GAS SCRUBBER

Abstract: A gas scrubber incorporates an exhaust fan and two separate impeller zones (3, 4). A number of impellers (10, 11, 12) in a first (3) of the zones, which is a spray-induction zone, have blade angles which vary from one impeller to the next alternately to accelerate and decelerate contaminated air drawn through a spray passage by the exhaust fan (14). Water supplied to the impeller blades in the spray-induction zone is thus transformed into a fine and turbulent mist spray. The second impeller zone (4) is in a spray elimination chamber, and the spray elimination impeller (13) acts to deflect air issuing from the spray passage through more than 90<o>. Spray issuing from the spray passage impinges on the blades of the spray eliminating impeller and is thrown radially (A) outwardly against the walls of spray elimination chamber. The gas scrubber requires no high pressure jets for spray generation, and no long and tortuous passages for spray elimination, and can operate using recycled water.

Description: TITLE;- "GAS SCRUBBER" DESCRIPTION Technical Field This invention relates to gas scrubbers for removing particulate and vapour contamination from air and gases. Background Art Gas scrubbers are known, and involve the creation of a fine mist spray to wet any- particulate matter entrained in the air or to dissolve any vapour ^ contaminants, followed by ^ the removal of the heavier wet particles and the spray generally from the air. Gas scrubbers therefore comprise two distinct zones, namely a spray-inducing zone and a spray separation zone. ■ c Spray induction has in the past been achieved by means of high pressure spray jets, by means of revolving lat or. undulating discs to which a ilm of water is applied, or by creating excessive turbulence in the air over the surface of some water, to provide a so-called self—induced spray. The use of high pressure jets is commercially disadvantageous because of their tendency to block with dirt:, so that such, spray inducers require a constant supply of clean fresh, water rather than recycled waste water. The use of a self-induced spray is more reliable in that it uses no high pressure jets and can use recycled water, but the fineness of the spray so produced is rather limited. Spray elimination has in the past been based on the general concept that if the moist air is passed along a tortuous path., each change of direction of the air will result- in some of the water droplets or noist particles entrained therein being carried b their greater inertia on a straight line path into contact with a wall or baffle en -which they condense or coalesce. It is cc-mo therefore for spray eliminators to car. rise a number of zig-zag baffles together defining a non-linear or tortuous path for the moist air- Such ■ a spray eliminator or mist separator is disclosed for example in the gas scrubber of Swedish Patent Specification No. 210236. Its efficiency is however limited, as is indicated by the high moisure content of the gas issuing from such a scrubber. The Invention The invention- provides a gas scrubber comprising means for drawing air successively through a spray passage and a spray separation chamber of larger cross-section than the spray passage, characterized in that spray-inducing means in the spray passage (3) comprise an axial array of axial flow impellers (10, 11, 12) on a common drive shaft (9) each for assisting the flow of air through the spray passage in a first direction towards the spray separation chamber (4) , and means for supplying water to the blades of the impellers to create a fine mist spray in the spray passage; and spray-separating means in the spray separation chamber comprise a further axial flow impeller or impellers (13) , in axial aliαrτent with the impellers of the spray-inducing means, for inducing air flow in a direction opposite to the ' first direction and deflecting the spray and air issuing from the spray passage outwardly against side walls of the spray separation chamber- It is perhaps surprising that the invention, in utilizing very similar components for spray generation and spray elimination, achieves a highly efficient gas scrubber- However extremely high efficiencies can be achieved according to the invention both in the removal of contamination from the air and in the removal of spray and moisture from the air in the spray separation chamber. In addition, a ■ number of other significant advantages are obtained as discussed in detail below. Dealing first with the operation of the spray-inducing means in the spray passage, it will be appreciated that the spray-inducing means provided according to the invention affords advantages over the spray nozzles, rotating discs and self-induction methods of the prior art. Water can readily be supplied to the impeller blades at a location immediately upstream of the impeller blades. One method is to supply the water axially upwardly through a centre bore in the drive shaft for the impeller blades, to spill out into an annular trough immediately upstream of the respective blade- - Rotation of the trough result in the water being flung out as a continuous coherent film which is immediately drawn upwardly by the air flow into the impeller blade path where it is broken into very fine droplets. A further very marked increase in the efficiency of this ' mist creation is possible by having successive impellers on the drive shaft with different blade pitches, so that there is a constant pressure variation and resulting turbulence on air passing through the spray passage. Tests have shown that where .there are three or more impellers on the drive shaft, a particularly fine mist spray can be achieved by varying the pitch of the impeller blades on the different impellers so that the first impeller accelerates the air flow, the second decelerates it and the third accelerates it again preferably to a flow rate in excess of that induced at the first impeller. Not only is the mist issuing from such a spray passage fine and uniform, but also it is found that any particles of solid or liquid or any fumes contaminating the incoming gas are wetted to an extent not previously possible in a single pass through such a simple and reliabl spray-inducing means. The consequence of the above wetting of the gas contaminants is that the effective particle size and mass of each particle or droplet of contaminant is increased. Use is made of this increased mass in the spray elimination 2one which constitutes the second element of the gas scrubber of the invention. As the gas issues from the spray passage, it passes into the spray separation chamber which has a larger cross-sectional area than the spray passage. This slows down the gas flow to a rate at which the removal of the spray is optimized. The further impeller in the gas flow path has its blades set to induce air flow in a direction opposite to that of the spray-inducing impellers, and this is sufficient to deflect the gas particles through more than 90°, back generally towards but to the side of the spray passage. Droplets and moist particles in the gas stream, however, have a greater inertia and are not as readily deflected. They therefore impinge on the blad of the further impeller, to be flung radially outwardl against the side walls of the spray separation chamber where they coalesce into a film of dirty water running down the side walls back into th reservoir. No other spray elimination device is needed in the gas scrubber of the invention,- although the. provision of baffles in the spray separation-chamber, extending generally radially towards the axis of the further impeller, does help to encourage turbulence in the spray- separation chamber and reduce the cyclic movement of air around that chamber as induced by the further impeller. t is surprising that a gas scrubber- so inherently simple in design should have an efficiency in excess of far more complicated devices, but it has. been found that a gas scrubber according to the invention having only three impellers in the spray passage and only one - 8 - further impeller in the spray separation chamber is 98% efficient in removing contaminants from air down to a particle size of 0.5 microns. Different contaminants that can be removed in this way include dust particles, oil mists, solvents such, as paint fumes, and welding fumes. This is. quite a startling efficiency, and is accompanied by a number of other advantages including economy of manufacture, compact size, economy of running, a low humidity gain in the scrubbed air' and therefore a low heat loss in the air as it passes through the scrubber., the ability to cope with a large range of particle sizes, the ability to recycle the water used for scrubbing, and the avoidance of high pressure jets: which, are prone to blockage. . Best Mode In the accompanying drawings: Figure 1 is a schematic vertical section through a gas scrubber according to the invention; Figure 2 is a schematic horizontal section through the spray separation chamber; and Figure 3 is an enlarged section through one of the impellers in the -spray passage. ^ Referring first to Figure 1, the gas scrubber comprises a housing 1 defining a water reservoir 2, a spray passage 3 and a spray separation chamber 4. The reservoir 2 is divided into two compartments by an inverted weir 5, so that contaminated air can pass from an inlet 6 to the spray passage 3 over the surface of water in the reservoir while maintaining isolation from a sludge disposal chamber 7 on the other side of the inverted weir 5. Sludge disposal from the chamber 7 can be by any suitable means such as a belt conveyor 8. The spray passage 3 is cylindrical in shape, with a vertical axis which forms the axis of a drive shaft 9 described in greater detail below. On the drive shaft 9 are mounted three axial flow impellers 10,11 and 12 in the spray passage 3; one further axial flow impeller 13 in the spray separation chamber 4; and a radial flow paddle-bladed exhaust fan 14 in an exit port 15 - leading from the chamber 4. Means are provided, as described below, for supplying water o the impellers 10 to 12 so as to create in the spray passage a dense and fine mist spray of rapidly moving water droplets which impinge upon and thoroughly wet the contaminants in the air that is being supplied to the inlet 6. Rotation of the impellers 10 to 12 causes 'the spray to be thrown radially outwardly against the wall of the spray passage 3 where some of the spray coalesces and runs down the wall-back into the reservoir 2. However setting of the impeller blades at different pitches, so that the blades of impeller 11 are of a lesser pitch than those of impellers 10 and 12, and so that the blades of impeller 10 are of a lesser pitch than those of impeller 12, greatly assists the creation and maintenance of the fine mist spray by establishing a continuous variation in pressure along the spray passage 3, and maximizing the turbulence of the air passing through that passage. Advantageouslythe impeller blades of the impellers 10 to 12, and optionally also of impeller 13, are of an adjustable nature so that their blade angles can be varied to suit the particularparticle size and nature of the contaminants in the air to be scrubbed. The flow of water over the impeller blades and down the wall of the spray passage 3 is sufficient to maintain a constant cleaning or washing of the blades and wall in use, so that no problems of silting-up are encountered. Advantageously the blades of the three impellers 10,11 and 12 are angularly offset one from the other so that each blade of impeller 10 overlies a space between adjacent blades of impeller 11, and so on down the row of impellers. In this way large articles such as cigarette cartons thrown into the air intake tend to pass through the spray passage Z rather than jamming and blocking it. The -impeller 13 has its blades set on the opposite hand to those of the impellers 10 to 12, so that it tends to induce an air flow axially downwardly towards the spray passage 3. The differenc in cross-sectional areas of the spray passage 3 and the spray separation chamber 4 is in itself sufficient to slow down the air flow considerably, preferably to a linear flow rate of less than 800 feet per minute (about 400 cm per second) . The additional effect of the counter- -flow induced by the impeller 13 is that the air is deflected in the general direction of the arrow A, whereas water droplets or saturated contaminant particles entrained in the air flow strike the impeller blades and are thrown radially outwardly against the side walls of the chamber 4. There they coalesce into a film of water and contaminant particles to flow down the walls and back into the reservoir 2. As with the spray passage 3, the action is self-cleaning in that the impeller 13 and the walls of the chamber 4 are constantly washed in use. Figure 2 shows the spray separation chamber 4 in horizontal section, to illustrate how radial baffles 16 are provided to reduce the tendency of air to swirl in a vortex around the axis of the drive shaft 9, and to promote increased turbulence in the chamber 4. The air turbulence in the chamber 4 is also increased by making this chamber of square section rather than circular section. Figure 3 illustrates the means for supplying water to the impellers 10 to 12 in the spray passage 3. The drive shaft 9 is hollow, with a water supply passage 17 up its centre. Water is pumped from the reservoir 2 by a low-pressure pump (indicated generally as 18 in Figure 1 but not shown in Figure 3) which is capable of supplying a head of perhaps half a metre of water. The water so supplied is passed upwardly through a supply pipe 19 axially of the drive shaft 9, there being a clearance between the supply pipe 19 and the water supply passage 17. The pump capacity should be sufficient to ensure that the resulting water loss is adequately compensated, and the water flowing down the outside of the supply pipe does have the advantage that it lubricates the junction between the rotary shaft 9 and the stationary supply passage* 19. Beneath each of the impellers 10 to 12 (only 10 being shown in Figure 3) is an annular trough 20 clamped to the outside of the shaft 9 by grubscrews 21. Water passes from the supply - SW J passage 17 through radial bores 22 and fills to overflowing the annular space formed between the shaft 9 and the trough 20. Rotation of the shaft 9 causes the water spilling from the trough to be thrown outwardly as a thin and coherent film of water which is immediately drawn upwardly into the impeller blades by the air stream. The spacing L- etween the trough 20 and the impeller 10 (or 11 or 12) is not critical, and in practice it has been found that spacings of up to 1 cm are very effective to ensure that the water film is supplied directly into the blades of- the impeller. The gas scrubber described above has operated at very low power consumptions with an efficiency of 98* on particle sizes down to 0.5 microns. Tests on the scrubbed gas show a very low moisture uptake, this being reflected in the relatively small cooling effect which the unit has on gas passing through it. - It is believed that the efficiency could further be improved by increasing the number of impellers in the spray passage 3 and/or by augmenting the further impeller 13 with other auxiliary impellers in the spray separation chamber.

Claims: CLAIMS 1. A gas scrubber comprising means for drawing air successively through a spray passage and a spray separation chamber of larger cross-section than the spray passage, characterized in that spray-inducing means in the spray passage (3) comprise an axial array of axial flow impellers (10,11,12) on a common drive shaft (9) each for assisting the flow of air through the spray passage in a first direction towards the spray separation chamber (4) and means for supplying water to the blades of the impellers to create afine mist spray in the spray passage; and spray-separating means in the spray separation chamber comprise a further axial flow impeller or impellers (13) , in axial alignment with the impellers of the spray-inducing means, for inducing air flow in a direction opposite to the first•direction and deflecting the spray issuing from the spray passage outwardly against side walls of the spray separation chamber. 2. A gas scrubber according to claim 1, wherein the successive impellers (10,11,12) of the spray- -inducing means have mutually different blade angles, to promote turbulence and pressure variation. in the spray passage (3) and to encourage the creation of a fine mist spray. 3. A gas scrubber according to claim 2, wherein the impellers (10,11,12) of the spray-inducing means are at least three in number and a first (10) is set to accelerate the air flow through the spray passage, the second (11) is set to decelerate the air flow and the third (12) is set to accelerate the air flow to a rate in excess of that induced by the first impeller. 4. A gas scrubber according to any one of claims. 1 to 3 , wherein the impeller or impellers (13) of the spray-separating means are mounted on and driven by the drive shaft (9) for the impellers (10,11,12) of the spra -inducing means. 5. A gas scrubber according to any one of claims 1 to 4, wherein the impeller or impellers (13) of the spray-separating means is or are set to induce an air flow that is substantially equal to but the reverse of that induced by the nearest impeller (12) of the spray-inducing means. 6. A gas scrubber according to any of claims 1 to 5, wherein the side walls of the spray separation chamber are shaped to encourage air turbulence within the spray separation chamber (4) . 7. A gas scrubber according to claim 6, wherein baffles (16) are provided on the side walls of the spray separation chamber (4) to resist the cyclic movement of air around that chamber. 8. A gas scrubber according to any preceding claim, wherein the means for supplying water to the blades of the impellers (10,11,12) of the spray-inducing means comprises a conduit (17) extending axially of the drive shaft (9) and communicating with an annular trough (20) immediately upstream of each of the impellers of the spray-inducing means, and a pump (18) • for supplying water from a ' reservoir (2) beneath the spray separation chamber via the conduit to the annular troughs. 9. A gas scrubber for removing particulate and vapour contamination from air, comprising: a housing defining a spray passage, a " spray separation ' chamber and a water reservoir; a passage for contaminated- air from an inlet through the spray passage and spray separation chamber to an outlet; an exhaust fan at the outlet for drawing air through the gas scrubber along the said passage; spray-inducing means in the spray passage, comprising a plurality of axial flow impellers on a common hollow drive shaft, each impeller having a blade angle different from that of adjacent impellers along the drive shaft, means for supplying water from the reservoir axially upwardly through the drive shaft to a - zone immediately upstream of each impeller and means for supplying the said water radially outwardly from the said zones onto the blades of the impellers; spra -separating means " in the spray— -separation chamber, comprising a further axial flow impeller on a portion ' of the drive shaft projecting into the spray-separation chamber, the blades of the further impeller being set on the opposite hand to those of the spray-inducing impellers, to deflect air issuing from the spray passage through more than 90 and to deflect spray issuing from the spray passage radially outwardly onto the walls of the spray separation chambers; and means for returning water collecting on the side walls of the spray separation chamber to the reservoir. 10. A gas scrubber according to claim 9, -wherein the exhaust fan is a radial flow paddle-bladed fan on the same drive shaft as the impellers, and the spray-inducing impellers are a series of three in number and are designed to accelerate the air flow through the spray chamber, then to decelerate that air flow, then to accelerate it again to a flow rate in excess of that created by the first-mentioned impeller of the series.

Inventors: NOTT DENNIS LAWSON
Applicants: DENKEN LTD, NOTT DENNIS LAWSON
Application year: 1981
Main classification: B01D47
IPC codes: B01D47, B01D50