US2971578A - Burner apparatus - Google Patents

Description

Feb. 14, 1961 G. R. DAVIS BURNER APPARATUS 4 Sheets-Sheet 1 Filed Oct. l0. 1956 A wu m V.

R W. A

BY ze Feb. 14, 1961 G. R. nAvxs BURNER APPARATUS 4 Sheets-Sheet 2 Filed OOC. l0, 1956 INVENTOR.

GLEN R DAVIS 4 Sheecs-Sheefl 5 Filed 0013. lO, 1956 INVENTOR. GLEN R. DAVIS ATTORNEY FIG.

Feb. 14, 1961 G. R. DAvls BURNER APPARATUS 4 Sheets-Sheet 4 Filed OCT'. 10. 1956 FIG. 7

INVENTOR. GLEN R. DAVIS WM Me?,

A 7' TOR/VE Y United States PatentO BURNER APPARATUS Glen R. Davis, Tulsa, Okla., assignor to Pan American Petroleum Corporation, a corporation of Delaware Filed Get. 10, 1956, Ser. No. 615,105

Claims. (Cl. 15S- 109) My invention relates to an improved burner design. More particularly, it pertains to a novel burner design especially adapted for use at relatively high temperatures over extended periods of time. Burners of the design herein contemplated nd application in combustion charnbers of the type employed for the partial oxidation of light hydrocarbons, such as, for example, natural gas or methane .to produce synthesis gas utilized in the production of synthetic liquid fuels, ammonia, methanol and the like.

In the preparation of hydrocarbon synthesis gas by direct combustion of methane or natural gas at elevated temperatures and pressures, rather severe requirements are placed on the thermal characteristics of the combustion chamber employed for the production of such gas and particularly on the burner utilized in the operation. Thus, at the pressures employed, i.e., 250 to 350 p.s.i., it is usual to have temperature zones of hom 2200 F. to 3500 F. throughout the chamber.

The conversion of methane to synthesis gas by combustion with oxygen may take place in successive steps. ln the rst step, methane may combine with oxygen in a ratio of 1:1 in accordance with the following equation:

This reaction is highly exothermic and raises the temperature of the products to about 3500 'F. If additional methane is available, i.e., if the feed-ratiov of oxygen to methane is less than 1:1, the excess methane may then be reformed by the steam and carbon-dioxide in the product gases in accordance With either or both of the following equations:

Both of'these reactions are endothermic and will cause the temperature of the products to drop to an equilibrium value.

The relative proportions of CO, CO2, H2 and H2O ,in the products are set by the water gas shift reaction. Thus, as the iinal temperature of the methane conversion reactions reaches equilibrium the CO and CO2 will adjust `to equilibrium proportions in accordance with the following equation:

Prior to my invention, a number of different types of burners were employed in combustion chambers used to prepare synthesis gas. However, much diiculty was experienced in attempts to operate these combustion chambers for long periods. rIlle problems encountered in previous attempts to produce synthesis gas yand to obtain adequate burner performance were concerned chieily with proper mixing of the preheated gas, suitable oxygen to methane ratio in the feed gas, procurement of adequate cooling of the critical burning areas, and prevention of burner failure due to the fusion of the gas and/or cooling channels.

2,971,578 Patented Feb. 14, 1961 pice One .particular type of burner previously proposed for use in combustion chambers of the aforesaid type, consisted of two concentric metal tubes, the end of the inner tube being recessed slightly within the outer tube. The latter curved inwardly at the end to form with the inner tube what was considered to be the correct annulus area. In the opera-tion of this burner, the methane was heated to a temperature of from about 400 to 600 F. higher than the oxygen and hence, when the preheated methane was supplied through the outer tube, the latter expanded lengthwise causing .an increase in the terminal ow area between the inner land outer tubes. This vreduced the velocity of the methane lat the point of mixing and resulted in internal burning and flame erosion, causing the `burner to fail. When methane was supplied through the inner tube the latter expanded, closing off the ow of oxygen through the outer tube. Another disadvantage of this design resided in the fact that the llame produced during operation was in contact with the burner tip or face, thus placing considerable stress on the burner structure. The yaccumulation of heat under such circumstances often contributed to fusion of the burner tip and subsequent failure thereof.

A -further equally serious difficulty with the abovementioned burner design or with any design in which the central gas channel comprises so much more area than the total area of the surrounding air or oxygen channels is that the gas (methane) ilowing through the central channel of relatively large diameter tends to penetrate the space above the burner surface a substantial distance without achieving proper mixing with the -air or oxygen, thus resulting in temperatures and other conditions favoring 'cracking of the methane to carbon. This follows from the fact it is Well known that the distance a jet of gas penetrates a still mass of another gas is proportional to the diameter of the jet. Otherwise expressed, the greater the diameter of the stream of gas being jetted into the still body of gas, the greater will be the penetration of the former into the latter.

Other burner designs employed a series of small channels ,through which methane was introduced and thereafter combined with oxygen flowing through a recessed centrally spaced channel. The preheated methane frequently carried small particles ofY carbon which had a tendency to plug one or more of the gas channels, causing the burner ame to be deected and resulting in the erosion of the burner face. Also welding on the face cooling water channels.

of burners previously employed was subject to cracking and carburization while in service. Burners of this type also tend to hold the flame in contact with Iat least a portion of the burner tface, causing a shorter burner life for.

the reasons outlined above.

In connection with my investigations which resulted in the burner design of my invention, designs developed by others were also considered. Such designs, however, involved the use of internal baffles or packing glands in the Structures of this sort are objectionable because of the likelihood of leaks into other parts of the system. Also, in the aforesaid burner designs some of the hot metal surfaces were left uncooled allowing high thermal gradients to develop which reduce the life expectancy of the equipment.

An object of my invention is to provide a compact design for use in a multiple burner unit having a novel and effective means for supplying gases to each of the burners and efficient means for securing adequate cooling of metal surfaces subjected to high temperatures. It is another object of my invention to provide a burner design wherein the occurrence of gas leaks or mechanical failures therein can be readily detected. It is a further object of my invention to provide a burner design having minimum stresses therein due to the high temperatures encountered during operation. It is still another object of my invention to provide means by which the individual gas streams supplied to the burner are separated by a conned stream of. liquid coolant.

Y Figure 1 is an isometric view partly in section illustrat-v ing one embodiment or" my burner design.

.Figure 2 is a vertical view primarily in section of another embodiment of my burner design.

Figure 3 is a plan view partly in `section showing, in a schematic fashion, an arrangement of several burner units of my design within a hollow combustion chamber of the typenow commonly used.

Figure 4 is a vertical sectional view of a burner body taken in a. plane perpendicular to the horizontal axis of the embodimentshown in Figure 2.

Figure 5 is a, fragmentary sectional view of can 46, shown in Figure 4, illustrating in further detail the interior structure of said can and the manner in which cooling water and oxygen flow therethrough.

Figure 6 represents a horizontal sectional view of Figure 5 taken along line A-A.

Figure 7 is another horizontal sectional view of Figure 5 taken along line B-B.

Referring now in detail to Figure l of the drawings, the burner assembly comprises an outerrconduit 2 carrying burner bodies 4, hereinafter referred to for convenience as caus, having a suitable refractory material 6 placted therebetween to, protect the remainder of the apparatus from intense heat. A second conduit 8 is located within and concentrically spaced with respect to conduit 2 thereby defining a` passageway 10 leading to annular slot 12 which is adjacent the exterior of can 4 and surrounds the remainder of theburner body. Centrally located in can 4 is a channel 13 terminating as a channel 14 on the top surface of said can and communicating with conduit 16 Within and concentrically spaced with respect to conduit 8*. symmetrically spaced about channel 14 are channelsv 18 which communicate, via annular slot 19, with passageway 20 formed by placing conduit 22 within and concentrically spaced with respect toconduit 8. In place of channels 18 a continuous slot about channel 14 may be employed; or such slot may be in the fonn of segments symmetrically spaced about channel 14 as shown in copending application Serial No. 522,022, tiled July 14, 1,955, by J. D. Hagy et al. In operation of the lburner gas such as natural gas or methane may ow through both ends of central conduit 16 Aand into channel 13. Oxygen or oxygen-enriched air ilows into the unit through both ends thereof via pipes` 24 and 26, through passageway 20,

slot 19 and channels 18. Water employed for cooling the unit flows through conduit 2 by means of pipe 28 into passageway 10 and annular slot 12. The water orV other suitable coolantthen followsa path through the can as indicated by the arrows this being over a barrier 30 down through a second annular slot 32 surrounding channel 13. The water then llows out of the burner via passageway 34 between conduits 16 and 22. In cooling each of the cans only a portion of the entire water stream flowing through passageway 10 rises up throughY annular slot 12 and liows on into passageway 34 via slot 32.

next can where an equal volume of water passes therethrough in a similar fashion. Combined cooling water from all cans eventually Hows into passageway 34 and out the apparatus through pipe 36. While the apparatus shown in Figure l employs a slot 19 connecting channels 18 with oxygen passageway 20, this also may be accomplished by the use of individual channels connecting channels 18 with passageway 20. Accordingly, in the present description and claims, where reference is made to an annular slotY connecting said jets to said passageway, the expressions annular slot or slot is intended to include individual channels.

In the embodiment shown in Figure 2, methane or natural gas, for example, isintroduced into the apparatus The remainder of the cooling water` continuingin passageway 10 flows tov theV through flanged inlet port 38 into conduit 40 lined with an innersleeve or shroud 42 supported by spacers 44 thereby creating a static vapor spaced adjacent the Walls of conduit 40. The presence of such space in the conduit serves to protect the surface thereof from high gas temperatures which create considerable stress in the metal tending to shorten the life of the equipment. Gas in conduit 40 is forced into cans 46 throughvchannel 48 thereof lined with shroud 49. Shroud 42 is closed 01T at the end opposite that into which gas is introduced, by means of plate 50. This plate may form a tight seal with shroud 42 and may be anchored to the walls of conduit 40. Oxygen enters chamber 52 through hanged port 54 and is forced through passageway 56 into annular slot 58 of cans 46. Passageway 56 is likewise lined with metal shrouds 60 and 61 supported by spacers `62. Oxygen owing through slot 58 mixes with gas from channel 48 at a point above the face of cans 46. Water or other suitable coolant for operation of the zburner is introduced through flanged port 64 into chamber 66 which is in communication with passageway 68. Water passes into annular slot 70 of cans 46. In this particular embodiment water is supplied to the system under conditions such thatV only a portion of the total volume of water originally in passageway 68 iiows up through slot 70 of each can, passes over a barrier 30 (see Figure l) and down a second annular slot 74. After the last canin the series has been cooled the water remaining in passageway 68 flows through' openings 76 into chamber 78 formed by closure 80 which is welded to the ends of conduits 84 and 86 and by closure 81 welded to the ends of conduits-40, 83 and 84. By this structure it is possible to flow coolant in contact with closure thereby serving to protect closure 89 from the intense heat of the gas generator combustion zone. The side or" closure 31 facingthe hot gas in conduit 40 is protected by means of vapor space 90 between cover plate 50 and ,closure 3l. The Water in chamber 78, then passes into a hollowU annular housing 88 having openings 92. Housing 88 rests on closure 81 whichv contains channels 94 through which water flows into passageway 96. As water flows through passageway 96 the water employed to cool the individual cans is combined with the water in passageway 96 by low through annular slot 74. TheseV combined quantities of water are then taken through passageway'95 linto chamber 93 and out the system through pipe 100.

Figure 3 is a schematic plan view of a combustion chamber 102 lined with a suitable refractory material 104. Burner units comprising cans 46 and outer conduits 86 are radially spaced about the combustion chamber and anchored thereto at openings 106.

Figure 4 is a vertical sectional View of a burner body taken in a plane perpendicular to the horizontal axis of the embodiment shown in Figure 2 showing in particular the structure and relationship of shroud 49 to channel 4S. Shroud 42 in conduit 40 is rendered secure and rigid by means of anchor welds 43. Annuler slot 58 and channel 48 terminate into channels 59 and channel 51, respectively.

In Figure 5 the manner in which. coolant contacts the outside of the burner face and, hows between slot 58 and channel 48 is illustrated by the arrows. In this embodiment annular slots 70 A'and 74 Vare connected by ducts 53 shown by dotted ilines passing back of channels 59.

Figure 6 is a horizontal sectional view of Figure 5 taken along line A A in which the structural relationship of ducts 53 connecting slots 70 and 74 lto channels 59 is more clearly shown. The arrows indicate the flow ofv coolant from slot 70 to slot 74. Channels 59 are drilled through metal blocks 63 which are an integral part of can 46. Blocks 63 also serve as a barrier between ducts 53, therebyv maintaining separate coolant flow between jets 59.

YFigure 7 is a horizontal sectional view of Figure v5 as dotted lines) passing therethrough connecting slots 70 and 74 at the upper ends thereof.

In the burner designs shown, the jets through which oxygen is passed may vary in number from about 4 to about 12 and from about 1/16" to about 1A" I.D. While the arrangement of these channels about the single centrally located gas channel need not follow any particular pattern, such channels should be evenly or symmetrically spaced about the central channel so that the resulting ame will not be deflected from its vertical axis. The extended axes of the channels should intersect at a common point on the extended vertical axis of the central gas channel and define an angle therewith which may vary from about 5 to about 30 in order to secure adequate mixing of oxygen with gas. The distance between the oxygen channels and the gas channel should be such that impingement of the two streams is approximately 0.5" to about 6" and preferably from about 2" to about 4" from the face of the burner.

' In a burner body design of the type discussed herein rand which is more Vfully described and claimed in the copending application S.N. 522,022 of James D. Hagy and Harry A. Waughtal, filed July 14, 1955, it is possible to provide gas and oxygen channels vof various diameters giving a rather wide range of area ratios. With gas channels having a large diameter and employing oxygen channels in combination therewith of sufficient diameter, difficulty in proper mixing of the gases leading to cracking, carbon formation, etc. is encountered. With gas channels smaller than about .3" a definite possibility of plugging exists. This may be caused from corrosion and Vsubsequent scaling of the channel walls or from the accumulation of entrained particles in the gas stream due to cracking resulting from preheating the gas. The presence of such solid materials (carbon, etc.) in the gas channels causes deflection of the ame which inevitably results in severe erosion ultimately causing the burner to fail.

For satisfactory operation the diameter of the oxygen channels and the diameter of the gas channel should be such that the ratio of the gas channel area to the total area of the oxygen channels lies within a range of from about 1:1 to about 3:1 and preferably within a range f from about 1.75:1 to about 2.5:1. Typically, the oxygen channel diameter (LD.) may range from a minimum of 0.06" to about 0.3 while the gas or methane channel may vary from about 0.3 to about l". Satisfactory combinations of gas and oxygen channel diam eters using eight oxygen channels are: oxygen channel diameters of 0.14" and 0.2, and methane channel diameters of 0.62 and 0.75", respectively.

The lower limit of the oxygen channel diameters, i.e., 0.06 is likewise considered important because of possible corrosion problems which might occur during operation of the burner. The tendency of the gas channel to scale or erode and cause subsequent plugging is not generally considered to be as great as that of the oxygen channels. Owing to the high temperatures to which oxygenmay be preheated, i.e., to about 600 F., foreign materials, such as gasket material, valve lubricants, etc., readily react under said conditions and tend to cause a plugging ofthe oxygen channels if they were smaller than about 0.06. In fact, for that reason, I ordinarily prefer to employ oxygen channel diameters ranging from about 0.15 to about 0.20".

Gas velocities employed in operation of burners of the type contemplated herein should be such that the velocity of the gas through the burner ports is about 200 feet per second although gas velocities in general may vary from about 50 to about 300 or 400 feet per second. Velocities in the lower range are determined by the degree of mixing that can be obtained while the higher velocities are limited by the pressure drop produced with gas or oxygen ports of a given diameter. At temperatures of the order of about 2400 to about 2600 F. pressures of about 300 or 400 p.s.i. and with a feed gas having an oxygen to methane ratio of about 0.6, a burner having the aforesaid dimensions operated in a highlysatisfactory manner over an extended period of time, producing only insignificant amounts of carbon.

In addition to the foregoing burner design the composition of the combustion chamber feed gas is an important factor in the production of a gas suitable for use in synthesis of the type previously mentioned. With combustion chamber feed gases having an oxygen to methane ratio below 0.58, the increase in carbon production is quite rapid. Accordingly, I prefer to employ feeds having an oxygen to methane ratio of about 0.6 to about 0.7 for maximum output of hydrocarbon synthesis gas with a minimum of carbon formation. These ratios, of course, Will vary with the use of higher molecular weight hydrocarbons.

It is likewise desirable and important to preheat both oxygen and methane. The former may be heated up to temperatures of 600 F., although it has been found satisfactory to utilize high pressure (300 to 400 p.s.i.) oxygen at compressor discharge temperatures which are in the neighborhood of 250 to about 300 F. Preheating ofthe hydrocarbon (methane) may be carried out up to temperatures of from about l000 to about 1200 F. Pilot plant tests of the maximum methane preheat temperatures have indicated that temperatures as high as 1200 F. may be employed without adverse effects from colting. Preheating of the oxygen and/ or methane serves very materially to increase the methane conversion.

One of the outstanding features of the burner design of my invention resides in the fact that at all times prior to actual mixing of the reactant gases the individual gas streams are kept apart from one another by means of a separate stream of liquid coolant. While this design is advantageous in protecting metal surfaces from deteriora'- tion due to exposure to high temperatures such design is likewise highly useful in determining the existence of gas leaks throughout any portion of the equipment. Thus, if a mechanical failure or leak develops inside a can, escaping gas or oxygen will pass into one of the separate adjacent Water channels where it may be safely conducted away from the combustion chamber. The occurrence of such failure can be readily detected by the presence of gas bubbles in the outlet water stream.

The materials employed in constructing a burner of the design contemplated herein may be selected from a relatively wide range of metals such as for example low carbonsteel, stainless steels, Inconel, nickel, and the like. The conduits and cans employed in the burner assembly are preferably fabricated from relatively thin metal sheet material having thicknesses ranging typically from about Ve to about 'e716 of an inch. The shrouds or sleeves placed inside the conduits carrying gas and oxygen are likewise quite thin, being generally in the neighborhood of about 1;(42 of an inch in thickness. In one commercial design of the burner of my invention the following dimensions were employed in the conduit assembly. The outer conduit carrying cooling water into the system was 7.5" O.D. The next conduit carrying oxygen was 6.6 O.D. The nert largest conduit was 4.5 O.D., while the central tube carrying gas was 3.5 O.D. The shroud protecting the inner walls of the central gas conduit was 3.0" O.D., while the large and small shrouds in the oxygen channel were 5.9" O.D. and 4.83" O.D.

Although the burner design of my invention has been described herein with particular reference for use in combination with a special burner body or can design, it will be appreciated that my design for distributing combustible gases and coolant to a burner i-s clearly applicable for use in combination with other burner body designs such as,` for example, that described in U.S.

, 7 s 2,725,933, granted to L. P. Gaucher, or U.S. 2,732,257, granted to H. S. Cress.

I claim k p l. In a burner apparatus the combination comprising a burner body having means for receiving two separate and confined gas streams and means for receiving a separate and confined coolant stream, individual means for maintaining each of said streams separate until ejected from said burner body and means within said body for effecting cooling thereof during operation, apparatus for separately supplying coolant and said gas streams to said burner body and for withdrawing coolant therefrom com- Y prising a first conduit having said burner body mounted thereon, a second conduit within and concentn'cally spaced with respect to said first conduit defining a first passageway between said conduits, said passageway being connected with said cooling means, a third conduit within and concentrically spaced with respect to said second conduit defining a secon-d passageway between said second and third conduits, said second passageway being connected with one of said individual means, and a fourth conduit connected with the other of said individual means, said fourth conduit being within and concentrically spaced with respect to said third conduit defining a third passageway therebetween which is connected with said cooling means.

2. In a burner apparatus the combination comprising a rst, a second, a third and a fourth conduit, concentrically arranged within one another, all closed at the same end and defining a first, a second and a third passageway between said first and vsecond conduits; said second and third conduits; and said third and fourth conduits, respectively; said first and second conduits having a common closure and said second, third and fourth conduits having a -separate common closure, the latter lying in front of said first-mentioned closure defining a hollow space therebetween and extending the diameter of said second conduit, a plurality of spaced burner bodies mounted on said rst conduit, a channel through each of said bodies, one end of said channel. terminating at the face of said bodies and the other being in communication with the interior of said fourth conduit, a first cooling means surrounding with said third passageway, a chamber surrounding said first `cooling means and communicating with said second passageway, a plurality of channels extending from said chamber through each of said bodies to said face and symmetrically spaced about said channel, the extended axes of said plurality of channels intersecting at a common point on the extended axis of said channel, a second cooling means adjacent the face of each of said bodies and surrounding said chamber, said second cooling means communicating with said first passageway, a duct within each of said bodies between each of said channels connecting said first and said second cooling means, said duct being adjacent said face of each of said bodies, an opening in said separate common closure connecting said hollow space with said third passageway, an opening in said second conduit connecting said hollow space with said first passageway, and inlet means on said fourth conduit located at the end thereof opposite said closures. 3. The burner apparatus of claim 2 in which the internal diameter of jets (2) ranges from about 0.14 to about 0.2 and the internal `diameter of jet (l) ranges from about 0.62" to about 0.75.

4. In a burner apparatus the combination comprising a burner body having a face, said body being mounted on a first conduit, a second conduit within and concentrically spaced with respect to said first conduit defining a first passageway between said first and second conduits, a third conduit within and concentrically spaced with respect t said second conduit defining a second passageway between said second and third conduits, a fourth conduit said channel and communicating within and concentrically spaced with respect to said third conduit defining a third passageway between said third and fourth conduits, a channel through said body, one end of said channel terminating at said face of said body and the other end opening into said. fourth conduit, a first cooling means surrounding said channel and communicating with said third passageway, a chamber surrounding said iirst cooling means and communicating with said second passageway, a plurality of channels extending fromsaid chamber through said body to said face and spaced about said channel, the extendedA axes of said plurality of channels intersecting at a common point on the extended axis of said channel, a second cooling means adjacent said face and surrounding said chamber., said second cooling means communicating with said first pa-ssageway, and a plurality of ducts within said body and adjacent said face connecting said first and second cooling means. k

5. The burner apparatus of claim 4 in which said second conduit is lined with a thin metal sleeve spaced equidistant from the walls of said conduit.

6. The burner apparatus of claim 4 in which the internal diameter of jets (2) ranges from about 0.06" to about 0.25 and the diameter of jet (l) ranges from about 0.3 to about 1".

7. In a burner apparatus the combination comprising a burner body having a face, said body being mounted on a first conduit, a second conduit within and concentrically spaced with respect to said first conduit defining a first passageway between said first and second conduits, a third conduit within and concentrically spaced with respect to said second conduit defining a second passageway between said second and third conduits, a fourth conduit within and concentrically spaced with respect to said third conduit defining a third passageway between said third and fourth conduits, a channel through said body, one end of said channel terminating at said fa'ce of said. body and the other end opening intosaid fourth conduit, a first cooling means surrounding said channel and communicating with said third passageway, a chamber surrounding said first cooling means and communieating with said second passageway, a plurality of channels extending from said chamber through said body to said face and spaced symmetrically aboutv said channel, the extended axes of said plurality of channels intersecting at a common point on the extended axis of said ,channel, a second cooling means adjacent said face and surrounding said chamber, said second coolingmeans communicating with said rst passageway, and a Vplurality of ducts within said body and adjacent said face connecting said rst and second cooling means.

8. The burner apparatus of claim 7 in which said `fourth conduit is lined with a thin metal sleeve spaced equidistant from the walls of said conduit.

9. The burner apparatus of claim 7 in which the internal diameter of jets (2) ranges from about 0.14" to about 0.2 and the internal diameter of jet (l) ranges from about 0.62 to about 0.75.

l0. In a burner apparatus the combination comprising a plurality of spaced burner bodies each having an individual face, said bodies being mounted on a rst conduit, a' second conduit within and. concentrically spaced with respect to said first conduit defining a first passageway between said first and second conduits, a third conduit within and concentrically spaced with respect tosai'd second conduit defining a second passageway betweenv said second and third conduits, a fourth conduit within and concentrically spaced with respect to said third. conduit defining a third passageway between said third and fourth conduits, a channel through each of said bodies, one end of said channel terminating at said face of each of said bodies and the other end thereof opening into said fourth conduit, a rst cooling means surroundingsaid channel and communicating with said third passageway, a chamber surrounding said rst cooling means and communicating with said second passageway, a plurality of channels extending from said chamber through each of said bodies to said face and spaced about said channel, o the extended axes of said plurality of channels intersectv ing at a common point on the extended axis of said channel, a second cooling means adjacent said face and surrounding said chamber, said second cooling means communicating with said first passageway, ducts within each of said bodies and adjacent said face connecting said rst and second cooling means.

and a plurality of 10 References Cited in the le of this patent UNITED STATES PATENTS Snee Mar. 23, isaacs et al. May 10, Rava June 9, Wolf Sept. 24, Gee Feb. l5,

FOREGN PATENTS France May 8, Germany Sept. 9,

UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No', 2,971!578 February 14, 1961 Glen R. Davis 1t is hereby certified/that error appears in Jmbe above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4 line 3v for "spaced" read space mg Column ,V line 64, for "nert" read next me; Column 7, line 8, after usaid" insert gas Signed and sealed this 8th day of August 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

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