GB1592490A

GB1592490A - Blue-flame oil burner

Info

Publication number: GB1592490A
Application number: GB50523/77A
Authority: GB
Original assignee: Deutsche Forschungs und Versuchsanstalt fuer Luft und Raumfahrt eV DFVLR
Current assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Priority date: 1977-01-08
Filing date: 1977-12-05
Publication date: 1981-07-08
Also published as: DE2700671A1; CH628724A5; YU309177A; NL7713861A; SE7713044L; NL176705B; IE46538B1; IE772479L; CA1080112A; AU511042B2; ATA813477A; FR2377002B1; FR2377002A1; DK146664B; NO774006L; DK543577A; ES465796A1; LU78704A1; AT383203B; NO145998C

Description

(54) BLUE-FLAME OIL BURNER (71) We, DEUTSCHE FORSCHUNGS- und VERSUCHSANSTALT für LUFT- und RAUMFAHRT e.V., of Linder The, 5000 Köln 90, Germany, a German Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention concerns a blue-flame oil burner, that is an oil burner intended to operate with a blue flame and having provision for recirculation of part of the combustion gases and having an oil-atomising device, a wall containing an orifice arranged downstream of the outlet of the oil-atomising device, a mixing tube arranged at a distance downstream of the orifice and coaxial with it and a flame-tube around the mixing tube.

Blue-flame oil burners require that the oil reaching the point of combustion is completely vaporised before it reaches that point. The operation of an oil burner with a blue flame has the advantage that the burner is able to operate with very small excess of air over that required for complete combustion so that practically stoichiometric combustion takes lace. Since combustion takes place with very small excess of air a very hot flame is produced which utilises the energy content of the fuel optimally and leads to improved heat transfer.In addition, the waste gases in comparison with waste gases from an optimally adjusted burner with a yellow flame contain extremely little harmful material (soot, NOx, SO3) Because of the known advantages of an oil burner in which oil is burnt in the vaporised state with a blue flame, there has been no lack of attempts to bring oil burners with blue flames on to the market.

In a known blue-flame type of oil burner an orifice plate is arranged in a double-walled combustion chamber downstream of the oil nozzle. Oil is sprayed through the orifice plate into an elongate mixing tube and, with the aid of air which enters the tube simultaneously through the orifice, partial combustion takes place with an excess of combustible material.

These combustion gases are returned and again enter the end of the mixing tube opposite the orifice plate at the same time as the spray of fuel. In this known burner the inner wall is provided with holes through which the greater part of the air of combustion enters in the form of a secondary air flow. (Proc. World Petr. Congr., 7th Volume 7, Sect. on Application and New Uses, Part 1, p 119 ff).

An oil burner with a blue flame with recirculation is also known in which the orifice plate is situated in a tube which extends so far in the upstream direction that the nozzle and the nozzle support are situated completely within the tube. With such a burner the flame should burn immediately behind the orifice plate and part of the combustion gas should be recirculated through an annular space between the tube and an outer tube (ibid.).

An oil burner is also known in which oil is sprayed directly into a tube which is surrounded by an outer recirculation tube connected to the inner tube by means of bores (German Federal Republic Patent Specification No. 1,064,188).

It is an object of the invention to design a blue-flame oil burner incorporating features of advanced conventional technology and which has a high operating reliability.

This object is solved according to the invention by the provision of an oil burner of the kind in which vaporised fuel is capable of burning with a blue flame and having provision for recirculation of part of the combustion gases within a flame-tube, the oil burner comprising the flame-tube, a wall extending transversely of the flame-tube and defining the upstream end thereof, the wall having therein an orifice through which air enters the flame-tube, the orifice being the only air inlet into the flame-tube, an oil atomising device positioned upstream of the wall and arranged to discharge an oil spray through the orifice into the flame-tube, and a mixing tube positioned within the flame-tube co-axially of the orifice and having its upstream end spaced axially from the wall by a distance such that the peripheral area of the space between the wall and the upstream end of the mixing tube and defined by an imaginary upstream extension of the inner peripheral wall of the mixing tube to the wall is at least equal to the difference between the internal cross-section area of the upstream end of the mixing tube and the cross-sectional area of the orifice, the mixing tube having a length, L, and an internal diameter, D, where the mixing tube is of circular cross-section, or where it is not of circular cross-section, an internal cross-sectional area at the upstream end thereof equal to the area of a circle having a diameter, D, such that the ratio L/D is between 1.0 and 1.75, and the flame-tube having an equivalent ratio L/ D of between 2.0 and 5.0 Preferably the ratio L/D of the mixing tube is equal to or is approximately 1.5.

Preferably the ratio L/D of the flame-tube is between 2.5 and 3.0.

If desired, one or both of the mixing tube and the orifice may be of non-circular crosssection and in that case the internal cross-sectional area of the mixing tube is between 1.5 and 3.0 times the cross-sectional area of the orifice.

The internal diameter of the flame-tube is approximately 2.0 to 2.5 times the internal diameter of the mixing tube.

The flame-tube may have a substantially constant cross-section throughout its length. For example, the flame-tube may be designed to be cylindrical.

A special problem with blue-flame oil burners is the reliable ignition of the flame in the variable starting conditions which occur in practice, for example relighting a burner which is still hot. Reliable ignition is achieved in that the bumer is started with an air pressure below that corresponding to stoichiometric air-fuel mixture flow. The supply of air is then increased, after the opening of an oil supply valve, until the quantity of air supplied is slightly greater than the stoichiometric value.

Operation in the manner described is preferably achieved in that the air is supplied by a throttle valve having an air flap movable to an open position by driving means operable by time switch means. For example, an electric or hydraulic motor may be provided as the driving means for the air flap, the motor being arranged to be automatically connected to the air flap by a reduction gear mechanism.

It is also possible to use an electrically excited magnetic device or a hydraylic cylinder in each case with time-controlling damping members.

It may also be convenient when starting-up either in conjunction with or instead of throttling the air supply to the burner to operate with a reduced rate of supply of oil compared with that corresponding to stoichiometric air-fuel mixture flow.

By way of example an embodiment of the invention is illustrated in the accompanying drawing and is described in detail in the following with reference to the drawing, which shows a longitudinal section through an oil burner according to the invention together with its control and supply devices.

The burner 2 illustrated includes a chamber 4 in which a pressure atomising nozzle 6 is mounted in a known manner at the end of an oil supply pipe 8. Oil is fed into the pipe 8 by an oil pump 10 which is driven by a motor 12 which also drives a blower rotor 14 in a known manner. The pump 10 delivers oil through a hand-operated throttle valve 16 and an electromagnetically-operated shut-off valve 18 into the oil supply pipe 8 which carries the atomising nozzle 6. The blower 14 delivers air through a duct 20 into the chamber 4 through a throttle valve 22 having an air flap 24 which can be adjusted by an electric motor 26. A pair of ignition electrodes 30 is carried by a support 28 mounted on the oil supply pipe 8 and is connected to an ignition transformer 32.

At a distance L3 from the mouth of the pressure atomising nozzle 6 there is a wall 34 having an orifice 36 therein coaxial with the nozzle. At a distance from the orifice 36 there is a mixing tube 38 which is attached to the wall 34 by mounting rods 40. The mixing tube 38 is situated inside an outer tube 42 which forms a flame-tube. In this example, the orifice 36 is circular and the mixing tube 38 and the flame-tube 42 are cylindrical but they need not be. Where the flame-tube 42 is not of cylindrical form it may or may not have a substantially constant cross-section throughout its length. The mixing tube 38 instead of being cylindrical may have a'cross-sectional area varying along its length.

The diameter D3 of the orifice 36 and the arrangement and dimensions of the mixing tube 38 and of the flame-tube 42 are in a predetetermined critical relationship to each other which will be discussed in detail in the following.

The atomising nozzle 6 should be adjustable in the axial direction in order to vary the through-put of air and the oil cloud input and thus make it possible to achieve the optimum composition of the mixture.

The diameter D3 of the orifice 36 should be chosen in such a way that at a delivery pressure of the blower 14 which corresponds to the delivery pressure normal in commercial oil burners, the air flow through the orifice 36 in the wall 34 has a predetermined velocity, the orifice 36 being the only passage for combustion air.

The internal diameter D1 of the mixing tube 38 is at least 1.25 and at maximum 1.7, and is preferably 1.4 to 1.45, times the diameter D3 of the orifice 36. Alternatively, the ratio of the internal cross-sectional area of the mixing tube 38 is between 1.5 and 3.0 times the crosssectional area of the orifice 36 where the mixing tube and/or the orifice are not of circular cross-section. The length L1 of the mixing tube 38 is between 1.75 and 1.0 D1 and preferably L1 = 1.5 D1 where D1 is the internal diameter of the mixing tube which is of circular cross-section. Where the mixing tube is not of circular cross-section, D1 is the diameter of a circle having an area equal to the internal cross-sectional area at the upstream end of the mixing tube.

The distance L4 of the mixing tube 38 from the wall 34 is chosen in such a way that a flow cross-section, radially of the mixing tube 38, which is at least 1 to 3 times the difference between the cross-sections of the orifice 36 and of the mixing tube 38 is produced between the upstream end of the mixing tube 38 and the wall 34. In other words, the peripheral area of the space between the wall 34 and the upstream end of the mixing tube 38 and defined by an imaginary upstream extension of the inner peripheral wall of the mixing tube 38 to the wall 34 is at least one to three times the differece between the internal cross-sectional area of the upstream end of the mixing tube 38 and the cross-sectional area of the orifice 36.The internal diameter D2 of the flame-tube 42 is approximately twice to 2.5 times the internal diameter D1 of the mixing tube 38 and the ratio of the length L2 to the diameter D2 of the flame-tube 42 is between 2:1 and 5:1 and is preferably between 2.5:1 and 3:1.

A spray cone of approximately 60 to 80 degrees is convenient for the pressure atomising nozzle 6. Particularly good results were obtained with nozzles with hollow spray cones. When the oil burner is to be started, the motor 12 is first switched on in the normal way. At the same time, the air flap 24 is closed unless it had already been closed during shutting off of the oil burner. A short time later voltage is applied to the ignition electrodes 30 and the electromagnetically-operated valve 18 is subsequently opened so that oil supplied by the oil pump 10 reaches the pressure atomising nozzle 6. At the same time, air is supplied by means of the blower 14 to the chamber 4. The mixture produced is ignited by the ignition electrodes 30. A flame is thereby produced in that part of the flame-tube 42 situated in front of the downstream end of the mixing tube 38.At the same time that ignition occurs, the air flap 24 is moved into the operating position by the motor 26. The diameter of the orifice 36 is such that the air inside the mixing tube 38 has a velocity such that drops of atomised oil do not form on the inner wall of the mixing tube 38 when the air flap 24 is open. By means of the core stream of air, oxygen-poor combustion gas is sucked through the annular space around the mixing tube 38 and through the upstream end of the mixing tube 38 or through openings in the rear wall of the mixing tube at its upstream end. These hot combustion gases surround the core stream and give up heat thereto.An additional criterion for the dimension of the orifice 36 and the air velocity is that the air velocity in the mixing tube 38 should be greater than the velocity of propagation of the flame in order to ensure that the flame burns at a distance from the downstream end of the mixing tube 38. The stated operating conditions give the result that the distance between the end of the mixing tube 38 adjacent the wall 34 and said wall 34 is small as possible. This distance must be chosen in such a way that the recirculating gases flowing into the mixing tube 38 are as loss-free as possible and gre able to mix with the oil cloud/air mixture of the core stream for the purpose of heat exchange. With experimental burners the following dimensions have been found to be favourable:1.Using a pressure atomising nozzle giving a throughput of 0.5 to 0.65 gal/hr and Diameter of the orifice D3 23.5-26 mm Internal diameter of the mixing tube D1 35 mm Internal diameter of the flame-tube D2 76 mm a very stable blue flame was produced over the whole range of throughput.

2. Using a pressure atomising nozzle giving a throughput of 0.65 to 0.9 gal/hr, the diameter D3 Of the orifice 36 was increased to 25-31 mm with the remaining dimensions kept contstant. Once again a stable blue flame was produced over the whole range of throughput.

A series of models developed for fabrication had the following principal dimensions (all measurements in mm): Model Dl D2 D3 L1 L2 L4 Nozzle 60"H gal/hr 1 36 77 23.5 57 225 9 0.5 2 36 77 26 57 225 9 0.65 3 36 77 29 52 225 14 0.75 In all the models the distance L3 of the nozzle mouth from the upstream face of the wall 34 was 2 mm.

Maintenance of the following combustion data was established for all models on the test bench: CO2 = 15 % (approximate theoretical maximum) Soot = 0 CO = 0.01% With blue flames monitoring of the flame cannot be carried out optically. To guarantee a reliable automatic operation of the blue-burning flame monitoring is possible by means of an ionisation detector 44 which is connected in known manner to a control device 46 by means of which, when the flame is extinguished, the supply of oil is cut off by closing the valve 18 and the motor 12 is switched off. After the flame has been produced the ignition electrodes are switched off by the control device in known manner.

It has been shown in practice that it is essential that the mixing tube 38 should be designed with as small a volume as possible, and thus, for given dimensions, with as small a wall thickness as possible. This ensures that the mixing tube 38 will glow bright red, so that a large portion of the heat of the recirculating combustion gases is transferred to the mixture of air and oil drops in the form of radiant heat, so that, once again, the complete vaporisation of the oil without droplets impinging on hot surfaces is ensured. With the aforesaid ratio L1:D1 for the mixing tube it is ensured that the mixing tube is surrounded over its whole length by oxygen-poor combustion gases so that with mixing tubes of heat resistant steel it is not possible for any marked scaling or oxidation of the mixing tube to occur.The mixing tube may also be made from heat resistant ceramic material. The support of the mixing tube 38 can be effected, instead of by the mounting rods 40, by radial arms which should then be mounted on the inside of the flame-tube 42.

With stoichiometric combustion in the flame-tube 42 practically no free oxygen is present.

This is a reason why the flame-tube 42 may be made of heat resistant steel without risk of wear due to scaling or oxidation. Alternatively, the flame-tube 42 may be made from a heat resistant ceramic material or a steel tube having a heat resistant ceramic coating may be used.

It is possible to arrange for the flame tube to be cooled, for example, by water which is to be heated by the oil burner. In this case the flame-tube may, for example, form part of the heat exchange system of the boiler. With cooled flame-tubes, it is not necessary to use highly heat resistant materials.

Combustion in the oil burner described is to a large extent independent of the size and shape of the combustion chamber of the boiler. Under very unfavourable conditions, resonance phenomena may, however, occur during the ignition phase. These can be prevented by providing the wall of the flame tube 42 with a few holes in the region of the flame front. The development of noise may also be caused by a change in the flame cross-section. For example to reduce noise, a conical expansion of the flame-tube may be provided or the flame-tube, downstream of the flame zone, may be given a star-shaped cross-section.

A special problem with oil burners burning with a blue flame is that of guaranteeing reliable ignition under all working conditions. Reliable ignition is achieved by, at the moment of ignition, supplying the burner with less than the stoichiometric air quantity required relative to the normal load. For this purpose the air flap 24 of the throttle valve 22 is moved to a suitable starting position by the motor 26 so that on fresh ignition only, a suitably adjusted less than stoichiometric air quantity is supplied to the chamber 4. After the oil has ignited, the air flap 24 is opened further by the motor 26, whereby the quantity of air required for stoichiometric combustion is allowed to pass through the throttle valve 22. The motor 26 may be switched on at the same time that the electro-magnetically-operated valve 18 is opened, a finite time delay necessary being produced by means of a step-down or reduction gearing and, if necessary, accessory electrical control elements. This method of operation has the advantage that the air quantity is increased continuously. Switching on of the motor may also occur in dependence of the detection of a flame by means of the ionisation detector 44. For adjustment of the air flap 24, it would also be possible to provide an electro-magnetic device with known means of delay. A delay could be ensured for both an electro-magnetic device and the motor by means of a semi-conducting resistor.It has been established that, in general, in order to achieve reliable ignition, it is sufficient to start the burner with less than stoichiometric air quantity for a duration of about 3 to 5 seconds. It has been established that within this time, a stable recirculation flow is achieved with simultaneous heating of the mixing tube to the operating temperature. During starting with less than the stoichiometric air quantity, as described, no formation of soot was observed. Probably the reason for this is that the amount of air present at any time within the combustion chamber of the boiler is sufficient to ensure that the necessary oxygen for complete combustion is available. Depending on the conditions in the boiler at a particular time it may be necessary to increase the time allowed before further opening of the air flap occurs to 6 to 10 seconds.

From a general point of view it is important for reliable ignition, to start the burner with an air velocity less than the air velocity at full load. Conveniently, it has been shown that during starting an excess pressure of 23 to 25 mm head of water during full air-fuel mixture flow.

If a hydraulically-operated motor is used, it is possible to employ the oil pressurised by the pump 10 as the pressure fluid. Under certain circumstances it may also be convenient to provide means by which it is possible to start the burner with, in addition to the throttling of the air stream, a throttled stream of oil as well. Then the stream of oil is raised to full flow in the same time period as that stated hereinbefore for the air stream. It is possible in this case, to keep the period of the less than stoichiometric operation short during starting and in the ideal case to maintain stoichiometric conditions oc combustion even during starting the burner. It is, a necessary condition that even with a decreased flow of oil, the atomisation of the oil is not impaired.Up to a certain size, larger oil drops are acceptable since the walls, in particular the wall of the mixing tube, the temperature of which is essentially equal to the temperature of the boiler water (about 70-90"C) during the starting process, have a certain storage capacity, that is to say larger drops of oil can settle on the walls at first and form an oil film there which will evaporate once full air-fuel mixture flow has been achieved.

In order to be able to operate the burner with maximum air-fuel mixture flow under varying conditions with as constant an excess of air as possible, that is as accurately stoichiometrically as possible, it is convenient to provide means by which it is possible to adjust the flow rates of air and/or oil automatically in response to momentary changes in operational conditions, in particular air pressure, air temperature and/or the burner draught.

With pressure atomising nozzles having a high throughput, the fine atomisation necessary for vaporisation is often no longer achievable. For burners of high power it may therefore be convenient to arrange a plurality (at least two) of nozzles at a distance from one another, either parallel or at an angle to each other, where the cross-section of the orifice 36 in the wall 34, the mixing tube 38 and, where necessary, the flame-tube 42 are geometrically adapted in a suitable manner.

The orifice 36 may, as illustrated, be a simple stamped-out opening in a disc-shaped wall 34. The orifice 36 may, however, be formed so as to project for a short distance in the form of a tube with an angular or a rounded-off transition to the wall 34.

WHAT WE CLAIM IS:- 1. An oil burner of the kind in which vaporised fuel is capable of burning with a blue flame and having provision for recirculation of part of the combustion gases within a flame-tube, the oil burner comprising the flame-tube, a wall extending transversely of the flame-tube and defining the upstream end thereof, the wall having therein an orifice through which air enters the flame-tube, the orifice being the only air inlet into the flame-tube, an oil atomising device positioned upstream of the wall and arranged to discharge an oil spray through the orifice into the flame-tube, and a mixing tube positioned within the flame-tube co-axially of the orifice and having its upstream end spaced axially from the wall by a distance such that the peripheral area of the space between the wall and the upstream end of the mixing tube and defined by an imaginary upstream extension of the inner peripheral wall of the mixing tube to the wall is at least equal to the difference between the internal crosssectional area of the upstream end of the mixing tube and the cross-sectional area of the orifice, the mixing tube having a length, L, and an internal diameter, D, where the mixing tube is of circular cross-section, or where it is not of circular cross-section, an internal cross-sectional area at the upstream end thereof equal to the area of a circle having a diameter, D, such that the ratio L/D is between 1.0 and 1.75, and the flame-tube having an equivalent ratio L/D of between 2.0 and 5.0.

2. An oil burner as claimed in Claim 1 in which the ratio L/D of the mixing tube is equal to or is approximately 1.5.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. if necessary, accessory electrical control elements. This method of operation has the advantage that the air quantity is increased continuously. Switching on of the motor may also occur in dependence of the detection of a flame by means of the ionisation detector 44. For adjustment of the air flap 24, it would also be possible to provide an electro-magnetic device with known means of delay. A delay could be ensured for both an electro-magnetic device and the motor by means of a semi-conducting resistor. It has been established that, in general, in order to achieve reliable ignition, it is sufficient to start the burner with less than stoichiometric air quantity for a duration of about 3 to 5 seconds.It has been established that within this time, a stable recirculation flow is achieved with simultaneous heating of the mixing tube to the operating temperature. During starting with less than the stoichiometric air quantity, as described, no formation of soot was observed. Probably the reason for this is that the amount of air present at any time within the combustion chamber of the boiler is sufficient to ensure that the necessary oxygen for complete combustion is available. Depending on the conditions in the boiler at a particular time it may be necessary to increase the time allowed before further opening of the air flap occurs to 6 to 10 seconds. From a general point of view it is important for reliable ignition, to start the burner with an air velocity less than the air velocity at full load. Conveniently, it has been shown that during starting an excess pressure of 23 to 25 mm head of water during full air-fuel mixture flow. If a hydraulically-operated motor is used, it is possible to employ the oil pressurised by the pump 10 as the pressure fluid. Under certain circumstances it may also be convenient to provide means by which it is possible to start the burner with, in addition to the throttling of the air stream, a throttled stream of oil as well. Then the stream of oil is raised to full flow in the same time period as that stated hereinbefore for the air stream. It is possible in this case, to keep the period of the less than stoichiometric operation short during starting and in the ideal case to maintain stoichiometric conditions oc combustion even during starting the burner. It is, a necessary condition that even with a decreased flow of oil, the atomisation of the oil is not impaired.Up to a certain size, larger oil drops are acceptable since the walls, in particular the wall of the mixing tube, the temperature of which is essentially equal to the temperature of the boiler water (about 70-90"C) during the starting process, have a certain storage capacity, that is to say larger drops of oil can settle on the walls at first and form an oil film there which will evaporate once full air-fuel mixture flow has been achieved. In order to be able to operate the burner with maximum air-fuel mixture flow under varying conditions with as constant an excess of air as possible, that is as accurately stoichiometrically as possible, it is convenient to provide means by which it is possible to adjust the flow rates of air and/or oil automatically in response to momentary changes in operational conditions, in particular air pressure, air temperature and/or the burner draught. With pressure atomising nozzles having a high throughput, the fine atomisation necessary for vaporisation is often no longer achievable. For burners of high power it may therefore be convenient to arrange a plurality (at least two) of nozzles at a distance from one another, either parallel or at an angle to each other, where the cross-section of the orifice 36 in the wall 34, the mixing tube 38 and, where necessary, the flame-tube 42 are geometrically adapted in a suitable manner. The orifice 36 may, as illustrated, be a simple stamped-out opening in a disc-shaped wall 34. The orifice 36 may, however, be formed so as to project for a short distance in the form of a tube with an angular or a rounded-off transition to the wall 34. WHAT WE CLAIM IS:-

1. An oil burner of the kind in which vaporised fuel is capable of burning with a blue flame and having provision for recirculation of part of the combustion gases within a flame-tube, the oil burner comprising the flame-tube, a wall extending transversely of the flame-tube and defining the upstream end thereof, the wall having therein an orifice through which air enters the flame-tube, the orifice being the only air inlet into the flame-tube, an oil atomising device positioned upstream of the wall and arranged to discharge an oil spray through the orifice into the flame-tube, and a mixing tube positioned within the flame-tube co-axially of the orifice and having its upstream end spaced axially from the wall by a distance such that the peripheral area of the space between the wall and the upstream end of the mixing tube and defined by an imaginary upstream extension of the inner peripheral wall of the mixing tube to the wall is at least equal to the difference between the internal crosssectional area of the upstream end of the mixing tube and the cross-sectional area of the orifice, the mixing tube having a length, L, and an internal diameter, D, where the mixing tube is of circular cross-section, or where it is not of circular cross-section, an internal cross-sectional area at the upstream end thereof equal to the area of a circle having a diameter, D, such that the ratio L/D is between 1.0 and 1.75, and the flame-tube having an equivalent ratio L/D of between 2.0 and 5.0.

3. An oil burner as claimed in Claim 1 or 2 in which the ratio L/D of the flame-tube is

between 2.5 and 3.0.

4. An oil burner as claimed in any preceding claim in which one or both of the mixing tube and the orifice are of non-circular cross-section and the internal cross-sectional area of the mixing tube is between 1.5 and 3.0 times the cross-sectional area of the orifice.

5. An oil burner as claimed in any preceding claim in which the internal diameter of the flanlc-tohe is approximately 2.0 to 2.5 times the internal diameter of the mixing tube.

6. An oil burner as claimed in any preceding claim in which the distance between the atomising device and the wall is equal to or is approximately 2 mm.

7. An oil burner as claimed in any preceding claim in which the flame-tube has a substantially constant cross-section throughout its length.

8. An oil burner as claimed in any preceding claim in which the flame-tube is cylindrical.

9. An oil burner as claimed in any preceding claim in which the mixing tube has a cross-sectional area varying along its length.

10. An oil burner as claimed in any preceding claim in which the air to the upstream side of the transverse wall is supplied by a throttle valve having an air flap movable to an open position by driving means operable by time switch means.

11. An oil burner as claimed in Claim 10 in which the driving means for the air flap is an electric or hydraylic motor arranged to be automatically connected to the air flap by a reduction gear mechanism.

12. An oil burner as claimed in any preceding claim in which means are provided by which either or both the rate of air flow and the rate of oil flow can be altered in dependence of any one or more of the following:- air pressure, air temperature and burner draught.

13. An oil burner as claimed in any preceding claim in which the flame-tube is watercooled.

14. An oil burner as claimed in any preceding claim in which the flame-tube forms a part of a heat exchange system of a boiler in which the oil burner is to be installed.

15. An oil burner as claimed in any preceding claim in which the oil atomising device is an oil spray nozzle with a hollow spray cone.

16. An oil burner as claimed in Claim lincluding control means whereby the burner is started with a low air pressure compared with that corresponding to stoichiometric air-fuel mixture flow.

17. An oil burner as claimed in Claim 16 including control means whereby the burner is started with a smaller rate of flow of air compared to that corresponding to stoichiometric air-fuel mixture flow.

18. An oil burner as claimed in Claim 10 or 11 including control means by which the burner is started with the air flap in a partly-closed position and the supply of air is increased, after the opening of an oil supply valve, by the continuous opening of the air flap until the quantity of air supplied is slightly greater than the stoichiometric value.

19. An oil burner as claimed in Claim 1 including control means whereby the burner is started with the rate of supply of oil reduced compared with that corresponding to stoichiometric air-fuel mixture flow.

20. An oil burner constructed and arranged substantially as described herein and shown in the accompanying drawing.