US3186132A | 1965-06-01 | |||
US4993200A | 1991-02-19 | |||
US20120058711A1 | 2012-03-08 | |||
EP0160353A1 | 1985-11-06 | |||
US4563840A | 1986-01-14 | |||
US3186132A | 1965-06-01 | |||
US4993200A | 1991-02-19 | |||
US20120058711A1 | 2012-03-08 | |||
EP0160353A1 | 1985-11-06 | |||
US4563840A | 1986-01-14 |
Claims 1 . A blow-suction housing for an abrasive blasting apparatus, comprising a jacket (7) having the shape of a housing with one open end, a blast nozzle (6), a suction opening in the jacket (7) of the housing, a suction hose (9) connected to the suction opening, a gasket (20) attached to the jacket (7), a flow channel (18) formed between the jacket (7) and the gasket (20), and the lower edge (7a) of the jacket, characterized in that a channel structure (17) is arranged on top of the flow channel (18), forming an air duct (17a), via which air can flow from the outside (21 ) to the flow channel (18), into the housing. 2. The blow-suction housing according to claim 1 , characterized in that the air flow rate in the flow channel (18) and the gap (26) can be arranged so high, typically 40 to 50 m/s, that the travel direction of particles flying outwards is turned to a movement inwards into the blow-suction housing. 3. The blow-suction housing according to any of the claims 1 to 2, characterized in that the flow channel (18) or the air duct (17a) comprises one or more bends. 4. The blow-suction housing according to any of the claims 1 to 3, characterized in that the flow channel (18) or the air duct (17a) comprises one or more protrusions (23) inside the channel or duct. 5. The blow-suction housing according to any of the claims 1 to 4, characterized in that the flow channel (18) or the air duct (17a) comprises one or more recesses (24) inside the channel or duct. 6. The blow-suction housing according to any of the claims 1 to 5, characterized in that the inner surface of the flow channel (18) or the air duct (17a) is coated, in part or in whole, with a sound absorbing coating (27). 7. The blow-suction housing according to any of the claims 1 to 6, characterized in that the lower edge (7a) of the jacket can be fitted against the surface (8) to be blasted so that the air flow effective in the gap (26) formed between the lower edge (7a) and the surface (8) to be blasted entrains the static particles (28) on the surface (8) to be blasted, and conveys them into the housing (15). 8, The blow-suction housing according to any of the claims 1 to 7, characterized in that the gasket (20) is made of a resilient material. 9. The blow-suction housing according to any of the claims 1 to 8, characterized in that the channel structure (17) and the gasket (20) are fastened to the jacket (7) of the blow-suction housing by a resilient joint (22). |
The invention relates to a blow-suction housing for an abrasive blasting apparatus, comprising a jacket having the shape of a housing with one open end, a blast nozzle, a suction opening in the jacket of the housing, and a suction hose connected to the opening, a gasket attached to the jacket, a flow channel formed between the jacket and the gasket, and the lower edge of the jacket.
Abrasive blasting is commonly used for cleaning surfaces, to be coated with paint, for example. In abrasive blasting, grit grains are blown at a high speed against a surface to be cleaned. The grains impinging on the surface at a high speed remove debris, such as rust and rolling scale, from the surface. In the blasting, compressed air is used for conveying the grains from a tank via a hose to a blast nozzle. The mixture of compressed air and grit grains is discharged from the nozzle at a high speed which may be as high as 150 m/s. The grit grains coming out of the nozzle, and the debris and dust being removed from the surface to be cleaned, are spread widely into the environment. Having impinged on the surface to be cleaned, and bounced from it primarily sideways, the grit grains may still have a speed of, for example, 50 to 80 m/s.
Various solutions have been developed for restricting the spread of dust and grains into the environment. Generally, housings of various sizes have been used, the abrasive blasting being confined in them. The housing is placed tightly against the surface to be cleaned, and the mixture of grit grains and dust is collected from the housing by suction. For the suction, a suction hose connected to an exhauster is fastened to the opening in the housing.
The grit grains blown from the nozzle to the inside of the housing cause wear of the housing, due to the high speed of the grains. The housing should be tight so that the grit grains cannot escape uncontrollably from the housing. At the same time, however, it has to be possible to draw air into the housing, so that the compressed air and the grit grains blown to the inside of the housing can be removed and conveyed back to the blasting apparatus. Normally, the tightness problem is solved by providing a brush or a brush-like element as a gasket between the housing and the surface to be blasted, to prevent the grain from escaping but to let in air between the bristles.
Document JP 1 1207624 discloses an abrasive blasting apparatus where blasting is carried out inside a housing, and the grit grains are sucked up via a suction hose back to the blasting apparatus. The housing comprises a brush which seals the housing against the surface to be cleaned. Air is drawn into the housing through the brush.
Document EP 0160353 presents a blow-suction housing of an abrasive blasting apparatus, where a brush is fastened to the vacuum housing in a flexible manner. Thanks to the flexible joint, the brush is flexibly movable if the surface to be sealed has a variable roughness. When the blow-suction head is moved manually, the brush adapts to the variations in distance.
Both above-mentioned solutions involve several problems. Within the blasting head, the grains bounce back and forth, wearing the bristles. After only a short time of use, the bristles on the inside are worn so much that the sealing is impaired. Secondly, because there is a strong negative pressure in the blast housing, the negative pressure and the air flowing through the bristles bend the bristles inwards, whereby a free space opens up between the surface to be blasted and the bristles. Grit grains may escape through this open space. The strong negative pressure in the housing also makes the blow-suction head heavy to move, because the negative pressure draws the housing against the surface to be cleaned.
As an approach to the problem of sealing the blow-suction head, document US 5833521 presents a structure where compressed air is blown into the blow-suction head. The compressed air constitutes an air curtain through which the grit grains cannot escape. Disadvantages of this approach include a high consumption of compressed air and the fact that the structure is not tight. Because the grit grains move at a speed as high as 80 m/s and higher within the blow-suction head, the thin curtain of compressed air is not sufficient to confine them within the housing. This is particularly true in the case of grit grains of metal, having a high density and thereby also a high kinetic energy.
It is an aim of the invention to present a blow-suction housing for an abrasive blasting apparatus, by which problems involved in the prior art can be eliminated. The aims of the invention are achieved with a blow-suction housing which is characterized by what will be presented in the independent claim. Advantageous embodiments of the invention will be presented in the dependent claims.
The blow-suction housing for an abrasive blasting apparatus according to the invention comprises a jacket having the shape of a housing with one open end, a blast nozzle, a suction opening in the jacket of the housing, a suction hose connected to the opening, a gasket fastened to the jacket, a flow channel formed between the jacket and the gasket, and the lower edge of the jacket. The blow- suction housing is designed so that when it is installed onto a surface to be blasted, a gap is left between the lower edge of the jacket and the surface to be blasted. A channel structure is mounted onto the flow channel in the jacket of the housing, to form an air duct, via which air can flow from the outside to the flow channel and into the housing.
In a preferred embodiment of the blow-suction housing for an abrasive blasting apparatus, the velocity of air flowing in the flow duct and in the gap can be arranged so high, typically 40 to 50 m/s, that the direction of movement of particles flying outwards is changed to a movement inwards into the blow-suction housing.
In another preferred embodiment of the blow-suction housing for an abrasive blasting apparatus, the flow channel or air duct comprises one or more bends. Alternatively, or in addition, the flow channel or air duct may comprise one or more protrusions or one or more recesses inside the channel or duct.
In a third preferred embodiment of the blow-suction housing for an abrasive blasting apparatus, the inner surface of the flow channel or air duct is coated, in part or in whole, with a sound-absorbing coating.
In yet another preferred embodiment of the blow-suction housing for an abrasive blasting apparatus, the lower edge of the jacket can be fitted against the surface to be blasted so that the stream of air effective in the gap forming between the lower edge and the surface to be blasted, entrains particles lying on the surface to be blasted, and conveys them into the housing.
In yet another preferred embodiment of the blow-suction housing for an abrasive blasting apparatus, the gasket is made of a resilient material.
In yet another preferred embodiment of the blow-suction housing for an abrasive blasting apparatus, the channel structure and the gasket are connected by a flexible joint to the jacket of the blow-suction housing.
In the following, the invention will be described in more detail with reference to the appended drawings, in which
Fig. 1 shows an abrasive blasting apparatus comprising a blow-suction housing according to the invention; Fig. 2 shows a blow-suction housing according to the invention in a cross-sectional view; and
Fig. 3 shows a preferred embodiment of the blow-suction housing according to the invention. Figure 1 shows an abrasive blasting apparatus comprising a blow-suction housing according to the invention. The blasting apparatus comprises a pressure vessel 1 containing grit grains 2. The size of the grit grains is generally 0.1 to 0.7 mm. A grain valve 3 is provided under the pressure vessel, for adjusting the amount of grains supplied to blasting. Compressed air is introduced in the apparatus by a com- pressed air valve 4 and a piping connected to it. The grit grains flowing from the pressure vessel 1 via the grain valve 3 are mixed with a stream of compressed air under the grain valve 3, and the mixture of compressed air and grains is conveyed via a blast hose 5 to a blast nozzle 6. From the blast nozzle 6, the mixture 16 of compressed air and grit grains comes out at a high speed into the blow-suction housing. Within the housing, the grit grains impinge on the surface 8 to be blasted and clean it by removing debris from the surface. When the grit grains impinge on the surface 8 to be blasted, their travel direction is changed to follow the surface to be blasted, whereby they hit the jacket 7 of the blow-suction housing, particularly its lower part and gasket 20 (Fig. 2). A suction hose 9, in which a negative pressure is caused by an air extractor 14, is connected to the jacket 7 of the blow-suction housing. Because of the negative pressure, air flows from the blow-suction housing to the suction hose. The grit grains and the debris removed from the surface 8 are conveyed via the suction hose to a cyclone 10. In the cyclone, dust and other fine particles are separated from each other. The re-usable grains flow back to the pressure vessel 1 , and the fine particulate is conveyed via the channel 1 1 to a filter 12 where dust and other debris are separated from the air flow. The dust 13 is collected in the lower part of the filter. Clean air is discharged from the filter via an air extractor 14.
Figure 2 shows a blow-suction housing according to the invention in a cross-sec- tional view. The jacket 7 of the blow-suction housing may be a cylindrical body of revolution, or it may have, for example, a rectangular shape. A blast nozzle 6 is provided in the upper part of the blow-suction housing, for injecting a grain jet 16 towards the surface 8 to be blasted. Around the jacket 7 of the blow-suction housing, a gasket 20 is attached, its lower edge being lower than the lower edge 7a of the blow-suction housing. The gasket is off the housing jacket 7 so that an air flow channel 18 is formed between the gasket and the housing. A gap 26 is formed between the lower edge 7a of the housing and the surface 8 to be blasted, via which gap air will flow from the flow channel 18 into the housing 15. The gasket 20 may be rigidly fixed to the jacket 7, or it may be fastened in a flexible way by springs or rubber elements so that the springs or corresponding resilient elements press the gasket against the surface 8 to be blasted.
Air will flow via the flow channel 18 into the gap 26 and further into the housing 15. The flow channel 18 is formed in the gap between the gasket 20 and the jacket 7 of the blow-suction housing. The gasket 20 is designed so that air will flow from above the gasket 20 and further from the space between the gasket 20 and the housing downwards, and via the gap 26 between the lower edge 7a of the housing and the surface 8 to be blasted, into the housing 15.
The flow channel 18 may be short or long. A separate channel structure 17 is arranged on top of it, forming an air duct 17a. Via the air duct 17a, air is conveyed to the flow channel 18. Fresh air is introduced via an inlet 21 in the channel structure 17. The air duct 17a and the flow channel 18 comprise a bend or bends 23 in the air flow direction, whereby grains flying out of the blow-suction head collide with these bends. In the collisions, the outwards movement of the grains is reduced or stopped completely. Thus, the air flow in the air duct 17a and the flow channel 18 will convey the granular particles back into the blow-suction chamber 15. The channels also comprise protrusions 24 which help to prevent the granular particles from flying out along a rectilinear trajectory. Furthermore, the walls of the channels comprise recesses 25 which trap the airborne granular particles.
A loud noise is produced by abrasive blasting, because the air and the grain jet 16 are expelled from the blast nozzle 6 at a high speed of, for example, 150 m/s. The inside of the air duct 17a and the flow channel 18 can be coated by a sound absorbing material 27. In this way, the level of noise emitted into the environment can be significantly reduced. Furthermore, the level of noise emitted outside the housing can be reduced by installing, for example, an air-permeable brush-like gasket outside the inlet 21 (Fig. 3).
The gasket 20 is made of a resilient material, such as rubber, whereby the gasket is pressed tightly against the surface 8 to be blasted even where the surface is uneven. Because the negative pressure in the blow-suction housing is low, due to the open air duct 17a and flow channel 18, the gasket 20 remains in constant contact with the surface 8 to be cleaned. It is not bent by the negative pressure so that an open space would be formed between the surface 8 and the gasket 20, through which space the grains could fly straight out. The gasket 20 is normally made of a material that is impermeable to air and resistant to wear. The gasket 20 is attached by bars 19 or corresponding members to the air duct 17a or the housing jacket 7. A gap 26 is left between the lower edge 7a of the blow-suction housing and the surface 8, through which gap air will flow into the housing. The gap is dimensioned so that the air flow rate in the gap is so high that granular particles 28 lying on the surface 8 will be entrained in the air flow. Typically, the air flow rate in the gap 26 is in the order of 40 to 50 m/s. At this rate, the air flow will convey the granular and dust particles from the surface 8 to be blasted to the inside of the blow-suction housing 15, from where they are conveyed by the air flow further into the suction hose 9.
Figure 3 shows a preferred embodiment of the blow-suction housing according to the invention. In this embodiment, the gap between the lower edge of the housing and the surface to be blasted has been made longer. By the longer gap, it is possible to increase the distance that the escaping grains have to travel in the direction opposite to the air flow direction. The longer travel distance will efficiently reduce the velocity of the grains flying outwards, and thereby improve the sealing of the blow-suction head. In the embodiment of Fig. 3, the lower edge 71 of the housing is designed to beflow technically advantageous, to reduce the pressure loss caused by the flow. Figure 3 also shows a brush-like gasket 30 arranged outside the channel system, reducing the noise level outside the blasting head.
The abrasive blasting apparatus operates as follows: When abrasive blasting is on, a mixture of grains and air is ejected at a high speed from the nozzle into the blow- suction housing, and the grains impinge on the surface to be cleaned. The speed of the grains may be, for example, 150 m/s when they are ejected from the nozzle. The output speed depends on e.g. the structure of the nozzle, the blasting pressure applied, and the mass flow rate of the grit grains. When the grains impinge on the surface to be blasted, they bounce primarily sideways. Thus, their speed is signifi- cantly reduced, but the speed may still be, for example, 50 to 70 m/s. A small part of the grains will fly out via the gap below the lower edge of the housing, in the direction of the surface to be blasted, and impinge on the resilient gasket. At the same time, air is sucked up from the inside of the housing, and replacement air is introduced between the housing jacket and the gasket, along the air duct, to the gap between the lower edge and the surface, and further to the inside of the housing. The gap between the lower edge of the housing jacket and the surface to be blasted is dimensioned so that the air flow rate in the gap is in the order of 40 to 50 m/s or higher. The size of the gap may be, for example, 10 to 20 mm. When the grains fly outwards via the gap, their velocity is significantly reduced by the air flowing in the opposite direction. The velocity of the grains is reduced by 40 to 70% in the gap, depending on the size and the specific weight of the grains. For example, if an aluminium oxide grain has the size of 0.2 mm and a velocity of 50 m/s upon entering the gap, its velocity will be reduced to a value of 28 m/s when it comes out of the gap and impinges on the gasket. When the grain escaped through the gap impinges on the gasket, its velocity will drop to zero or at least decrease further to a significant extent. At this stage, most of the grains will be entrained in the air flow back into the housing. From the housing, they are conveyed by the air flow further along the suction hose to the blasting apparatus. A small proportion of the grains will bounce further outwards in the air duct.
The air duct between the housing jacket and the gasket is provided with mechanical stops and bends so that when these are hit by the grains, their outward movement will be stopped. In the air duct, the air flow rate is designed such that the grains will be entrained in the air flow back to the blow-suction housing and further via the suction hose to the blasting apparatus. As an additional sealing outside the gasket, for example a brush may be provided, replacement air being drawn through the brush into the flow channel. Thus, a large amount of replacement air will flow into the blow-suction housing along a separate open channel. The introduction of replacement air into the blow-suction head via the separate channel will not bend the gasket inwards and in this way open up a gap between the gasket and the surface to be blasted, but the gasket remains tight all the time. The separate replace- ment air duct is arranged so that the air flowing in it will prevent grains from escaping the blasting head. The air flow rate in the gap between the lower edge of the housing and the surface to be blasted is dimensioned such that static grains lying on the surface to be cleaned are entrained in the air flow. The static grains are thus sucked up from the surface to the inside of the blow-suction housing, and they cannot escape the blow-suction housing when the housing is moved across the grains. Thanks to this arrangement, no dust, grit grains or other impurities are left on the surface to be blasted. Furthermore, the blow-suction housing according to the invention has the advantage that the negative pressure inside the housing is considerably lower than in solutions of prior art. This makes it easier to move the hous- ing.
The structure according to the invention is suitable for both small and large nozzles 6, or several nozzles. The diameter of the blasting head according to the invention may vary according to the use. For example, measured at the gasket 20, the diameter may be 100 mm, or in large vacuum suction heads, for example, 250 mm. The tightness of the blow-suction housing according to the invention is good even with a high rate of grain flow from the nozzle to the blow-suction housing. The blow- suction housing according to the invention can be moved faster than housings of prior art across the surface 8, without leaving grit grains on the surface to be blasted. The force needed to move the blow-suction housing is lower than the force needed to move housings of prior art, because the negative pressure within the housing is lower.
Some advantageous embodiments of the blow-suction housing according to the invention have been described above. The invention is not limited to the solutions described above, but the details of implementing the invention may vary within the scope of the claims.
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