CN212883164U

CN212883164U - High stability quartz glass atomizer

Info

Publication number: CN212883164U
Application number: CN202021578728.XU
Authority: CN
Inventors: 樊宏
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-08-03
Filing date: 2020-08-03
Publication date: 2021-04-06
Anticipated expiration: 2030-08-03

Abstract

The utility model discloses a high stability quartz glass atomizer belongs to atomizer technical field, has solved current atomizer poor stability, has produced the carbon deposit easily and has leaded to clean inefficiency and with the poor problem of test pipeline junction leakproofness. The high-stability quartz glass atomizer comprises an outer tube, a central tube, a liquid inlet tube, an air inlet tube and a plug-in connector, wherein the outer tube, the central tube, the liquid inlet tube and the air inlet tube are all quartz glass tubes; the outer pipe comprises a nozzle with a tapered structure, and the pipe wall of the nozzle is in an arc structure; the tail end of the central pipe is connected with a liquid outlet of the liquid inlet pipe, the plug-in connector is detachably arranged at a liquid inlet of the liquid inlet pipe, an annular sealing ring is arranged on the outer wall of the plug-in connector, and a solution to be measured enters the central pipe through the liquid inlet pipe; the central tube is arranged in the outer tube, the central line of the central tube is superposed with the central line of the outer tube, and the space between the outer wall of the central tube and the inner wall of the outer tube forms an annular gas circuit. The utility model discloses an atomizing is efficient, difficult jam, and stability is high.

Description

High stability quartz glass atomizer

Technical Field

The utility model relates to an atomizer technical field especially relates to a high stability quartz glass atomizer.

Background

At present, modern advanced element detection and analysis instruments such as inductively coupled plasma atomic emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS) and the like are widely applied in various industries. The pneumatic atomizer is an important key component on the precision instrument, and mainly has the main function of atomizing a sample solution in a liquid path branch pipe into fine aerosol by utilizing high-speed airflow in the gas path branch pipe on the atomizer so as to be introduced into an analysis instrument such as ICP-OES or ICP-MS for element component determination.

The existing concentric atomizer is made of common glass materials, carbon deposition is easily generated at the nozzle of the glass atomizer in the testing process, a traditional chemical reagent cleaning mode is adopted, the cleaning time is long, and the cleaning efficiency is low. Because the pipe diameter of the central capillary of the atomizer is very thin, and the central capillary made of common glass has small hardness and is not high-temperature resistant, carbon deposition in the atomizer can not be removed by adopting a high-temperature combustion mode. In addition, the liquid inlet joint and the air inlet joint of the existing atomizer have poor sealing performance, are easy to leak liquid or air, and have poor stability.

SUMMERY OF THE UTILITY MODEL

In view of the above analysis, the present invention aims to provide a high stability quartz glass atomizer for solving the problems of poor stability, easy carbon deposition generation, low cleaning efficiency and poor sealing performance at the connection part of the test pipeline of the existing atomizer.

The purpose of the utility model is mainly realized through the following technical scheme:

a high-stability quartz glass atomizer comprises an outer tube, a central tube, a liquid inlet tube, an air inlet tube and a plug-in connector, wherein the outer tube, the central tube, the liquid inlet tube and the air inlet tube are all quartz glass tubes; the outer pipe comprises a nozzle with a tapered structure, and the pipe wall of the nozzle is in an arc structure; the tail end of the outer pipe is hermetically connected with the outer wall of the liquid inlet pipe or integrally formed; the tail end of the central pipe is connected with a liquid outlet of the liquid inlet pipe, the plug-in connector is detachably arranged at a liquid inlet of the liquid inlet pipe, an annular sealing ring is arranged on the outer wall of the plug-in connector, and a solution to be measured enters the central pipe through the liquid inlet pipe; the central tube is arranged in the outer tube, the central line of the central tube is superposed with the central line of the outer tube, and a space between the outer wall of the central tube and the inner wall of the outer tube forms an annular gas path; the side wall of the outer tube is provided with an air inlet pipe, and carrier gas enters the annular air passage through the air inlet pipe and is sprayed out from the nozzle.

Furthermore, the outer pipe also comprises a main pipe section connected with the tail end of the nozzle, and the main pipe section is in reducing transitional connection with the nozzle.

Further, the port of the central tube is positioned in the nozzle, and the port of the central tube is retracted into the nozzle orifice of the nozzle by a distance of 1-2 mm.

Further, the inner diameter of the nozzle decreases linearly from the rear end of the nozzle to the nozzle port.

Furthermore, the nozzle comprises an inner diameter reducing section and an inner diameter straight pipe section, the inner diameter reducing section is provided with an inwards concave channel, the inwards concave channel is formed by enclosing inwards concave curved surfaces, the inner diameter straight pipe section is provided with a straight channel, the diameter of the straight channel is unchanged, and the inner diameter of the nozzle is reduced in a nonlinear mode from the tail end of the nozzle to the port of the nozzle and then is kept unchanged.

Furthermore, the inner diameter of the main pipe section is 3.5-4.5mm, and the outer diameter is 5.5-6.5 mm; the inner diameter of the nozzle port is 0.3 mm; by the tail end to the head end of center tube, the pipe diameter of center tube is by thick even attenuate, and the tail end external diameter of center tube is 2mm, and the tail end internal diameter of center tube is 1.5mm, and the head end external diameter of center tube is 0.4mm, and the head end internal diameter of center tube is 0.2 mm.

Furthermore, the plug-in connector comprises a front section, a middle section and a rear section which are connected in sequence, and the front section, the middle section and the rear section are all of hollow structures and are coaxially arranged; the front section is of a cylindrical structure, and the outer diameter of the front section is equal to the inner diameter of the liquid inlet pipe; the middle section is of a circular truncated cone structure, the minimum outer diameter of the circular truncated cone is equal to the outer diameter of the front section, a rubber layer is arranged on the peripheral surface of the circular truncated cone, the middle section is in reducing transitional connection with the front section, and the maximum outer diameter of the circular truncated cone is smaller than or equal to the outer diameter of the rear section.

Further, the periphery of the rear section is provided with anti-skidding threads.

Furthermore, the plug connector is a Teflon plug connector, the plug connector is connected with a Teflon tube, and the Teflon tube is connected with the sample bottle.

Further, along the carrier gas flow direction, the periphery of intake pipe is equipped with spacing arch of second and first spacing arch, and the peak of first spacing arch is higher than the peak of the spacing arch of second.

Compared with the prior art, the utility model discloses can realize one of following beneficial effect at least:

a) the utility model provides a high stability quartz glass atomizer adopts the quartz material to make, and quartzy expansion coefficient is low, and material property is stable, and hardness is big, and purity is high, and surface finish is good, not only can prevent that salt from taking place stifled, can also clear away the intraductal carbon deposit of feed liquor through the mode of high temperature burning, convenient operation, clean effectual, shorten conventional scavenging period greatly, promote efficiency of software testing greatly, and can thoroughly avoid disturbing the test result because of wasing thoroughly in addition, improved the test result accuracy.

b) The utility model provides a high stability quartz glass atomizer, the spout department pipe wall of nozzle is the circular arc structure, has promoted the atomization efficiency of atomizer.

c) The utility model provides a high stability quartz glass atomizer, the anterior segment that the plug connects set up ring type seal, middle section and set up to the round platform structure, and double seal structure has promoted sealed effect by a wide margin, prevents the weeping, has promoted the job stabilization nature of atomizer.

d) The utility model provides a high stability quartz glass atomizer adopts the teflon plug to connect and teflon pipe, can avoid signal interference, improves the reliability of test result.

e) The utility model provides a high stability quartz glass atomizer, intake pipe are equipped with the second grade anti-drop structure that is used for preventing the air guide hose landing, are equipped with first spacing arch and the spacing arch of second promptly, and first spacing bellied peak is higher than the spacing bellied peak of second, and after the air guide hose cover was established in the intake pipe, utilize between first spacing arch and the spacing arch of second to bind the piece, can effectively prevent air guide hose landing in the testing process.

The utility model discloses in, can also make up each other between the above-mentioned each technical scheme to realize more preferred combination scheme. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and drawings.

Drawings

The drawings are only for purposes of illustrating particular embodiments and are not to be construed as limiting the invention, wherein like reference numerals are used to designate like parts throughout the drawings.

FIG. 1 is a schematic structural view of a high stability quartz glass atomizer in an embodiment of the present invention;

fig. 2 is a schematic structural diagram of a first structural nozzle in an embodiment of the present invention;

fig. 3 is a schematic structural diagram of a nozzle with a second structure in an embodiment of the present invention.

Reference numerals:

1-an outer tube; 2-a central tube; 3-a liquid inlet pipe; 4, an air inlet pipe; 5-a nozzle; 51-circular arc structure; 52-a straight channel; 53-concave channel; 6-annular gas circuit; 7-a first limit projection; 8-a second limiting bulge; 9-plug-in connector; 10-ring seal ring; 11-Teflon tube.

Detailed Description

The following detailed description of the preferred embodiments of the invention, which is to be read in connection with the accompanying drawings, forms a part of this application, and together with the embodiments of the invention, serve to explain the principles of the invention and not to limit its scope.

The utility model discloses a specific embodiment of a high-stability quartz glass atomizer, which comprises an outer tube 1, a central tube 2, a liquid inlet tube 3, an air inlet tube 4 and a plug connector 9, as shown in figure 1; wherein, the outer tube 1, the central tube 2, the liquid inlet tube 3 and the gas inlet tube 4 are all quartz glass tubes. The outer pipe 1 comprises a nozzle 5 with a tapered structure, and the tail end of the outer pipe 1 is hermetically connected with the outer wall of the liquid inlet pipe 3 or integrally formed; the tail end of the central tube 2 is connected with the liquid outlet of the liquid inlet tube 3, the plug-in connector 9 is detachably arranged on the liquid inlet of the liquid inlet tube 3, and the solution to be measured enters the central tube 2 through the liquid inlet tube 3; the central tube 2 is arranged in the outer tube 1, the central line of the central tube 2 is superposed with the central line of the outer tube 1, and a space between the outer wall of the central tube 2 and the inner wall of the outer tube 1 forms an annular gas path; the lateral wall of outer tube 1 is equipped with the intake pipe installing port, and intake pipe 4 is sealed to be located the intake pipe installing port, and the carrier gas gets into cyclic annular gas circuit through intake pipe 4 and is spouted by nozzle 5.

The existing atomizer is made of borosilicate glass, has high impurity content and poor surface smoothness, contains micro bubbles, and can further influence the surface smoothness after being drawn, thereby influencing the high salt resistance of the atomizer and being easy to cause salt blockage. The atomizer of the embodiment is made of quartz glass, has low impurity content, high hardness and good surface smoothness, does not contain or contains a very small amount of micro-bubbles, can keep better surface smoothness after being drawn, is beneficial to preventing salt blockage, and improves the working stability of the atomizer.

In this embodiment, the plug connector 9 is a teflon plug connector, the plug connector 9 is connected with a teflon tube 11, and the teflon tube 11 is connected with a sample bottle. The plug-pull connector 9 and the Teflon tube 11 made of Teflon materials can avoid signal interference and improve the reliability of a test result.

Further, the plug-in connector 9 comprises a front section, a middle section and a rear section which are connected in sequence, wherein the front section, the middle section and the rear section are all hollow structures and are coaxially arranged; the front section is of a regular cylinder structure, the outer diameter of the front section is equal to the inner diameter of the liquid inlet pipe 3, and the periphery of the front section is provided with an annular sealing ring 10; the middle section is a circular truncated cone structure, the minimum outer diameter of the circular truncated cone is equal to the outer diameter of the front section, a rubber layer is arranged on the outer peripheral surface of the circular truncated cone, the middle section is in reducing transitional connection with the front section, the maximum outer diameter of the circular truncated cone is smaller than or equal to the outer diameter of the rear section, and anti-skidding threads are arranged on the outer periphery of the rear section, so that an operator can conveniently hold the circular truncated cone. After an operator holds the rear section and inserts the plug-in connector 9 into the liquid inlet pipe 3, the front section of the plug-in connector 9 is in sealing contact with the inner wall of the liquid inlet pipe 3 through the annular sealing ring 10, because the middle section is in a round table structure, the minimum radius of the round table is equal to the inner diameter of the liquid inlet pipe 3, and along with the continuous extension of the front section, when the minimum radius department of middle section round platform reachd the inlet of feed liquor pipe 3, along with the operator continues to exert the insertion force, deformation can take place for the rubber layer on round platform surface, get into the inlet of feed liquor pipe 3, at this moment, plug joint 9 has two sealings with the inner wall of feed liquor pipe 3, one is the inner wall sealing contact of the ring packing ring 10 of anterior segment and feed liquor pipe 3, another is the inner wall sealing contact of the rubber layer on round platform surface and feed liquor pipe 3, the two-stage seal design, sealing effect has been promoted, the sample tube accident drops in having avoided the test process, the operational reliability of atomizer has been guaranteed.

In this embodiment, the carrier gas enters the gas inlet pipe 4 through the gas guide hose, the gas inlet pipe 4 is provided with an anti-drop structure for preventing the gas guide hose from slipping off, the anti-drop structure is a two-stage limiting protrusion structure, and includes a first limiting protrusion 7 and a second limiting protrusion 8, the highest point of the first limiting protrusion 7 is higher than the highest point of the second limiting protrusion 8, specifically, in the flow direction of the carrier gas, the periphery of the gas inlet pipe 4 is provided with the second limiting protrusion 8 and the first limiting protrusion 7, and the concave space between the second limiting protrusion 8 and the first limiting protrusion 7 is used for binding the binding piece. After the air guide hose is sleeved on the air inlet pipe, the air guide hose is wrapped by the first limiting bulges 7 and the second limiting bulges 8, the first limiting bulges 7 and the second limiting bulges 8 are bound by binding pieces such as clamps and metal binding wires, and the air guide hose can be effectively prevented from sliding down in the test process.

One of the factors influencing the stability of the signal measured by the concentric glass atomizer is the problem of blockage caused by the accumulation of salt in the nozzle, and when the solution with high salt content is atomized, the salt in the solution can be gradually separated out to slowly block the gas path gap, so that the flow rate and the pressure of gas are changed, the atomization efficiency is influenced, and the drift of the measured signal is caused. In severe cases, the carrier gas connection tube is disconnected, and analysis needs to be interrupted to clean the atomizer. Whether salt blockage easily occurs or not mainly depends on the retracting distance of the central pipe port (the liquid outlet of the central pipe) from the outer pipe port (the nozzle of the nozzle 5), the smaller the retracting distance of the central pipe port, the higher the possibility of salt blockage is, and the larger the retracting distance of the central pipe port is, the atomization effect is influenced. In this embodiment, the port of the central tube is located in the nozzle 5, and the port of the central tube is retracted into the nozzle orifice of the nozzle 5 by a distance of 1-2 mm.

It has been found that the taper of the atomiser nozzle 5 is related to atomisation efficiency, and that a blunt cone angle provides a higher atomisation efficiency than a sharp cone angle, because at a blunt cone angle the gas flow path contracts more rapidly, resulting in a greater gas flow rate, and hence an improved atomisation efficiency.

In this embodiment, the nozzle of the nozzle 5 is passivated, the pipe wall of the nozzle 5 is in the arc structure 51, and in the manufacturing process, the nozzle of the nozzle 5 is ground by using a high-precision grinding machine and sintered at a high temperature, so as to obtain the arc structure 51 with a blunt taper angle.

Further, the nozzle 5 includes two structures: the nozzle of the first structure, from the tail end of the nozzle to the nozzle port, the inner diameter of the nozzle 5 is linearly reduced, as shown in fig. 2; the nozzle of the second structure comprises an inner diameter reducing section and an inner diameter straight pipe section, wherein the inner diameter reducing section is provided with an inward concave channel 53, the inward concave channel 53 is surrounded by an inward concave curved surface, the inner diameter straight pipe section is provided with a straight channel 52, the diameter of the straight channel 52 is unchanged, the inner diameter of the nozzle 5 is reduced in a nonlinear mode from the tail end of the nozzle to the port of the nozzle and then is kept unchanged, and the length of the straight channel 52 is 1-2mm, as shown in figure 3.

The port of the central tube 2 is flush with the lower end face of the straight channel 52, carrier gas in the atomizer with the structure firstly passes through the concave channel 53 with the concave curved surface in the annular gas path and enters the straight channel after being accelerated by the concave curved channel, the pipe wall at the nozzle of the nozzle 5 is in the arc structure 51, the contraction of the gas flow channel is faster, larger gas flow velocity can be formed, and the atomizer has better atomization effect.

It can be seen from the Poiseuille equation that the amount of solution lifted is directly related to the radius of the central tube 2 and the pressure difference and inversely related to the viscosity of the liquid and the length of the central tube 2. Wherein, the solution lifting amount refers to the volume of the lifting liquid in unit time, and directly influences the solution amount reaching the plasma torch, and the solution lifting amount is a parameter related to the sensitivity. The minute changes of the flow rate and pressure of the carrier gas and the conditions of the viscosity and density of the sample solution cause the change of the lifting amount of the solution to cause the fluctuation of the analysis signal intensity, so that the stability is poor. Because the atomizer will match with the fog room, guarantees that the nozzle is in the tangent line entrance of whirl fog room, and the length of atomizer is generally fixed, consequently, this embodiment reaches the lifting volume that will require through the pipe diameter of control center tube 2, the pipe diameter of outer tube 1, and in this embodiment, the concrete structure and the parameter of center tube 2, outer tube 1 are: the outer pipe 1 also comprises a main pipe section connected with the tail end of the nozzle, the main pipe section is in variable diameter transition connection with the nozzle 5, the inner diameter of the main pipe section is 3.5-4.5mm, the outer diameter is 5.5-6.5mm, the inner diameter of the nozzle 5 is gradually reduced from the tail end of the nozzle to the port of the nozzle, and the inner diameter of the port of the nozzle is 0.3 mm; by the tail end of center tube 2 to head end, the pipe diameter of center tube 2 is by thick even attenuate, and the tail end external diameter of center tube 2 is 2mm, and the tail end internal diameter of center tube 2 is 1.5mm, and the head end external diameter of center tube 2 is 0.4mm, and the head end internal diameter of center tube 2 is 0.2 mm. Above-mentioned parameter setting can promote solution lift volume, and then guarantees sensitivity to test, and the wall thickness more than or equal to 0.2mm of center tube 2 is big than the thickness of the center capillary wall of current atomizer, has increased the intensity of center tube.

During implementation, carrier gas enters the annular gas path 6 through the gas inlet pipe 4 and is sprayed out through the nozzle 5, the carrier gas is accelerated in the annular gas path, negative pressure is formed at the nozzle of the nozzle 5, a solution to be tested enters the central pipe 2 through the liquid inlet pipe 3 under the action of the negative pressure and is sprayed out through the port of the central pipe 2, the accelerated carrier gas collides with the solution sprayed out from the port of the central pipe 2 in the nozzle, the solution to be tested is atomized, and atomized fog-shaped particles enter the fog chamber until analysis and test are completed.

Compared with the prior art, the high-stability quartz glass atomizer provided by the embodiment is made of quartz, the quartz has low expansion coefficient, stable material property, high hardness, high purity and good surface smoothness, can prevent salt blockage, can remove carbon deposition in a liquid inlet pipe in a high-temperature combustion mode, is convenient to operate and good in cleaning effect, greatly shortens the conventional cleaning time, greatly improves the testing efficiency, can thoroughly avoid interference of a testing result due to incomplete cleaning, and improves the accuracy of the testing result; the pipe wall at the nozzle of the atomizer is arranged to be in an arc structure, so that the atomization efficiency of the atomizer is improved; the front section of the plug-in connector is provided with the annular sealing ring, and the middle section of the plug-in connector is provided with the circular truncated cone structure, so that the double sealing structure greatly improves the sealing effect, prevents liquid leakage and improves the working stability of the atomizer; meanwhile, the Teflon plug-in connector and the Teflon tube are adopted, so that signal interference can be avoided, and the reliability of a test result is improved. The secondary anti-falling structure is used for preventing the air guide hose from slipping down and arranged on the periphery of the air inlet pipe, the secondary anti-falling structure comprises a first limiting protrusion and a second limiting protrusion, the highest point of the first limiting protrusion is higher than the highest point of the second limiting protrusion, when the air guide hose is sleeved on the air inlet pipe, the air guide hose is bound by the binding piece between the first limiting protrusion and the second limiting protrusion, the air guide hose can be effectively prevented from slipping down in the test process, and the working reliability of the atomizer is improved.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention should be covered by the present invention.

Claims

1. The high-stability quartz glass atomizer is characterized by comprising an outer tube (1), a central tube (2), a liquid inlet tube (3), an air inlet tube (4) and a plug-in connector (9), wherein the outer tube (1), the central tube (2), the liquid inlet tube (3) and the air inlet tube (4) are all quartz glass tubes;

the outer pipe (1) comprises a nozzle (5) with a tapered structure, and the pipe wall of the nozzle (5) at the nozzle opening is in an arc structure (51);

the tail end of the outer pipe (1) is hermetically connected with the outer wall of the liquid inlet pipe (3) or integrally formed;

the tail end of the central tube (2) is connected with a liquid outlet of the liquid inlet tube (3), the plug-in connector (9) is detachably arranged on a liquid inlet of the liquid inlet tube (3), an annular sealing ring (10) is arranged on the outer wall of the plug-in connector (9), and a solution to be tested enters the central tube (2) through the liquid inlet tube (3);

the central tube (2) is arranged in the outer tube (1), the central line of the central tube (2) is superposed with the central line of the outer tube (1), and a space between the outer wall of the central tube (2) and the inner wall of the outer tube (1) forms an annular gas circuit;

the side wall of the outer tube (1) is provided with an air inlet tube (4), and carrier gas enters the annular air passage through the air inlet tube (4) and is sprayed out from the nozzle (5).

2. The atomizer according to claim 1, characterized in that the outer tube (1) further comprises a main tube section connected to the end of the nozzle, the main tube section being connected to the nozzle (5) in a variable diameter transition.

3. A high stability quartz glass atomizer according to claim 1, characterized in that the port of the central tube (2) is located inside the nozzle (5), the port of the central tube (2) being set back into the orifice of the nozzle (5) by a distance of 1-2 mm.

4. A high stability quartz glass atomizer according to claim 1, characterized in that the inner diameter of the nozzle (5) decreases linearly from the nozzle tail end to the nozzle port.

5. The high stability quartz glass atomizer according to claim 1, wherein the nozzle comprises an inner diameter reducing section having a concave channel (53) and an inner diameter straight tube section having a straight channel (52), the diameter of the straight channel (52) being constant, the inner diameter of the nozzle (5) decreasing non-linearly from the nozzle end to the nozzle port and then remaining constant, the inner diameter of the inner diameter reducing section having a concave curved surface (53).

6. The high stability quartz glass atomizer of claim 2, wherein the inner diameter of the primary tube segment is 3.5-4.5mm and the outer diameter is 5.5-6.5 mm;

the inner diameter of the nozzle port is 0.3 mm;

by the tail end of center tube (2) to head end, the pipe diameter of center tube (2) is by thick even attenuate, and the tail end external diameter of center tube (2) is 2mm, and the tail end internal diameter of center tube (2) is 1.5mm, and the head end external diameter of center tube (2) is 0.4mm, and the head end internal diameter of center tube (2) is 0.2 mm.

7. The high-stability quartz glass atomizer according to claim 1, wherein the plug-in connector (9) comprises a front section, a middle section and a rear section which are connected in sequence, and the front section, the middle section and the rear section are all hollow structures and are coaxially arranged;

the front section is of a cylindrical structure, and the outer diameter of the front section is equal to the inner diameter of the liquid inlet pipe (3);

the middle section is of a circular truncated cone structure, the minimum outer diameter of the circular truncated cone is equal to the outer diameter of the front section, a rubber layer is arranged on the peripheral surface of the circular truncated cone, the middle section is in reducing transitional connection with the front section, and the maximum outer diameter of the circular truncated cone is smaller than or equal to the outer diameter of the rear section.

8. The high stability quartz glass atomizer of claim 7, wherein the outer periphery of the rear section is provided with non-slip threads.

9. The high-stability quartz glass atomizer according to claim 1, wherein the plug (9) is a teflon plug, the plug (9) is connected with a teflon tube (11), and the teflon tube (11) is connected with the sample bottle.

10. A high stability quartz glass atomizer according to claim 1, characterized in that the gas inlet tube (4) is provided at its periphery with a second limiting protrusion (8) and a first limiting protrusion (7) in the flow direction of the carrier gas, the highest point of the first limiting protrusion (7) being higher than the highest point of the second limiting protrusion (8).