CA2015524A1 - Clean window for processing enclosure - Google Patents

Abstract

RD-19,148 CLEAN WINDOW FOR PROCESSING ENCLOSURE
ABSTRACT OF THE DISCLOSURE
A viewport capable of providing sustained clear viewing is provided. The clear viewing is the result of maintaining a unidirectional flow of purge gas from the lens of the port toward the chamber on which the port is mounted.
An annular flow channel surrounding the viewpath distributes purge gas to a porous inner wall of the annular channel.
Purge gas passes through the porous inner wall and flows unidirectionally toward the chamber.

Description

2 ~

RD~
Typed: ~/2/69 CLEAN ~NDOW FO~ PROCESSING ENC~OSURE

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention relates to two copendin~
applications as follows: Serial Nos. 07/376,094 and 07/376,095, filed July 6, 1989. The texts of these related applications is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention does not relate to powder production in which the product sought from the production is the powder. Rather, it relates to dealing with a by-product of powder production and with fine powder produced as a by-product of melt processing. The by-product is a cloud of very fine particles which forms in apparatus where material is formed, processed, or melted.
There are a number of material processing ; procedures which are conducted inside of an enclosure or container, either because the materials being processed must be protected from the atmosphere or there is fear that materials being processed will be dispersed through a processing plant and essentially contaminate what would otherwise be clean areas within the plant.
Such material processing procedures include high - 25 intensity heating of metals, plasma spray deposit, plasma heating, transferred arc heating, spray depositing of metals, gas atomization of metals, electron beam heating of metals, and other material processing procedures.
One such material proce~sing method involves the formation of fine powder. One process in current use for , ' ~ ` .;
, 2 ~ 2 ~

RD-'9 1~8 Typed: 8/2/89 atomizing materials into fine powder involves the atomizatlon of a flowing stream of liquid or molten material by a stream of gas which impinges on the liquid stream to disperse and atomize it. The present invention is not concerned with formation of powder by gas atomization.
However, such atomization is accompanied by the production of extremely fine powder as, for example, a fine fog or cloud of metal powder formed from molten metals which are atomized through the action of streams of gas impinging on a stream of the molten metal.
Another such material processing procedure is the use of the powder in a spray deposit process, either where the spray is formed from a liquid metal or where the spray is formed by flowing a powder through a plasma flame to cause lS the powder particles to melt into a molten spray.
The present invention is not concerned with this material processing procedure but is concerned with a fog or cloud of very fine particles which forms as a by-product of such material processing procedure~
There are numerous other materials processing techniques which involve the formation of vapors or fine powder or the utilization of vapor or fine powder in forming articles and which form fogs or clouds of very fine particles as a by-product of such material processing procedure.
It is often desirable in many of these operations to have some visual access to the key phenomena which accompanies the material processing. However, such visual access can be diminished or altered or obscured by such fogs or clouds of very fine particles. Numerous deslgns have been proposed and developed to permit viewing of the interior of a processing chamber while the processing is in progress so that certain processing criteria can be observed, sensed, or controlled.

2~1 3 _ 3 _ RD-l 9, 14~
Typed: 8/2/89 -~ ~hen fine particle clouds are generated within an enclosure, as a by-product of processing or reaction of materials which is taking place, the actual critical phenomena within the enclosure can be obscured by the particulate cloud formed within the chamber. One of the characteristics of such clouds as formed within the processing chambers is a tendency of the particles to deposit on the inner surfaces of the chamber including depositing on the inner surface of a window in a wall of the chamber. Such deposit of particulate material can interfere with viewing and can obscure the observation from the chamber exterior.
In severe cases, the observation of the chamber interior may be entirely prevented because of occlusions of particulate material on the inner surface of the window.
Numerous schemes have been devised for maintaining the inner surface of such viewing windows clear. However, it has been observed that many of these schemes have limited success so that they can increase the time during which viewing can take place but over a protracted period the particulate material gradually occludes on the window and obscures the viewing or other transmission of light throùgh the window.
In the past, attempts which have been employed to keep viewing ports of such apparatus clean have involved the use of a gas curtain, or gas jets, or of a moving film, or of shutters. However, it has been found that these techniques have given mixed results in maintaining clean viewing ports.
Although these techniques have been of limited help in maintaining clean viewing ports, there are other problems which are developed through their use. One such problem relates to the use of a gas curtain or a gas jet. Such use generally requires large gas flows or high velocity gas flows. Such high velocity or high volume of gas flow in time creates highly turbulent and low pressure regions in the -2~1~524 R~)-14, 1 48 Typed: 3/2/89 vicinity of the viewing port. This can lead to eddy flow of gas, to particle entrainment and to impaction onto the viewing windows even though high volume of gas and high velocity of gas is employed to prevent such deposit.
By contrast, the moving film technique operates on the principle of simply allowing the deposit of the particulate matter but to permit this deposit only on a transparent film. If deposit occurs, the film is then indexed to move the particle-bearing film from the path of view and to replace it with a fresh clear film which can then receive add~tional particulate deposit. Alternatively, a clear transparent film is continuously moved past the viewing port to provide a clear transparent section at the viewing port location even though there is a continuous deposit of particulate matter on the film as it passes the viewing port.
This clear film technique requires a large degree of mechanical hardware operating in a dynamic mode and has the potential of being very cumbersome in size and in operation.
Further, the film introduces another element into the optical path and may be partially opaque to light signals moving along the path as, for example, light in the infrared spectrum.
The shutter technique is simply a mechanical way in which a viewing port is physically isolated from the chamber where the finely divided or vaporous material is being processed. Through the use of a shutter, the viewing port is isolated but the shutter is opened when observations are required and it is closed after they have been made. One problem with the shutter technique is that it does not permit continuous observation. Further, it has the deficiency that for the brief times that the shutter is open and particulate matter does deposit on the viewing lens, there is no mechanism for removing the deposited material and it simply accumulates with each opening of the shutter.

201~2~

R3-l9.148 Typed: 8/2/89 BRIEF STATEMENT OF THE INVENTION

It is, accordingly, one object of the present invention to provide a means by which a viewing can be accomplished on a continuous basis into an enclosure in which vaporous or finely divided material is produced or is processed.
Another object is to provide an optical path through the wall of an enclosure in which vaporous or finely divided material is produced or processed.
Another object is to provide a viewing port for an apparatus in which material is occluded on the apparatus wall where the viewing port remains free of such occlusions.
Other objects will be, in part, apparent and, in part, pointed out in the description which follows.
In one of its broader aspects, the object of the invention can be achieved by providing a clean window mechanism in the wall of an enclosure, the inner surface of which is subject to becoming coated with a finely divided material. The first requi~ite of this mechanism is to provide an opening through the wall of the enclosure to define a view path for the enclosure. A transparent window is disposed at the outer-most portion of the view path. An inner and outer generally annular set of walls are extended around an outer portion of the view path to define between the walls an annular gas flow plenum. The inner annular wall of the plenum is formed of a porous materlal to permit flow of gas there through. The window is mounted at the outer end of the annular gas flow plenum. The annular gas flow plenum and window are mounted in the opening of the enclosure to close the opening to the passage of ambient gas into or out of the enclosure. Gas supply means are provided for supplying gas to the plenum tP induce a flow of gas through : ' :

2 ~

RD-l 9, 148 Typed: 8/2/89 the porous inner wall into the internal view path of the enclosure whereby the deposit of material on the window is substantially precluded or prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

The explanation which follows will be understood with greater clarity if reference is made to the accompanying drawings in which:
FI~URE 1 is a axial section of the clean window of the subject invention;
FIGURE 2 is a vertical section of a window similar to that of Figure l but mounted at an angle for convenience of viewing; and FIGURE 3 is a schematic illustration of a window as used in connection with a gas supply and other apparatus.

DETAILED DESCRIPTION OF THE INVENTION

One of the objects which the inventors had in forming a structure adapted to keep a view window clear is to avoid and overcome the tendency of gases in the region of the view window to move in a turbulent fashion and to form eddies which result in a backflow of the particulate or vapor ; 25 material which is found to deposit on a window. In contrast to such turbulent flow, the inventors sought to provide a smooth unidirectional flow of a cleaning gas away from the window. After extensive testing of gas flow in relation to the window, the inventors found that a smooth unidirectional flow is feasible employing an apparatus as illustrated in Figure 1.
; Referring now to Figure 1, an apparatus for providing a clean window for an enclosure in which processing or production of finely divided or vaporous material takes :, ~ ;,. , .~

~ o ~

~-1 9, 148 Typed: 8/2/89 place is illustrated. The viewing apparatus 10 is mounted on an enclosure such as a chamber or tank 12. The viewing apparatus 10 is mounted over an opening 14 in the tank wall.
A collar 16 and flange 18 are permanently attached to the tank wall as by the weld 20. A spool piece 22 having a lower flange 24 and an upper flange 26 is mounted over flange 18 by the bolts 28. An orifice plate 30 may be mounted between flange 18 and flange 24 to permit a control of gas flow ,from within a chamber 32 and from within the hollow center of spool 22 to the chamber 60.
Alternatively, this orifice plate may be omitted as a further alternative an orifice plate such as 42 having a descending collar such as 43, shown in phantom, may be employed. An annular plenum 34 is formed between the sidewalls of the spool 22, that is, between a porous inner wall 36 and an impervious outer wall 38. Gas may be supplied to plenum 34 through the gas inlet pipe 40 extending through the impervious side wall 38.
The orifice plate 30 has a restricted orifice 42 which has a diameter significantly smaller than that of the center opening 32 of spool 22 as is evident from the figure.
O-rings 44 and 46 provide a seal between the flange at 18 and orifice plate 30 and between flange 24 and orifice plate 30, respectively. The proportions of the parts illustrated in the Figure 1 correspond to those of parts actually used in an apparatus successfully used experimentally and found capable of keeping the view window 50 clear for extended periods of time.
An O-ring 48 disposed in a conforming well in flange 26 provldes a seal between flange 26 and the rim of transparent window member 50. The window 50 is held in place by an upper anr.~lar plate 52 and by a set of bolts 54 extending between the upper annular plate 52 and the flange 26.

:

201~2~

~3D-19, 148 Typed: 8/2/89 The operation of the device involves the supply of a gas through the port 40 to the annular chamber 34 to permit gas to enter the central chamber 32 through the porous wall 36. As the porous wall 36 extends all the way around central chamber 32 the gas inflow is from all sides. After entering chamber 32 the gas flows down through orifice 42 in orifice plate 30 to provide a continuous gentle unidirectional flow of gas from chamber 32 into the interior of the tank 12, only a portion of the wall 13 of which is illustrated in Figure 1.
This flow of gas into the enclosure 12 defined by wall 13 prevents or greatly diminishes any deposit on window 50 of particulate matter or condensation of any vapor generated or processed within the processir.g chamber 60 within enclosure 12.
The gas supplied through port 40 to plenum 34 and thence to chamber 32 may be any gas commonly used in gas atomization or in operation of a plasma spray apparatus and may typically be helium gas or argon gas or a mixture of such gases in various proportions for chamber 32 operation at atmospheric or at reduced pressure. For example, such an apparatus has been successfully operated with gas chamber pressures of one third atmosphere to above one atmosphere.
The supply gas pressure must be great enough to overcome pressure drops in the system and to create a positive gas flow into chamber 32 and hence into tank 12.
The use of an orifice plate 30 or an extended tube such as 43 may permit operation of the viewport with reduced gas flow.
An alternative form of the apparatus is illustrated in Figure 2. In the apparatus of Figure 2, the view port is designed to permit the viewing to be done at an angle to the surface wall 113 of enclosure 112. Although the structure of the device of Figure 2 is quite similar to that of Figure 1, there are some differences and these differences are pointed .

:

2 ~

Typed: 8/2/89 - out in the description which follows. In this description where the structure is similar to that of Figure 1, the same numbers are used with an adder of 100 units. Thus, the window in Figure 1 is 50 and the window in Figure 2 is 150.
rhe tank wall to which this structure of Figure 2 is mounted is 113 and the tank wall to which the structure of Figure 1 is mounted is 13. In general, there is a view mechanism 110 which is made up of a spool 122. An angled pedestal 116 supports the spool 122 in place and mounts the spool over an opening 114 in the wall 113. The pedestal 116 is mounted to the wall 112 ~y welding or any convenient mounting means such as by braising, bonding, etc. The spool is made up of an upper flange 126 and a lower flange 124 having an outer wall 138, and an inner porous wall 136, extending therebetween.
lS Inner wall 136 surrounds an inner chamber 132 to which gas is delivered by flow through the porous wall 136. Between the inner porous wall 136 and outer wall 138, an annular chamber 134 receives and distributes the gas supplied through a gas inlet port 140.
The bolts 128 anchors flange 124 to the pedestal support 116. The bolts 154 anchors flange 126 to plates 152 and 153. The plates 152 and 153 hold lens 150 therebetween.
There are some differences in the structures which concern the angular viewing of the tank interior. In the structure of Figure 2, it is evident that the opening 151 is located eccentrically in the plate 152 and this is done to permit a line of sight in a more advantageous position for viewing of the interior 160 of the enclosure within wall 112.
Another difference is structure concerns the flange 124. It is evident that the interior chamber 132 is eccentric to the flange 124 or, conversely, the flange 124 is mounted eccentrically with respect to the other elements of spool 112, and particularly the wall members 136 and 138. Here again, the eccentricity of the chamber 132 is provided to .
- , 2 ~ 4 RD-~l9.l4~
Typed: 8/2/89 improve viewing of the enclosed space 160 within the tank enclosed by wall 113 but such eccentric mounting is not critical to practice of the present invention.
In operation, a gas is supplied through the inlet port 140 to the annular plenum 134 and is flowed through the porous inner wall 136 to the view zone 132. The uniform flow of gas into the view zone or central chamber 132 and from it through the opening 114 into the interior 160 of the enclosure 112 within wall 112 precludes or greatly inhibits and reduces the backflow of vapor or finely particulate matter from the enclosure 160 up to the under surface of lens 150.
Turning now to Figure 3, a schematic illustration of the way in which the apparatus of the present invention may be employed is provided. An enclosure 220 is provided for processing of some materials, which processing results in the production of finely divided material or vapors of the material. The window mechanism of the present invention 210 is mounted in one wall 212 of the enclosure 220. Viewing through the window 210 permits the observation of a processing station 230 within the enclosure 220. The processing may be, for example, a plasma torch 232 shown schematically extending through a seal 216 in a wall 214.
Alternatively, such a window 210 may be used in connection with infrared sensing as described in U.S. Patent 4,656,331, assigned to the same assignee as the subject invention.
The plasma 234 may cause deposit of very finely divided material ln a molten state onto a rotating drum 236 supported in rotary fashion on shaft 238 from a variable speed motor 240, supported on motor mount 242. The supply of gas and powder to the plasma gun 232 is not illustrated as it forms no part of this invention. Gas is supplied to the clean window mechanism 210 through the gas inlet port 240 RD-~. 148 Typed: 3/2/89 ~ from a gas tank 242 equipped with a conventional gauge 244 and flow control valve 246. The flow of gas occurs along the gas piping 248 to the gas inlet port 240 of the clean window mechanism 210 of the schematically illustrated Figure 3.
The plasma torch processing of material is only one of a num~er of possible processings, as explained above, for which the clean window of the present invention provides a useful viewing or sensing structure. Other processing may include, for example, high intensity melt processing generally, grinding or pulverizing, or gas atomization of liquid metal, or a spray forming of a deposit on a receiving surface such as the rotating surface 236 illustrated in Figure 3.
The foregoing provides a general description of the structure and means by an optical access can be provided from the exterior of a chamber enclosing an atmosphere containing vapors or finely divided material. There are many alternatives which may be provided, both in the structure and in the processing which takes place within such an enclosure.
Some of these details may be made apparent by consideration of the following examples.

Ex~oe~E-l:
A window structure as described with reference to Figure 1 was fabricated. The two concentric cylinders 36 and 38 were 4.75 inches in length with flanges welded to either end to permit the mounting of the window to an opening in a furnace wall. The inner cylinder conslsted of sintered 304 stainless steel of nominal 10 micron porogity and l/16 inch thickness. The porous metal was obtained as a sheet, identified as Mott metallurgical part No. 1100. The porous metal sheet was cold-rolled and butt-seam welded. This formed the inner cylinder 36 of the apparatus and an outer cylinder of 304 stainless steel was also provided with an .

2~1~52~

RD-19, 1.48 Typed: 8/2/89 inside diameter of 5 inches. This provided a 3/16 inch annular gap between the inner cylinder 36 and the outer cylinder 38. A 1/4 inch fitting was welded to the outer cylinder as the purge gas feed port 40. The window was operated with a cylindrical extension tube of 4.5 inch internal diameter and a 5 inch length (including flange thic~ness) with the cylindrical extension extending down in the manner of the collar 43 of Figure 1. The apparatus was also operated successfully without use of any extension tube or orifice plate.
Stainless steel is not necessary as the construction materials for the window tubing and other materials can be used equally well. In forming the window, welding is preferred for the gas tight joints inasmuch as braising or soldering causes wicking of the braise solder and the closure of appreciable portions of the porous metal.
The function of the porous metal in the above example, as in the structures described above, is to uniformly distribute the gas flow to the window chamber 32 so that eddy swirls and gas jets are avoided and do no~ entrain chamber gas and particles. It is also to minimize turbulence and consequent entrainment. In the test geometry of the above example, the flow restriction provided by the porous metal served as a pressure drop distributor so that gas could be introduced at one location 40 and nevertheless flow into the chamber 32 from all sides. It will be understood, of course, that the porous metal as employed in the above examples served very well but is not the only porous material or the only porosity which may be employed in practicin~ the present invention as other porous materials and other porosities can be employed succe~sfully in carrying out the construction and operation of a clean window structure as described above. For example, a set of fine screens or a - ~ .. ..

20~ 2~

RD-12~148 Typed: 8/2/89 pack of granules set between screens can serve as the porous material.
It was discovered from testing of the structures described in Example 1, that the tested designs were highly effective in avoiding turbulence in the window chamber and in the optical path extending through the window chamber. It is possible to increase or reduce the length of the chamber depending on the actual diameter of the chamber and on the pressure of gas employed in the enclosure 60. Generally, the larger the diameter of the chamber 32, the greater the length of a cylindrical extension such as 43 for satisfactory operation of the clean window. In other words, for larger diameters, there is a requirement of larger length to diameter aspect window chamber ratios. One reason is that where two different gases of two different densities are employed there is an opportunity for buoyancy induced inversions. Such inversions can give particulate matter in enclosure 60 access to the window. To avoid such inversions, the higher aspect ratios should be employed. Where such higher aspect ratios are employed lower gas flow to chamber 32 can often be employed.
One source of such inversions is the introduction of a cold gas into the annular plenum 34 and into the chamber 32 to mix with a ho~ gas in enclosure 60. Generally, it would be preferred to have the temperature of the entering gas approximately the same as that of the enclosure gas but this is often not practical for several reasons.
Another source of such inversion is the use of a lighter gas such as helium in connection with a heavier chamber atmosphere such as nitrogen or arqon. Generally, smaller diameter of windowq are preferred where smaller aspect ratios are available. Such arrangements have a tendency to reduce buoyancy-induced inversions.

~ ~ ' .
, 2 ~ 4 R~-19.148 Typed: 8/2/a9 In applications where a low profile on the furnace exterior is sought, it is possible ~o recess the gas diffusion annulus such as 43 of Figure 1 into the enclosure such as 60.
Gas flows of 4 SCFM of argon or 1 SCFM of helium were demonstrated to be adequate to keep windows clear during our experimental investigation where the window was mounted to an apparatus where plasma arc melting was carried out within the enclosure or where rapid solidification plasma deposition was carried out within the enclosure. These flows have not adversely affected the operation of the furnaces with which they are used.
Lower gas flow is believed to be possible with helium gas for vertically-oriented windows having long cylindrical downwardly descending extensions such as 43 of Figure 1. It is also believed that the increase in gas flow, in order to avoid or prevent deposit of vapor or particulate matterJ is substantially larger for windows of larger diameter.

A gas purged window, as illustrated in Figure 1, was built for use on a low pressure tank in which rapid solidification plasma deposition (RSPD) processing was carried out. The inner porous tube had a diameter of 4.5 inches and the outer tube had an ID of 5.0 inches. The tube length was about 5 inches and it was closed at the top by a sapphire window having a 1/4 inch thickness. An extension cylinder extended about 5 inches into the RSPD tank. The axis of the tubes and, accordingly, the optical path through the tubes was set at about 65 from vertical. The RSPD
process was run in the tank with a variety of materials and torch gas compositions and deposition rates. Some of the " ' ;, ; ,"
' 2~1~5~

RD-l~. 148 Typed: 8/2/89 conditions were chosen to create a very dirty tank gas. Both argon and helium purge gases were used.
Argon purging was effective at a threshold flow rate of about 2.4 to 2.8 SCFM. It was estimated, based on these results, that for very clean runs, flows as low as 2.0 SCFM may have been effective. The effectiveness of the runs was based on physical and visual inspection of the windows after the runs had been completed and by observing the penetration of chamber gas and smoke into the window well during the test. The overall appearance, as well as the apparent mechanisms for fowling a wlndow of this geometry, are very similar to the plasma arc melting facility discussed with reference to Example 1. The chamber gas flow could be well defined by observing the scattering of particulate matter from the window chamber. For argon gas flow, threshold flows are based on the minimum flow needed ~o stop eddies of particulate bearing chamber gas before such gas reaches the window. It was observed that the outflow of smoke from the window chamber was unidirectional.
Helium effectively purged the window well at flows less than .6 SCFM. This is consistent with the experience recited in Example 1 a~ove. In the RSPD facility, stable stratification of particulate matter was very evident in the window well. Based on this evidence, it was concluded that it is likely that the window would work nearly as well without the cylindrical extension if helium gas were used as the purge gas. This result is consistent with the results recited in Example 1 which indicate that buoyancy forces are critical in determining purge gas flow rates.
It was determined that control of gas density to minimize density differences of purge gas versus chamber gas is advantageous. The chamber pressure for the RSPD actions of Example 2 was maintained at about 250 torr for all of the runs which were made.

`~ .

.

2 ~

Bn~l9Ll~8 Typed: 8/2/89 In place of the sapphire lens of the apparatus of Example 2, a lens of quartz or Pyrex~, or, in fact, of conventional window glass may be employed.

- .
, : . -.
.

Claims (10)

1. A clean window mechanism for use in the wall of an enclosure on the inner surface of which finely divided material occludes, which comprises, an opening through the wall of the enclosure defining a view zone, a transparent window disposed at the outermost portion of said zone, inner and an outer generally annular walls extending around an outer portion of said view zone and defining therebetween an annular gas flow plenum, the inner annular wall being porous, said window being mounted at the outer end of said view zone, said annular gas flow plenum and window being mounted in the opening of said enclosure to close said opening to passage of ambient gas to or from said enclosure, and gas supply means for supplying gas to said plenum to induce a flow of gas through said porous inner wall into the view zone whereby deposit of material on said window is prevented.

2. The mechanism of claim 1, in which said window is a window selected from the group consisting of window glass, Pyrex?, glass, quartz, sapphire, and arsenic trisulfide.

3. The mechanism of claim 1, in which there is an orifice plate below the annular gas flow plenum to restrict the backflow of chamber gas toward said window.

RD-19,148 Typed: 8/2/89

4. The mechanism of claim 1, in which there is descending collar extending from below said gas flow plenum into said enclosure.

5. The mechanism of claim 1, in which the porous inner wall is porous media.

6. The mechanism of claim 1, in which the flow of gas induced by said porous inner wall is unidirectionally away from said window.

7. The mechanism of claim 1, in which the inner and outer walls are attached to end flanges and have the general form of a spool.

8. The mechanism of claim 1, in which the purge gas employed is selected from the group consisting of helium, argon, and mixtures thereof.

9. The mechanism of claim 1, in which the porous inner wall is porous stainless steel.

10. The invention as defined in any of the preceding claims including any further features of novelty disclosed.