JP3758001B2 - Apparatus and method for efficient haze containment by hood - Google Patents

Description

[0001]
The present invention relates to a breathable enclosure containing steam and preventing its diffusion. Such an enclosure is commonly known as a fume hood. More specifically, the present invention relates to a fume hood that can be opened to allow access to the interior. Here, there is a possibility that the smoke may unintentionally leak outside the hood due to the opening. The present invention relates most particularly to a fume hood that controls the speed of entry and the volume of make-up air.
[0002]
Fume hoods are well known in industrial and scientific facilities where it is desirable to prevent the diffusion of volatile substances from the workplace, especially toxic substances, and to prevent inhalation of toxic substances by those who handle them. ing. Hoods can vary in size, from devices that contain only the hands or arms of an operator, to large devices and large equipment that can be accommodated by one or more people. Typically, a hood takes air that does not contain toxic substances into the hood and at a predetermined (which can be changed from time to time) from the inside of the hood to a safe discharge point away from the hood. An enclosure with an exhaust system for exhausting air and fumes. Closed enclosure openings, such as doors and sashes that can be slid vertically, generally allow air to enter the hood and as needed by the person and equipment involved in the operation performed in the hood Can be involved.
[0003]
A common goal in using a fume hood is to prevent backflowing and leaking of fumes from the door when the door is open. One strategy is to increase the airflow through the hood while the door is open. However, an increase in airflow can increase turbulence in the hood, which can cause the air that is actually smoky to be expelled from the door on a single occasion. Furthermore, the air introduced into the hood and then exhausted into the atmosphere may be adjusted for human comfort at some cost, so a high flow hood is a waste of energy. It is expensive to drive.
[0004]
US Patent No. 4,741,257 issued to Wiggin et al. On May 3, 1988, measures the air pressure inside and outside the hood and regulates the flow of air into the hood to maintain a constant pressure differential A system is disclosed. If the hood door or sash is open or closed, rebalance the differential pressure so that the air velocity through the hood “face” or empty area, regardless of the position of the sash. To keep it constant, the system increases or decreases the incoming air flow. The underlying assumption of this control system is that the optimal control method is to keep the air velocity through the face of the hood constant. This assumption is not necessarily accurate because it does not address what is the optimal differential pressure and thus what is the optimal airflow through the hood to prevent backflow. The flow governed by this system can waste conditioned air. Although energy may be saved by reducing the overall volume, the study of trace gases made by the inventor has shown that the fume hood is inherently safer by using variable volume fixed face speed control. Showed that it should not. Fume hoods that employ different designs and have different dimensions may have different optimum flow rates and flow rates, which also include absolute and fluctuating pressure outside the hood, airflow and activity at the front of the hood, Susceptible to environmental conditions, including inner loads, as well as the temperature of make-up air entering the hood.
[0005]
A typical upper exhaust hood basically has two zones. It is a second chamber that may have a lower surface velocity working chamber and a vortex that provides an exhaust system above it. It has been found that the strength and stability of the vortex is substantially more important in controlling the backflow than the surface velocity of the air entering the hood. If the flow velocity is very small, the vortices are unstable and do not appear clearly, and backflow may occur through the open sash. If the airflow is too fast, the vortex may collapse or be destroyed and replaced by a violent airflow movement that causes the fumed column to erupt from the hood face. When the airflow is properly adjusted, a smooth vortex is formed in the upper chamber, which efficiently captures the fumes from the lower working chamber, carries the airflow to the outlet, and reverses the fumes through the face. To prevent.
[0006]
There is an important differential pressure between the vortex chamber and the outside of the hood at an important location on the side wall of the vortex chamber, which can be determined by the method described later. This differential pressure varies with the volume of air flowing through the hood and the percentage of the face open area. A stable vortex forms a laminar flow, and under these conditions the sensor placed in the critical position in the vortex detects minimal pressure fluctuations between the interior of the vortex chamber and the exterior of the hood. This condition indicates the optimal airflow through the hood and the optimal differential pressure. However, under conditions where the vortex is unstable, the sensor detects slight pressure fluctuations associated with intense or chaotic flow through the upper chamber. The dynamic control system of the present invention alters the flow of make-up air from the lower chamber to the vortex chamber to form a stable vortex, thereby minimizing the perceived pressure change. In order to achieve this, feedback sensing of pressure fluctuations is used.
[0007]
Measurement of vortex pressure at the boundary wall requires a differential pressure transducer that can measure velocity pressure in real time on the order of .000015 inches on a water level gauge. Because the membrane mass and distance traveled are too large and the reaction time is too long, the known mechanical differential pressure fluctuation sensors are inadequate. Known hot-wire resistance temperature devices (RTD's) or thermistors cannot provide sufficient gain in the first stage for real-time measurements.
[0008]
It is a primary object of the present invention to provide an improved apparatus and method that minimizes turbulence in the swirl chamber of a fume hood.
[0009]
It is a further object of the present invention to provide an improved control means that minimizes counterflow escape of air containing fumes from the fume hood.
[0010]
It is a still further object of the present invention to provide an improved responsive sensor that detects differential pressures as low as .000015 "wc or less.
[0011]
It is a further object of the present invention to provide an improved apparatus and method for maximizing the fume hood's fume content while minimizing the amount of make-up air used within a period of time. Is the purpose.
[0012]
Briefly, the system according to the present invention has a responsive differential pressure sensor located at a critical and determinable location on the wall of the vortex chamber above the working chamber. In the operating state, due to the low pressure of the vortex chamber, there is a slight continuous flow of air that enters the vortex chamber from the outside of the hood through the sensor. When the system detects a very small and very fast change in pressure in the vortex chamber, the system assumes that the vortex has become unstable, and changes the airflow entering and exiting the vortex to reduce turbulence and stabilize the vortex A drive signal for adjusting one or more movable plates is generated. This sensor may be used in an uncontrolled manner as part of an alarm system to alert a hood operator of a potentially dangerous backflow condition.
[0013]
The sensor has a pair of matched linear thermal Zener diodes on opposite legs of a resistive bridge that functions as a signal converter. One of the diodes is coupled to a heat sink and provides a fixed heat loss bias between the sensors that mechanically unbalances the matched sensor and prevents electronic fluctuations. The bridge circuit has a two-stage output amplifier. The sensors are shielded by exactly the same metal tube pieces, which are separated from the diodes by rubber spacers. It reacts differentially to cooling by air passing over the thermal diode. The air velocity is calibrated to directly show the variation in the differential pressure between the inside and outside of the hood. For example, a 2 volt output of the bridge can be matched to an amplification variation of .000015 "wc.
[0014]
A linear diode coupled with a fixed thermally actuated mechanical structure allows the measurement of slight pressure changes at the walls of the vortex chamber, indicating vortex integrity and stability. The new inlet nozzle and aerodynamic pickup geometry do not accept similarly sized undesirable energy waves from noise sources outside the hood, such as building HVAC pulsations and wind.
[0015]
There is no universal sensor optimum position that can be applied to all hoods. Based on the height and depth of the vortex chamber and the ratio of the surface air velocities measured at the two differently sized openings in the face, I am able to determine the optimal position and math for any hood. Got the formula.
[0016]
The bridge circuit, including sensors and amplifiers, is a vortex differential pressure transducer that sends real-time signals to a dedicated analog controller with a proportional integration algorithm and an adaptive gain algorithm. The output of the controller is sent to an actuator that changes the position of the bypass air damper in the hood to change the volume of make-up air entering the vortex chamber. This control system looks for a damper position where a minimum pressure fluctuation is sensed by the sensor, ie indicating the presence of a stable vortex. A second damper at the exit from the vortex chamber may also be operated by the control system to help obtain adequate air flow through the vortex chamber. If the minimum position cannot be obtained within the operating range of the control damper, the output signal activates the exhaust damper of the hood and causes the exhaust fan to turn down so that the system is within the control range of the control damper.
[0017]
This technique is particularly useful for controlling multiple smoke hoods that are coupled to a single exhaust system. It is impossible to adjust the central exhaust system to optimally work with multiple hoods at various locations within a building. The discovery of how vortices can be optimized by manipulating dampers in the hood to protect the fume hood user is a very important advance in terms of operator safety It is. If for some reason the vortex cannot be stabilized by operating the exhaust damper and bypass damper, the system can be modified to issue an unsafe condition alarm. This alarm is not based on the surface speed as disclosed in the prior art, but on the captured quality of the smoke hood. This technique can also save energy because a lower face speed is used when the hood face area is small (the door is almost closed).
[0018]
The above and other objects, features and advantages of the present invention, as well as presently preferred embodiments, will become more apparent upon reading the following description with reference to the accompanying drawings.
[0019]
FIG. 1 illustrates a typical prior art fume hood having a housing 12 that includes various elements that control or direct the movement of air into, within, and from the hood. ing. A door or sash 14 that can be slid vertically allows the opening or face 16 to be adjusted for entry and exit of the operator and the air hood. The inside of the hood has a working chamber 18 and a vortex chamber 20 above it, and may have a working light 21. The exhaust chimney 22 includes a fan 24 (not shown in FIG. 1) that draws air into the hood from the face 16 and the inlet 17 that contacts the floor and expels the air from the opening in the housing 12. . Airflow from the entrance of the first hood enters mainly from the face 16, flows toward the rear plate 26, and goes up to the angled plate 28. Angled plate 28 helps to rotate the airflow into the cylindrical vortex 30. The hood has a second air flow path to provide an airflow through the hood floor. The plates 26 and 28 are located away from the rear wall 32 and form a passage 33. Plates 26 and 28 are also away from the upper portion 34 and form an outlet 36 from the vortex chamber 20. In addition, the plate is also away from the bottom 38 and forms a lower opening 40. The plate 26 may have an intermediate hole 42 that leads to the passage 33.
[0020]
FIG. 2 illustrates a hood 10 that has been modified in accordance with an embodiment of the present invention. As described in more detail below and shown in FIG. 9, the sensing stage 44 of a dynamic differential pressure transducer 45 having a pair of matched thermal Zener diodes disposed on opposite legs of an electronic bridge circuit. Is located outside the hole in the side wall 46 of the hood and senses a slight change in the air flow through the hole due to a very slight change in pressure in the vortex chamber to indicate air turbulence. Airflow turbulence is an unstable and undesirable condition in which vortices are absent or incompletely formed, and the fumes may escape from the face of the hood. If such a change is sensed, the signal due to the thermally induced electrical imbalance between the pre-diodes includes a signal amplifier, preferably the first of the converter located outside the hood. Sent to stage 2. The amplified signal in real time is sent to a feedback controller 50, preferably an analog computer, shown schematically in FIG. 10, having a proportional integral and adaptive gain algorithm. The signal may also be sent to alarm 51, which may have a variable threshold discriminator that provides a predetermined alarm limit. The output of controller 50 is sent to actuator 52, preferably a servo stepping DC motor actuator. The actuator moves the outlet damper 54 and the bypass damper 56 in synchronism by means of a connecting chain or cable 58. To get more air into the vortex, the bypass damper 56 is moved to further limit the bypass air passing through the bottom slot 40 and the outlet damper 54 is moved to open the outlet slot 36 further. Conversely, to reduce the air flowing into the vortex, the bottom slot is opened and the outlet slot 36 is squeezed to bypass the air to the passage 33. The system seeks a state where the change in signal from sensor 44 is zero, which indicates that there is a stable vortex in the vortex chamber. If a zero condition is not obtained, the system assumes that the total airflow through the hood is inadequate and the second actuator 53 has a throttle damper 55 in the exhaust chimney to change the total airflow. Operate.
[0021]
The correct position of the pressure sensor on the side wall of the hood can be determined for any hood as shown in FIG. 3 by the following procedure.
[0022]
First, all inlets, such as floor sweep bypasses, are sealed, allowing only air to pass through the face. Open the sash to the maximum release position.
[0023]
Second, the hood displacement is set to an average face speed of 100 feet per minute, for example by changing the speed of the exhaust fan or adjusting the exhaust damper.
[0024]
Third, close the sash 50% and measure the average face speed.
[0025]
Fourth, divide the 100 values obtained in the third step, to obtain a pure hood index number called R ^2.
[0026]
R ² = 100 fpm / average face speed fpm (1)
Fifth, measure the height A and depth B of the vortex chamber at the middle of the top and front, respectively, and calculate the radius C having the origin O at the orthogonal point of A and B by the following formula Use measurements to do this.
[0027]
C = {√ (2.98xAB / π)} / 2 (2)
Sixth, the distance D from the origin O to the upper end of the opening F of the face is measured, and C is subtracted from this value to obtain X.
[0028]
X = D-C (3)
Seventh, taking Z = X / 2, the placement factor b is calculated by the following equation.
b = Z (R ² )
Eighth, a sensor is disposed at a distance of (b + Z) along the line D from the opening F. The structure of the sensing element is shown in FIGS. The first thermal diode 60 and the second thermal diode 62 are electrically identical in their response to heat and are connected to respective legs 64 and 66 of the electronic bridge 45 as shown in FIG. Each diode is shielded by a length of thin-walled metal tube 70 and 72, said shielding tube, for example, a 0.220 inch long, 0.094 inch outer diameter, 0.006 inch wall thickness brass tube, etc. Are exactly the same in size and material. Each tube is insulated from the diode by a rubber spacer 74 and has a uniform air gap between the sensor glass beads and the shield tube. Diode 60 is a reference diode in the bridge. The tube 72 around the diode 62 is mechanically attached by soldering to the anode lead of the diode, providing a mechanical heat sink 73 to the diode 62 as a sensing diode. Heat is dissipated faster from the diode 62 than from the diode 60. A balanced current bridge attempts to maintain a constant current through both legs of the bridge, but air passing across the diode creates a resistance-current imbalance and generates a carrier output signal from the bridge.
[0029]
This mechanically induced electrical disturbance provides several advantages. First, the mechanically induced imbalance never changes. As a result, there is no drift over time between the two sensors that may have been experienced with known electronic imbalance techniques. Second, the combination of a linear thermal diode and a metal shield tube provides high gain sensing at low temperatures and can prevent soil burn-in that can occur with hot wire thermal resistance temperature devices. Third, the diode used in this configuration can sense differential pressures as low as 0.000015 inches of the water level gauge (WC) and can deliver the amplified output from a DC 2 volt bridge.
[0030]
The thermal diode having this shield is fixed to the respective leads 76 and 78 in the window 80 of the sensor housing 82. These leads are connected via cable 84 to the input leads of second stage 48 having first and second amplifiers 86 and 88 and other known resistors of an electronic bridge. The end of the window is preferably chamfered so that the depression angle α is about 60 degrees. The window throat 90 is preferably about 0.322 inches wide.
[0031]
As shown in FIGS. 7 and 8, the sensor housing 82 is disposed in a recess in a mounting base 92 that is attached to the outer surface 93 of the side wall 46 of the hood adjacent to the side wall hole 94. The mounting base 92 is preferably substantially circular and surrounds the hole 94. Since the noise in the form of pressure pulses generated outside the hood is sensed by the diode and can be attributed to false signal generation, the pedestal 92 effectively removes irrelevant pulses from outside the hood. An annular nozzle that can be damped is preferred. In the preferred embodiment, the diode is located a distance 96 between 0.2 inches and 3.4 inches from the inner surface 98 of the sidewall 46, and the hole 94 has a diameter 95 of 0.690 inches. Starting from the end of the well 91, the nozzle of the mounting base 92 has a radius of curvature 97 of 0.600 inches, an opening diameter 100 of 1.5 inches, and a depth of the mounting base 92 to the recessed holes 91 of 0.388 inches. Curved outward to point. Beyond a diameter of 100, the surface of the mounting bends outward, and the expected operating pressure range up to a diameter of about 102 times the diameter of the opening, which is about 4 times the diameter 95 and the installation depth is 0.450 inches FIG. 6 shows a substantially parabolic inlet curvature, which is an approximate copy of the square root function of the air flow in the air versus the fume hood differential pressure. Outside the diameter 102 location is a planar annular surface 103 that is 0.031 inches wide and substantially parallel to the sidewall 46.
[0032]
Since the control of the fume hood vortex is a time-varying non-linear process, it is controlled in real time by the use of an adaptive algorithm. The self-adaptive gain controller 50 shown in FIG. 10 compensates for various loop gains and takes the place of a human fume hood operator that adjusts the controller at different sash positions. The symbols in FIG. 10 are generally accepted names for the components of the controller 50 and describe the control algorithm. The control algorithm can be implemented in either an analog or digital control loop (shown). From the viewpoint of response speed, analog control is preferable to digital control.
[0033]
An adaptive controller coordinates system braking without compromising real-time control requirements. When the vortex is stable, no braking is necessary and the proportional band is narrow. However, when disturbance occurs, the band is widened by dispersing the deviation in the frequency band. If the oscillations are normal, they pass through the high frequency channel. The signal is then rectified and the result is sent to the positive and negative deviation adapters. The integrator responds proportionally to the deviation by increasing the proportional band of the controller. At the same time, the low frequency band of the deviation is amplified by the gain K, rectified and sent to the integrator. When there is an offset, drift or reaction delay, the integrator reduces the proportional band of the controller and returns the vortex to its initial position.
[0034]
The vortex differential pressure transducer 45 can also be used as a part of a stand-alone hood alarm system, as shown in FIG. For hoods that do not have the vortex control device of the present invention, it is important for the operator to know when the hood is not under proper airflow control and a backflow of fumes may occur. The output signal of the converter 45 may be sent directly to an alarm device 51 having a critical discriminator that distinguishes the alarm signal from the carrier signal.
[0035]
From the foregoing, it will be apparent that an improved device is provided and optimally a method for dynamically containing fumes in the hood. In the present apparatus and method, differential pressure is sensed at certain critical locations in the vortex chamber, and the airflow passing through the vortex chamber is varied under control to maintain a firm vortex. Changes and modifications to the apparatus and methods described herein in accordance with the present invention will be apparent to those skilled in the art. Therefore, the above description should be understood as illustrative and should not be construed in a limiting sense.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view of a prior art hood without vortex sensing and active control.
FIG. 2 is a longitudinal sectional view similar to FIG. 1 showing vortex sensing and control elements according to the present invention.
FIG. 3 is a schematic cross-sectional view of a fume hood vortex chamber illustrating the correct placement of the vortex pressure sensor.
FIG. 4 shows the installation of the thermal diode in the sensor.
FIG. 5 is a plan view of the sensor.
FIG. 6 is a cross-sectional view of the sensor along line 6-6 in FIG.
FIG. 7 is a cross-sectional view of a sensor disposed for operation on a nozzle member extending through the side wall of the vortex chamber.
FIG. 8 is a cross-sectional view of the nozzle inlet curve of FIG. 7 showing dimensions that prevent the nozzle from receiving unwanted pressure waves from outside the vortex chamber.
FIG. 9 is a design diagram of a dynamic eddy pressure transducer including the sensor shown in FIGS.
FIG. 10 is a design diagram of an adaptive gain controller that converts an input signal from the vortex pressure converter into an output signal to a fume hood baffle plate adjusting servo mechanism and an exhaust damper.
FIG. 11 is a schematic longitudinal sectional view of a hood provided with a fume hood performance alarm including the eddy pressure sensor and the converter of the present invention.

Claims (17)

In a fume hood having an adjustable opening leading to the working chamber, a vortex chamber above the working chamber, and an exhaust system having a fan, the vortex chamber is variably bypassed through the passage to the exhaust system. Dynamically controlling the amount of airflow passing through
a) Vortex differential pressure transducer,
b) an electronic controller that receives the amplified signal from the vortex differential pressure transducer, processes the signal, and provides a controller output signal;
c) an actuator responsive to the output signal from the electronic controller; and
d) changing the amount of air passing through the vortex chamber by changing the amount of air that is in the bypass passage and is operable by the actuating device and passes through the bypass passage to the exhaust system; A system with a trigger. The converter is
a) a sensor for detecting a change in the pressure difference between the vortex chamber and the outside of the hood, which is arranged in the passage of the vortex chamber and is arranged close to a hole penetrating the wall of the vortex chamber; and
2. The system according to claim 1, comprising an electronic balance bridge comprising an operational amplifier for amplifying the signal from the sensor. The system according to claim 2, further comprising a nozzle inlet outside the hole in the wall of the vortex chamber. The sensor has first and second thermal reaction diodes disposed in a passage of air flowing through the hole to a vortex chamber, the diodes being connected by electrical leads to opposite legs of the electronic balance bridge. And a first and a second annular shield insulated from the conductor, respectively, the first shield being thermally conductively attached to the conductor of the first diode to form a heat sink from the first diode. 4. The system according to claim 3, wherein the first diode is a signal diode and the second diode is a reference diode. 5. The system according to claim 4, further comprising a sensor housing having a window with opposing inlet ends chamfered with the diode being exposed to an airflow passing through the aperture and having an inlet depression angle of about 60 degrees. The nozzle is substantially annular, the hole has a diameter of about 0.7 inches, the sensor housing is disposed in the recess of the nozzle, and the curvature of the annular nozzle has three regions. The first region starts at the end of the well and has a radius of curvature of about 0.600 inches to a point where the nozzle opening diameter is about 1.5 inches and the nozzle depth to the well is about 0.388 inches. Curved outwardly, the second area is operated from the first area to the nozzle opening diameter position which is about 4 times the diameter of the hole and the depth of the nozzle to the recess is 0.450 inch. Curved outward with a substantially parabolic curve that is an approximate copy of the square root function of air flow versus differential pressure within the pressure range, the third region extending from the right second region, approximately 6. A system according to claim 5 which is a 0.031 inch substantially planar annular surface. 2. The apparatus further comprises a second damper that can be actuated opposite to the bypass damper by the actuating device, disposed in an outlet slot from the vortex chamber to the exhaust system, and changing an opening area of the outlet slot. By the system. A second actuating device responsive to the output signal from the electronic controller, and a second actuating that can be actuated by the second actuating device and disposed in the exhaust system to reduce the amount of exhaust by the exhaust fan The system according to claim 1, further comprising a device. Feedback where the amplification of the signal from the vortex differential pressure transducer is inversely proportional to the stability of the vortex chamber and the control system controls the amount of air passing through the vortex chamber to minimize the amplification of the signal The system according to claim 1 which is a control system. 10. The system according to claim 9, further comprising an alarm that can be activated by the eddy differential pressure transducer if the amplification of the signal exceeds a predetermined limit. The optimum position of the hole in the wall of the vortex chamber is such that F is the position of the upper end of the face opening leading to the working chamber and O is the height of the vortex chamber at the respective midpoints of the top and front of the vortex chamber. The following calculation C = {√ (2.98xAB / π)} / 2 = in the vertical plane along the line of length D connecting F and O when the orthogonal point between the depth A and the depth B is taken Radius from O X = DC Along line D Z = X / 2
R ² = Air velocity of 100 feet per minute through the wide open face of the hood divided by the average air velocity when the face opening area is reduced by 50 %
b = Z (R ² )
The system according to claim 1, characterized in that it is at a distance of (b + Z) from F using b and Z calculated by . The system of claim 1, wherein the electronic controller uses a proportional integral and adaptive gain algorithm programmed in processing the signal. The system according to claim 11, wherein the electronic controller is an analog computer. A bridge that operatively reacts to temperature changes in opposing legs, the magnitude of the reaction being directly related to the velocity of air passing over the opposing legs, the air velocity being in the opposing legs Directly related to the resulting dynamic differential pressure,
a) first and second thermal reaction diodes disposed in the air passage, the resistance value of which varies with temperature, and is coupled to opposite legs of the electronic balance bridge by electrical conductors;
b) first and second diode shields disposed around the first and second diodes, respectively, wherein the first diode is a reference diode and the second diode is a signal diode; The second shield is attached to the second diode conductor so as to be thermally conductive to form a heat sink from the second diode ;
c) first and second operational amplifiers in a feedback loop that control the voltage output of the bridge proportional to the resistance difference between the first and second diodes;
Having a sensor device. 15. A sensor device according to claim 14, wherein the first and second diodes have substantially the same thermal response. 15. A bridge according to claim 14, wherein the voltage output is variable between 0 and 2 volts. A method of optimizing the performance of a fume hood having a vortex chamber above a work chamber by dynamically optimizing vortex stability in the vortex chamber,
a) In the vortex chamber exposed to the vortex , F is the position of the upper end of the face opening leading to the working chamber, and O is the height A of the vortex chamber at the respective midpoints of the top and front of the vortex chamber. Along the line of length D connecting F and O, where
C = {√ ( 2.98xAB / π)} / 2 = radius from O in the vertical plane
X = D-C along line D
Z = X / 2
R ² = Air velocity of 100 feet per minute through the wide open face of the hood divided by the average air velocity when the face opening area is reduced by 50 %
b = Z (R ² )
Measure the change in the pressure difference between the inside of the vortex chamber and the outside of the hood at a position at a distance of (b + Z) from F using b and Z calculated by
b) A method comprising the steps of changing the flow of air entering the vortex chamber through the working chamber until the change in the differential pressure is minimized.

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