Note: Descriptions are shown in the official language in which they were submitted.
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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
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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.
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-~ ~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
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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.
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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
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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
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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.
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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
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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
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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
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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
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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
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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.
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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
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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.
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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.
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