US 3516771 A
Description (OCR text may contain errors)
Jun 23, 1910 J. F. RENDWA 3,516,17
BURNER FOR SPECTROSCOPIC USE Filed Au 1, 1968 52 1e G02 52 4?; e4 70 a0 3d 44 62 INVENTOR 50 as v 24 20 14 68 P W-m ATTORNEYS United States Patent US. Cl. 431-126 11 Claims ABSTRACT OF THE DISCLOSURE the flow stream upstream of the restriction. The nature of the restriction is such that the gas velocity through it exceeds the burning velocity of the combustion gas. The volume of the flow passage between the restricted portion and the outlet slot is maintained small to reduce the effects of flashback.
This invention relates to a burner useful with spectroscopic instruments and, more particularly, to a burner useful in atomic absorption spectroscopy.
BACKGROUND OF THE INVENTION Many different types of analyses utilize a flame to impart energy to a sample to the point where the sample either emits or absorbs characteristic radiation. Alternatively, flames have been used as light sources to provide characteristic monochromatic radiation. Regardless of the particular usage, most of these systems have certain features in common. These common features include the nebulization of a sample prior to introduction into the flame.
Whether the sample is sprayed directly into the flame or is mixed with a combustible gas prior to the flame, one of the problems encountered has-been the problem of thorough atomization or nebulization of the sample- Even with the best nebulizing techniques, the nebulized particles often either tend to recombine or are not thoroughly nebulized to begin with. In either event, the resulting larger particles tend to produce discontinuous reaction bursts. These discontinuous reaction :bursts distort the analysis.
There is also the ever present problem of the so-called flashback which refers to an explosion of the combustible gas within the confines of the burner itself. As a minimum, this explosion extinguishes the flame. In the extreme, it can damage the burner and even nearby laboratory workers. The combustible gas often is rn'nred with the combustion gas at the very last minute to reduce the flashback hazard. This results in poor mixing and degraded combustion. Some of these burners have a relatively large fuel consumption, are limited in the fuels and oxidants that can be used, are both physically and opti- 3,516,771 Patented June 23, 1970 cally noisy, or provide a relatively short optical path. A longer optical path often is desirable in the case of atomic absorption spectroscopy. Many of these prior art burners have the additional problem of being easily clogged and relatively diflicult to clean.
Accordingly, it is an object of this invention to obviate many of the disadvantages of the prior art burners.
Another object of this invention is to provide an improved burner having a reduced flashback tendency, and which is relatively easy to clean.
SUMMARY OF THE INVENTION A preferred embodiment of the invention introduces a stream of nebulized sample droplets entrained in a combustion supporting gas into a flow passage leading to an exit slot in a burner. A portion of the flow passage immediately upstream of the exit slot is restricted to the degree that the flow velocity of the gas and droplet mixture exceeds the burning velocity of the combustion gas used. Immediately upstream of the restricted portion, the combustion gas is introduced radially inward into the flow stream from points about the periphery of the flow passage. The peripheral introduction enhances the uniformity of the resulting combustion mixture. Furthermore, the volume of the flow passage between the restricted portion and the exit slot has a reduced volume to limit the effects of flashback. In one embodiment of this invention, a nebulizing nozzle directs a stream of droplets entrained in a combustion supporting gas horizontally into a mixing chamber. A baffle is disposed in the stream of droplets to incur turbulence of the stream and permit larger droplets to drop to the bottom of the mixing chamber. The burners flow passage communicates vertically with the mixing chamber such that the finely nebulized droplets move upwardly from the mixing chamber to the exit slot of the burner.
DESCRIPTION OF THE DRAWINGS The novel features of this invention which are considered characteristic are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:
' FIG. 1 is a pictorial view of the burner of this invention including the mixing chamber for a combustion sup? porting gas and nebulized sample and the elements forming a flow passage in which the combustible gas is com} bined with the combustion supporting gas and nebulized sample;
FIG. 2 is a plan view of the upper portion of the burner forming the flow passage showing the details of the exit slot;
FIG. 3 is a sectional View taken through the section line 33 of the upper portion of the burner illustrated in FIG. 1; I
FIG. 4 is a bottom plan view of the upper portion of the burner; and v p FIG. 5 is a cross-sectional elevation view of the entire burner including the burner head as Well as the nebulizing chamber.
3 7 DESCRIPTION OF THE PREFERRED EMBODIMENT The pictorial drawing of FIG. 1 illustrates quite clearly a burner which includes a nebulizing chamber 10 and a head portion 12 in which the combustible gas is mixed with a combustion supporting gas and the sample droplets. The details of the nebulizing chamber 10 are perhaps seen most clearly in the cross-sectional view of FIG. 5. The nebulizing chamber 10 includes a hollow cylindrical member 14 which has disc shaped end caps 16 and 18, respectively, which close either end of the cylinder 14. Each of the end caps 16 and 18, respectively, may be flanged as at 20 to aid in their proper positioning within the cylinder 14. The end caps 16 and 18 may be welded in position as desired. In some instances a press fit will be suflicient to provide the desired seal. The lefthand end cap (in the drawing) 18 has a central bore 22 formed along the axis of the cylinder 14 to accommodate a nebulizer 24.
The nebulizer 24 is made up of two parts, a base member 30 which has a central bore 32 and a counterbore 34. A sample input tube 36 is positioned in the central bore 32. The base member 30 has a second counterbore 38 in which a nebulizer head 40 is heated and welded if desired. The nebulizer head 40 is in the form of a hollow cylinder which is seated within the second counterbore 38 and has a central bore 42 of roughly the same diameter as the counterbore 34 of the base member 30. Set screws 44, positioned radially at 120 angles about the nebulizer head 44, serve to position the sample input tube 36 such that it is located directly in the center of the orifice 46 of the nebulizer head. The end portion of the central bore 42 is tapered to form the orifice 46. An annular peripheral groove 50 is formed in the exterior of the nebulizer head 40 to accommodate a sealing O-ring 52 which seals the interior of the nebulizer chamber 10. Finally, the base member 30 is bored radially to accommodate a conduit 54 which transports a combustion supporting gas into the counterbore 34 for passage about the sample input tube 36 and through the orifice 46 to nebulize the fluid sample as will be described.
The highthand (in the drawing) end cap 16 also has a central axial bore 60 which is threaded to accommodate a threaded spoiler adjusting rod 62. A spherical spoiler or bafile 64 is secured to the end of the spoiler adjusting rod 62. A knurled knob 66 permits the spoiler adjusting rod 62 to be rotated and thereby adjust the axial position of the spoiler 64 with respect to the orifice 46 of the nebulizer head 40. A drain tube 68 is welded in the bottom portion of the cylinder 14. The drain tube 68 is connected to a drain trap (not shown).
- The upper side of the cylinder 14 has a radial opening 70 formed therein. A hollow cylindrical neck 72, cut to conform to the cylindrical surface of the cylinder 14, is welded to the outer portion of the cylinder 42 at the orifice 70. A tubular tapered neck portion 74 is adapted to be inserted into the chamber neck 72.. An internal annular groove 76 formed in the chamber neck 72 is adapted to accommodate an O-ring 78 which provides a seal for the neck portion 74. The burner head 12 is welded to the top of the neck 74. The tapered neck portion 74 is flared as may be seen in FIGS. 1 and 3 to fit within a slot in the lower portion of the burner head 12 and thereby provide a flow passage for the nebulized sample, as will be described, upwardly through the burner head 12 to an exit slot 80.
The burner head 12 consists essentially of three parts a burner base 84 and a pair of side portions 86. The side portions 86 are removably secured together as by screws or bolts 88 (FIG. 3) and in turn are attached to the burner base 84 by an additional set of screws 90 (FIG. 2). The burner base has a central, vertically disposed slot or flow passage 94 formed therein that is generally rectangular in cross-section. The abutting side members 86 together form a flow path 96 in the form of a slot-type channel. The flow passage 94 narrows in the upper portion of the burner head to end in the exit slot where the flame 81 forms and is flared at the lower portion to have the same cross-sectional area and shape as the slot 94.
A sealing gasket 102 may be positioned between the side members 86 and the burner base 84 to provide a seal and also to raise the side members 86 away from the base 84 sufliciently to provide a small continuous slit-like opening 104 between the slot-like flow path 94 in the burner base and an annular trough 106, seen most clearly in the illustration of FIG. 4, which is formed in the upper portion of the burner base 84. The slit-like opening is a continuous annular slit that permits the combustible gas to be introduced radially into and from the entire periphery of the flow stream of combustion supporting gas and nebulized sample. The thickness of the slit is maintained small so that the combustible gas can flow at a greater velocity than the flow stream. This enhances thorough mixing. A conduit 108, connected from a source of combustible gas such as hydrogen or acetylene is Welded in a bore 110 to communicate with the closed trough 106 and supply a combustible gas thereto. The cross-sectional area of the trough 106 must be suflicient to permit the combustible gas to completely fill the slits 104 such that the combustible gas is entering the flow stream peripherally at all points substantially simultaneously and at the same rate.
To further aid in mixing the combustible gas with the flow stream a mixing rod 98 is positioned along the length of the flared portion of the flow path 96. The mixing rod 98 is in the form of a cylindrical rod having a pair of end studs which are inserted into the notches formed in the side portions 86 and most clearly seen in the cross-sectional view of FIG. 3. The mixing rod 98 is formed to have a diameter slightly less than the smaller dimension of the rectangular cross-section of the flow passage 96 at the point where the mixing rod is positioned. This provides a restricted portion through which the combustible gas along with the nebulized sample and combustion supporting gas must flow thereby to increase the velocity of the combustible gas to a point exceeding that of the burning velocity of the gas. This aids in preventing flashback from the exit slot from propagating beyond the mixing rod 98. For this purpose several sizes of mixing rods may be supplied to accommodate fuels having different burning rates.
In operation a combustion supporting gas such as air or oxygen is passed through the orifice 46 of the nebulizer at a relatively high velocity. The velocity must be sufiicient such that it will nebulize and entrain sample supplied to the sample input tube 36 and direct a nebulized or atomized stream of the sample and the combustion supporting gas into the nebulizer chamber 10. This stream of nebulized sample is directed against the spoiler 64 whose axial position within the cylinder 14 may be adjusted by rotation of the knob 66. The spoiler functions to create turbulence within the stream such that heavy droplets have a better chance of combining with each other at an early stage and dropping to the bottom of the chamber 14 from which they may be drained through the drain tube 68 to a trap.
Those properly entrained droplets which do not fall to the floor are swept upwardly through the neck 74 into the burner head 12. As the flow stream passes through the slot-type passage 94 in the lower portion of the burner head 12 and reaches the top of the burner base, it is then combined with a combustible gas fed through the supply tube 108 to the enclosed trough 106. The enclosed trough 106 completely surrounds the slot-type passage 94 through which the flow stream 'of nebulized sample is passing such that combustible gas is entrained by and mixed with the nebulized sample at all peripheral points of the passage 94 simultaneously. The slit must be narrow so that the velocity of the combustible gas flow therethrough is greater than the velocity of the flow stream through the passage 94. This enhances mixing.
Immediately thereafter, the mixture of combustible gas, nebulized sample, and combustion supporting gas passes around the mixing rod 98. In doing so the restriction in the flow passage afforded by the mixing rod 98 causes the velocity of the mixture to exceed that of the burning velocity of the gas employed. The diameter of the mixing rod is selected so that this condition prevails. There is a turbulence area immediately downstream of the mixing rod where the flow streams, split or divided by the mixing rod, recombine. This turbulence enhances thorough mixing of the gases and nebulized sample making up the flow stream immediately prior to the exit slot where combustion occurs. From this point the gases continue along upwardly through the flow path to the exit slot 80 whence the flame is formed to cover the entire length of the exit slot. This permits a long flame section which is particularly useful in atomic absorption spectroscopy.
The particular advantages of this invention are many. For one, in the event there is a flashback from the exit slot 80, the flame cannot travel beyond the mixing rod 98 since the velocity of the passage about the mixing rod exceeds the burning velocity of the flame. By maintaining the volume of the flow passage 96 between the exit slot 80 and the mixing rod 98 relatively small, the magnitude of the explosion caused by the flashback is reduced.
Secondly, the additional mixing afforded by the mixing rod insures uniform mixing of the combustible gas with the nebulized sample and combustion supporting gas. This mixing is enhanced by the peripheral introduction of the combustible gas to the flowing stream. Additionally, the horizontal direction of the initial nebulized sample stream greatly facilitates the dropping out of the heavier, poorly nebulized sample droplets. Having been initially horizontally directed the selected droplets must be sufliciently light to turn a 90 angle and be directed upwardly through the neck 74 into the burner head 12. This provides a very thorough selection basis for selecting only those droplets which are thoroughly nebulized before they are introduced into the flame. This prevents the uneven flashing which often occurs in a spectral burner of this type.
The construction of the burner permits its ready disassembly. This ready disassambly is particularly enhanced by the use of the O-rings. The disassembly is necessary to permit thorough cleaning of the burner and the burner head 12 itself is completely disassembled by the simple removal of six screws 88 and 90. All of the materials em ployed in this burner preferably are stainless steel or similar material which cannot only withstand heat but are not chemically reactive with the sample. Clogging of the burner is unlikely. The burner has a low fuel consumption rate, can be used with a Wide variety of fuels, and is relatively quiet both optically and physically.
While only a few gases are specifically described, it is to be understood that virtually any of the known combustible and combustion gases may be used.
There has thus been described a relatively unique spectral flame burner which is easy to disassemble, is subject to reduced flashback damage and insures thorough mixing of the fuel with the combustion supporting gas.
It will be obvious that various modifications may be made in the apparatus and in the manner of operating it. It is intended to cover such modifications and changes as would occur to those skilled in the art as far as the following claims permit and as far as consistent with the state of the prior art.
What is claimed is:
1. A burner apparatus comprising:
a droplet separating chamber,
a nebulizing nozzle assembly directing a stream of droplets entrained in a combustion supporting gas horizontally into said chamber,
means including a baffle disposed in said chamber in the path of said stream for deflecting said stream away from the horizontal,
a burner assembly,
a vertically disposed conduit communicating with said chamber for passing said droplet stream to said burner assembly,
said burner assembly having a body portion defining a flow passage and an exit slot for said droplet stream,
mixing means in said body portion for mixing a combustion gas with said droplet stream, and
means downstream of said mixing means for restricting said flow passage, whereby the flow velocity of said droplet stream and said combustible gas through the restricted portion exceeds the burning velocity of said combustible gas.
2. An apparatus according to claim 1 which also includes means for adjusting the position of said baffle along the axis of said stream.
3. An apparatus according to claim 1 wherein said mixing means for mixing a combustible gas with said flow stream includes means defining a peripheral slit opening inwardly into said flow passage.
4. An apparatus according to claim 3 wherein said slit is oriented to open radially into said flow passage and dimensioned to increase the flow rate of said combustible gas over that of said droplet stream.
5. An apparatus according to claim 3 wherein said slit extends continuously about said flow passage.
6. An apparatus according to claim 1 wherein said flow passage is substantially rectangular in cross section.
7. An apparatus according to claim 6 wherein said restricting means includes a partition positioned in said flow passage adapted to divide said flow passage, whereby said droplet stream with said combustible gas is split into two streams and again recombined prior to passage through said exit slot.
8. An apparatus according to claim 6 wherein said restricting means includes a partition positioned in said flow passage adapted to divide said flow passage, Whereby said droplet stream with said combustion gas is split into two streams and again recombined prior to passage through said exit slot.
9. An apparatus according to claim 8 wherein said partition is a cylindrical rod.
10. An apparatus according to claim 7 wherein said mixing means for mixing a combustible gas with said flow stream includes means defining a peripheral slit opening inwardly into said flow passage.
11. A burner apparatus comprising:
a body portion defining a flow passage having a generally rectangular cross section, and an exit slot, said body portion including:
a base member defining the upstream portion of said flow passage, said base member having a planar surface on its downstream side and an annular trough in said planar surface about said flow passage, and
a pair of upper members having mating surfaces defining the downstream portion of said flow passage and said exit slot, said upper members being secured to said planar surface of said base member, thereby to complete said flow passage and define with said planar surface a peripheral slit continuously connecting said trough with said flow passage;
means for introducting a stream of nebulized sample droplets entrained in a combustion supporting gas into said flow passage;
means for introducing combustible gas into said trough for passage through said peripheral slit to mix combustible gas with said stream of nebulized sample droplets; and
rod means positioned in said flow passage between said References Cited UNITED STATES PATENTS Axdorfer 431354 Graham 43 l-346 Vallely 431-354 Heylman et al.
v 8 1,785,804 12/1930 Adams 431 4 2,714,833 8/1955 Gilbert 431 4 XR FOREIGN PATENTS 140,576 4/1920 Great Britain. 5 367,747 8/1930 Great Britain.
FREDERICK L. MATTESON, JR., Primary Examiner R. A. DUA, Assistant Examiner US. Cl. X.R.