Abstract:
The invention provides dust collecting apparatus  500  for use in collecting airborne dust adjacent to a moving surface, such as a paper web  511.  The apparatus comprises an elongate duct  503  extending in a direction transverse to the direction of movement of the moving surface, the duct having along its length at least one opening  504  permitting entry of air into the duct, and air extracting means in fluid communication with the duct. The apparatus is positionable adjacent to the web so that the apparatus and the moving surface define a “mouth”  514  into which is received at least a proportion of a layer of dust laden air  515  adjacent to and moving with the moving surface. In the preferred embodiment, the opening is shaped and positioned so that air enters the duct in a direction approximately tangential to a wall  507  of the duct and so that air in the duct moves both rotationally about the length of the duct and longitudinally along the duct. That is, a vortex motion is induced in the air entering the duct. This aids in keeping the dust entrained so that it is less likely to settle in, and so to foul, the duct. The opening may vary in width along the length of the duct, or may be one of several openings of varying sizes, so that a uniform (or other desired) distribution of mass flow rate of air per unit length of duct may be obtained along the duct. Air guiding and constraining formations  9, 10  may be provided upstream and downstream of the duct.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The invention relates to improved dust removal apparatus for controlling airborne dust in the vicinity of moving services and particularly moving webs, with particular application to machinery for handling paper.  
       BACKGROUND ART  
       [0002]     Machinery that provides for the continuous transport of dry webs of paper over multiple rollers and other components where the web&#39;s direction changes can be the cause of substantial dust generation. Dust that accumulates on the machinery can interfere with correct operation, lead to product quality problems in some circumstances, hinder maintenance, and may also present a fire hazard. Dust that is transferred into the air can also represent a fire hazard, and additionally can be breathed by workers.  
         [0003]     Much effort has been directed to the development of dust hoods for sucking dust laden air from parts of such machines. However, such devices are themselves imperfect in operation and can require substantial power consumption.  
         [0004]     In some applications it is important to remove entrained dust from air in the vicinity of a moving web and to remove dust from the web itself. This is commonly done by dust hoods and the like that use a combination of directed jets of air provided by a compressor and suction applied near the moving web.  
         [0005]     Such installations are usually provided at critical points in the path of a moving web, for example near doctor blades that crepe (separate) a web of paper from the surface of a so-called Yankee drum. However, there are often locations in paper making and handling machinery where significant dust is generated by, for example, being entrained in a “boundary layer” of air moving with the web close to its surface or by being thrown off the web near rollers and the like, where it is undesirable or difficult to provide elaborate, costly, high-power consuming and bulky dust removal equipment.  
         [0006]     There is thus a need for a method of controlling dust at such locations without these disadvantages or in which they are reduced. The present invention provides apparatus and methods which address this problem.  
         [0007]     It is believed that the concepts are not limited in their application to paper processing machinery only.  
       STATEMENT OF THE INVENTION  
       [0008]     The invention provides in a first aspect dust collecting apparatus for use in collecting airborne dust adjacent to a moving surface, the apparatus comprising:  
         [0009]     an elongate duct extending in a direction transverse to a direction of movement of a moving surface and spaced apart from the surface, the duct having along its length at least one opening permitting entry of air into the duct; and  
         [0010]     air extracting means in fluid communication with an interior space of the duct and adapted to draw air therefrom,  
         [0011]     wherein said apparatus is positionable adjacent to the moving surface so that the apparatus and the moving surface define an air inlet into which is received at least a proportion of a layer of dust laden air adjacent to and moving with the moving surface at least some of said air being drawn into the duct.  
         [0012]     The invention emphasizes collection of airborne dust rather than the active dislodging of dust from a moving web surface, and this can lead to less power consumption, through for example avoidance of the use of compressed air, and the advantageous use of the momentum of air that is moved by the traveling web itself and (as discussed below) parts of the machinery.  
         [0013]     In the preferred embodiment, the or a said opening is shaped and positioned so that air enters the duct in a direction approximately tangential to an inner surface of the duct and so that air in the duct moves both rotationally about the length of the duct and longitudinally along the duct. That is a votex motion is induced in the air entering the duct. This aids in keeping the dust entrained so that it is less likely to settle in, and so to foul, the duct.  
         [0014]     A scroll-like arrangement may be used for the duct in this case. Specifically, the or a said opening of the duct when seen in section transverse to the length of the duct may comprise a flow passage defined on one side by a first wall extending inwardly of the duct to a free edge of the first wall and on an opposite side by a second wall extending outwardly of the duct, one side of the first wall partially defining the inner surface of the duct.  
         [0015]     The said duct may be formed from a tubular member having a wall in which a longitudinal cut is made and a part of the wall is deformed inwardly to form the said first wall. This is useful as the construction of the duct can be comparatively low cost.  
         [0016]     Preferably, the or a said flow passage is elongate in the lengthwise direction of the duct and of varying width along its length.  
         [0017]     Instead of one slot-like opening, the duct may have a plurality of said openings. Members of the said plurality of openings may be of varying sizes, again so as to provide for control of the flow rate distribution.  
         [0018]     Preferably, the first wall is so shaped and sized and the air flow rate in the duct is able to be so chosen that in use in a specified position of the duct relative to the moving surface that air passing the free edge of the first wall in the said flow passage and air passing the said free edge inside the duct travel in substantially the same direction as seen in section transverse to the length of the duct. That is, air flowing past the edge, or “lip” of the first wall should preferably not have to turn sharply when passing beyond the lip. This can be achieved through testing or computation fluid dynamics simulation, and is found to work well.  
         [0019]     Desirably, the or each said opening is so sized and proportioned that in use in a specified position of the duct relative to the moving surface and with a specified air flow rate in the duct a specified distribution of air flow rate per unit duct length is obtained along the length of the duct, preferably a constant air flow rate per unit duct length. This can be achieved in designing the apparatus for a given application by testing, or by computer simulation.  
         [0020]     The air extracting means may be connected to the duct at either end or both ends of the duct. Alternatively, the connection may be made at an intermediate point along the length of the duct although this is generally less convenient. Air may be drawn from the duct by the extraction means tangentially or axially.  
         [0021]     The dust collecting apparatus may comprise a downstream formation having an edge that is elongate in the direction along the length of the duct the edge being positionable adjacent to the moving surface and in use of the apparatus being passed by a point on the moving surface after the point passes a said opening of the duct. (That is, the term “downstream” is here being used to refer to the direction of movement of the moving surface.)  
         [0022]     The apparatus may further comprise an upstream formation elongate in the direction along the length of the duct, the upstream formation and the moving surface defining a space therebetween and in use of the apparatus air being drawn from the said space into the duct, the upstream formation being encountered by a point on the moving surface before the point passes a said opening of the duct. The use of an upstream formation can give more flexibility in actual installations.  
         [0023]     In one embodiment, the upstream formation has a leading edge defined by upper and lower surfaces that diverge backwardly therefrom. The said lower surface may extend from the said leading edge to a point on the duct adjacent to a said opening.  
         [0024]     It has been found particularly desirable that the dust collecting apparatus further comprise movable support means whereby the apparatus is movable relative to the moving surface. This can greatly assist when access for maintenance (including threading of paper webs, for example, where the application is to paper-handling machinery) is required or to allow correct operation of the apparatus for different modes of operation of the machine to which it is fitted.  
         [0025]     The movable support means preferably comprises mechanical actuators supporting opposite ends of the apparatus.  
         [0026]     The movable support means is preferably operable to move the apparatus towards and away from the moving surface, and optionally to rotate the apparatus about an axis that extends parallel to the length of the duct.  
         [0027]     The invention provides in a further aspect a dust collecting installation in equipment in which a continuous web is transported along its length, the installation comprising dust collecting apparatus according to any of the forms disclosed herein and a surface of the web being the said moving surface, wherein the apparatus is so sized proportioned and positioned and the air extraction means is operated at such an air flow rate that a specified proportion of dust entrained in air moving with the moving surface is drawn into the duct. This proportion may be high, for example more than about 80%, preferably about 90% and more preferably approximately 100%. A user may measure the thickness and dust mass distribution of the moving “boundary layer” of air close to a moving web and provide an installation tailored to extract most of the dust in that layer. This may include providing that the rate of air flow into the duct per unit duct length is approximately constant across the width of the web.  
         [0028]     A dust collecting installation can be provided wherein in the said equipment the web passes over a cylindrical roller and the dust collecting apparatus is on the opposite side of the web from the roller, the installation including a guide formation having a cylindrical surface concentric with and facing inwardly towards an axis of rotation of the roller and extending from a leading edge around said axis to said apparatus so that air moving with the surface and dust entrained therein are guided around said roller and drawn into the duct. The guide formation may conveniently be formed from sheet material.  
         [0029]     A dust collecting installation can be provided wherein in the said equipment the web passes over a cylindrical roller and the dust collecting apparatus is on the same side of the web as the roller, the installation being characterized in that the or a said duct opening is so positioned that a jet of air generated where the web first contacts the roller augments air flow into the or a said opening of the duct. This use of such windage generated by the machinery is an extension of the invention&#39;s use of the momentum of the “boundary layer”.  
         [0030]     The upstream formation, where provided, may define a leading edge elongate in the direction along the duct length and the apparatus may be so positioned that the minimum distance between the leading edge and the moving surface is greater than the minimum distance between the downstream formation edge and the moving surface. In effect such an installation may be visualized as “swallowing” the dust-laden boundary layer. The said minimum distance between the leading edge and the moving surface may be a specified proportion of the height of a boundary layer of air moving with the moving surface.  
         [0031]     In a further aspect, the invention provides a method for limiting dust concentration in machinery in which a web is transported along its length comprising the steps of providing and operating a dust collecting installation according to any one of the embodiments disclosed herein at at least one position along the length of the web. The invention allows for the possibility at reasonable cost of providing dust-control installations at multiple positions along the web path.  
         [0032]     It is to be explicitly understood that not all the concepts described herein and believed to be inventive are set out above. Others are set out in the following detailed description. 
     
    
       [0033]     In order that the inventive concepts may be better understood there will now be described, non-limitingly, certain preferred embodiments as shown in the attached Figures, of which:  
         [0034]      FIG. 1  is a side view of part of a machine handling a continuous web with dust thereon;  
         [0035]      FIG. 2  is a perspective view of a hood that embodies inventions described herein;  
         [0036]      FIG. 3  is a cross-sectional view of the hood of  FIG. 2 , the section being taken at station “AA”;  
         [0037]      FIG. 4  is a cross-sectional view of the hood as shown in  FIG. 3 , the section being taken at station “BB” and omitting certain parts;  
         [0038]      FIG. 5  is a schematic side view of the leading edge of the hood and web of  FIG. 2 , with dust conventration and velocity gradients near the web shown;  
         [0039]      FIG. 6  is a cross-sectional sideways-looking view of a further hood leading section and a web;  
         [0040]      FIG. 7  is a cross-sectional sideways-looking view of a still further hood leading section and a web;  
         [0041]      FIG. 8  is a cross-sectional view (looking transversely across a web) of a further dust removal apparatus according to the invention;;  
         [0042]      FIG. 9  is a cross-sectional view (looking transversely across a web) of a further hood in association with rollers and the web;  
         [0043]      FIG. 10  is a cross-sectional view (looking transversely across a web) of a still further hood in association with rollers and the web;  
         [0044]      FIG. 11  is a cross-sectional view (looking transversely across a web) of a inventive guide in association with a roller and the web;  
         [0045]      FIG. 12  is a cross-sectional view (looking transversely across a web) of yet another hood in association with rollers and the web. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0046]      FIG. 1  is a side view of a part of a machinery installation  100  of a type where the present invention may be applied. The installation  100  could be part for example of a machinery installation for making multi-ply toilet tissue. A web  101  of paper passes sequentially over three rollers  102 ,  103  and  104 , changing direction at each stage. A typical distribution of airborne dust around the web is shown very approximately (i.e. not exactly) using dots and possible contours of equal dust concentration, as follows. Arrows  105  show dust in “boundary layers” of air that move with the web  101  along unsupported lengths of the web  101 . Arrows  106  show where dust is thrown outward from the web  101  as it changes direction. Arrows  107  show jets of air generated where the web  101  and surfaces of each of rollers  102 ,  103  and  104  approach each other to form internal corners. This general pattern has been established by testing.  
         [0047]      FIGS. 2-4  show a dust removal hood  1  installed adjacent to three generally cylindrical rollers  2 ,  3  and  4  that guide a moving web  5  of paper in machine section  6 . Rollers  2  and  4  are fixed in position, and roller  3  is an idler secured at each end on radius arms  600  pivotally mounted to a fixed frame  7  of machine section  6  so that roller  3  can be moved to take up slack in, and apply tension to, web  5  in known manner.  
         [0048]     Hood  1  is not intended necessarily to remove both airborne dust and dust embedded in web  5 , but rather to remove or reduce airborne dust, except where dust embedded in web  5  happens to be dislodged where it is working, for example by being thrown off as mentioned above in relation to  FIG. 1 .  
         [0049]     The use of this particular example is not intended to limit the scope of the inventions here disclosed. The inventions are considered to be potentially applicable to many machinery installations where a moving web carries dust on its surface and/or embedded in it and/or entrained in moving air adjacent to its surface. It is believed that the present inventive concepts may be applicable in applications other than paper processing and manufacture.  
         [0050]     Unless removed, dust on or around webs such as web  5  may represent a respiratory or fire hazard, or may collect in undesirable quantities on or around the machine section  6 . By various mechanisms, dust on or in the web  5  may be transferred into the surrounding air as the web passes over rollers  2 ,  3  and  4 .  
         [0051]     Hood  1  is mounted above rollers  2  and  4 , and extends between them and lengthwise along them.  
         [0052]     The construction of hood  1  between its ends is best seen in the cross-sectional view of  FIG. 3 . An outer cover  8  has a front section  9  and a rear section  10  meeting arranged in a shallow inverted “V” formation. Secured below cover  8  is a duct  12 , of substantially circular cross-section that extends longitudinally of hood  1  and that has an elongate slot  13  in its wall  11 . The slot  13  has a width that varies along the length of duct  12 , and is adjacent to a lower surface  14  of rear section  10  of outer cover  8 . A lower wall  15  extends rearwardly from a leading edge  21  and is secured to wall  11 .  
         [0053]     End plates  16 , parallel to each other, are secured to opposite ends of outer cover  8  and of lower wall  15 . Duct  12  is secured to and extends through each end plate  16 . Slot  13 , however, only extends between end plates  16 , so that outside them duct  12  is simply a closed circular duct. The distance between end plates  16  is slightly greater than the length of rollers  2  and  4  so that rollers  2  and  4  can in use of hood  1  be positioned partially within hood  1 .  
         [0054]     Air and entrained dust is sucked from under hood  1  through duct  12 . To this end, a flexible hose  17  is secured to duct  12  in known manner at one end of duct  12  and connects hood  1  to the inlet of a suitable blower or fan (not shown). A blanking plate  18  is provided at the other end of duct  12 , although if required in a particular application, it would of course be possible to provide instead of plate  18  a second hose (not shown) similar to hose  17 .  
         [0055]     Depending from a rear edge  18  of rear section  10  of cover  8  is a wad catcher plate  19 , whose lower edge  20  is in use positioned adjacent to an upper surface of web  5  where it passes over roller  4 . Provision of a wad catcher  19  integral with hood  1  is advantageous in that dust accumulation on the front face of wad catcher  19  is limited. Referring to the toilet paper manufacturing application ( FIG. 1 ) wad catcher plate  19  would be used in particular to remove dust or lumpy material from the side of web  5  that is to be joined to another ply. In other applications, a wad catcher plate  19  might not be required.  
         [0056]     Lower wall  15  and front section  9  meet at an acute angle at a leading edge  21  of hood  1 . Leading edge  21  is in use of hood  1  positioned adjacent to an upper surface of web  5  where it passes over roller  2 .  
         [0057]     It will be noted that lower wall  15 , front section  9  of upper cover  8  and wall  11 , being connected, together define a closed shape so that hood  1  inherently has substantial torsional stiffness, a desirable feature. The duct  12  by itself, with or without cover  8 , would of course provide much less torsional stiffness due to the presence of slot  13 .  
         [0058]     Hood  1  is supported as follows. Saddles  22  are provided to support duct  12  at each end of hood  1  where it protrudes beyond end plates  16 . Each one of saddles  22  is able to be raised and lowered as required, using one of two actuators  23 . Each actuator  23  is a screw jack type operated by an electric motor. Such actuators are available commercially, and particularly suitable ones are able to provide close control of the position of a load such as hood  1 . Actuators  23  are secured to parts of the fixed frame  7 .  
         [0059]     By means of actuators  23 , hood  1  can be raised above rollers  2  and  4  sufficiently far for access when web  5  is to be threaded through machine section  6  and for general maintenance and/or cleaning. Thereafter hood  1  can be lowered accurately to, and held in, a working position of hood  1 , as shown in  FIG. 3 , wherein leading edge  21  and the wad catcher lower edge  20  are adjacent to web  5 .  
         [0060]     Although in the interests of clarity not shown in FIGS.  2  to  4 , parallel blocking plates may be secured to frame  7  and placed close to each end of rollers  2 ,  3  and  4  to ensure that airflow into duct  12  through slot  13  is substantially in planes parallel to the direction of motion of web  5  with little airflow entering between runs  24  and  25  of web  5  near the web&#39;s edges. That is, hood  1  in its working position, the upper surface of web  5 , and the blocking plates define a nearly-closed space  26  in fluid communication with duct  12 . The gaps  27  and  28  between edges  21  and  20  and web  5  of course allow some air flow into space  26 .  
         [0061]     In the sectional view of  FIG. 4 , rear section  10  of cover  8  is omitted, so that the whole of duct  12  can be seen. This Figure best shows the variation in width of slot  13  required to obtain a uniform air flow rate into duct  12  across the width of web  5 .  
         [0062]     Generally, it is found that a moving web  5  (especially of dry paper toilet-type tissue) causes a body of air to move lengthwise with the web  5 , that body of air carrying a burden of dust. As shown in  FIG. 5 , the concentration of dust will vary with distance from each surface of web  5 . There will also be a progressive reduction of lengthwise velocity of the air with increased distance from the web surface, i.e. there will be a moving “boundary layer” of dust laden air. The working position of hood  1  is chosen so that the leading edge  21  is spaced from web  5  by a distance so chosen that a suitably large proportion of the dust entrained in the boundary layer passes into the hood. The flow rate of airborne dust into the hood through gap  27  is the product of the mean velocity and concentration to the height of the gap. Note that additional dust is usually carried in the web itself, and a proportion of this is in general expelled into space  26 , for example when the web  5  passes over roller  3 . The acuteness of the angle between the front section  9  of cover  8  and lower wall  15  is provided to limit any tendency to develop a region of stagnant flow at the front of hood  21 .  FIG. 6  shows the condition to be avoided as far as possible, a web  29  passing below a hood  30  with a wall  31  at its leading end that extends normally to web  29 . Dust can accumulate in a stagnant region  32  developed in front of hood  30 , all the more so when a small gap  33  between hood  30  and web  29  is chosen to minimize the required air flow in the hood  30 .  
         [0063]     Lower wall  15  is so shaped that in the working position of hood  1  there is only limited variation in the distance between web  5  and lower  15  in the region  34  behind leading edge  15 . This lessens deceleration of air after it passes through gap  27  by comparison with the deceleration that would happen if hood  1  did not include lower wall  15  (as is the case in hood  30 ). Such deceleration could also lead to undesirable accumulation of dust under hood  1 .  
         [0064]     The arrangement (as seen in cross-section) of duct  12  with its inlet slot  13  positioned under rear section  10  of cover  8  promotes vortex flow of air within duct  12  superimposed on the longitudinal flow of air within duct  12 . Arrow  35  in  FIG. 3  shows the rotational direction of the vortex flow. Such vortex flow is advantageous, in tending to draw dust towards the center of duct  12  and away from the walls, where it might otherwise tend to accumulate. It is thought that where cyclone-type separator is provided downstream of the duct  12 , vortex flow in duct  12  may be advantageous in enhancing the dust-separating effectiveness of the cyclone separator.  
         [0065]     Note that hoods based on the principles set out herein may be made to suit other parts of web-transporting machinery. For example,  FIG. 9  (which is comparable for interpretation purposes to  FIG. 3 ) shows a cross-section of a hood  70  suitable for dust removal on the inner side of a web  71  passing over a roller  72 . A duct  73 , having a lengthwise slot  74  is again provided, and a formation  75  that is elongate and extends parallel to the length of roller  72  supports duct  73 . Formation  75  is also shaped to guide air into duct  73  as indicated by arrows in  FIG. 9 . It has been found that in at least some circumstances a “jet” of air is forced away from the point where web  71  converges with roller  72 , this jet being represented by arrow  76 . Slot  74  is positioned so that this jet flows substantially directly into slot  74 , and is compatible with the rotational direction direction of the vortex flow induced in duct  73  by the positioning of slot  74 . Generally, hood  70  is arranged so that the vortex flow induced in duct  73  is compatible with the flow around the exterior of duct  73  and around roller  72 , thus limiting flow losses and consequently the power requirements for clearing the dust laden air.  
         [0066]      FIG. 10  is a comparable view of another hood  80  operating on a similar principle. In this case, a duct  81  is placed closer to web  82  on the exit side of roller  83 .  
         [0067]     It is of course not desirable to provide a complete dust removal hood at every conceivable location on a large machinery installation, on cost and accessibility grounds. It has been found that at some locations it is beneficial to provide a simple shaped air flow guide formed from sheet material at locations adjacent to moving webs, and particularly where webs pass over rollers. As an example,  FIG. 11  shows a side view of a web  200  passing over a roller  201 , with a guide  202  formed from sheet material supported adjacent to web  200 . Guide  202  is elongate and extends along the length of roller  201 . Guide  202  is shaped and positioned so that there is an approximately constant gap  203  between web  200  and guide  202 . It has been found that guide  202  can in at least some applications limit the otherwise noticeable tendency of dust to spread outwardly (in larger quantities than elsewhere along web  200 ) at roller  201 . Without any intention to be held to a particular explanation, it is thought that air carried along by the moving web  200  is assisted to flow around the curved path in gap  203  by guide  202 .  
         [0068]     A guide such as guide  202  may be combined with a hood. As an example,  FIG. 12  shows in cross-section a web  90  passing over two rollers  91  and  92 , with a hood  93 , of the same type as hood  80 , being provided for dust removal around roller  92 . A guide  94  is provided to guide dust laden air moving with web  90  over roller  91  so that it can be sucked into duct  94  of hood  93 .  
         [0069]      FIG. 8  shows a simpler hood  500  than those discussed above, and that should be found suitable in many applications. Hood  500  is shown in cross-section and extends transversely to the direction of travel (shown by arrow  501 ) of a web  502 . Hood  500  comprises a duct  503  generally of tubular form but with a tapering flow passage  504  by which air and dust are sucked into an inner space  505  of duct  503 . As in the other hoods described above, flow passage  504  is positioned so that air enters space  505  tangentially, encouraging a vortex flow pattern in the direction indicated by arrow  506  superimposed on the axial flow created by an air exhausting means (not shown, and of any suitable known type). Duct  503  has a wall  507  that is substantially circular and smooth inside, except for a first wall section  508  that blends with wall  507  and curves inwardly into space  505 , ending at a free edge or lip  509 , and a second wall section  510  that extends wall  507  downward (as shown) in a tangential direction. Second wall section  510  extends almost to web surface  511  and is fitted (as an option) with a wad collector  512 . The gap  513  between wad collector  512  and surface  511  is smaller than gap  514  between wall  507  and surface  511 , so that gap  514  forms a “mouth” into which dust entrained in boundary layer  515  passes.  
         [0070]     The first wall section  508  is found to be able to provide better performance of the hood  500  than if it is absent. First wall section  508  is preferably shaped and sized (using results from suitable testing, computer simulation or the like) so that air inside the space  505  and passing lip  509  (indicated as to its direction by arrow  516 ) and air in passage  504  passing lip  509  (indicated as to its direction by arrow  517 ) travel in approximately the same directions, parallel to first wall section  508  at lip  509 . This condition is believed to work well.  
         [0071]     It will be appreciated that hood  500  can be very simply constructed. Duct  503  could for example be formed from a tube (for example an extrusion, preferably an aluminum alloy extrusion) by cutting lengthwise and bending the wall  507  inward (form the position shown in chain-dotted line) to form wall section  508 . Second wall section  510  is then secured to the unbent part of wall  507  to obtain the shape shown in  FIG. 8 . The width of passage  504  can be set to suit the application, and even bent so as to vary in width along its length to provide for a desired variation of mass flow rate of air per unit length of duct along the duct length, for example a constant rate.  
         [0072]     Hood  500  has been described by reference to a cross-section, however, it should be understood that its ends may be treated similarly to the other hoods described, for example hood  1 . For example an end may have a blanking plate or a exhaust connection, and may be supported by a mechanical actuator (all not shown).  
         [0073]     As with all the other hoods described herein, by suitable design, including choice of air extraction rate, substantially all or a specified proportion of the “boundary layer air can be drawn into the space  505 , with its entrained dust for removal.  
         [0074]     Many variations and extensions of the concepts set out herein will be readily apparent to persons skilled in the art and may be made without departing from the spirit or scope of the present invention.  
         [0075]     For example, referring back to hood  1 , it is believed that in at least some circumstances a rounded leading edge may be preferable to a sharp leading edge such as leading edge  21 .  FIG. 7  shows a front part of a hood  60  having a rounded leading edge  61 , positioned over a moving web  62 . Streamlines  63 ,  64  and  65  are shown, representing flow generated by motion of the web  62 . Streamline  63  represents flow that stays outside hood  60  and streamline  65  shows flow passing into the hood  60 . Streamline  64  is a streamline that ends at a stagnation point  66  on edge  61 . It is thought that for example where a variable web-to-edge gap  67 , or a variable suction rate is required to deal with different conditions or product types, leading edge  61  may be less prone than a sharper one to accumulate dust on the exterior of hood  60  and may reduce energy losses.  
         [0076]     Another variation appears also to have potential importance. In the various hoods described herein, elongate slots are provided whereby air enters a duct that forms part of a hood assembly. These may be of variable width. For example in hood  1 , air enters elongate duct  12  through a slot  13  that extends substantially along its length. Similarly, as other examples, hood  70  has a slot  74  by which air enters duct  73 , and hood  50  has a slot  174  by which air enters duct  175 . It is desirable in many cases, particularly where there are no end plates in use such as end plates  16  of hood  1  or end-positioned blocking plates (not shown) as mentioned above in relation to hood  1 , that the slot width vary along its length. See  FIG. 4 , where a variable width is shown in respect of slot  13  of hood  1 . Variable width slots such as slot  13  can be quite expensive and difficult to provide. However, it has been found possible to provide more simply and cheaply made slots with only a surprisingly small degradation of performance. This can be done by approximating a single elongate slot with a plurality of shorter slots arranged lengthwise of the duct in question and separated by webs. By way of example,  FIG. 13  shows a hood la that is the same in every respect to hood  1  except that it has a modified slot arrangement.  FIG. 13  is the same view of hood la as  FIG. 4  is of hood  1 , with identical item numbers used for identical parts for convenience. Instead of the single duct  13  of hood  1  there is an array of shorter slots  13   a  separated by webs  176 . Each slot  13   a  has a constant width (i.e. in the peripheral direction of duct  12 ), but these widths vary from slot  13   a  to slot  13   a , so that the effect of the variable width of slot  13  is approximated. It has been found that satisfactory performance can be obtained, with much easier and cheaper fabrication. It is of course possible to make the lengths of such multiple slots differ one from another, and to make the widths of individual slots variable within their own length, still with some dividends in ease and simplicity of fabrication, and potentially with improved performance. There is no requirement to limit such arrays of slots to quadrilateral slots: individual slots may be of differing shapes, for example an individual slot that is part of an array could have semicircular ends (not shown). Still further, a variable (or, for that matter, constant) width elongate slot can be approximated by an array of openings that are not sufficiently elongate to amount to slots, for example an array of circular holes. This general principle can be applied to slots and air passages generally of hoods according to the invention where such slots would otherwise be made with variable width along a substantial length.  
         [0077]     The use of multiple slots or openings described by reference to  FIG. 13  is also able to apply to the simple hood  500 .  
         [0078]     Still further variations will readily suggest themselves to persons skilled in the art that remain within the spirit and scope of the inventions as described herein.  
         [0079]     In this specification, the words, “comprise”, “comprises”, and “comprising”, used in relation to a specified set of integers, elements or steps are to be taken as meaning that the integers, elements or steps are present, but not as precluding the possibility of other integers, elements or steps being present.