US3705784A

US3705784A - Liquid fuel burner having reduced thermal stress and rapid start-up time

Info

Publication number: US3705784A
Application number: US105212A
Authority: US
Inventors: Robert Reichhelm; Albert Morrison Jr
Original assignee: Burnham Corp
Current assignee: FATTIBENE ARTHUR T; REICHHELM JENIFER; Burnham Corp
Priority date: 1971-01-11
Filing date: 1971-01-11
Publication date: 1972-12-12
Anticipated expiration: 1989-12-12
Also published as: GB1382111A; ES398664A1; CA948094A; DE2129077B2; DE2129077A1; DE2129077C3; AU470263B2; CH555514A; BE777884A; IT946433B; AU3776572A; BR7200112D0; JPS5235890B1; NL7200414A; NL158605B; FR2122196A5

Abstract

A burner for burning gasified liquid fuel such as oil, in which the fuel is gasified by being mixed with hot burned gases drawn from the combustion chamber, is provided in which the normal thermal stresses found in such burners due to the return of the hot burned gases for gasifying are substantially reduced and, at the same time, start-up time is reduced. Minimization of the thermal stresses is accomplished by utilizing the heat of vaporization of the oil or other fuel to cool the hot returning gases, and so limit thermal expansion, by having the gasification of the fuel commence close to the hot gas outlet of the burner, rather than at a more remote distance, such as a gasification chamber. This permits elimination of the gasification chamber, which normally has been required to provide a preheating area for the oil, and so further reduces stresses. With such elimination, rapid start-up is provided by preliminarily burning ungasified fuel introduced by the air and fuel system of the principal unit.

Description

Unite States Patent Reichhelm et a1.

[ 5] Dec. 12, 1972 [54] LIQUID FUEL BURNER HAVING REDUCED THERMAL STRESS AND RAPID START-UP TIME [72] Inventors: Robert Reichhelm, Lancaster; Al-

bert Morrison, Jr., Leola, both of [73] Assignee: Burnham Corporation, Irvingtonon-Hudson, N.Y.

[22] Filed: Jan. 11, 1971 [21] Appl. No.: 105,212

[52] US. Cl ..431/l16 [51] Int. Cl ..F23m 9/06 [58] Field ofSearch ..43l/115, 116,215

[56] References Cited UNITED STATES PATENTS 3,361,183 1/1968 Reichhelm ..431/1 16 2,688,360 9/1954 Haynes et al.. ..43l/1l5 1,492,674 5/1924 Chapman ..43l/115 Primary Examiner-Edward G. Favors Attorney-Bryan, Parmelee, Johnson & Bollinger [57 ABSTRACT A burner for burning gasified liquid fuel such as oil, in which the fuel is gasified by being mixed with hot burned gases drawn from the combustion chamber, is provided in which the normal thermal stresses found in such burners due to the return of the hot burned gases for gasifying are substantially reduced and, at the same time, start-up time is reduced. Minimization of the thermal stresses is accomplished by utilizing the heat of vaporization of the oil or other fuel to cool the hot returning gases, and so limit thermal expansion, by

I having the gasification of the fuel commence close to the hot gas outlet of the burner, rather than at a more remote distance, such as a gasification chamber. This permits elimination of the gasification chamber, which normally has been required to provide a preheating area for the oil, and so further reduces stresses. With such elimination, rapid start-up is provided by preliminarily burning ungasified fuel introduced by the air and fuel system of the principal unit.

14 Claims, 3 Drawing Figures PATENTED DEC 12 I972 FIGI INVENTORS. ROBERT REICHHELM BY ALBERT MORRISON JR.

FIG. 3

TTORNEYS LIQUID FUEL BURNER HAVING REDUCED THERMAL STRESS AND RAPID START-UP TIME BACKGROUND OF THE INVENTION This invention relates to a burner in which a liquid fuel, such as oil, is first gasified, mixed with air, and then burned. Gasification occurs by mixing the fuel with hot burned gases coming from the burner chamber.

Burners using ga'sified fuel are not new, although the techniques for gasification, and the success of the techniques, has varied.

Gasification systems are shown in various of the patents of the present inventor, Reichhelm, such as U.S. Pat. Nos. 2,197,347, and 3,308,868 where a portion of the oil is burned and used to gasify the remainder; U.S. Pat. Nos. 2,735,481 and 3,320,743 where the fuel is gasified by being mixed with heated air; and U.S. Pat. No. 3,361,183, where gasification is obtained by recirculating a portion of the combustion gases into admixture with the fuel and a Venturi is used to draw in the gasified fuel and the hot combustion gases. In addition, multiple burners, such as those adapted to burn different fuels, or to burn the same fuel in different ways, have existed. Examples of these are Reichhelm U.S. Pat. No. 3,308,868 and French patent In burners being used for gasifying fuel by mixing oi with hot products of combustion, such as that shown in Reichhelm U.S. PatpNo. 3,361,183 or in Reichhelm copending application Ser. No. 56,482, filed July 20, 1970, the gasification of the fuel is shown taking place in a separate chamber. This chamber has hot products of combustion entering the chamber at one inlet, liquid fuel entering it at another, and a mixture ofthe two being drawn out at an outlet port after gasification; It then goes into a mixing chamber for mixing with compressed air and into a burning chamber for burning. This structure, having a gasification chamber, can preheat for starting for the use of an electric heater around the gasification chamber. The structure, however, also results in undesirable thermal stresses due to the fact that the hot combustion gases must pass from the burning chamber to the gasification chamber. This method of preheat also takes a relatively long start-up time.

SUMMARY OF THE INVENTION The inherent heat of vaporization of a gasifiable fuel, such as oil is used to reduce thermal stresses within a gasification type burner by reducing the temperature of the components most subject to stress. This is accomplished by injecting the liquid fuel into the hot combustion gas stream at an earlier point in the systems operation so that its heat of vaporization will be taken up sooner and so serve to reduce the temperature of the returned gases before thermal expansion occurs. This permits elimination of the gasification chamber, further reducing thermal stresses and also adding to compaction.

The fuel and air supply system used for the above is also integrally used to provide a start-up system for a cold burner using the fuel without gasification.

DESCRIPTION OF THE DRAWINGS The invention is best understood through reference to the drawings, as follows:

FIG. 1 is a schematic drawing showing flow and operation of the unit.

FIG. 2 is a perspective view of the burner.

FIG. 3 is a side elevation of the burner partially cut away in a section to show portions of the burning chamber and return gas duct.

DETAILED DESCRIPTION OF THE INVENTION Structure port 14. Port 14 leads from chamber rearwardly of the unit in the general direction of burner chamber inlet or throat 11, through the refractory material to an interconnecting point 15 where it leads to interconnecting means or duct 17, normally a metal pipe. The purpose of outlet 14 is to carry hot, burned gases, i.e., the products of combustion, from the end of burner chamber 10 back towards the mixing chamber 6, the gases being used to gasify or vaporize liquid fuel.

Mixing chamber 6 mixes the gasified fuel with combustion air and feeds it into the burning chamber 10. The air, under pressure, enters mixing chamber 6 through air inlet duct 20 and air inlet 7. Duct 20, as it approaches mixing chamber 6 is constructed to form a Venturi 22 within chamber 6 to create a suction in line 17. This suction serves to draw the hot burned gases from burning chamber 10 through outlet 14 into chamber 6.

The liquid fuel to be burned comes from a fuel source inlet 25. Source 25 branches to form fuel supply system 7. Branch 26 passes through metering valve 27 to an inlet 28 in interconnecting duct 17. Fuel for use in the start-up system, to be described below, follows duct 30 through valve 31 and duct 32 to a nozzle 33 located just within the Venturi construction 22 in air inlet 20.

It should be noted that the oil inlet 28 into the interconnecting duct 17 is positioned proximate to the exit from the refractory material 5 near point 15. Preferably, it should be located as close to the refractory material as is practical. The oil may be fed by either mechanical or gravity pressure, as is desired. The oil is mixed with the hot burned gas in interconnecting duct 17 at oil inlet 28. Duct 17 then carries the mixture of oil and hot combustion gases to mixing chamber 6; the mixture enters chamber 6 at burned gas and fuel inlet An electrically operated igniter plug 35 is located at the burning chamber inlet 11 or slightly toward the mixing chamber from that point. It has been found that by having the igniter 35 positioned slightly outside of, but before, the burning chamber, the plug has less tendency to become fouled by products of combustion.

It will be noted that the above structure permits elimination of the gasification chamber, (compare chamber 14 in Reichhelm U.S. Pat. No. 3,361,183). It will also be noted that the oil has been mixed with hot gas at a point much closer to the point of exit of hot gases from the refractory material (point 15; compare the liquid fuel inlet 21, FIG. 1, of the Reichhelm patent). If the gasification chamber is eliminated, however, the electric preheater is also eliminated (compare heater 18 of Reichhelm U.S. Pat. No. 3,361,183); in its stead a completely distinct but integrally operating start-up unit identified by circuit 30 has been provided. This start-up unit is operatively associated with fuel supply system 7.

This start-up system is integral with the overall system using and the same fuel source. It is adapted for sequential operation, i.e., operation of the start-up followed by operation of the principal gasifying system. The start-up unit has liquid fuel passing from source 25 through duct 30, valve 31, and duct 32 to nozzle 33 within, or proximate to, Venturi 22.

Operation Operation of the liquid fuel burner will be first described in the operating stage, and then the start-up aspects considered.

In operation, oil or other liquid fuel capable of being gasified enters the system from source 25 passes through duct 26 and valve 27 and is fed into the hot combustion gases at the intersection point of duct 28 and interconnecting duct 17. Meanwhile hot gases, a portion of the total volume of combustion gases, have been taken from combustion chamber through hot gas discharge port 14 passing through refractory block 5 to point 15 and then into interconnecting duct 17 and into mixing chamber 6. Air under pressure enters through air inlet 20 and passes through Venturi 22 into mixing chamber 6. The passage of the air through Venturi 22 creates a suction which draws the hot burned combustion gases through inlet 14 and duct 17 into mixing chamber 6. These hot combustion gases (mixed with and carrying gasified oil) are mixed with the air in chamber 6 and pass through burning chamber inlet 11 to be burned in chamber 10. Electrical plug 35 pro vides ignition. Most of the combustion gases are then discharged through discharge opening 12 with only a small portion returning through outlet 14.

As noted above, the oil is injected into interconnecting duct 17 as close to the source of hot gas as is practical. Since the oil, as with any vaporizable liquid, requires heat of vaporization, the heat energy necessary for the gasification of the liquid fuel comes from the hot burned gases from duct 14. Thus the gasification of the oil serves to cool the hot gases. By placing inlet 28 close to the source of hot gas, the cooling takes place early in interconnecting pipe 17. Thus pipe 17, a gas delivery pipe, does not reach the usual red-hot temperature normally found in prior art structures. Reduction of the temperature of the duct 17 reduces its thermal expansion (the expansion primarily being in a longitudinal direction), and so reduces thermal stresses in the area between mixing chamber 6 and refractory material 5. This is particularly important at points of junction.

Interconnecting duct 17 must have sufficient length after injection of the oil to allow adequate time for gasification of the fuel, if a gasification chamber is not used. Determination of adequate length relative to quantities of returned gas used can readily be made by observing the color of the flame in burner chamber 10. If blue, adequate gasification has taken place; if yellow is present, it is likely that some of the fuel has remained liquid;

If duct 17 has sufficient length after inlet 28, relative to the volume and temperature of hot return gases in duct 17, substantially complete gasification will take place in duct 17 prior to the oil reaching mixing chamber 6. Under these circumstances and if there remains sufficient heat energy in the mixture entering inlet 13, additional liquid fuel may also be injected at nozzle 33 for enhanced heat capacity of the overall burner. This, then, would result in two-stage operation if all oil is gasified, but will have yellow in it if there is too much oil from nozzle 33 (acceptable under some circumstances).

Though gasification using hot, burned gases only appears to be preferable, the presence of air with the hot gas in duct 17 is not objectionable provided it does not cool the gas to the point of incomplete gasification of fuel (i.e., yellow flame) and provided that premature ignition does not occur.

When the burner unit is cold and needs to be started, feedback of combustion gases will not, of course, serve to gasify the oil being fed into the system. Consequently, some other method of preheating is necessary. In the present invention the start-up system is an integral part of the total system since it uses the same air supply, Venturi, and fuel supply. The oil passes through

ducts

30 and 32 through the nozzle 33 and is picked up by the air entering through the air inlet duct 20. It will be ignited by plug 35 and will burn in burning chamber 10. This start-up operation is essentially full operation of the burner, not just a pilot light, but uses ungasified fuel and so is less efficient.

Valves

27 and 31 are used to determine which oil supply system is used. At start-up, valve 31 is open and valve 27 closed, allowing the oil to flow to nozzle 33. Once the warm-up period is over, valve 27 is opened to permit oil to flow into interconnecting duct 17, and valve 31 is closed. If desired,

valves

27 and 31 may be combined at inlet source 25 and may also be operated automatically by a sequence timing control. If, after start-up, two-stage gasification is to be used, both valves be left open by an amount that will give desired combustion balance.

It has been found that a unit can be started in under half a minute to the point where the hot burned gases can vaporize the oil. One unit built has had a preheating time of approximately 16 seconds.

What is claimed is:

1. A liquid fuel burner for burning a gasifiable liquid fuel such as oil and having reduced thermal stresses, including a burner made of refractory material and having a fire chamber, said chamber having a fuel mixture inlet port, a burned gas removal port, and a hot burned gas feedback port,

a mixing chamber for mixing said fuel and air, said mixing chamber being connected to said inlet port,

a burned gas and fuel inlet to said mixing chamber,

means directly interconnecting said burned gas removal port to said burned gas and fuel inlet,

an air inlet to said mixing chamber for directing air under pressure into said mixing chamber, said air inlet including a Venturi for drawing burned gas and fuel into said chamber through said interconnecting means and said burned gas and fuel inlet, and

a gasifiable liquid fuel inlet connected to said interconnecting means proximate to said refractory material,

whereby said fuel will be gasified by hot burned gas as it passes through said interconnecting means and the heat of. vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses therein.

2. A liquid fuel burner as set forth in claim 1 in which there is insufficient air present in said interconnecting means to support combustion of said gasified fuel prior to its reaching said fuel mixture inlet port.

3. A liquid fuel burner as set forth in claim 1 including ignition means positioned proximate to, but before, said fuel mixture inlet port, for igniting said mixture of said fuel and air.

4. In a liquid fuel burner for burning a gasifiable liquid fuel such as oil which includes a burner made of refractory material and having a fire chamber therein, said fire chamber having a fuel mixture inlet port, a burned gas removal port and a hot burned gas feedback port, a source of air under pressure, a mixing chamber for mixing said fuel and air and being connected to said inlet port, that improvement for reducing thermal stresses in hot gas feedback including means directly interconnecting said hot burned gas feedback port with said mixingchamber,

means for introducing said gasifiable liquid fuel into said interconnecting means, so proximate to said refractory material that said fuel will be gasified by said hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses.

5. A liquid fuel burner improvement as set forth in claim 4 including a metering valve associated with said gasifiable liquid fuel inlet to control the rate of flow into said inlet.

6. In a liquid fuel burner for burning a gasifiable liquid fuel such as oil which includes a burner made of refractory material and having a fire chamber therein, said fire chamber having a fuel mixture inlet port, a burned gas removal port and a hot burned gas feedback port, means for connecting said hot burned gas feedback port to said fuel mixture inlet port, a source of air under pressure, a mixing chamber for mixing said fuel and air and being connected to said inlet port, and

means for introducing liquid fuel into said connecting means so that it may be gasified prior to combustion, that improvement for start-up of said burner including:

a liquid fuel nozzle disposed in said air source,

ducts interconnecting said nozzle with said liquid fuel introducing means, and

control valve means for controlling flow of said liquid fuel to said nozzle or to said connecting means,

whereby said burner may be started by use of said nozzle to inject liquid fuel into said air for burning in said fire chamber prior to use of gasified fuel.

7. A liquid fuel burner improvement as set forth in claim 6 including a sequence timing control for said control valve means.

8. A liquid fuel burner for burning a gasifiable liquid fuel such as oil and having reduced thermal stresses, including a burner made of refractory material and having a fire chamber, said chamber having a fuel mixture inlet port, a burned gas removal port, and a hot burned gas feedback port,

a burned gas and fuel inlet to said mixing chamber,

means interconnecting said burned gas removal port to said burned gas and fuel inlet,

said interconnecting means being a metallic pipe running between said burned gas feedback port and said burned gas and fuel inlet,

whereby said fuel will be gasified by hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses therein.

9. A liquid fuel burner as set forth in claim 8 in which said gasifiable liquid fuel inlet is sufficiently proximate to said refractory material relative to the length of said interconnecting pipe and the temperature of said burned gas so as to gasify said fuel as it passes through said interconnecting means sufficiently to substantially avoid appearance of yellow in the flame in said fire chamber.

10. A liquid fuel burner for burning a gasifiable liquid fuel such as oil and having reduced thermal stresses, including a burner made of refractory material and having a fire chamber, said chamber having a fuel mixture inlet port, a burned gas removal port, and a hot burned gas feedback port,

a burned gas and fuel inlet to said mixing chamber,

an air inlet to said mixing chamber for directing air under pressure into said mixing chamber, said air inlet including a Venturi for drawing burned gas and fuel into said chamber through said interconnecting means and said burned gas and fuel inlet,

and a starter unit for creating a tire in said fire chamber when starting said burner so as to produce hot burned gas for feedback and gasification, said starter unit including a nozzle for inserting said fuel in its liquid form into said air inlet,

11. A liquid fuel burner as set forth in claim 10 including fuel supply for said fuel inlet and for said nozzle, and control valve means for controlling flow of said fuel to said nozzle or said fuel inlet sequentially.

12. A liquid fuel burner for burning a gasifiable liquid fuel such as oil and having reduced thermal stresses, including a burner made of refractory material and having a fire chamber, said chamber having a fuel mixture inlet port, a burned gas removal port, and a hot burned gas feedback port,

a burned gas and fuel inlet to said mixing chamber,

and means for inserting additional liquid fuel into said mixing chamber at said air inlet to provide a second stage of liquid fuel gasification, said second stage utilizing heat energy remaining in said hot burned gas entering said mixing chamber,

13. In a liquid fuel burner for burning a gasifiable liquid fuel such as oil which includes a burner made of refractory material and having a fire chamber therein,

said fire chamber having a fuel mixture inlet port, a burned gas removal port and a hot burned gas feedback port, a source of air under pressure, a mixing chamber for mixing said fuel and air and being connected to said inlet port,

that improvement for reducing thermal stresses in hot gas feedback including means interconnecting said hot burned gas feedback port with said mixing chamber, means for introducing said gasifiable liquid fuel into said interconnecting means, so proximate to said refractory material that said fuel will be gasified by said hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses, and a starter unit to provide initial hot burned gas for feedback, said starter unit including means for inserting ungasified fuel into the air stream in said mixing chamber. 14. In a liquid fuel burner for burning a gasifiable liquid fuel which. includes a fire chamber and a mixin chamber for mixing air with said liquid fuel after gasi lcation and for feeding same to said fire chamber, that improvement including duct means interconnecting said fire chamber and mixing chamber to carry hot combustion gases from said fire chamber to said mixing chamber, said duct means interconnecting said burner and said mixing chamber by substantially the most direct path, said duct means having a fuel inlet area for receiving said liquid fuel prior to gasification, and said fuel inlet area being positioned sufficiently close to said firing chamber and sufficiently far from said mixing chamber as to permit substantial gasification of said fuel in said duct means before said fuel as carried by said hot combustion gases reaches said mixing chamber and as to permit the heat of vaporization of said oil to reduce the temperature of said duct means.

Claims

1. A liquid fuel burner for burning a gasifiable liquid fuel such as oil and having reduced thermal stresses, including a burner made of refractory material and having a fire chamber, said chamber having a fuel mixture inlet port, a burned gas removal port, and a hot burned gas feedback port, a mixing chamber for mixing said fuel and air, said mixing chamber being connected to said inlet port, a burned gas and fuel inlet to said mixing chamber, means directly interconnecting said burned gas removal port to said burned gas and fuel inlet, an air inlet to said mixing chamber for directing air under pressure into said mixing chamber, said air inlet including a Venturi for drawing burned gas and fuel into said chamber through said interconnecting means and said burned gas and fuel inlet, and a gasifiable liquid fuel inlet connected to said interconnecting means proximate to said refractory material, whereby said fuel will be gasified by hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses therein.

4. In a liquid fuel burner for burning a gasifiable liquid fuel such as oil which includes a burner made of refractory material and having a fire chamber therein, said fire chamber having a fuel mixture inlet port, a burned gas removal port and a hot burned gas feedback port, a source of air under pressure, a mixing chamber for mixing said fuel and air and being connected to said inlet port, that improvement for reducing thermal stresses in hot gas feedback including means directly interconnecting said hot burned gas feedback port with said mixing chamber, means for introducing said gasifiable liquid fuel into said interconnectiNg means, so proximate to said refractory material that said fuel will be gasified by said hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses.

6. In a liquid fuel burner for burning a gasifiable liquid fuel such as oil which includes a burner made of refractory material and having a fire chamber therein, said fire chamber having a fuel mixture inlet port, a burned gas removal port and a hot burned gas feedback port, means for connecting said hot burned gas feedback port to said fuel mixture inlet port, a source of air under pressure, a mixing chamber for mixing said fuel and air and being connected to said inlet port, and means for introducing liquid fuel into said connecting means so that it may be gasified prior to combustion, that improvement for start-up of said burner including: a liquid fuel nozzle disposed in said air source, ducts interconnecting said nozzle with said liquid fuel introducing means, and control valve means for controlling flow of said liquid fuel to said nozzle or to said connecting means, whereby said burner may be started by use of said nozzle to inject liquid fuel into said air for burning in said fire chamber prior to use of gasified fuel.

8. A liquid fuel burner for burning a gasifiable liquid fuel such as oil and having reduced thermal stresses, including a burner made of refractory material and having a fire chamber, said chamber having a fuel mixture inlet port, a burned gas removal port, and a hot burned gas feedback port, a mixing chamber for mixing said fuel and air, said mixing chamber being connected to said inlet port, a burned gas and fuel inlet to said mixing chamber, means interconnecting said burned gas removal port to said burned gas and fuel inlet, said interconnecting means being a metallic pipe running between said burned gas feedback port and said burned gas and fuel inlet, an air inlet to said mixing chamber for directing air under pressure into said mixing chamber, said air inlet including a Venturi for drawing burned gas and fuel into said chamber through said interconnecting means and said burned gas and fuel inlet, and a gasifiable liquid fuel inlet connected to said interconnecting means proximate to said refractory material, whereby said fuel will be gasified by hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses therein.

10. A liquid fuel burner for burning a gasifiable liquid fuel such as oil and having reduced thermal stresses, including a burner made of refractory material and having a fire chamber, said chamber having a fuel mixture inlet port, a burned gas removal port, and a hot burned gas feedback port, a mixing chamber for mixing said fuel and air, said mixing chamber being connected to said inlet port, a burned gas and fuel inlet to said mixing chamber, means interconnecting said burned gas removal port to said burned gas and fuel inlet, an air inlet to said mixing chamber for directing aiR under pressure into said mixing chamber, said air inlet including a Venturi for drawing burned gas and fuel into said chamber through said interconnecting means and said burned gas and fuel inlet, a gasifiable liquid fuel inlet connected to said interconnecting means proximate to said refractory material, and a starter unit for creating a fire in said fire chamber when starting said burner so as to produce hot burned gas for feedback and gasification, said starter unit including a nozzle for inserting said fuel in its liquid form into said air inlet, whereby said fuel will be gasified by hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses therein.

12. A liquid fuel burner for burning a gasifiable liquid fuel such as oil and having reduced thermal stresses, including a burner made of refractory material and having a fire chamber, said chamber having a fuel mixture inlet port, a burned gas removal port, and a hot burned gas feedback port, a mixing chamber for mixing said fuel and air, said mixing chamber being connected to said inlet port, a burned gas and fuel inlet to said mixing chamber, means interconnecting said burned gas removal port to said burned gas and fuel inlet, an air inlet to said mixing chamber for directing air under pressure into said mixing chamber, said air inlet including a Venturi for drawing burned gas and fuel into said chamber through said interconnecting means and said burned gas and fuel inlet, a gasifiable liquid fuel inlet connected to said interconnecting means proximate to said refractory material, and means for inserting additional liquid fuel into said mixing chamber at said air inlet to provide a second stage of liquid fuel gasification, said second stage utilizing heat energy remaining in said hot burned gas entering said mixing chamber, whereby said fuel will be gasified by hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses therein.

13. In a liquid fuel burner for burning a gasifiable liquid fuel such as oil which includes a burner made of refractory material and having a fire chamber therein, said fire chamber having a fuel mixture inlet port, a burned gas removal port and a hot burned gas feedback port, a source of air under pressure, a mixing chamber for mixing said fuel and air and being connected to said inlet port, that improvement for reducing thermal stresses in hot gas feedback including means interconnecting said hot burned gas feedback port with said mixing chamber, means for introducing said gasifiable liquid fuel into said interconnecting means, so proximate to said refractory material that said fuel will be gasified by said hot burned gas as it passes through said interconnecting means and the heat of vaporization of said fuel will cool said hot burned gases, thereby reducing the temperatures thereof and reducing thermal stresses, and a starter unit to provide initial hot burned gas for feedback, said starter unit including means for inserting ungasified fuel into the air stream in said mixing chamber.

14. In a liquid fuel burner for burning a gasifiable liquid fuel which includes a fire chamber and a mixing chamber for mixing air with said liquid fuel after gasification and for feeding same to said fire chamber, that improvement including duct means interconnecting said fire chamber and mixing chamber to carry hot combustion gases from said fire chamber to said mixing chamber, said duct means interconnectIng said burner and said mixing chamber by substantially the most direct path, said duct means having a fuel inlet area for receiving said liquid fuel prior to gasification, and said fuel inlet area being positioned sufficiently close to said firing chamber and sufficiently far from said mixing chamber as to permit substantial gasification of said fuel in said duct means before said fuel as carried by said hot combustion gases reaches said mixing chamber and as to permit the heat of vaporization of said oil to reduce the temperature of said duct means.