EP3374695B1

EP3374695B1 - Scraper

Info

Publication number: EP3374695B1
Application number: EP16781194.2A
Authority: EP
Inventors: Thomas Frank COHN; Alexander Michael POPE; Richard Krispin STRONG
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2015-11-13
Filing date: 2016-10-12
Publication date: 2022-02-16
Anticipated expiration: 2036-10-12
Also published as: WO2017081439A1; EP3374695A1; GB201520104D0

Description

The present invention relates to a scraper device. The invention finds particular use in the combustion of exhaust gases from processes, such as those from the semiconductor industry.
Preventing or limiting the emission of hazardous gases exhausted from industrial processes to the atmosphere is now a major focus of both the scientific and industrial sectors. In particular the semiconductor industry, where the use of process gases is inherently inefficient, has set its own targets for reducing the amount of gases exhausted to the atmosphere from fabrication plants. Examples of compounds which it is desirable to destroy are those from etch processes such as fluorine, SF₆, NF₃ or perfluorocarbons (CF₄, C₂F₆ etc.), or from deposition processes, such as PECVD (plasma enhanced chemical vapour deposition) using silane (SiH₄) or TEOS (Tetraethylorthosilicate).
One method of destroying, or abating, gases such as silane from an exhaust gas stream uses a generally cylindrical combustion chamber to which gases to be destroyed are conveyed via one or more nozzles at a first inlet end. The nozzle arrangement supplies a fuel gas, such a methane or propane, to the exhaust gases via a plurality of inlets concentrically surrounding the exhaust inlet nozzles to produce a fuel rich exhaust gas mixture which is then ignited via a pilot flame such that a flame forms in the combustion chamber in the presence of air. The combusted exhaust gases are then conveyed from the combustion device at the second, outlet, end of the cylindrical combustion chamber.
The destruction of gases such as silane or TEOS produces silica, SiO₂, which can deposit on the internal surfaces of the combustion chamber, including around the nozzle arrangement and reduce the efficacy or operating lifetime of the equipment.
It is known to provide a scraper located within the combustion chamber, said scraper comprising a first cylinder located proximate the nozzle arrangement/inlet end; and a second cylinder located distal to the inlet end. The scraper is provided with n scraper elements connected to and extending longitudinally through the combustion chamber between the first and second cylinders at substantially equally positions around the respective circumferences of the cylinders; and a scraper actuation device for rotating the scraper about the longitudinal axis of the combustion chamber by at least 360/n degrees such that, on rotation of the scraper, substantially all of the internal walls of the combustion chamber are scraped by the scraper elements.
However, it has been found that known scraper elements are mechanically weak and, more importantly, can cause the combustion chamber to reach undesirable temperatures due to their configuration. It is the object of the present invention to overcome these problems whilst still providing sufficient scraping of the internal surfaces of the combustion chamber. Other scraper arrangements are known from JP2004028546 and US2005226790 .
In a first aspect the present invention provides a scraper for scraping the internal surfaces of a cylindrical combustion chamber comprising: a first cylinder; a second cylinder; a plurality of scraper elements connected to and extending between the first and second cylinders at substantially equally positions around the respective circumferences of the cylinders; wherein, the scraper elements are V-shaped with the apex, or crease, of the V directed axially outward such that, in use, the scraper element scrapes deposits from the internal surfaces of the combustion chamber into which it is located, and wherein the internal angle, Θ, of the V-shaped scraper element is between 120 degrees and 160 degrees.
By forming the scraper elements in a V-shape with an internal angle, Θ, of the V at its apex between 120 degrees and 160 degrees, particularly around 140 degrees, a particularly efficient scraping of the internal surface of the combustion chamber can be achieved, whilst observing little effect on the internal wall temperature of the combustion chamber. This is surprising because when other similarly shaped scraper elements are used the internal walls of the combustion chamber have been shown to excessively overheat.
In a second aspect the invention provides a combustion device comprising the scraper.
Other preferred and/or optional aspects of the invention are defined in the accompanying claims.
In order that the present invention may be well understood, embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which:

Figure 1A is a cross section of the combustion device comprising the scraper according to the present invention.
Figure 1B is an enlarged section of Fig 1A illustrating the distance between scraper element and second cylinder and the internal wall of the combustion chamber.
Figure 2A is a schematic of the scraper according to the invention.
Figure 2B is an illustration of the location of the scraper elements around the circumference of the scraper cylinders.
Figure 2C is a cross section of the V-shaped scraper elements of the invention.
Figure 3 is an illustration of the scraper according to the invention.
Figure 4 is an illustration of the scraper actuation support device.
Figure 5 illustrates the pass through slot for the actuation device.
Figures 6A and 6B illustrate the connection between the actuating mechanism and the scraper.
Figure 7 illustrates an alternative connection between the actuating mechanism and the scraper.

Referring first to Fig 1A, a partial cross section of the scraper according to the invention located inside a combustion chamber of a combustion device is shown. The combustion device 100 comprises a nozzle arrangement (not shown) which conveys exhaust gases to the combustion chamber 71 via a first inlet opening 10. The exhaust gases inlet nozzles are coaxially surrounded by a series of nozzles that introduce a fuel gas to the combustion chamber. The fuel gas/exhaust gas mixture is ignited via a pilot flame (not shown) to produce a flame extending into the combustion chamber 71. Air is conveyed to the chamber along a plenum chamber 78 surrounding the combustion chamber. The air enters the combustion chamber 71 through the plurality of louvre type orifices 40 in a lower section of the chamber 71 in a contra-flow direction to the fuel gas/exhaust gas mixture. The air supports the combustion of the fuel gas/exhaust gas mixture.
This type of combustion device 100 is highly efficient at destroying PECVD type gases such as silane and TEOS producing silica as a by-product of the combustion. Although such combustion devices are arranged to reduce the formation of silica deposit on the nozzle arrangement or on the combustion chamber walls, 7, over time deposits build up and make the device inefficient and/or require servicing.
It is known to provide a scraper arrangement within the combustion chamber comprising first and second cylinders and a plurality of scraper elements connected to and extending perpendicularly between said first and second cylinders. However as described above, the presence of known scraper elements in such a scraper device can cause unwanted temperature rises in the combustion chamber.
The scraper device according to the invention 200 is shown in more detail in the collection of Figures 2 and 3. The scraper comprises a first cylinder 16 and second cylinder 14 and a plurality of scraper elements 6 connected, by fixing means such as bolts, rivets or welds, to and extending between the first and second cylinders 16, 14 at substantially equally positions around the respective circumferences of the cylinders 16, 14. The scraper elements 6 can comprise a reinforced section 6A at the distal ends of the elements 6 where they are connected to the cylinders 14, 16 to prevent the connections between the scraper elements 6 and cylinders 14, 16 from being mechanically affected by hard deposits during rotation.
The scraper elements 6 are V-shaped along their length 6B and connected to the cylinders 14, 16 with the apex, or crease, of the V is directed axially outward from the cylinders such that when located in the combustion chamber, and rotated about the central longitudinal axis of the chamber, the apex of the V 6C scrapes the internal surfaces of the combustion chamber walls. The internal angle, Θ, of the V can be between 120 and 160 degrees but, as described, 140 degrees gives both an excellent scraping action whilst causing little change in the internal wall 7 temperature of the combustion chamber.
The scraper 6 illustrated in the figures is formed from a 2 mm thick piece of suitable metal, such as, steel which has an internal angle of 140 degrees. The distance between the distal ends 6A, 6B of the V-shaped scraper element is 20 mm, with the distance from the apex 6C to the distal ends of the V 6A, 6B being 10.6 mm. The scraper 200 shown comprises six scraper elements 6 but more or fewer elements located around the scraper are also suitable, for example 4 or 8. In use the scraper mechanism is rotated, forwards and backwards, about the longitudinal axis of the pump in an arc at least 360/n, preferably slightly more, where n is the number of scraper elements distributed evenly around the scraper.
It has also been found that by forming a gap 13 of between 2 mm and 6 mm, more preferably between 4 mm and 5mm, and ideally 5 mm, between the outermost edge of the apex 6C of the V-shaped elements 6 and the internal surface of the combustion chamber wall 7, when at room temperature, the internal wall temperature, scraper efficacy and exhaust gas destruction efficiency within the chamber, in use, can be kept at desirable levels. Normally scraper blades are required to work as close as possible to the wall which they are employed to work, thus it is surprising that by moving the scraper edge away from the wall it is designed to scrape still achieves a desirable level of scraping whilst improving destruction efficiency.
Similarly it has been found that by forming a gap of between 3 mm and 6mm, ideally around 5 mm, between the outer edge of the second cylinder, which will be located proximate the outlet to the combustion chamber, little change to the chamber wall temperature is observed. The second cylinder, which is located proximate the combustion chamber outlet, generally requires greater clearance than that of the first cylinder.
The first cylinder 16, located proximate the combustion chamber inlet also comprises a plurality of substantially axial spokes 25 which, on rotation of the scraper about the longitudinal axis 70 of the combustion chamber scrape the nozzle arrangement to clear it of any deposit. The number of spokes 25 is ideally at least equal to the number of scraper elements such that, on rotation of the scraper, the whole of the nozzle arrangement is cleared of deposit.
The aforementioned scraper has been shown to provide high mechanical strength, excellent scraping of the internal wall surfaces of the combustion chamber, whilst preventing over heating of the combustion chamber walls compared to known scraper devices.
The obvious location to drive and support the scraper device would be from a central location at the top or bottom of the scraper, however this means that the nozzle arrangement must be designed such that the actuating mechanism doesn't interfere with the gas flows into the combustion chamber 71. Therefore the actuation means for the scraper uses a drive mechanism that converts linear motion to rotary motion via a link arm located external to the combustion chamber.
Figure 4 illustrates how the scraper 200 is supported for rotation about the longitudinal axis of the combustion chamber. The scraper is supported by a series of "top-hat" shaped bearings located around the circumference of the combustion chamber upon which the upper part 16A of the first cylinder 16 is supported. The bearings have a smaller circumference portion 204 upon which the lower surface of the upper part 16A of the first cylinder 16 is rested to support the weight of the scraper 200, within the combustion chamber 71, and allow its rotational movement therein. The bearings also comprise a second larger diameter cylinder 206 (with respect to the first bearing cylinder 204) which is of large enough diameter that it extends out to support the outer surface of the upper cylinder part 16A, and thus the scraper 200, from tangential movement. The bearing arrangement show comprises six bearings 204, 206 located at substantially equal positions around the circumference of the internal wall of the combustion device, but as little as four may be considered with this type of bearing arrangement.
This bearing arrangement has been shown to prevent the seizures that were shown to occur with known bearing arrangements. The bearings material is chosen to ensure that the surfaces in rolling contact are different grades of stainless steel to minimise galling. The bearing cage is stainless steel 316, the bearing wheel stainless steel Nitronic 60 and the baring shaft stainless steel 316. Nitronic 60 stainless steel is particularly suited to anti-galling applications but its anti-galling properties significantly reduce at temperatures above 600°C. It is surprising that in the operating environment of up to 800°C very little wear is seen on the rolling elements.
To prevent powder from accumulating on the bearings and potentially causing premature seizure a shield 210 is placed on the inside of the bearings. The shield may also act to block direct thermal radiation from the flame in the combustion chamber and therefore reduce the operating temperature of the bearings. To prevent powder build up on the shield it can be fitted loosely in to the scraper 200 to allow it to rattle during the movement of the scraper and thus shake off deposits.
The drive mechanism 302 attaches to the outer circumference of the scraper 200 via a drive plate 308 and through a slot 304 in the side of the combustion chamber wall, as shown in Figures 5A and 5B. In order to limit the flow of air into the combustion chamber 71 a baffle arrangement 306 is provided which extends beyond the outer extents of the slot 304. The baffle 306 moves with the drive mechanism 302 and scraper cage 200 such that the slot 304 always remains covered throughout the full travel of the drive mechanism/scraper cage. The slot 302 is as narrow as possible to minimise the passage of air into the combustion chamber.
An earlier, undisclosed, design of a drive plate was supported by two pins running in arc shaped rails on the outer surface of the combustion chamber walls. The pins rotated within holes in the drive plate. The linear actuator acted to move the drive plate and the interaction between the pins and rails plus the shape of the rails caused the drive plate to move in the required 60° arc. However, it was found that the pins would quickly wear on the rails and the drive plate would rest on the slot, causing further wear. The wear caused the drive plate to seize. Therefore a method to control the movement of the drive plate while preventing the drive plate from rubbing on the slot was required.
The design of the fixed drive plate 308 illustrated in Figures 5A/B and Figures 6A/6B removes all of the rubbing elements between the linear drive and the scraper cage 200. The drive plate 308 is rigidly attached to the scraper cage 200 within the combustion chamber 71 and passes through the centre of the slot 304 to connect to the linear drive 302. This removes all possibility of wear of the drive mechanism. The tolerance stack up of the parts means that the position of the drive plate 308 relative to the slot 302 cannot be guaranteed therefore raising the possibility of the drive plate rubbing, and seizing against, the edge of the slot 304. The embodiment illustrated mitigates this by allowing the connection to the scraper cage 200 to be adjusted after the scraper cage has been installed into the combustion chamber.
The connection comprises of a central spar 310 between the upper and lower rings 16B, 16A of the first cylinder 16. A slot (not shown) in the spar 310 allows a front plate 314 and back plate 315 to be rigidly attached to the spar but allowing adjustment post installation of the scraper 200 within the combustion chamber. The scraper 200 is inserted into the flame tube after which the central screw 316 that holds the front and back plates 314, 315 to the spar can be loosened, the plates 314, 316 adjusted to line up with the centre of the slot and the screw tightened. The drive plate 308 is then attached rigidly to the cage by two screws 317 which pass through the drive plate, front plate and spar to engage with a threads in the back plate. The act of tightening these screws holds the drive plate rigidly to the scraper cage.
To prevent loosening of the screws 317 from the repeated compression, and tension, the drive force will subject them to when rotating the scraper 200, several methods can be employed. The drive plate 308 is shaped with two outer forks 318 to prevent it "yawing" about the front plate and thus minimising the tension and compression the screws. In addition a grub screw 319 is tightened against the thread of each screw 317 to prevent their loosening.
An alternative drive plate design, as illustrated in Figure 7, further improves and overcomes the problem of transferring the linear drive to the scraper 200 without allowing the drive plate to wear on the slot. The drive plate comprises several sub-sections, namely, a tongue part 321 and a mating part 322 engaging around a flat spar 323 of the scraper cage. The spar 323 is located between and connected to the upper portion 16B and lower portion 16 A of the first cylinder 16. The fit of the spar 323 to a groove 324 retains the drive plate to the scraper 200 in 5 degrees of freedom, whilst allowing the drive plate to remain free in the vertical axis. This results in the connection of the drive plate to the scraper 200 being able to be made without adjustment and allows for droop of the scraper cage relative to the drive plate without the drive plate being pressed onto the lower edge of the slot which would cause unnecessary wear.
The drive plate is supported in the vertical direction by its connection to the linear actuator and a pair of bearings 325 fitted to a third component 326 of the drive plate. The bearings run on rails 327 which are attached directly to the scraper 200 (either by welding or other fixing means). Thus the tolerance stack up that controls the position of the bearings, and hence the position of the drive plate, relative to the slot is minimised thus ensuring that the drive plate cannot rub on (and seized against) the slot.

Claims

A scraper (200) for scraping the internal surfaces of a cylindrical combustion chamber comprising: a first cylinder(16); a second cylinder (14); a plurality of scraper elements (6) connected to and extending between the first and second cylinders at substantially equally positions around the respective circumferences of the cylinders; characterized in that, the scraper elements are V-shaped with the apex of the V directed axially outward such that, in use, it scrapes the internal surfaces of the combustion chamber into which it is located, and wherein the internal angle, Θ, of the V is between 120 degrees and 160 degrees.
A scraper for scraping the internal surfaces of a cylindrical combustion chamber according to Claim 1, wherein the internal angle, Θ, of the V-shaped scraper (6) is 140 degrees.
A scraper according to Claim 1 or Claim 2, wherein the distance between the distal ends of the V of the V-shaped scraper element (6) is between 15 and 30 mm.
A scraper according to Claim 3, wherein the distance between the distal ends of the V of the V-shaped scraper element (6) is 20 mm and the distance from the apex to each of the distal ends is 10.6 mm.
A scraper according to any preceding claim wherein the V-shaped scraper elements (6) are configured such that, when the scraper is located within the combustion chamber, the apex of the V is located between 2 mm and 6 mm from the internal surface of the combustion chamber at room temperature.
A scraper according to Claim 5 wherein the V-shaped scraper elements (6) are configured so, when the scraper is located within the combustion chamber, the apex of the V is located 5 mm from the internal surface of the combustion chamber at room temperature.
A scraper according to any preceding claim wherein the circumference of the second cylinder is such that, when the scraper is located within the combustion chamber there is a gap of between 3 and 6 mm, preferably 5 mm, between the outer wall of the cylinder and internal wall of the combustion chamber at room temperature.
A scraper according to any preceding claim wherein the first cylinder comprises a plurality of axially extending spokes (25) located within the cylinder extending outwardly from a central hub for, in use, scraping the nozzle arrangement of the combustion device into which it is located.
A combustion device (100) comprising a generally cylindrical combustion chamber (71) and at a first inlet end of the cylinder; a nozzle arrangement for conveying an exhaust gas to be combusted to the combustion chamber; and at the second distal end of the cylinder an outlet for conveying gas away from the combustion chamber; the combustion device further comprising a scraper (200) according to any preceding claim, said scraper located substantially within the combustion chamber, said scraper comprising the first cylinder located proximate the inlet end; and the second cylinder located distal to the inlet end; n scraper elements connected to, and extending longitudinally through the combustion chamber; and a scraper actuation device for rotating the scraper about the longitudinal axis of the combustion chamber by at least 360/n degrees.
A combustion device according to Claim 9, wherein the scraper is supported by a series of "top-hat" shaped bearings located around the circumference of the combustion chamber upon which the upper part 16A of the first cylinder 16 is supported and the actuation means for the scraper uses a drive mechanism that converts linear motion to rotary motion via a link arm located external to the combustion chamber.