Method and device for detecting and controlling the residual length of an electrode during the preparation of nanopowders

Abstract

The application relates to the technical field of process regulation, and particularly discloses a method and equipment for detecting and controlling the residual length of an electrode in a nanometer powder preparation process. In the method, the average arc voltage U of an electrode in a period t is recorded when the electrode is at different heights H, the average arc electric field intensity E corresponding to the different heights H is obtained H ; a relationship between the distance h of an electrode sheath gas outlet from a molten pool and the electrode sheath gas pressure P is simulated; the arc voltage U' and the electrode sheath gas pressure P of the electrode at a certain height H are recorded, so that the arc length L arc is calculated; the electrode sheath gas pressure P is substituted into the formula to calculate h; and the distance h of the electrode sheath gas outlet from the molten pool is deducted by the arc length L arc , so that the electrode length L ele is obtained. The application realizes uninterrupted online monitoring of material depth and electrode length under a non-stop state through a system instrument.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of process regulation, and more particularly, it relates to a method and device for detecting and controlling the remaining length of an electrode in a nanometer powder preparation process. BACKGROUND

[0002] Nanometer metal powder exhibits many properties that macroscopic materials do not have due to its special size, and its unique optical and electronic properties have great potential application value in the fields of electrochemistry, biomedicine, electronic information, and optics.

[0003] The preparation method of nanometer metal powder usually adopts evaporation condensation method. In the evaporation condensation method, the metal evaporation method includes electric explosion, radio frequency, and direct current arc plasma. Among them, the direct current arc plasma preparation method uses metal raw materials as the arc anode, tungsten, graphite, and other materials as the cathode, and a direct current power source maintains the discharge channel between the anode and the cathode to establish a direct current arc. In order to improve the yield of nanometer powder, the industry usually adopts the method of increasing the discharge strand number and increasing the arc current to input high energy. Therefore, the cathode bears extremely high energy density, the energy density is about 10 6 kW / m 2 , the current density is about 10 8 A / m 2 , and the temperature can reach 10 4 ℃, and the electrode has great ablation loss. Timely online monitoring of the remaining length of the electrode and compensating the suspension height are of great significance to maintaining the normal discharge of the direct current arc plasma system.

[0004] At present, there is no relevant literature in the industry for precise detection of the electrode length and the material depth. Based on this situation, the applicant urgently needs to develop an arc electrode remaining length detection and control method in a nanometer powder preparation process to make up for the blank of arc electrode control in the industry. SUMMARY

[0005] In order to make up for the blank of arc electrode control in the industry, the applicant provides an electrode remaining length detection and control method and device in a nanometer powder preparation process.

[0006] In a first aspect, the application provides an electrode remaining length detection and control method in a nanometer powder preparation process, which adopts the following technical solution:

[0007] The electrode remaining length detection and control method in a nanometer powder preparation process includes the following steps:

[0008] S1, when the system is just started, record the arc average voltage U in a period of time t at different heights H, and obtain the arc average electric field strength E corresponding to different heights H H, E H , H, U satisfy formula (1): E H =U / (H0-D0-a-L ele0 );

[0009] Wherein, a is the height position of the electrode clamped by the electrode lifting device, which should be recorded and taken as a preset value when the electrode is installed, H0 is the initial height of the electrode, D0 is the laying thickness of the material, and L ele0 is the initial length of the electrode;

[0010] The relationship between the distance h between the electrode sheath gas outlet and the molten pool and the electrode sheath gas pressure P is simulated, and h and P satisfy formula (2): h=A·P B +W, A, B and W are all empirical parameters;

[0011] S2, record the arc voltage U' and the electrode sheath gas pressure P of the electrode at a certain height H', so as to calculate the arc length L arc , L arc =U' / E H ;

[0012] The electrode sheath gas pressure P is substituted into formula (2), and h can be calculated;

[0013] The distance h between the electrode sheath gas outlet and the molten pool is deducted by the arc length L arc , that is, the length of the electrode L ele , that is, L ele =h-L arc .

[0014] Further, the electrode sheath gas pressure P is not less than 40kPa.

[0015] Further, the arc voltage U is the average value of x% quantile in a period of time t, and the value of x is 0-20.

[0016] Further, in the S1 step, the electrode height H is raised to the arc voltage U close to the no-load voltage of the direct current power supply.

[0017] Further, in the S1 step, the no-load voltage of the direct current power supply is not less than 200V.

[0018] Further, after the S2 step, the material depth D can be calculated according to formula (3), and formula (3) is D= H'-L arc -L ele -C, C is the distance between the electrode terminal in the initial state and the initial material height. In general powder preparation, high-frequency arc starting is adopted, so C can be taken as 2-5mm in the initial state.

[0019] In a second aspect, the application provides an electrode residual length detection and control device in a nanopowder preparation process, which adopts the following technical scheme:

[0020] The electrode residual length detection and control device in a nanopowder preparation process is specially used for the electrode residual length detection and control method in the nanopowder preparation process, and comprises:

[0021] An arc generator comprises an electrode base, an electrode fixed on the electrode base, and an electrode sheath gas pipeline provided on the electrode base, wherein an electrode sheath gas outlet of the electrode sheath gas pipeline is located at the top end of the electrode.

[0022] An electrode lifting device is fixed on the electrode base to control the electrode to be lifted in the vertical direction.

[0023] A height measurer is installed on the electrode lifting device to record the electrode height H.

[0024] A sheath gas flow controller is installed on the electrode sheath gas pipeline to record the electrode sheath gas flow and control the sheath gas flow.

[0025] An electrode sheath gas pressure gauge is installed on the electrode sheath gas pipeline to record the electrode sheath gas pressure P.

[0026] A voltage gauge is electrically connected to the electrode to record the arc voltage U.

[0027] Further, the electrode sheath gas points to the electrode tip, and the electrode sheath gas flow is not less than 50 SLPM.

[0028] Further, the electrode sheath gas flow is controlled to be 150-200 SLPM.

[0029] The application has at least the following advantages:

[0030] Firstly, the electrode length L ele By the indirect method, the applicant finds that the electrode length L ele The sum of the arc length L arc and the distance h between the electrode sheath gas outlet and the molten pool is the electrode length, so that the arc length L arc and the distance h between the electrode sheath gas outlet and the molten pool are calculated to obtain the electrode length.

[0031] The arc can be equivalent to multiple gas discharge channels in parallel to form multiple electric fields, so that the arc is simplified as a uniform electric field in the application. In the application, the average arc equivalent electric field strength E H is calculated according to the average arc voltage U under different electrode heights H through the uniform electric field formula. When the electrode is located at a certain height H', the corresponding average arc equivalent electric field strength E HAccording to the arc voltage U', the arc length L can be calculated arc ;

[0032] Since the electrode length is short, the distance between the electrode sheath gas outlet and the anode material liquid surface is close, and a large back pressure is formed after the electrode sheath gas hits the anode material liquid surface. Therefore, there is a relationship between the distance h between the electrode sheath gas outlet and the molten pool and the electrode sheath gas pressure P. Through multiple experiments, the applicant obtains the empirical relationship between h and P. By substituting the electrode sheath gas pressure P into formula (2), the size of h can be obtained.

[0033] According to the above formula, the error rate between the residual length of the electrode and the actual residual length of the electrode is reduced to within 10%, and the accuracy of indirect detection is high. The measurement parameters of the application are obtained through the system self-instrument of the voltage meter, height measuring device and electrode sheath gas pressure meter installed in the original process, so as to realize uninterrupted online monitoring of the material depth and the electrode length without stopping. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The structure diagram of the arc electrode residual length detection and control device in the preparation process of the nano powder.

[0035] Figure 2 The specific structure diagram of the arc generator. Figure 1

[0036] BRIEF DESCRIPTION OF DRAWINGS: 1, arc generator; 11, electrode base; 12, electrode; 13, electrode sheath gas pipeline; 14, electrode sheath gas outlet; 2, electrode lifting device; 3, height measuring device; 4, sheath gas flow controller; 5, voltmeter. DETAILED DESCRIPTION

[0037] The nano metal powder is prepared by using a direct current arc plasma method. In the direct current arc plasma method, tungsten, graphite and other materials are used as cathodes, and the cathode electrode discharges to evaporate the metal into metal vapor. At present, in order to increase the yield of nano metal powder, the industry chooses to increase the arc current and increase the input energy, but too high temperature is easy to cause the electrode to produce ablation. Therefore, the applicant needs to obtain the length of the remaining electrode in time and accurately, and at the same time adjust the suspension height of the electrode to maintain normal discharge parameters.

[0038] ​There is no related technology in the field for accurate measurement of electrode height. The applicant considers transferring electrode detection methods from other fields to this field: in a high-temperature environment, the electrode length detection method generally uses image recognition technology, for example, CN117824778A, which uses camera recognition technology in combination with a displacement sensor to measure the distance from the electrode to the arc starting point, and uses an arc length empirical formula to measure the arc furnace molten steel level. However, this image recognition technology is not suitable for the preparation of nano metal powder, for the following reasons: first, the high-temperature arc causes the metal raw material to evaporate, and the reactor is filled with high-concentration metal particles, which interfere with the camera visual positioning and affect the electrode length measurement. Second, this method requires the camera and the electrode calibration height to have high collimation. However, the structure of the direct current arc plasma system is compact, and it is difficult to arrange a camera that meets the requirements. Finally, this method can only measure the material height / depth, but cannot measure the electrode length.

[0039] Based on the above situation, the applicant has found through a large number of studies that the remaining electrode length L ele and the material depth D can be indirectly obtained by constructing an algorithm based on the detection table provided by the nano powder production system.

[0040] The following embodiments are implemented in the following arc electrode remaining length detection and control device, and the following will be described in conjunction with the drawings Figures 1-2 of the specification.

[0041] An arc electrode remaining length detection and control device in a nano powder preparation process, referring to Figure 1 , the nano powder preparation process is carried out in an evaporation chamber, the top of the evaporation chamber is provided with an arc generator, and the bottom is provided with a crucible, the crucible serves as an anode, and the crucible is used to hold the material to be evaporated. The arc generator excites a high-temperature plasma arc stably, so that the material in the crucible evaporates, and the material forms a molten pool. The vapor state material cools extremely fast to obtain nano powder.

[0042] Referring to Figure 2 , the arc generator includes an electrode base, an electrode, and an electrode sheath gas pipeline, the electrode is fixedly connected to one end of the electrode base and serves as a cathode; the electrode sheath gas pipeline is opened around the electrode base, and the electrode sheath gas outlet of the electrode sheath gas pipeline is opened at one end of the electrode base close to the electrode. The gas flow of the electrode sheath gas outlet flows along the outer periphery of the electrode, and the electrode sheath gas flows towards the tip of the electrode.

[0043] Referring to Figure 1 and Figure 2 , the electrode base is fixedly connected with an electrode lifting device, and the electrode lifting device controls the moving distance of the electrode in the vertical direction by driving the electrode base. The height measurer is fixedly connected to the electrode lifting device, and the height measurer can automatically record the height H of the electrode according to the moving distance of the electrode lifting device.

[0044] Referring to Figure 1 and Figure 2 , an electrode sheath gas flow controller is installed in the electrode sheath gas pipeline, the electrode sheath gas flow controller is used to adjust the size of the electrode sheath gas flow, and the electrode sheath gas flow is not less than 50 SLPM. If the electrode sheath gas flow is too low, the restraining force of the electrode sheath gas on the arc is weak, the arc is easy to drift laterally, and the arc voltage is unstable. Although the measurement method of the present application does not require an upper limit value of the electrode sheath gas flow, too high electrode sheath gas flow can easily lead to unstable operation of the nanometer powder production process, so the electrode sheath gas flow is controlled to be 150-200 SLPM. At the same time, the electrode sheath gas pressure P is controlled to be not less than 40 kPa, the arc is bound by high pressure, so that the arc is maintained stable.

[0045] Referring to Figure 1 Because the electrode length is short, the electrode sheath gas outlet is close to the material liquid surface (melt pool), and after the electrode sheath gas hits the material liquid surface, a large back pressure is formed, so there is a certain relationship between the electrode sheath gas pressure P and the distance h between the electrode sheath gas outlet and the melt pool. Based on this situation, an electrode sheath gas pressure gauge is fixedly connected at the electrode sheath gas outlet to record the electrode sheath gas pressure P.

[0046] Referring to Figure 1 The input end of the electrode is connected to a direct current power supply, and a voltmeter is built into the direct current power supply, and the arc voltage U is automatically read by the control system. The no-load voltage of the direct current power supply is not less than 200V, and by setting a high voltage setting, a larger arc length can be achieved to eliminate the influence of cathode and anode voltage drop on arc voltage measurement.

[0047] Example 1

[0048] A method for detecting and controlling the remaining length of an arc electrode in a nanometer powder preparation process is performed according to the following steps:

[0049] S1, a tungsten electrode with a diameter of 10 mm and an initial length of 140 mm is prepared, and the tungsten electrode is installed on an electrode base; the arc current of the tungsten electrode is set to 100 A, the flow of the electrode sheath gas is 150 SLPM, and the laying thickness D0 of the material is 45 mm.

[0050] Because the behavior of the arc is affected by electromagnetic force and aerodynamic force, the airflow around the arc is heated to produce strong turbulence; at this time, the arc tends to take the shortest discharge path between the cathode and the anode due to electromagnetic force constraint, and discharge in the hot gas cluster due to the lower breakdown voltage of the hot gas; therefore, under the influence of the airflow, the arc cannot take the shortest discharge path, but is curved and stretched with the turbulence, resulting in an increase in arc voltage. Therefore, a percentile needs to be set to eliminate the influence of bending and stretching. If the percentile value is set too high, the average value of the statistics will include higher voltage values corresponding to more arc deformation, resulting in a larger measured arc length; therefore, the percentile is usually set below 20%.

[0051] Based on the above situation, the embodiment sets to record 10 arc voltage values per second, the recording period t is 20 s, and the 5% lower percentile is taken, and the no-load voltage of the DC power supply is 250 V;

[0052] When the system is just started, record the average arc voltage U of a period of time t at different heights H, and obtain the arc average electric field strength E corresponding to different heights H H , E H , H, and U satisfy formula (1): E H =U / (H0-D0-a-L ele0 );

[0053] Wherein, a is the height position of the electrode lifting device clamped on the electrode, which should be recorded and taken as a preset value when the electrode is installed, a=0 when the electrode lifting device is clamped near the electrode sheath gas outlet, and a=0 in the embodiment; H0 is the initial height of the electrode, D0 is the laying thickness of the material 45mm, and L ele0 is the initial length of the electrode 140mm.

[0054] At the same time, simulate the relationship between the distance h between the electrode sheath gas outlet and the molten pool and the electrode sheath gas pressure P, and h and P satisfy formula (2): h=A·P B +W; A, B and W are all empirical parameters, and the unit of h is mm and the unit of P is kPa; at this time, the unit of W is mm, B has no unit, and the unit of A is mm×kPa^(-B); when the measurement accuracy requirement is not high, W is 0mm; therefore, according to experience, the embodiment regresses formula (2) to be h=1.2×P 1.13 .

[0055] S2, record the arc voltage U' and the electrode sheath gas pressure P of the electrode at a certain height H', and calculate the arc length L arc , L arc =U' / E H ;

[0056] Substituting the electrode sheath gas pressure P into formula (2), h can be calculated;

[0057] The distance h between the electrode sheath gas outlet and the molten pool minus the arc length L arc That is, the electrode length L ele L ele =hL arc ;

[0058] When the system is first started, the electrode height is raised to H, which is 287.20 mm. The average arc voltage U at the 5th percentile within 20 seconds is recorded as 186 V. Subtracting the initial electrode length L... ele0 After setting the material thickness to 140mm and the laying thickness D045mm, the corresponding electric field strength E is calculated. H It is 18.2V / cm;

[0059] The arc voltage U' at an electrode working height H' = 174 mm is read as 20 V. The arc length L is then calculated. arc It is 11mm;

[0060] At the same time, the electrode sheath gas pressure P was read as 65 kPa, and substituted into formula (2) h=1.2×P 1.13 In the calculation, h is found to be 134 mm;

[0061] Subtract the arc length L from h arc Obtain the electrode length L ele It is 123mm;

[0062] S3. Measure the distance C between the electrode terminal and the initial material height before starting the system;

[0063] After the system is started, the material depth D can be calculated according to formula (3), which is D=H'-L. arc -L ele -C, where C is 2mm;

[0064] The H and L calculated in step S2 arc and L ele Substituting the calculated value into formula (3), the material depth D is calculated to be 38mm;

[0065] Measured electrode length L after system shutdown ele The thickness is 133mm, and the material depth D is 45mm.

[0066] Conclusion: The error rate between the measured value and the calculated theoretical value of the electrode length is 7.5%, and the error rate between the measured value and the calculated theoretical value of the material depth is 15.6%.

[0067] The applicant found that the measured value was lower than the actual value, the main reason was that the electric field strength E used in the measurement did not consider the voltage drop near the electrode, resulting in the calculated arc length L arc was higher than the actual value, so that the electrode length L ele and the material depth D were underestimated. However, the measurement error value was less than 10mm, which had practical significance in the direct current arc plasma system.

[0068] Example 2

[0069] A method for detecting and controlling the residual length of an arc electrode in the preparation process of a nano-powder, according to the following steps:

[0070] S1, continue to use the residual electrode of example 1, set the arc current of the tungsten electrode in this example to 120A, and the flow rate of the electrode sheath gas to 150SLPM; on the basis of example 1, continue to start the system; since the system will continue to supplement the material during operation, the laying thickness of the material is maintained at 45mm;

[0071] This example sets to record 10 arc voltage values per second, the recording period t is 20s, and takes 5% lower quantile, the no-load voltage of the direct current power supply is 250V;

[0072] S2, raise the electrode height to H, H is 283.67mm, record the arc average voltage U of 5% quantile in 20s as 205V, deduct the initial length L ele0 133mm of the electrode and the laying thickness D045mm of the material to calculate the corresponding electric field strength E H is 19.4V / cm;

[0073] Read the arc voltage U' of the electrode working height H'=160mm as 37V, calculate the arc length L arc as 19mm;

[0074] At the same time, read the electrode sheath gas pressure P as 59kPa, substitute into formula (2) h=1.2×P 1.13 , calculate h as 121mm;

[0075] Deduct the arc length L arc from h to get the electrode length L ele as 102mm;

[0076] S3, substitute the calculated values of H, L arc and L ele in step S2 into formula (3), calculate the material depth as 37mm;

[0077] After shutting down the system, the actual electrode length L ele110mm, and the material depth D is 45mm.

[0078] Conclusion: the error rate between the measured value of the electrode length and the calculated theoretical value is 7.2%, and the error rate between the measured value of the material depth and the calculated theoretical value is 17.8%.

[0079] Example 3

[0080] A method for detecting and controlling the residual length of an arc electrode in the preparation process of a nano-powder is performed according to the following steps:

[0081] S1, continue to use the residual electrode of Example 2, set the arc current of the tungsten electrode in this example to 150A, and the flow rate of the electrode sheath gas to 200SLPM; on the basis of Example 2, continue to start the system; since the system will continuously supplement the material during operation, the laying thickness of the material is maintained at 45mm;

[0082] This example sets to record 10 arc voltage values per second, the recording period t is 20s, and takes 5% lower quantile, the no-load voltage of the direct current power supply is 250V;

[0083] S2, raise the electrode height to H, H is 260.67mm, record the arc average voltage U of 5% quantile in 20s as 205V, and deduct the initial length L of the electrode ele0 110mm and the laying thickness D of the material 045mm, the corresponding electric field strength E is calculated as H 19.4V / cm;

[0084] Read the arc voltage U' of the electrode working height H' = 182mm as 80V, and calculate the arc length L arc as 41mm;

[0085] At the same time, read the electrode sheath gas pressure P as 68 kPa, substitute into formula (2) h = 1.2 x P 1.13 , and calculate h as 141mm;

[0086] Subtract the arc length L from h arc to obtain the electrode length L ele as 100mm;

[0087] S3, substitute the calculated values of H', L arc and L ele in formula (3) and take C as 2mm, and calculate the material depth as 39mm;

[0088] After the system is turned off, the measured electrode length L ele is 103mm, and the material depth D is 44mm.

[0089] Conclusion: the error rate between the measured value of the electrode length and the calculated theoretical value is 2.9%, and the error rate between the measured value of the material depth and the calculated theoretical value is 11.1%;

[0090] The measurement error of Example 3 is significantly lower than that of Example 1 and Example 2. The main reason is that the electrode sheath gas is increased, and the electrode sheath gas has a constraint effect on the arc, so that the calculated arc length L arc The deviation of the distance between the electrode terminal and the material is smaller.

[0091] The technical features of the above-described embodiments can be combined in any manner. In order to make the description simple, all possible combinations of the technical features in the above-described embodiments are not described, however, as long as the combinations of the technical features do not contradict, they should be considered as the scope of the description.

[0092] In addition, the above-described embodiments only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the scope of protection of the patent of the present application should be subject to the appended claims.

Claims

1. A method for detecting and controlling the residual length of an electrode in the process of preparing nano-powder, characterized in that: The method comprises the following steps: S1, when the system is just started, record the average voltage U of the arc in a period of time t at different height H, obtain the average electric field strength E corresponding to different height H H , E H , H, U meet formula (1): E H =U / (H0-D0-a-L ele0 ) Wherein, a is the height position of the electrode lifting device clamping the electrode, which should be recorded and taken as a preset value when the electrode is installed, H0 is the initial height of the electrode, D0 is the laying thickness of the material, L ele0 is the initial length of the electrode; A relationship between a distance h of the simulated electrode sheath gas outlet from the molten pool and an electrode sheath gas pressure P, h, P satisfying formula (2): h=A·P B + W, A, B and W are all empirical parameters; S2, the arc voltage U' and the electrode sheath gas pressure P at a certain height H' of the recording electrode, so as to calculate the arc length L arc , arc L H = U' / E The electrode sheath gas pressure P is substituted into formula (2) to calculate h; The distance h of the electrode sheath gas outlet from the molten pool minus the arc length L arc i.e. the electrode length L ele i.e. L ele = h - L arc .

2. The method of claim 1, wherein the method further comprises: determining the remaining length of the electrode based on the measured current and the measured voltage. The electrode sheath gas pressure P is not less than 40 kPa.

3. The method of claim 1, wherein the method further comprises: determining the remaining length of the electrode based on the measured current and the measured voltage; and controlling the electrode based on the determined remaining length of the electrode. The arc voltage U is the average value of x% quantile in a period of time t, and the value of x is 0-20.

4. The method of claim 1, wherein the method further comprises: determining the remaining length of the electrode based on the measured current and the measured voltage; and controlling the electrode based on the determined remaining length of the electrode. In the S1 step, the electrode height H is raised to make the arc voltage U close to the no-load voltage of the direct current power supply.

5. The method of claim 1, wherein the method further comprises: determining the remaining length of the electrode based on the measured current and the measured voltage. After the S2 step, the material depth D can be calculated according to formula (3), formula (3) being D = H'-L arc -L ele -C, C being the distance between the electrode terminal and the initial material height in the initial state.

6. An apparatus for detecting and controlling the remaining length of an electrode in the process of manufacturing nano-powder, characterized in that, The electrode remaining length detection and control method for the nano-powder preparation process of any one of claims 1-5 comprises: An arc generator comprises an electrode base, an electrode fixed on the electrode base, and an electrode sheath gas pipeline opened on the electrode base, wherein the electrode sheath gas outlet of the electrode sheath gas pipeline is located at the top end of the electrode; An electrode lifting device is fixed on the electrode base to control the electrode to rise and fall in the vertical direction; A height measurer is installed on the electrode lifting device to record the electrode height H; A sheath gas flow controller is installed on the electrode sheath gas pipeline to record the electrode sheath gas flow and control the sheath gas flow; An electrode sheath gas pressure gauge is installed on the electrode sheath gas pipeline to record the electrode sheath gas pressure P; A voltage gauge is electrically connected to the electrode to record the arc voltage U.

7. The apparatus for detecting and controlling the residual length of the electrode in the nano-powder production process according to claim 6, wherein the apparatus further comprises a controller for controlling the driving of the electrode according to the detected length of the electrode. The electrode sheath gas points to the electrode tip, and the electrode sheath gas flow is not less than 50 SLPM.

8. The apparatus for detecting and controlling the residual length of the electrode in the nano-powder production process according to claim 7, wherein the apparatus further comprises a controller for controlling the driving of the electrode according to the detected length of the electrode. The electrode sheath gas flow is controlled to be 150-200 SLPM.

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