19th September 2002, 2:04
Rules of thumb for flow area ratio
Is there any rules of thumb to ensure that powder flow is not restricted after installation of magnetic grids or screening elements? I know that it depends on flow properties of the material. If I don't have any details of flow characteristic of the material - except that it's poor, what should I do to ensure that installation of this new equipment would not cause other problems such as bridging. FYI, my knowledge on powder handling is very basic.
20th September 2002, 9:02
The first point is to distinguish between confined and unconfined flow situations. Confined flow is where the bulk solids fills the whole cross section of the flow channel, as with a hopper or full chute. This is the most difficult to deal with as the bulk is constrained and cannot deform without interacting with material alongside. Bridging, in these circumstances, depends upon the strength of the powder in relation to the size and form of the flow channel. The correct technical procedure is given by shear testing of the powder and applying a formula by Andrew Jenike. This process is described in an document prepared by the Working Party for the Mechanics of Particulate Solids of the European Federation of Chemical Engineers and published by the Institution of Chemical Engineers as ‘A Standard Shear Cell Testing Technique’. Basically, this is hard going for the newcomer to powder handling. Ajax has prepared a summary of flow problems and background to the flow of solids. If you forward your address, we will be pleased to send a copy and some concise notes on various associated subjects of bulk handling.
In relation to the passage of material past grids or through screens, the material is normally in the form of an unconfined flow stream. Obviously lumps must not clog the apertures but, as regards build-up and blocking, the main problem of flow stoppages on these fitting is usually that a surge of material is deposited that acts for an instance as an unconfined mass with sufficient ‘body’ to span across the openings. Should this back up in the feed channel it will be more difficult to clear. To prevent this occurring you should examine the nature of the flow stream. If there is any likelihood of an erratic flow rate, then consideration should be given to either attenuating the irregularities. One method is to break up the flow with dispersing devices, such as chutes with pegs or blades that spread or delay some of the flow. Also ensure that the flow is spread out over as much of the area as possible. This can be done by a chute with a very slightly bowed surface that terminates in a saw-tooth edge to discharge the material over an area, rather than a focused stream or ‘waterfall’.
Fine powder will always tend to stick in square corners, so if in any doubt make chutes and transfer point with radius corners. Blockages that occur on a small scale, say on a magnetic grid where powder will hold up on the bars, can usually be dislodged with a small amount of vibration.
A very useful tool for anyone concerned with powder flow is a wall friction tester. This offers a means to quantify the angle of slope essential for flow and allows comparisons to be made to select the optimum contact surface. There is no unique ‘low friction’ material with powders, as it depends on the nature of the sulk solid as much as the contact surface. See www.ajax.co.uk – powder testers.
My final advice is to carefully consider the technical ability of any supplier when purchasing equipment for solids handling. It is not just a matter of good engineering or low cost. Problems in operation can immensely outweigh the full purchase cost of apparently items such as chutes, diverter valves, small hoppers and similar crucial sections on a flow route. It is a good idea to carry out a flow audit, as a inaugural part of any project review, to carry out due diligence to the operational integrity of the system. If you require any guidance on this Ajax can offer some help. Also helpful to one wanting to know more about powder and bulk is a ‘Glossary of Terms in Powder and Bulk Technology’ published by the British Materials Handling Board. At £ 15 plus £ 5 o/s postage. This covers the technical terms with much useful ancillary information.
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