WO2007080373A1

WO2007080373A1 - Apparatus and method for injecting a liquid chemical reagent into a mixing chamber

Info

Publication number: WO2007080373A1
Application number: PCT/GB2007/000010
Authority: WO
Inventors: Shane Richard Wootton
Original assignee: Shane Richard Wootton
Priority date: 2006-01-07
Filing date: 2007-01-03
Publication date: 2007-07-19
Also published as: GB0600259D0; GB2433904A

Abstract

Apparatus (301) for injecting a liquid chemical reagent into a mixing chamber (203) to mix with a second chemical reagent. The apparatus comprises an outlet chamber (407) having an outlet duct (306) configured to allow liquid chemical reagent to be ejected from the outlet chamber through the duct, and a throttle member (414) having a head end part (416) configured to move relative to the outlet duct to control the ejection. Positioning means are configured to apply a positioning force to a rear end of the throttle member (414) such that the positioning force acts against force applied to the throttle member by pressure of liquid chemical reagent in the outlet chamber thereby positioning the throttle member. The apparatus also has a channel (415) configured to allow liquid chemical reagent to pass into a pressure chamber (451) at the rear of the throttle member so that hydraulic pressure of the reagent in the pressure chamber provides at least a portion of the positioning force, and control means (423) for controlling the pressure of liquid in the pressure chamber.

Description

APPARATUS AND METHOD FOR INJECTING A LIQUID CHEMICAL REAGENT INTO A MIXING CHAMBER

Background of the Invention The present invention relates to apparatus for injecting a liquid chemical reagent into a mixing chamber, and a method of injecting a liquid chemical reagent into a mixing chamber.

High pressure mixing systems are known for the production of potyurethane in which chemical reagents are brought into contact under high velocity having been released from respective high pressure injectors.

An example of a prior art injector 801 is shown in Figure 8 located within a mixing head 802 (shown in dashed outline) and a second similar injector 803 (shown in dotted outline).

The injector 801 comprises a nozzle 804 having an aperture 805 though which reagent is ejected, and a throttle member 806 moveable into or out of the aperture to provide a required constriction and hence operating pressure of reagent. The position of the throttle member is determined by a forwards force from a compressed spring 807 and a rearwards force on the front end of the throttle member generated by pressure of liquid reagent passing through the nozzle. The force applied by the spring 807, and consequently the pressure of the liquid reagent, is adjustable by manually adjusting a screw-threaded element 808. For example, as the element 808 is moved forward to further compress the spring 807, the operating position of the throttle member 806 is moved forwards causing further constriction of the nozzle outlet and increased pressure in the liquid reagent.

A problem with such an injector is that it requires manual adjustment if adjustment is made to the flow rate of the reagent. If manual adjustment is not made, then the resulting variation in pressure of the liquid reagent can cause poor mixing or failure of the system. A second problem with such an injector occurs when particulate contaminants become trapped at the outlet of the. nozzle. The contaminant causes further constriction of the nozzle outlet and consequently causes the pressure of the reagent to rise above tolerable limits. Thus, if such a pressure rise is observed by an operator, production of material has to be stopped while the throttle of the injector is manually opened to flush the contaminant out.

A third problem with such injectors is that any change in viscosity causes a change in operating pressure of the reagent even when the flow rate through the injector remains constant.

Brief Summary of the Invention

According to a first aspect of the present invention, there is provided apparatus for injecting a liquid chemical reagent into a mixing chamber to mix with a second chemical reagent, said apparatus comprising: an outlet chamber having an outlet duct configured to allow liquid chemical reagent to be ejected from said outlet chamber through said duct; a throttle member having a head end part configured to move relative to said outlet duct to control said ejection; positioning means configured to apply a positioning force to a rear end of said throttle member such that said positioning force acts against force applied to said throttle member by pressure of liquid chemical reagent in said outlet chamber to position said throttle member; a channel configured to allow liquid chemical reagent to pass into a pressure chamber at the rear of said throttle member so that hydraulic pressure of said reagent in said pressure chamber provides at least a portion of said positioning force; and control means for controlling the pressure of liquid in said pressure chamber.

According to an aspect of the present invention, there is provided a method of injecting a liquid chemical reagent into a mixing chamber to mix with a second chemical reagent, said method comprising the steps of: ejecting liquid chemical reagent from an outlet chamber through an outlet duct; positioning a throttle member by applying a positioning force to a rear end of said throttle member which acts against force applied to said throttle member by pressure of liquid chemical reagent in said outlet chamber; allowing liquid chemical reagent to pass through a channel into a pressure chamber at the rear of said throttle member so that hydraulic pressure of said reagent in said pressure chamber provides at least a portion of said positioning force; and controlling the pressure in said pressure chamber.

Brief Description of the Several Views of the Drawings Figure 1 shows apparatus used for the production of a material by high pressure mixing of chemical reagents;

Figure 2 shows a schematic diagram of the apparatus of Figure 1 ; Figure 3 shows an injector 301 embodying the present invention in partial cross-section, along with a partial view of the mixing head 104 in dashed outline;

Figure 4 shows the injector 301 of Figure 3 in a partial cross- sectional front view

Figure 5 shows the injector 301 of Figure 3 in a partial cross- sectional isometric view; Figure 6 shows the injector 301 of Figure 3 in a normal stable operating configuration;

Figure 7 shows the injector 301 of Figure 6 after an increase in flow rate of reagent; and

Figure 8 shows an example of a prior art injector 801.

Written Description of the Best Mode for Carrying out the Invention Figure 1

Apparatus used for the production of a material by high pressure mixing of chemical reagents is shown in Figure 1. The apparatus comprises a first storage container 101 containing a first liquid chemical reagent, and a second storage container 102 in which a second liquid chemical reagent is stored. In the present example the apparatus is configured to manufacture polyurethane, and consequently the first storage container 101 contains polyol and the second container 102 contains an isocyanate.

Pumping devices 103 provide for circulation of the chemical reagents to a mixing head 104 at high pressure. Reacted polyurethane is dispensed from the head 104 and the overall operation of the device is controlled by a control unit 105. The mixing head 104 is suspended from a boom 107, which also supports electrical cabling, hydraulic pipes, and pipes carrying the two chemical components to the mixing head 104. A remote control unit

109 mounted on the mixing head 104, is provided to allow a human operator, such as operator 108, to input commands to the control unit 105.

In the present example, the mixing head 104 is manoeuvred by the operator 108 into position to dispense the polyurethane into each of the moulds 110 and 111. The mould 110 requires material to be dispensed from the mixing head at a different rate to the mould 111, but, as will be explained below, this change in dispensing rate is facilitated by the structure of the injection apparatus within the mixing head.

Figure 2

The apparatus of Figure 1 is shown in the schematic diagram of Figure 2. The mixing head 104 has a pair of injectors 201 and 202 which, during use, inject a respective one of the two chemical reagents into a mixing chamber 203. The mixing chamber 203 has an associated production piston 204 (shown in dotted outline in Figure 2), which is moveable by a hydraulic system. The production piston is moveable between: a forward position in which it totally occupies the mixing chamber and causes injected liquids to be re-circulated back to their respective storage containers via grooves in the production piston; and a retracted position in which it allows injected chemical reagents to mix.

One end of the mixing chamber 202 is open to a purging chamber

205. The purging chamber has an associated purging piston 206 (shown in dotted outline) which is also moveable by the hydraulic system. The purging piston is moveable between: a forward position in which it occupies the purging chamber 205; and a retracted position in which it allows mixed reagents to enter from the mixing chamber and pass through an open end of the purging chamber to exit the mixing head, as indicated by arrow 207.

When the apparatus is not producing material, chemical reagents stored in storage containers 101 and 102 are circulated around a circuit at low pressure. Thus, the first chemical reagent stored in storage container 101 is circulated by a first pumping device 103A through pipe 208A, pipe 209A, a first stream distributor valve 210A, and back to the container 101 through pipe sections 211A and 212A. Similarly, the second chemical reagent stored in storage container 102 is circulated by a second pumping device 103B through pipe 208B₁ pipe 209B through a second stream distributor valve 210B, and back through pipe sections 211 B and 212B.

Fluid pressure gauges 213A and 213B are located on pipes 209A and 209B respectively and measure the respective pressures of the chemical reagents as they leave pumping devices 103A and 103B. The pressure gauges provide a digital signal indicative of the measured pressures back to the control unit 105, as well as providing a visual display for a user 108 of the apparatus.

To generate material in the mixing head, the stream distributor valves 210A and 210B are switched to prevent flow through pipe sections

211 A and 211 B and to allow the chemical reagents to be circulated to the dispensing head 104 via pipe sections 214A and 214B respectively.

In the present example, fluid flow meters 215A and 215B are located on pipes 214A and 214B respectively, and provide a signal to the control unit 105 indicative of the rate of flow of reagents along said pipes. However, the meters 215A and 215B are optional, and the apparatus can be operated without their use.

Initially, the production piston 204 is located in its forward position and consequently, the first chemical reagent is supplied to the injector 201, passed along a groove in the production piston 204 and returned via pipe sections 216A and 212A to the container 101. Similarly, the second chemical reagent is supplied to the injector 202 and returned via pipe sections 216B and 212B via a second groove in the production piston 204.

Due to the reagents being forced through the injectors, the pressures in the supply lines, monitored by meters 213A and 213B, increases. For a given flow of reagents, as measured by meters 215A and 215B, the actual pressures depend upon a positional setting of components within the injectors 201 and 202 respectively. As with conventional injectors, the positional setting of each injector is manually adjustable. Typically, for successful mixing of reagents, pressures of 150 Bar may be used.

However, once the injectors 201 and 202 are adjusted, a degree of variation to the flow rate may occur without substantially affecting the pressure in the supply line.

Once the required positional setting has been made to the injectors, production of material can commence. The production piston 204 is retracted, and reagents fed to the head 104 via pipes 214A and 214B are injected into the mixing chamber 203 (as indicated by arrows 208) where they collide to produce the reacting mixture.

Because a degree of variation to the flow rate of reagents may occur without substantially affecting the pressure in the supply line, flow rates may be purposely be adjusted without the need to make manual adjustment to the positional settings of the injectors. Thus, for example, material may be supplied to the mould 110 at a first rate and the flow rate adjusted before supplying material to the second mould 111 without the need to manually adjust the positional settings of the injectors. Figure 3

An injector 301 embodying the present invention is shown in partial cross-section in Figure 3 along with a partial view of the mixing head 104 in dashed outline. A second injector similar to injector 301 is also located within the mixing head 104, but for the purposes of clarity it is not shown in

Figure 3.

The injection apparatus 301 is located within the head 104 and maintained in position by an external screw-thread 302 on the injector 301 and a mating internal thread on the head 104. An inlet channel 303 is provided in the mixing head 104 to provide a passageway to a cavity 304 which surrounds a portion of the injector containing its inlets 305. Thus, in use liquid reagent is supplied to the inlets 305 of the injector 301 via the inlet channel 303 and cavity 304.

An outlet duct 306 at a front end of the injector 301 is located to one side of the production chamber 203 of the mixing head 104. A similar outlet on the second injector (not shown) is arranged on the opposite side of the production chamber such that during production, when the production piston is retracted, liquid reagents ejected from the outlet collide within the chamber. While the production piston 204 is in its forward position, grooves

307 formed along its side direct reagent from the injector outlets to a, respective outlet duct formed in the mixing head. For example, reagent from injector outlet 306 is directed to outlet channel 308. From the outlet channels the reagents are re-circulated back to the respective storage containers.

Figure 4

The injector 301 of Figure 3 is shown alone in the partial cross- sectional front view of Figure 4 and isometric view of Figure 5. Several of the components making up the injector 301 are similar to those forming conventional injectors such as those shown in Figure 8. Thus, the injector 301 includes a nozzle member 401, a sleeve connector 402, an injector body 403 and a linkage member 404 which are similar to components in a conventional injector. The nozzle member 401 has a circular groove 405 in its rear face from which extend a number of passageways 406 to an outlet chamber 407. A central cylindrical bore 408 extends from the rear face of the nozzle member to the outlet chamber, while the outlet duct 306 extends from the front face of the nozzle member to the outlet chamber. The sleeve connector 402 has the form of a hollow cylinder which has several radially spaced holes 409 through its wall thereby defining inlets 305. The sleeve connector 402 is positioned around the nozzle member 401 and has an inner shoulder 410 which abuts a corresponding shoulder on the outside of the nozzle member 401. The sleeve connector also has an internally threaded portion 411 which mates with a threaded portion of the body 403 so that the sleeve connector and nozzle member are rigidly fixed to the body. The body 403 has a central protrusion 412 at its front end which abuts the rear face of the nozzle member 401 such that an annular gap exists around the protrusion between the nozzle member 401 and body 403 to provide a passageway from the holes 409 in the sleeve connector to the circular groove 405.

The bore 408 of the nozzle member 401 contains the main body 413 of a throttle member 414. The main body 413 is generally cylindrical in shape and is dimensioned to be a good sliding fit within the bore 408. However, material has been ground from the main body 413 along the whole of its length to provide it with a flat face, and consequently a channel 415 exists between the flat face and the bore 408. In the present case, the main body has also been ground down at its rear face to produce a second flat face at 45 degrees to the rear face such that it widens the rear end of the channel 415. A head portion 416 of the throttle member 414 is formed at the front end of the main body 413. The head portion has a cylindrical tip that is dimensioned to be a good fit within the outlet duct 306.

A spigot formed on the rear end of the throttle member 414 extends through a bore 451 in the injector body 403 and into a hole in the linkage member 404.

A helical positioning spring 417 is located between the rear of the linkage member 404 and a spacer 418 which is itself held in place by a body extension 419. The helical spring 417 and spacer 418 are chosen such that the spring is placed in compression and consequently applies a light force to the rear of the throttle member 414 via the linkage member 404.

The body extension 419 has a male threaded portion 420 configured to mate with an female thread formed in the bore of the injector body 403. A shoulder 421 is provided next to the threaded portion 420 to trap a seal 422 between it and the end face of the injector body. Thus, the body extension provides a stop for one end the helical spring 417 and, with the seal 422, seals the bore of the injector body. In addition, as will now be described it also provides a housing for a pressure relief valve 423. A bore 424 extends through the whole length of the body extension, and comprises a narrow bore portion 425 connected to a wide bore portion 426. At the rear end 427 of the body extension 419, the wide bore portion 426 is threaded and contains an adjuster screw 428. The adjuster screw has a threaded portion that mates with the thread of the bore, and a plain cylindrical portion that is a good sliding fit within the non-threaded part of the wide bore portion. The cylindrical portion of the adjuster screw is provided with an annular groove which retains an o-ring 429 to provide a seal between the screw and the bore.

Where the wide bore portion 426 meets the narrow bore portion 425, the bore is provided with a flat circular face 430 which forms a valve seat on which may rest the valve 423.

The valve 423 is in the form of a cylinder 431 having a concentric circular ridge 432 formed on one face and a spigot 433 extending from the other face. The cylinder 431 has a diameter which allows the valve to slide within the bore 424 and also allow liquid reagent to pass between its cylindrical face and the bore. The spigot 433 and a similar spigot extending from the adjuster screw 428 are located within either end of a helical valve spring 434. This spring may be of the type generally used in conventional injectors. The valve spring 434 is placed in compression by tightening the adjuster screw 428. Consequently, the valve spring 434 tends to push the circular ridge 432 of the valve against the valve seat 430 thereby closing off the end of the narrow bore portion 425.

An aperture 435 is provided through the wall of the body extension to provide a drain from the wide bore portion. This aperture may be open to the atmosphere to allow the measurement of leakage flow during testing, but in the present embodiment a pipe connects it to the relevant low pressure return pipe, such as pipe 216A or 216B, so that the reagent is recycled.

During operation, high pressure liquid chemical reagent passes through the inlet holes 409 in the sleeve connector 402, though the gap between the injector body 403 and the nozzle member 401 and into the circular groove 405 of the nozzle member. The reagent then passes from the circular groove down the passageways 406 to the outlet chamber 407. If the injector has not been previously used the throttle member 413 is pushed hard back by the pressure of reagent in the outlet chamber 407, so that its rear face rests against the front face of the injector body 403 as shown in Figure 4. Consequently, the outlet duct 306 is fully opened allowing reagent to pass through easily. However, a portion of the reagent passes up the channel 415 between the throttle member 413 and the bore 408 of the nozzle member 401, it fills the bore 451 of the injector body passes through the narrow bore portion 425 of the body extension 419 and pushes against the relief valve 423. Consequently, pressure builds within the reagent in the bore 451 of the injector body and urges the throttle member 414 forwards. This forward movement tends to move the head portion 416 of the throttle member towards the outlet duct 306 to close it.

Consequently, pressure increases in the outlet chamber 407 to resist further forward movement of the throttle member 414. The throttle member 414 therefore assumes a position such that the force applied to its front end by the pressure of reagent in the outlet chamber balances the force applied to its rear end by the pressure of reagent in the injector body added to the light force provided by the positioning spring 417.

As pressure in the injector body rises further it eventually reaches a threshold value where it provides sufficient force to the pressure relief valve 423 to lift it from its seat. Reagent then leaks around the valve to into the wide bore portion 426 and out of the aperture 435.

The threshold pressure at which the valve opens depends upon the force applied by the valve spring 434 which itself depends upon the degree to which the spring is compressed. As the compression of the spring is adjustable by adjustment of the adjuster screw 428, the adjuster screw provides a means of adjusting the threshold pressure.

Once the pressure in the injector body has reached the threshold value and the pressure relief valve has opened, the pressure of reagent in the bore of the injector body is unable to rise substantially further. Thus, the throttle member 414 takes up a stable position where the force on its front end due to liquid pressure of reagent passing through the outlet chamber

407 equals that due to the liquid pressure in the bore 451 of the injector body when added to the force from the positioning spring 417. In this stable situation, the flow of reagent along the channel 408 is equal to the flow escaping through the pressure relief valve 423 and therefore liquid pressure in the bore 451 is stable. The injector will then be in a configuration as illustrated in Figure 6.

Figure 6

The injector 301 of Figure 4 is shown in a normal stable operating configuration in Figure 6. Thus, the pressure relief valve 423 is open, and the throttle member 414 is in an intermediate position between fully forward and fully retracted. Consequently, the majority of liquid chemical reagent entering the outlet chamber 407 is ejected through the outlet duct 306 while a small proportion passes along channel 415 into the injector bore 451. Meanwhile reagent leaks past the pressure relief valve 423 at a similar rate to that leaking along the channel 415 so that the pressure in the bore remains constant and the position of the throttle member remains stable.

If the flow of reagent to the injector 301 is increased to a higher rate, the pressure of reagent in the outlet chamber 407 tends to increase. However, since the pressure of reagent in the bore 451 of the injector body

403 is held substantially stable by the action of the pressure relief valve 423, the throttle member 414 is forced slightly backwards, and the outlet duct 306 becomes less constricted so that the liquid pressure in the outlet chamber increases very little. When the position of the throttle member 414 is once again stabilised, just as before, the force on its front end due to liquid pressure in the outlet chamber 407 equals that due to the liquid pressure in the bore 451 when added to the force from the positioning spring 417. The main difference now is that the throttle member 414 is more retracted to allow for the greater flow rate. It may be noted that that the pressure in the outlet chamber 407 has increased slightly, because the positioning spring is now more compressed and therefore exerting more force on the throttle member. However, this increase in pressure is only slight because the positioning spring is very light and the forces it applies are very small when compared to those generated by the pressure of liquid reagent. For example, forces applied to the front and rear of the throttle member by liquid pressure are typically in excess of a hundred times greater than those exerted by the positioning spring.

Figure 7

The injector 301 of Figure 6 is shown in Figure 7 after an increase in flow rate of reagent, as described above. Thus, when compared to Figure 6, the throttle member of Figure 7 is more retracted. It may be noted that further increases in flow rate would cause further retraction of the throttle member 414 without a substantial rise in reagent pressure within the outlet chamber 407 or as measured by the relevant pressure meter 213A or 213B. Of course, when the throttle member 414 is fully retracted, further increases would then cause substantial rises in pressures.

From the configuration of Figure 7, if the flow rate of reagent to the injector 301 is reduced, the pressure in the outlet chamber 407 initially decreases, and, as the throttle member 414 is free to move, the pressure in the liquid reagent in the bore 451 correspondingly drops. If the drop is sufficiently large, the pressure relief valve closes. In any event, the force due to the pressure in the outlet chamber remains equal to the force due to the pressure in the bore 451 when added to the force in the positioning spring 417. Consequently, the pressure of liquid reagent in the output chamber remains just above that of reagent in the bore and consequently a portion of the reagent in the outlet chamber flows along the channel 415 between the throttle member 414 and the bore 408 of the nozzle member 401. As the liquid flows along the channel the throttle member moves forward causing pressure in the outlet chamber to rise while pressure in the bore 451 correspondingly rises.

If the pressure relief valve was closed by the initial pressure drop, it will be reopened as the pressures increase. As before, the throttle member 414 will take up a stable position to correspond with the new increased flow rate when forces pushing it backward equal those pushing it forward, and when the flow of reagent along the channel 408 is equal to the flow escaping through the pressure relief valve.

Thus, in the new stable position the throttle member 414 will be moved forward when compared to Figure 7, but the change in pressure in the outlet chamber 407 (and pressure measured by meters 213A or 213B) will only be slightly reduced due to the slight reduction in compression of the positioning spring 417.

Returning to Figure 6, the effect of particulate matter blocking the outlet duct 306 may also now be understood. Such a blockage will tend to increase the liquid pressure in the outlet chamber 407 and so move the throttle member 414 back in a similar manner to that described for the increased rate of flow of reagent. However, once the throttle member 414 has retracted sufficiently to allow the blockage to pass through the outlet duct 306, the pressure in the outlet chamber 407 will drop. The position of the throttle member 414 will then be readjusted back to its normal stable position of Figure 6, by a similar process to that described above for the reduced rate of flow.

In the present embodiment, the pressure relief valve 423 is located in an extension 419 to the main body 403 of the injector 301. However, other embodiments are envisaged in which the bore 451 of the injector body is connected to a remote pressure relief valve by a high pressure pipe.

In addition, other embodiments are envisaged in which the channel 415 is produced by alternative means. For instance, rather than producing a flat along the throttle member 414, the channel may be produced by machining a groove along the throttle member or by drilling a fine hole through the throttle member. In a further alternative embodiment, the nozzle member is machined to produce a channel. However, in each embodiment a pressure chamber, such as the bore 451, contains liquid chemical reagent whose hydraulic pressure is used to provide a force to the rear of the throttle member and the pressure chamber is connected to the outlet chamber by means of a channel.

Claims

1. Apparatus for injecting a liquid chemical reagent into a mixing chamber to mix with a second chemical reagent, said apparatus comprising: an outlet chamber having an outlet duct configured to allow liquid chemical reagent to be ejected from said outlet chamber through said duct; a throttle member having a. head-end part configured to move relative to said outlet duct to control said ejection; positioning means configured to apply a positioning force to a rear end of said throttle member such that said positioning force acts against force applied to said throttle member by pressure of liquid chemical reagent in said outlet chamber to position said throttle member; a channel configured to allow liquid chemical reagent to pass into a pressure chamber at the rear of said throttle member so that hydraulic pressure of said reagent in said pressure chamber provides at least a portion of said positioning force; and control means for controlling the pressure of liquid in said pressure chamber.

2. Apparatus according to claim 1, wherein said control means comprises a pressure relief valve.

3. Apparatus according to claim 2, wherein said pressure relief valve is configured to open when said pressure in said pressure chamber rises above a threshold value.

4. Apparatus according to claim 2 or claim 3, wherein said pressure relief valve comprises a valve spring which acts to close said valve.

5. Apparatus according to any one of claims 1 to 4, wherein said liquid chemical reagent in said pressure chamber acts directly upon said throttle member.

6. Apparatus according to any one of claims 1 to 5, wherein said channel is configured such that liquid reagent is able to pass from said outlet chamber along said channel to said pressure chamber.

7. Apparatus according to claim 6, wherein said channel is defined by said throttle member.

8. Apparatus according to claim 7, wherein said channel is defined by a groove or flat on said throttle member.

9. Apparatus according to any one of claims 1 to 8, wherein said positioning means further comprises a positioning spring which tends to force the throttle member forward to close the outlet duct.

10. Apparatus according to any one of claims 4 to 9, wherein said positioning spring is substantially lighter than said valve spring.

11. Apparatus according to any one of claims 1 to 10, wherein said outlet chamber is located within a nozzle member having a bore terminating in said outlet chamber, and said throttle member is configured to slide within said bore.

12. Apparatus according to any one of claims 1 to 10, wherein said apparatus comprises a nozzle member having a bore terminating in said outlet chamber, and said throttle member has a main body configured to slide within the bore of said nozzle member, and a head portion at a front end of said main body configured to be a sliding fit within said outlet duct, such that said head portion is beatable within said outlet duct to close said duct or retractable from said outlet duct to open said duct.

13. A method of injecting a liquid chemical reagent into a mixing chamber to mix with a second chemical reagent, said method comprising the steps of: ejecting liquid chemical reagent from an outlet chamber through an outlet duct; positioning a throttle member by applying a positioning force to a rear end of said throttle member which acts against force applied to said throttle member by pressure of liquid chemical reagent in said outlet chamber; allowing liquid chemical reagent to pass through a channel into a pressure chamber at the rear of said throttle member so that hydraulic pressure of said reagent in said pressure chamber provides at least a portion of said positioning force; and controlling the pressure in said pressure chamber.