US2102152A - Premixing device for fluid fuel burners - Google Patents

Description

Dec. 14, 1937. J. w. HAYS 2,102,152

' PHEMIXING DEVICE FOR FLUID FUEL BURNERS Filed Jan. 25, 1933 4 Sheets-Sheet 1 Dec. 14, 1937. J. w. HAYS 2,102,152

PREMIXING DEVICE FOR FLUID FUEL BURNERS Filed Jan. 25, 1933 4 Sheets-Sheet 2 v DH B p :5 I Q INVENTOII I Dec. 14, 1937. w, A 2,102,152

PREMIXING DEVICE FOR FLUID FUEL BURNERS Filed Jan. 25,1933 4 SheefLs-Sheet 3 J K G a \-j I12 WIT 3 ES INVENTOR 7 Dec. 14, 1937. J. w. HAYS PREMIXING DEVICE FOR FLUID FUEL BURNERS 4 Sheets-Shet 4 Filed Jan 25, 1933 wn-N s s Patented Dec. 14, 1937 V STATES ears PREMEXING DEVICE FOR FLUID FUEL BURNER/S This invention relates to devices for mixing fuels, as for example liquid, gaseous or finely divided solid fuels with air, and is particularly suitable for use in heating systems of the flameless or surface combustion type.

Surface combustion heating systems operate on the principle that when fuelmixed with oxy gen is directed onto an incandescent surface,

combustion of the fuel takes place without flame on the surface.

While the principle of surface combustion has long been known, it has never been of any commercial importance, because of the fact that previously known systems have been unsafe and uncertain in operation. The greatest danger in the known systems has resided in flash backs and back'firing in the fuel delivering means. In these systems, the practice has been to mix the fuel and the oxygen at a place spaced from the combustion chamber and, pass the mixture through a restricted tube at high'velocity' and pressure into the combustion zone. The tube through which the mixture is injected sometimes becomes highly heated and ignites the fuel miX- ture causing explosions at the mixing stage or the source of fuel supply. Flashback screens and extremely high pressures on the fuels have been used in order to stop the explosions. Neither of these expedients has proven efficient in rendering the systems safe and practical.

An object of the present invention is the production of a construction which successfully eliminates the danger of explosions in heaters of all types wherein mixtures of fuel and air or other combustion supporting gases are fed to the combustion zone.

A further object of the invention is the production of a fuel and combustion supporting gas injecting device which may be readily used at either high or low fuel injecting pressures, with resulting economy of operation.

The objects of the invention have been achieved through a construction which allows the fuel and air and combustion supporting gas to be mixed only at or closely adjacent the point or zone of combustion. More specifically, this construction may suitable consist of one or more fuel injecting nozzles through which the fuel alone is injected into the combustion zone; Supplementing the fuel injecting means are inlets for combustion-supporting gas through which the gas is led and mixed with the fuel. The fuel nozzles and combustion supporting gas inlets may be of any suitable proportions for conducting the ele' ments of the combustible mixture into the combustion zone; preferably they are restricted in Size and are of a comparatively large number. Such an arrangement makes for good distribution of the fuel throughout the combustion zone, without the use of excessively high pressures on the fuel if a forced system of feed is used, or excessive draft, if an induced draft system of feed is used. Either a forced or induced draft may be used with my mixing device, but preferably afforced system is used with a surface combustion type of heater, because a more equally distributed combustion zone is formed. Induced draft has a tendency to cause the fuel mixture and resulting heat zones to follow the lines of least resistance, thereby causing highly heated channels or lanes rather than a uniformly heated zone.

With my mixing device, under no circum stances can explosions take place in the fuel inlets or conduits, because no combustion supporting gas is present in them, nor can explosions take place in the combustion-supporting gas inlet systems, because no fuel is present therein.

My fuel injecting and mixing device when used with a surface combustion heater is arranged externally of the combustion zone with the inlet nozzles in such a position that the fuel and the combustion supporting gas will be directed against the surface of the refractory mass in the combustion zone. The fuel and the gas will impinge upon the surface of the refractory mass and mushroom, spread and mingle, thereby producing a combustible mixture.

Any suitable means may be used for igniting the gas and fuel mixture, but the igniting means is preferably located at the point where the gas and the fuel mix, that is, immediately adjacent the surface of the refractory mass upon which the fuel impinges.

For a better understanding of my invention,

, reference may be had to the accompanying drawings, in which;

Figure 1 is a cross-sectional view of the mixing device without its housing.

Figure 2 is a plan view on the line, 2-2, Figure 1.

' Figure 3 is a plan view on the line, 3-3, Figure 1.

Figure 4 is a detail of the tube, N, Figure 1.

Figure 5 is the complete assembly of combustion tube with its accessories and the mixing device, all shown in cross-section.

Figure 6 is a plan view on the line, 6't, Figure 5.

Figure 7 is a plan view of the mixer and its accessories, together with the combustion tube and its accessories.

Figure 8 is an assembly elevation, without detail, showing mixer, combustion tube, etc., together with a. pressure blower.

In Figure 1, P is a gas-supply pipe through which gas is preferably furnished under a pressure in excess of the operating pressure and in excess of the air pressure. Should the static pressure of the gas be less than that of the air it is plainly to be seen that no gas could flow to the mixer. On the other hand, the air, owing to its much greater volume, (the proportions being about 10 of air to l of gas when natural gas is used) may have a pressure in ounces while the gas pressure may be in pounds. High gas pressures, when available, are to be desired because they contribute to the capacity of the combustion tube. C is a distribution chamber for the gas before it enters the nozzle tubes N. B is a bafile to insure distribution of the gas at common pressure to all of the tubes N, but it is not a basic part of my invention. The tubes N, are set gastight in the plate TP, and the latter is secured by the bolts L, to the flange of the detachable housing DH, of the chamber C. G is a gasket of any suitable material to make the chamber C gastight. V is a Venturi orifice plate with orifices O, of the well-known Venturi type. One of the offices of this plate is that of a retainer to hold the refractories, Re, in place when the apparatus is operated in a horizontal position. When the apparatus is operated in a vertical position with the draft and the mixer at the top, the plate may be dispensed with. While some mixture of the air and the gases is produced in the Venturi orifices the mixture is not completed until the streams of air and gas are mushroomed by impingement upon the exposed surfaces of the refractory materials. The nozzles of the tubes N, are aligned, as shown in the drawings, with the orifices O. The adjustable spacers S, are used to fix the nozzles N1 of the tubes N at the desired distances from the orifices 0. These spacers may be of any form to satisfy the requirements. In the drawings I show them as rods turned at each end to provide shoulders, one of the turned-down ends being threaded to screw into the plate V, and the other passing through TP to which it is secured by the nut Nt, as shown. W is a gasket of the washer type to prevent leakage of gas from C. ST is an equalizer, or straightener. Its use will be explained later in this specification. Should it be desired to change the distances of the nozzles NZ from the orifices O, the nuts Ni, may be removed and the spacer-washers SW, or some of them, shifted to the opposite side of TP, thereby bringing Nl into a closer or more distant position position with respect to O, as desired.

The above letters and numerals are used throughout the several drawings.

The complete assembly of mixer and combustion tube is shown in the drawings, Figure 5, in which CT is the combustion tube and Re the refractory material with which the tube is packed. Gr is a casting, in the form of a grid, which serves to retain the refractory material in the tube while providing sufficient space for the escape of the waste gases. Gr may be secured in place by any practicable means. J is a jacketing tube surrounding CT and spaced apart therefrom at any desired distance. Flat metal rings R are welded to J and CT so that the space Sp between the two tubes is completely enclosed, except for the inlet pipe In and the outlet pipe 015. The fluid to be heated, Fol, whether water, oil, vapor or whatnot, is caused to flow through Sp, preferably, as shown, in a direction contrary to the flow of the hot gases HG, through CT.

In the preferred form of construction, which is shown in the drawings, the tube J extends a short distance beyond one end of CT and forms a housing H, for the mixer, illustrated in Figures 1, 2, 3 and 4. Fl is a flange which is welded to J and at the extreme end thereof. Fl is provided with bolt-holes, corresponding to those in F and TP, and G1 is a gasket to render the mixer M air-tight. Air under the desired pressure, whether in ounces or in pounds, is admitted to M through the inlet A. ID is an igniting device, which may be a spark plug, as shown, or any other suitable contrivance.

My invention will be best understood if I next explain the method of its operation.

Air in sufiicient volume and at the desired pressure may be supplied by a suitable blower, a compressor or in any other manner. I prefer a pressure-volume blower, PB, which is indicated in Figure 8. Whatever means may be employed to provide a flow of air, it is highly desirable that the air be delivered in constant volume and at constant pressure.

The ordinary atmospheric gas burner receives its air supply through an opening cmmunicating direct with the outside air. The flow of air is produced by lowering the pressure in the furnace below that of the atmosphere, or by the inspirating effect of the gas flowing at a plus atmospheric pressure through an orifice or by both means combined. Burners of this type are sometimes referred to as open system burners because they are subjected to atmospheric pressure at the burner air inlet and to atmospheric pressure again at the stack outlet. My burner is subjected to plus atmospheric pressure at its inlet, applied to both the air and the gas and to plus atmospheric pressure again in the combustion zone which is packed with refractory materials amongst which the fuel is burned and a great increase in the gas volume takes place. My burner may, therefore, be said to be of the closed system type.

Gas is delivered through the pipe P at a pressure which should be constant and preferably in excess of that at which the air is supplied. The higher the gas pressure the better for the following reasons:

1. In order to secure the best possible distribution of the gas and the best possible air-gas mixture, I prefer to deliver the gas through a multiplicity of nozzles Nl. I have found that the best results are secured when the orifices or vents of these nozzles do not exceed g gnd or %l,ths of an inch in diameter. Such small openings under low pressure operation might become clogged with dust or other foreign material carried by the gases, but if the latter are under a considerable pressure head this will be less likely to happen.

2. The higher the pressure of the gases the greater the velocity of the air-gas mixture flowing through the orifices O, and, likewise, the greater the mushrooming effect when the mixture impinges upon the refractories Re. Such mushrooming, or spreading, promotes a thoroughness of air-gas mixture. The exposed refractories, i. e., those upon which the air and the gas impinge before ignition and upon and among which the mixing is completed, are main- I tained in a state of relative coolness by the air and gas currents.

3. Gas for industrial uses can usually be secured at high pressures without increase of cost. Pressure gas, therefore, represents an available amount of kinetic energy, costing nothing, and this should be utilized whenever possible. High pressure gas reduces the load on the air blower and thereby lowers the cost of operation;

I do not hold-myself as limited in any way to the number and spacing of tubes N, or orifices 0, shown in my drawings. The number required will depend upon the size of the combustion tube, the available gas pressure and the dimensions of the nozzle vents. confined to any other particular detail of any part of the mixing device shown and described herein. I prefer, as shown in Figure 4, to give the nozzle vents a Venturi effect, so far as practicable.

Air'is delivered to the chamber M through the inlet A. The air circulates in the space between the housing H and the nozzle tubesN, and from this space enters the N tube-bank from all sides. To avoid turbulence at, and an unequal distribution of air to, thenumerous nozzles, the straightener St may be used to straighten the air flow and break it up into a series of currents running parallel with the tubes N, which are severally aligned with the'orifices 0. S t serves to equalize the distribution of the air and air pressure and prevent short-circuiting to the nearest orifices. St is merely an orifice plate a with holes in alignment with the orifices 0. It is adjustable as to position by means of the spacers Si, which are washers of various thick-.

nesses, and the space desired may be arrived at by increasing. or decreasing the number of washers. I find, in practice, that very satisfactory results are secured by positioning the straightener approximately as shown in my drawings. The straightener may be considered as a desirable accessory, but by no means a basic part of my invention.

The air and gas are partially mixed in the orifices O. The gas, being under relatively high pressure, is projected at high velocity and in a diverging cone into each orifice, drawing air with it from the surrounding stream of air. The air, being under pressure, flows under its own velocity head into the gas stream. The velocity in the orifice is, therefore, a composite of the velocities of the air and the gas, and this velocity is accentuated by the Venturi effect of the orifice. Mixture is completed to a point of extreme thoroughness because of the multiplicity of the gasair stream, and because of the mushrooming effect when these streams impinge upon the broken surfaces of the refractory materials R. The gases are ignited in the vicinity of this point of impingement by means of an ordinary sparkplug, or other igniting device ID. It will be seen that a flash-back is impossible for the reason that the point of ignition lies in, or near to, the very locality where the gas and the air are brought together to form a combustible mixture. There is noreason for a screen or other antifiashback device, because there can be no flashback into an atmosphere consisting entirely of air or entirely of gas.

Immediately after ignition the mixture burns in the interstices of the refractory packing. at and beyond thepoint of ignition and in a short time the refractories become heated to a point of incandescence, whereupon the mixture burns Neither do I hold myself as without flame and a very high temperature is reached, the heat being produced, very largely if not entirely, in the radiant form. Combustion ispractically instantaneous. This being so, the capacity limits of such a combustion tube are fixed by the free area of the interstitial spaces of the refractorypacking and by the pressures used to force the gases into and through the tube.

The combustion'zone, or hot-spot, is indicated in Figure by the dotted lines, CZ. The transfer of heat from the incandescent refractories Re in CZ, to and through the tube CT, to the fluid Fl, is almost instantaneous, the heat being in the radiant form. CT may be packed with the refractories Re throughout its entire length, if considered advisable. The hot gases leaving CZ are baffled by these refractories, and brought into contact repeatedly with CT, giving up their heat as they flow. Furthermore, these gases heat the refractories themselves and the heat is transferred by radiation and conduction from one refractory element of the packing to another and finally to the tube CT, through which it is conveyed to the fluid in the space Sp. In this way a very rapid and very complete transfer of heat is effected.

The positioning of the combustion zone CZ is a very important consideration in all cases where such combustion tubes are employed. The advantages of counter-flow in effecting heat exchanges are well known. The fluid to be heated should, in all cases where possible, flow in a direction contrary to that of the hot gases. This is best accomplished, as in the present instance,

by fixing the location of the combustion zone at the extreme end of the combustion tube CT, and approximately adjacent to the place Where the heated fluid reaches its outlet. In this ,way the fiuid, when in its hottest state, is exposed to the highest temperature and when in its coolest state, on entering the heater, is exposed to the lowest temperature. Thus the highest heater capacities and efficiencies are obtained.

It can easily be seenthat if the combustion zone should be established at the opposite end of the combustion tube, or midway of the tube, there would be a marked loss of heat energy with the waste combustion gases, and this would be refiected adversely upon the heating device, both as to its efiiciency and its capacity.

Previous inventors, as already explained, have ignited the air-gas mixture at the discharge outlet of the combustion tube, relying upon a balancing of the velocities of mixture flow and of flame propagation to fix the location of the combustion zone, but I have found that the combustion zone is likely to remain fixed, at, or close to, the spot where it is first established. The tendency of the hot-spot to remain stationary is influenced to some extent by the state of therefractory packing, as to coarseness or fineness, by the length of the tube and by the temperatures attained in the hot-spot as well as by the velocities. There is an enormous increase in gas volume in the combustion zone, due to the high temperature, and this acts as a brake upon the flow of the gases through the tube.

My invention enables me, among other things. to fix the combustion zone at the forward end of the combustion tube, or at any other point where I may prefer to ignite the mixture, and to maintain said zone substantially in its original location and position.

I accomplish this by igniting the mixture at the forward end of the tube by a spark plug, as shown in my drawings, or otherwise.

While I have shown tubes in my drawings and referred to them as such in this specification, I do not hold myself as limited to tubes. Any form of chamber or receptacle may be used to hold the refractory materials or the fluid which is to be heated, whether that form be tubular or otherwise, as it is plain that I can vary the size, shape and materials of any and all of the structures shown and claimed without departing from the spirit of my invention.

I have found that under some circumstances, as already stated the orifice plate V may be omitted, but where the air pressures are low it is of a distinct advantage.

I have found that there is a distinct advantage in using a number of pipes N, as this gives a better distribution of the combustible mixture across the face of the refractories Re, in the tube CT. The higher the gas pressures the smaller the number of tubes N which are required.

I claim:

1. In a device for delivering a combustible mixture of a fuel and combustion-supporting gas to a furnace chamber containing an apertured refractory mass, the combination of a plurality of fuel injecting nozzles directed into the furnace adjacent said refractory mass, means for supplying fuel under pressure to said nozzles, means for directing a combustion-supporting gas into the furnace chamber adjacent the nozzles, whereby the fuel and the combustion-supporting gas impinge upon the refractory mass and are thoroughly mixed and means for igniting the mixture initially at the point of mixture.

2. In a device for delivering a combustible mixture of fuel and combustion-supporting gas to a furnace chamber containing an apertured refractory mass, the combination of a plurality of fuel injecting nozzles directed into the furnace chamber adjacent said refractory mass, means for supplying fuel under pressure to said nozzles to impinge the fuel on the surface of said refractory mass, means for directing a combustion-supporting gas around said nozzles and impinging it on the refractory mass, whereby the fuel and said gas will be mixed, and means adjacent the refractory mass for igniting the mixture of fuel and gas.

3. In a device for delivering a combustible mixture of fuel and combustion supporting gas to a furnace chamber containing an apertured refractory mass, the combination of a fuel distributing chamber, means for supplying fuel to said chamber under pressure, a plurality of fuel injecting pipes connected to said distributing chamber, nozzles on said pipes directed into said furnace chamber adjacent said refractory mass for impinging the fuel against said refractory mass, means for directing a combustion-supporting gas around said nozzles and impinging it on said refractory mass, whereby said fuel and said gas will be thoroughly mixed and means for igniting the mixture at the point of mixture.

4. In a device for delivering a combustible mixture of a fuel and a combustion-supporting gas to a furnace chamber containing an apertured refractory mass, the combination of a plurality of fuel injecting nozzles directed into said furnace adjacent said refractory mass, a plate between said nozzles and said refractory mass, apertures in said plate in alignment with said nozzles, means for delivering a combustion supporting gas around said nozzles and through said apertures, means for supplying fuel under pressure to said nozzles, whereby the fuel will be directed through said apertures, at least partially mixed with said gas, impinged upon said refractory mass and mixed, and means for igniting the mixture substantially at the refractory mass.

5. A flameless combustion heating device comprising a furnace chamber, an apertured refractory mass therein providing a surface for flameless combustion of fuel, means for directing a plurality of streams of fuel against said refractory mass, from a point exterior of the furnace chamber, means for directing a combustion-supporting gas against said refractory mass, whereby said fuel and gas are thoroughly mixed, and means for initially igniting the mixture at the point of mixture.

JOSEPH W. HAYS.

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