US2912918A

US2912918A - Blast room with uniform down-draft ventilation

Info

Publication number: US2912918A
Application number: US667766A
Authority: US
Inventors: William H Mead
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-06-25
Filing date: 1957-06-25
Publication date: 1959-11-17
Anticipated expiration: 1976-11-17

Description

Nov. 17, 1959 Filed June 25, 1957 O Q c a Q 0 Q 0 W. H. MEAD BLAST ROOM WITH UNIFORM DOWN-DRAFT VENTILATION a'o oo n ueoooooo 5 Sheets-Sheet l oflnovaoncu otn0o 1 INVENTOR. WILL/AM H. MEAD AT TOR/VEY Nov. 17,1959 w. H. MEAD 7' 2,912,918

BLAST ROOM WITH UNIFORM DOWN-DRAFT VENTILATION Filed June 25, 1957 5 Sheets-Sheet 2 Illll In uvvszvr WILL/AM fill/E40 WWW A T TOR/VEY.

Nov. 17, 1959 w. H. MEAD 2,912,918

BLAST ROOM WITH UNIFORM DOWN-DRAFT VENTILATION Filed June 25, 1957 5 Sheets-Sheet 3 /50 L 1 a \52 h 52 56- A INVENTOR.

I W/LL/A/MMMEAO BY @Ha AT TOR/VEY Nov. 17, 1959 w. H. MEAD BLAST ROOM WITH UNIFORM DOWN-DRAFT VENTILATION Filed June 25. 1957 5 Sheets-Sheet 4 INVENTOR.

WILL/AM H. MEAD ATTORNEY Nov. 17, 1959 BLAST ROOM WITH UNIFORM DOWN-DRAFT VENTILATION Filed June 25, 1957 w. H. MEAD 2,912,918

5 Sheets-Sheet 5 f W/LL/AM I'LMEAD L4 7' TOR/VE Y United States Patent wrrn UNIFORM DOWN-DRAF VENTILATION BLAST ROOM This invention relates to an abrasive blast room. More particularly, it relates to structure in a blast room for providing uniform down-draft ventilation and for improved withdrawal from the room of the used abrasive. This application is a continuation-in-part of my co-pending application Serial No. 57 7,375,'filed April 10, 1956, now abandoned. Y

The problem of ventilating a blast room has not been well understood by many people. Ventilation has nothing to do with the operators safety, because he is fed filtered air through a blast helmet and never breathes the air in the room anyway. The primary purpose of ventilation is to allow the operator to see what' he is doing. Since labor now represents 70% of the cost of blasting, considerable savings can be made 'by so improving the ventilation in the room that the blaster can clearly see the objects he is blasting; then he will neither underblast nor overblast them, nor will he have to pause from time to time, as he has to when the air clears only gradually.

'A secondary purpose of ventilation can be, and is in this invention, to provide air currents that carry the used abrasive and waste out of the room and convey them to reclamation and disposal apparatus.

Heretofore, blast-room ventilation has been provided by an airstream that enters the room through one or morerelatively small openings in its ceiling, usually one opening at about its center, and flows downwardly and outwardly to exits along the sides or ends of the room. This designresulted in what is termed side draft ventilation, since the flow is from the center of the ceiling toward the sides of the floor. The air near'the center of the floor therefore tended to move slowly and gave the operator inadequate visibility. The results were that the blast operator had to shut off the blast hose from time to time and let the air clear, that the blaster tended to guess rather than observe whether the blasting was adequate, and that the blaster was more likely to under-abrade or over-abrade the workpiece than he was to get the correct amount of abrasion.

The present invention provides uniform down-draft ventilation by providing for the entry of air from the ceiling in a uniform-distribution pattern and for its exit through openings spaced substantially evenly over the entire fioor area, preferably at about six-inch intervals. The ceiling is provided with permeable maze-like baffling that keeps abrasive from escaping therethrou gh while providing for uniform distribution and free entry of air. Thus, the inlet maze gives equalized air entry, and the floor gives equalized air exhaust. i

. Another problem with blast rooms heretofore in use has been their reliance on a deep pitat least half as deep as the width of the roomto collect the abrasive and waste by gravity. If the water table was low enough, the pit would remain dry, but it was in all events costly, presented obvious (but none the less serious) installation problems, and precluded adequate maintenance of'the 2,912,918 Patented Nov. 17, 1959 "ice 2 machinery located below ground level. The abrasive, waste, etc., were removed from the pit by moving buckets or screws. All such moving parts meant that breakdowns could be expected from time to time, due in part to the wear necessarily produced when abrasive materials are handled. a

This pit,- with its attendant problems, has been eliminated by an invention that can be called the -waflle floor, the basic principles of which are-described in the patent application by Ted Adamson Arnold, Serial No. 456,257, filed September 15, 1954, now Patent No. 2,839,338 (which does not relate to the visibility and ventilation problems); In'place of a single largedeep hopper in a pit, a waffle floor provides a large number of small shallow hoppers that empty into shallow ducts, through which air carries the abrasive and waste out of the room. The present inventionutilizes a greatlyimproved form of waffie floor in combination with the air-maze ceiling to obtain uniform down-draft ventilation, improved conveying away of abrasive and waste,

and other novel resultsl' e By'improving the efiectiveness of ventilation, the invention makes it possible to use less air' for ventilation, and this means smaller blower fans and less power consumption. Moreover, the'airstream that ventilates the room and carries off the waste abrasive can be put to work in a system for reclaimingthe abrasive from the waste, so that the abrasive can be re-used. Y Y Thus, the present inventionprovides true and uniform clown-draft ventilation by providing a maze for air intake at the top of the room in combination with an improved wafile floor. The'maze may comprise afalse ceiling, no real root being needed for the room-where the blast chamber is inside a large shed or other building covered with a roof.' Where the blastroom-must be a separate building, the maze may be at least a part of the roof of the roornf Airzmay besupplied-throhgh openings in the actual roof or through openings in a side wall between a false ceiling and the-true'one. In either event'the air is taken in and evenly distributed across the room, while the exit of paint flakes, dust,---dirt, and abrasive from-the ceiling is barred. f The present invention utilizes a waflle-type floor similar to that disclosed in the 'Arnold application but preferably having some important differences. 'Ducts below the floor conduct the abrasive, the abraded waste, arid the down-draft air out from the room, the down-draft air preferably being utilized for carrying the abrasive to a reclaiming point for collection there and redirection through the blasting hose. A very important part of this invention is a novel relationship between the sizes of the openings through the wafile floor from one'side of the room to another, along the lengthof the ducts. This, together with the size relationships of the duct crosssections along the length of the ducts, is important "in assuring uniform down-draft ventilation and proper'con- "veyance of the abrasive-waste mixture; 1

One object of the present invention is, therefore, to provide a blast room having uniform down-draft ventilation, affording greatly improved visibility 'to the blaster. 1 Another object of the inventionis to provide 'a blast room wherein the air enters from the ceiling and the down-flowing air is dispersed evenly over the room-.

Another object of the invention is to provide a novel waflle-type floor in a blast roomthat is able to provide an even distribution of the outlet air therethr'oughi" Another object of the invention is to provide ablast room having in combination" an air maze at the ceiling and a novel waflle-type 'floontoprovide foruniforii'i down-draft ventilation 'therebetweenand for collecting and conveying away the spent ahrasivei i Another object of the invention is to provide for conveyance of the air and entrained particles through the duct system below the floor at a controlled substantially uniform velocity. v Another object of the invention is to provide a ventilation system that will keep a blast-room floor free of debris, abrasive, etc. at all times.

Another object of the invention is to provide for more efficient air movement through a room, making possible the most economical use of blowing machinery, conduits, etc.

A general object of the invention is to cause the air Within a room or enclosure to circulate in such a manner as to provide both satisfactory visibility and the removal of the dust and abrasive for reclamation of the abrasive. This object is achieved in my blast room by causing uniform entry of air through the ceiling, causing the air to fiow downwardly with substantially uniform velocity at substantially all points within the room. The air entrains the foreign material to be removed and preferably passes through a perforated structural floor to the waffle floor beneath, the latter comprising a large number of small hoppers occupying substantially the full floor area. The small hoppers direct the air and entrained foreign material toward carefully sized orifice openings at the lower end toward which they converge. The orifice size for each hopper is carefully chosen so that an equal quantity of air will flow through each hopper, even though the vacuum available beneath each orifice may vary, due to the pressure drop resulting from the air flow in the sub-floor region.

By way of example, consider a mixture of air and refuse-laden abrasive as it leaves the hopper orifice at the upstream end of any duct. Further upstream from this first hopper orifice a small quantity of air is admitted to establish required fiow velocity in the duct. (This velocity is needed for conveying purposes, to prevent stoppage of the conveying action even if a hopper were to become completely filled by abrasive.) The mixture being considered joins the previously-admitted air and progresses downstream toward the next hopper. At the orifice below the next hopper where more air and abrasive are admitted to the stream, the duct area-is increased so as to maintain proper flow conditions. This action is repeated at each hopper until the last hopper at'the downstream end of the duct has been passed, at which time the duct reaches its maximum cross-sectional area, and the quantity of material conveyed within the duct is also at a maximum. As the pressure within the duct at the downstream end is substantially lower than at the upstream end (due to the pressure drop along the duct necessary for maintenance of a substantially constant conveying velocity) the orifices opening from the hoppers into the downstream end of each duct are proportionately smaller than the orifices: at the upstream end. A proper balance of duct pressure and hole size at each hopper results in equal flow through each hopper. The various floor ducts may connect at the downstream wall with apparatus for separating the air from the solid material, after which the usable abrasive may be returned to the system for re-use.

While the invention is discussed in connection with blast rooms, it also applies to other types of rooms requiring uniform down-draft ventilation. For example, paint-spraying rooms may employ the same principles. So may underground ore-crushing installations, the crushing machinery being set on a wafiie fioor to provide better air conditioning in the crushing area. The invention is not intended to be limited solely to abrasive blasting chambers.

Other objects and advantages of the invention will appear from the following description of the preferred form thereof presented in accordance with 35 USC 112.

In the drawings:

Fig. 1 is a view in perspective of a self-contained blastroom assembly embodying the principles of this inven tion. One wall is partly broken away to show the in terior.

Fig. 2 is a view in elevation and in section, on an en-' larged scale, of a room similar to that of Fig. 1 except for the ceiling and roof structure.

Fig. 3 is a further enlarged view in perspective and in section of a portion of an air maze bafiie arrangement used as a false ceiling for air intake in rooms not having a true roof.

Fig. 4 is a still further enlarged view in elevation and in section of one of the battle arrangements of Fig. 3.

Fig. 5 is an enlarged view in elevation and in section of one of the air-maze inlets of Fig. 2.

Fig. 6 is a fragmentary view in perspective of a portion of a waflie floor assembly, with parts broken away and shown in section.

Fig. 7 is a plan view of the floor with successive layers broken away to show the parts beneath.

Figs. 8a and 8b are views in elevation and in section of the lower portion of a blast room, showing the floor and the outlet duct, taken on the line 88 in Fig. 7a The left-hand end of Fig. 8b represents the same point as the right-hand end of Fig. 8a, so that these two figures are really one view broken in half in order to use a large scale.

Fig. 9 is a curve plotting static pressure against distance of the orifice from the upstream end.

Fig. 10 is an enlarged sectional view of a portion of Fig. 8 at the upstream end of the room.

Fig. 11 is a view in section taken along the line 1111 in Fig. 7.

The blast room 10, as shown in Figs. 1, 2, and 7, may comprise a chamber enclosed by

end Walls

11 and 12 and

side walls

13 and 14. The room 10 may have a roof 15, which is not essential in every instance, espe cially where the blast chamber is inside a larger build ing. In any event, the room 10 has an air inlet maze which may or may not include a roof 15 as a part thereof and does include a false ceiling. The room 10 preferably has a perforate floor 16 comprising wire meshor perforated steel plate or other perforate supporting surface for the blaster 17 and the objects 18 to be blasted, and a waffle floor 20 through which the spent abrasive and waste falls and through which the down draft of air leaves. Below the wafiie floor 20 is a series of ducts 21 which may carry the abrasive-and-waste-laden airstream into a pickup snout 22, whence the stream may be conducted through a conduit 23 into a concentrator 24 where, by methods well known in the art, a cyclone apparatus and other separating means settle out the reusable spent abrasive from the air, dust, and light Waste which would contaminate the spent abrasive. The spent abrasive may be recycled through a reclaimer 2S and blast generator 26 into a blast hose 27 held by the operator 17. The waste, dust, and air carried away from the concentrator.24 and reclaimer 25 may pass through another conduit 28 to a dust separator 29, which may be a bag filter, wet precipitator, or other apparatus for separating dust from the air. The dust is disposed of, while the clean air may either be exhausted into the exterior atmosphere or recycled to the room 10. Forced draft may be provided by a suitable blower (e.g., fan) in or adjacent the separator 29.

As stated previously, a true roof 15 is not an essential feature of the blast room 10. The roof 15 gives protection from rain, wind, and snow, Where the room 10 is outdoors, but many blast rooms are in factories, where no true roof 15 need be provided. Whether or not there is a roof, an air maze provides for adequate intake and distribution of air, preferably with sufficient back pressure to assure eventual uniform distribution. Where a roof 15 is used, air intakes may be provided by shielded chimneys 30 (Figs. 2 and 5) through the roof 15 or by openings 31 through the side wall 14 (Fig. l) or through one or more of the

walls

11, 12, and 13. Where, as in Fig. 1, the inlets 31 are through only one wall 14, it is preferred to slope the roof 15 so providing a trapezoidal space 32 above a closely-meshed screen 33, whose height diminishes as the distance from the inlets 31 increases, thereby aiding in achieving an even distribution of air to the air maze. The slope of the roof 15 in Fig. 2 is purely for shedding rain, snow, etc., but does not at all interfere with the even distribution of air.

The shielded chimney 3% of Figs. 2 and 5 preferably includes a cylindrical stack 34 with a flanged lower end 35 and a cylindrical cover 36 having a closed upper wall 37 spaced above the upper end of the stack 34 and open at the lower end. Suitable spacers 38 provide the support and spacing for the cover 36. Air enters at the opening 40 provided at the lower end of the cover 36 and around the stack 34, goes up in the space 41 between the cover 36 and stack 34, goes through the space 42 above the stack 34 and then vertically down the stack 34 and through an opening 43 into the space 32. Then it passes down through the screen 33. Abrasive is efiectively kept from getting out by the screen 33 and the baffled path through the chimney 36).

Where there is no roof 15, there may be a false ceiling 50, itself preferably comprising a series of airmaze baffles 51 providing a labyrinthine passage for the air therethrough and emptying it down into the room. By this means, the passage from the room of abrasive, dust, and other contaminating agents is kept at a minimum, and the air enters the room and follows a true down-draft path. In the preferred baffle structure 50 for a roofless room shown in Figs. 3 and 4 all the sheet metal baffles 51 are alike and are parallel to each other for substantially the full length of the room. At the top is a series of imperforate upper plates 52 spaced apart to provide longitudinal openings 53. Beneath each opening 53 are a pair of converging

inclined plates

54, 54 whose lower ends are spaced apart and secured together by fasteners 55. The plates 52 preferably have depending vertical flanges 56 to which are secured spacer strips 57 that are secured to the inclined plates 54. The bulk of the air passes down through the openings 53, is deflected up by the plates 54, passes over their upper ends 58 which are spaced below the plate 52, and is deflected by the plate 52 down into the room. Solid particles that find their way above the baflles 54 fall out through the space 59 at the lower end along with a minor proportion of the air.

The floor structure of the present invention is a very important feature thereof. The supporting floor 16 itself may comprise perforated steel plate or wire mesh or other perforate structure strong enough to support the operator 17 and the workpiece 18. For example, A steel plate with holes about /2" in diameter spaced to provide between about 45% tov 55% of the area open, is satisfactory. If desired, when heavy blasting work is to be done, the article 18 being blasted may be supported by a dolly wheeled into the room It) on one or more railroad-type rails (not shown) or other type of conveying equipment may be used. The perforate floor 16 is not an absolute essential. Rooms 10 set up for conveyor blasting need not have it, for example. But it is usually an important element of the room, not only for convenience in walking and for support, but also as a scalping plate to catch large flakes, etc., of waste and prevent them from plugging the hoppers.

The perforated floor 16 rests on the waffle floor 20. This floor 20 presents an improvement upon the Arnold patent application Serial No. 456,257 filed September 15, 1954. As stated in that patent application, the provision of a floor with a plurality of small hoppers entering the ducts is a vast improvement over the former construction where there was a single huge hopper. Such a single hopper meant excavating a pit below the room for the collection of abrasive. Such a room also was not able to avail itself of uniform down-draft ventilation since the floor merely served for the collection of abrasive and the ventilation Was taken care of by passing air from a single ceiling duct down and out between the side walls and the floor. This resulted in side-draft ventilation instead of down-draft ventilation.

In the present invention the waffle floor 20 comprises a series of hoppers 6t) and may be made up as a series of inverted ti-shaped members 61, the apex 62 of the V at the top comprising partitions between adjacent hoppers 6t), and the converging bottoms. 63 of two members 61 forming troughs between. them and spaced apart from each other. Then a series of individual folded diamondshaped inserts 64 may be welded across the space between each pair of inverted members 61 to complete the hopper assembly. The bottoms 63 of the hopper-forming members 61 and the bottoms 65 of the inserts 64' may be space-welded to an orifice plate 70 and the tops or apexes 62 may be welded to the perforate floor 16 at various spots.

Alternatively, the waflle floor 20 may be provided in a cast unit of any desired size and shape. For example, each cast unit may consist of six small hoppers 60 in one direction and four in the other direction, providing twenty four small hoppers 60 in all.

In any type of wafiie floor 2t), a typical size for the hoppers 60 is about six inches square at the top and about three and one-half inches high. Several openings 66 through the plate 16 open into each hopper 60, and

each hopper 60 slopes (preferably at about 4055) toward its bottom end. Hopper slopes of 45 to 52 have been found exceptionally satisfactory. The bottom of each hopper 60 may rest upon, and as said above, is preferably welded to, or otherwise secured to, the orifice plate 70, which also serves as the upper wall of the ducts 21, the other three walls of which are defined by a three-sided, initially open-top assembly that may be formed by bending steel plate members to provide side walls 71 and a bottom 'wall 72 and may then be welded to the plate 70 to close the ducts 21. Or, the threesided assembly may be welded together, if desired. The plate 70 is preferably welded to the side walls 71. Thus, the complete waflle floor assembly may have the ducts 21, orifice plate 7%,

hopper members

61 and 64, and perforate floor 16 all welded together into a unitary structure.

The proportioning of the ducts 21'and of the orifice openings through the plate 70 is important in achieving uniform down-draft and effective conveyance of the entrained particles, because proper proportioning will provide substantially the same velocity of air throughout the length of each duct 21. As shown best in Fig. 10, there is preferably an initial opening 73 at the side 13 of the room 10 farthest from the pickup snout 22. The opening 73 may be protected by a baflie 74 that may extend from the side wall 14, air entering the opening 73 by first passing down through plate openings 66 into the hopper 60a and then up through other openings 66 on the opposite side of the partition 74. The opening '74 leads into the upstream end of the duct 21. The end hopper 60a empties into the duct 21 through the largest orifice 75 of any hopper 6%} in the series, though it need not be greatly larger than the others and is smaller than the opening 73. As before stated, the orifice sizes are carefully calculated, as are the duct-shapes and cross-sectional areas past each orifice. The hole diameter is obtained from the following equation:

D: )458Q C /F D is the diameter of any particular orifice, in inches; Q is the desired volume rate of air to pass through the orifice in cubic feet per minute of free air; A C is the coefficient of entry of the air, a factor depending on the design of the hopper; and i where P is the static vacuum in the duct beneath the orifice, expressed in inches of water, relative to the pressure in the room.

In the design of the present invention, C has been determined experimentally to be about 0.90. It may vary somewhat if different shaped hoppers are used, but can be determined experimentally or can be derived from tables by those skilled in this art. Inserting this value and simplifying, the equation for the hoppers shown in the drawings becomes As examples of orifice size:

P in Q in D, inches inches cu. it./min.

1 2 20 0. s5 6 20 0. e5 6 15 0. 5s

Examples 1 and 2 would be at different points in the same room. Example 3 would be at the same position as Example 2 but in a room having a lesser volume flow rate.

Fig. 9 is a typical curve plotting the static vacuum P against the distance along a duct measured from the end farthest from the outlet. From such a curve the exact orifice diameters are calculated. For most uses, the diameters may be rounded off to the nearest sixteenth of an inch, so that, for example, in one room the entrance opening 73 may be 1%", the first two

orifices

75 and 76 may be in diameter, the next three

orifices

77, 78, and 79 may be 1 the next four /4", the next five and the next seven and so on.

The net result, of course, is that there will be the same flow through each hopper 60. Thus, in a room having a volume flow rate Q of 80 cubic feet of free air per minute per square foot, and where the hoppers 60 are 6" square at their upper end, every hopper 60 in the room will have 20 cubic feet of air pass through its orifice each minute.

In some installations, due to manufacturing tolerances, installation tolerances, and other factors, the theoretical values may not be fully accurate. So, after installation, each orifice may be measured for P, the static pressure by a manometer inserted therein from above. If corrections are necessary, the orifice opening may be reamed to widen it, or a washer may be inserted to narrow it.

- Where the first orifice 75 empties into the duct 21, near its upstream end, the duct 21 is narrow and shallow. The narrowness is provided by the nearly converged portion 80 of the side walls 71 and the shallowness may be provided by an insert plate 81. a The insert plate 81 is preferably stepped from a level about /3 the depth of the duct down to its full depth, each step 82 occurring below an orifice. The reason for stepping, rather than using a smooth slide, is that the increment of air through each orifice at this location is a sizable proportion of the air in the duct at that location. Thus, at the

second orifice

76, 50% as much air is added as was already in the duct; 33 /s% as much is added at the third orifice 77; 25% at the fourth; 20% added at the fifth, 16 /a% at the sixth. When the percentage has dropped below about 15%, stepping is no longer essential, and smooth tapering or sloping becomes both possible and economically more practical.

The duct 21 can be very narrow here, because no substantial amount of abrasive can get into the passage 73. The introduction of a certain amount of substantially abrasive-free air through the passage 73 at the duct end assures a flow of air horizontally through the full length of the duct 21 and enhances movement of abrasive from beneath the openings 75, etc. This means that abrasive and waste will always be moved by air moving horizontally through the duct 21, as well as by the air that enters the duct 21 along with the abrasive through each hopper 60. Yet the opening 73 is not made so large as to divert any large proportion of air, for that would cancel the down-draft effect. The proportions must be adjusted for the stated ends in view.

The narrow, shallow portion 83 of the duct 21 may continue past the first orifice 75 before the insert plate 31 steps down at 82; then it will step down for each orifice until it joins the normal duct bottom plate 72 of the initial portion 85 of the duct 21. In the drawing this portion 85 is shown extending approximately half the distance between the

side walls

13 and 14. The actual extent on any duct depends on the amount of air moved, etc. It may run the full width or any portion thereof. During this portion 85, the duct depth remains constant (i.e., the bottom plate 72 is parallel to the plate 70), while the width gradually increases as the side walls 71 flare apart from the narrow portion 8%), to a point 86 where the ducts 21 are separated from each other only by a single side wall 71-the maximum possible width. The side walls 71 are so narrow that nothing is to be gained by joining two or more ducts by removing some of the side walls. The remaining portion 87 of the duct 21 necessarily remains constant in width, and increases in depth, a sloping bottom wall portion 88 being provided as shown in Fig. 8. Thus, the initial depth of such a duct portion 87 may be approximately 2 /4" (the depth of the portion 85), and at the wall 14 it may be approximately 4 /2 deep in, say, 10 feet of length (see Fig. 8).

The shape of the ducts 21 is important in order to get uniform down-draft ventilation, for the same amount of air should leave each square foot of floor, while the air maze provides for the desired equal distribution of the entering air. The increase in cross-sectional area of the duct corresponds to the amount of air added at each

opening

75, 76, 77, etc. The width is increased first, to prevent piling up of abrasive beneath the openings and then, when a substantial flow of air is assured, the depth is increased. In this way, the air flows straight down from the maze to the ducts 21, without being diverted to some portions of the room more than others.

In operation, air enters from the exterior or the interior of a large building through the area 32 above the air maze and is dispersed evenly through the air maze down into the room. It leaves the maze and passes directly down through the room to the perforate floor 16 and waffle floor 20 into the ducts 21. The quantity of air moved through the room 19 and to be conducted from the room is kept uniform at all points within a short distance below the ceiling by an even distribution of outlet openings from the many hoppers 60 and by the increase in cross-sectional area of the ducts between the

walls

13 and 14. Thus, the air entering at the far side 13 of the room is conducted into a smaller area duct portion 83. The duct 21 is first deepened by the stepped portion 81 and then widened at constant depth until it reaches its maximum width. Then it is further deepened to its maximum depth at the discharge end 14 of the room.

Spent abrasive and waste is rapidly carried away from the object 18 by the down draft and falls through the perforate floor 16, carried in part by the down draft of air, clearing the work 18 as so that the operator 17 has continuous visibility of the workpiece 18. The abrasive falls through the perforate openings 66 into the hoppers 60 provided by the w'affie floor 20 and from thence into the ducts 21. The force of the down-draft ventilation is sufiicient not only to keep the work clean, but also to carry the abrasive away into the pickup snout 22 andfrorn there to the concentrator 24 and reclaimer 25, whence it may be redirected by compressed air to the blast hose 27. The forced ventilation provided also carries the dust and waste to a separator 29 where it is eliminated and then the clean air is exhausted into the atmosphere.

If desired, the circuit may be made a closed one instead, with the clean air being returned by a closed duct to the false ceiling area 32 above the air maze.

To those skilled in the art to which this invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting. For example, the waffie floor 20 may be made as a cast unit and may be a separate unit from the ducts 21 and the perforate floor 16.

It is not essential that the ducts 21 be rectangular in cross section or that they taper smoothly, as shown. They may have an individual straight section between successive openings and be stepped out at each opening, this being for many purposes an equivalent for taper but'also being less desirable in many instances. The ducts 21 may be ovoidal or of other shape, although the one described is generally preferred.

Further, it is not always essential that the ducts 21 first taper widthwise and then depthwise, though this is highly desirable. One reason why it is preferred is that the pickup snout 22 provides a single manifold receiving'the entire flow from all the ducts 21, and if unwanted expansion is to be avoided, the ducts should, at least by the time they enter the pickup snout, be substantially full width; otherwise there will be'a rapid expansion from a series of narrow ducts. Of course, there are also the other good reasons already stated for using the preferred construction.

I claim':

1. A blast room providing both uniform down-draft ventilation and air conveyance of the abrasive and waste, comprising a perforate ceiling providing air-inlet means taking in air in a uniform distribution pattern over substantially the entire area of said ceiling; a waffie-type floor comprising a perforate plate and a multiplicity of small hoppers beneath said perforate plate, each said hopper being shallow and occupying but a small fraction of said floor area, the upper end of said hoppers occupying substantially the entire area of said floor; and a plurality of ducts below said floor providing support means for said hoppers and into which said hoppers open for passage thereinto of the air passing down from said ceiling and the solids falling onto said floor, to conduct the same away from said room, each said duct extending from one side of the room to the other and beneath a single line of hoppers, means defining openings of reduced size at the lower end of said hoppers, graduated in size to transmit equal amounts of air into said duct, so that in each duct the air flow is controlled and the hoppers that open into different ducts do not directly affect each other.

2. The room of claim 1 wherein said hopper openings into said ducts are smallest in size near an outlet end of said ducts and are progressively larger the farther they are from that end.

3. The room of claim 1 wherein each duct has an exhaust outlet through which it conducts said air away from said room and wherein the cross-sectional area of said duct increases throughout its length, for uniform air velocity therethrough, each said duct having a section of constant depth and increasing width, in no case extending 10 greater than, the width of a hopper, followed by a second section of constant width and increasing depth.

4, The room of claim 3 wherein, said section of constant depth is preceded by a narrow portion of constant width stepped in depth as it passes below each hopper opening.

j 5. The room of claim 1 wherein said perforate plate has about half its area consist of its openings.

6. The room of claim 1 in which several of said perforate plate openings open into each said hopper.

7. The room of claim 1 wherein said means defining openings comprises an orifice plate substantially coextensive with said floor and forming a part thereof and constituting the upper wall of said plurality of ducts and providing part of the support means provided by said ducts.

8. In a room adapted to provide uniform down-draft ventilation, the combination of: a ceiling providing a series of air inlet means that distribute the air evenly over substantially the entire area of the room within a short distance below said ceiling and providing for substantially the entire intake of air into said room; a perforate floor having its openings evenly distributed across substantially its entire area; a wafiie floor supporting said perforate floor and having a large number of hoppers evenly distributed thereacross, each hopper sloping down toward an outlet opening; an orifice plate supporting said waffle floor and having orifices therethrough aligned with said hopper outlet openings; and a series of ducts extending across said room, mounted under and supporting said orifice plate, and into which said orifices open, the crosssectional area of said ducts increasing toward outlet ends thereof, but the width thereof never exceeding the width of a hopper, the relative increment corresponding to the number of orifices beyond which it extends at any point, said orifices being graduated in size along each duct, the smallest openings being adjacent said outlet ends to provide constant velocity of air therethrough, by providing for substantially identical air-intake through each orifice in said room.

9. The room of claim 8 wherein each said duct has a shallow narrow portion with smoothly diverging side walls and a constant depth until the side walls of adjacent ducts come together.

10. The room of claim 9 wherein each said duct also has, after its maximum width is obtained, a portion of smoothly increasing depth, provided by a bottom wall sloping away from an upper wall.

11. The room of claim 9 wherein said shallow narrow portion is preceded by a narrow constant-width portion with a stepped bottom wall, stepping down below successive orifices to handle smoothly the large percentage increases in air volume occurring at each of the first few orifices.

12. The room of claim 8 wherein said hopper openings are determined according to the equation where D is the diameter of the opening in inches, Q is the cubic feet per minute of free air to pass through each hopper, C the coefficient of entry for the air, and P the static vacuum in the duct directly below the hopper in inches of water, relative to the pressure in the room.

13. A waifle floor for a blast room, comprising a perforate plate; a multiplicity of small hoppers beneath said perforate plate, each said hopper being shallow and occupying but a small fraction of said floor area, the upper end of said hoppers occupying substantially the entire area of said floor; and a plurality of ducts below said floor into which said hoppers open for passage thereinto of air and solids falling onto said floor, to conduct the same away from said room, said hoppers having open- 11 ings of reduced size in their lower end, graduated in size to transmit equal amounts of air into said duct. I

14. A unitary floor for a blast room adapted to provide uniform down-draft ventilation and conveyance away from said room of the abrasive and waste particles in an airstream, comprising a perforate plate having its openings evenly distributed across substantially its entire area; a wafiie-like structure supporting and welded to said perforate plate and comprising a large number of small hoppers evenly distributed thereacross, each hopper sloping down toward an outlet opening; an orifice plate supporting and welded to said wafile-like structure and having orifices therethrough aligned with said hopper outlet openings; and a series of ducts extending across said room supporting and welded to said orifice plate and into which said orifices open, the cross-sectional area of said ducts increasing toward outlet ends thereof, but the width thereof never exceeding the width of a hopper, the relative increment corresponding to the number of orifices beyond which it extends at any point, said orifices being graduated in size along each duct, the smallest openings being adjacent said outlet ends to provide constant velocity of air therethrough, by providing for substantially identical air-intake through each orifice in said orifice plate.

15. A blast room providing both uniform down-draft ventilation and air conveyance of the abrasive and waste, comprising a perforate ceiling providing air-inlet means taking in air in a uniform distribution pattern over substantally the entire area of said ceiling; a wafile-type floor comprising a perforate plate and a multiplicity of small hoppers beneath said perforate plate, each said hopper being shallow and occupying but a small fraction of said floor area, the upper end of said hoppers occupying substantially the entire area of said floor; and a plurality of ducts below said floor into which said hoppers open for passage thereinto of the air passing down from said ceiling and the solids falling onto said floor, to conduct the same away from said room, said hoppers having openings of reduced size in their lower end, graduated in size to transmit'equal amounts of air into said duct, said hopper openings into said ducts being smallest in size near an outlet end of said ducts and progressively larger the farther they are from that end,

the size of said hopper openings being determined according to the equation 0.0458!) D= cut where D is the diameter of the opening in inches, Q is the cubic feet per minute of free air to pass through each hopper, C the coefiicient of entry for the air, and P the static vacuum in the duct directly below the hopper, expressed in inches of water, relative to the pressure in the room.

16. A wafiie floor for a blast room, comprising a perforate plate; a multiplicity of small hoppers beneath said perforate plate, each said hopper being shallow and occupying but a small fraction of said floor area, the upper end of said hoppers occupying substantially the entire area of said floor; and a plurality of ducts below said floor into which said hoppers open for passage thereinto of air and solids falling onto said floor, to conduct the same away from said room, said hoppers having openings of reduced size in their lower end, graduated in size to transmit equal amounts of air into said duct, said hopper openings being determined according to the equation where D is the diameter of the opening in inches, Q is the cubic feet per minute of free air to pass through each hopper, C, the coelficient of entry for the air, and P the static vacuum in the duct directly below the hopper, expressed in inches of water, relative to the pressure in the room.

References Cited in the file of this patent UNITED STATES PATENTS