirl device were found to give satisfactory operations:
alpha = 50°
h = 10 mm
d = 22 mm
[0027] The foregoing data were obtaincd experimentally and are typical practical characteristics
rather than optimum or unique values for satisfactory operation of the burner.
[0028] The fuel employed had the following approximate properties:
specific gravity 0.827 @ 10°C
viscosity 3.60 cS @ 20°C
distillation (ASTM)
10 vol.% distilled 175°C
50 vol.% distilled at 260°C
90 vol.% distilled at 3530C
[0029] The pri.or 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.
[0030] 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.
[0031] In figures 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 cntering from the air spaet 15 batween the insulation and the outer
casing 14. The resulting mixtore of fuel vapour and air passes out of the hole 19
in the horizontal top casing mamber 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.
[0032] 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.
[0033] 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.
1. A burner for burning a liquid fuel comprising a burner head (23) at which a mixture
of air (as hereinbefore described) 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 downsteam 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 3 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 (13) 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 inlet orifice
(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.