dear peter,

air pads or bin activators only fight the symptomps but not the cause! if you want to ensure massflow in your silo without clogging and other problems you have to care for a correct geometry of your silo in combination with a discharge aid, that feeds your material continouosly over a corect dimensioned outlet.

i invite you to take a look at the homepage www.geroldinger.com you´ll find a lot of hints regarding your difficulties choosing the menue "technology".

geroldinger also provides a discharge system called oszillomat, especially developed for demanding bulk materials and tasks, which could surely solve your tasks.

best regards

gottfried

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Pads with a porous membrane can clog with fine materials, but are particularly prone to the formation of deposits if the powder is hygroscopic, because pressurised air entering though the pads usually carries water vapour that can be released by the pressure drop. The type of air ‘button’ with a flexible rim that deflects under a supply pressure to allow air escape against the hopper wall is more likely to work better in these circumstances. Typical devices of this type, sometimes called ‘magic mushrooms’, are available from www.vibrair.uk.con, amongst other suppliers.

Bin activators are also fitted to overcome arching problems with hydrated lime. Both these and air pads are expedients, but common solutions, to allow the use of hoppers with more shallow wall angles and smaller outlet size demands than are necessary for mass flow. Hydrated lime settles to achieve a significant bulk strength and has high surface friction with most materials of wall construction, so these techniques may well offer the most economical choice for a storage facility. Neither method gives a ‘first-in, first-out’ flow pattern, but this does not appear to be a crucial need for most applications using this material.

Hydrated lime is a fine powdered material that becomes highly fluid when excessively dilated so it is prudent to control the air input volume, rather than its pressure, to avoid the over-supply of air beyond that necessary to weaken the material enough to discharge. ■

Fine powders such as hydrated lime respond much better to aeration than vibration. Aeration "loosens" the material, whereas with vibration there is the potential to "compact" the material. Also, since aeration is a less costly approach, it provides an economic benefit. As to the type of aeration system, air pads can be a problem as they are effectively an air filter, which can plug over time due to oil and dirt in the compressed air system. I would suggest a bin aerator such as the Dynamic Air Vibra-Jet (see http://www.dynamicair.com/products/binaeratork.html). This device uses a rubber seal to prevent powder from back-feeding into the air supply, and therefore will not plug over time. ■

I prefer aeration cones to aeration pads in your service. Select an aeration cone such as that made by Myerlin. Control the air supply pressure with a flow orifice to prevent very high air flows and resulting fluidization of the powdered material. Use dry air or N2 if the material is hygroscopic.

This will cost much less than other big changes.

Regards,

Tim Agarwal

Pneumatic Conveying Consultants

polypcc@aol.com ■

One of the key factors that does not come across in your intitial posting is what is the envisioned service life for the silo? Your solution cost should be viewed within the context of the operational life cycle for the vessel. If you can justify the expense I would strongly recommend considering the geometry of the vessel that you are using. The use of a conical vessel in conjunction with gypsum will result in a requirement for unrealistic convergent angles - this will apply even if you install a bin activator! If you can afford to consider a fully engineered solution (which is obviously what should be required for a medium to long term operational requirement), I would suggest the retention of the storage section of the silo, but replace the cone with a plane flow transition section (i.e. round to slot). In conjunction with this type of convergence we would often recommend the use of plenums that are set back from the cone, through which dehumidified air can be introduced. The function of the air in this type of application is to facilitate the dilation of the powder as it moves towards the outlet. Any intrusion into the flow channel (in a mass flow vessel) should be avoided at all costs, or the risk will exist that material will "hang up" and lead to flow problems.

A material such as hydrated lime requires first in, first out discharge. You must be aware that virtually all discharge aids are just that - discharge aids. They will not alter the flow pattern that develops once discharge is initiated. If your silo has regions of non moving material in it, there is a fairly good chance that you are going to get discharge or quality problems at some point in time!

Feel free to contact me if you wish to discuss further.

Regards

Richard Farnish, Consulting Engineer

The Wolfson Centre for Bulk Solids Handling Technology, Univ.Greenwich, London, UK

mailto:r.j.farnish@gre.ac.uk

http://www.bulksolids.com ■

I would like to say that aeration along the hopper wall of a bin does reduce the effective angle of wall friction and if this aeration is provided such that it covers the entire internal hopper surface it can change funnel flow to mass flow. It is obvious that correct design of this aeration system is critical.

Regards,

Amrit Agarwal ■

Dear Peter,

You also have the possibility to adapt a Sodimate Archbreaker under your silo. We are a french company specialized in quick or hydrated lime silo discharge with dosing screw in water & gaz treatment. The modification of the silo will not be important to adapt an archbreaker.

You can visit our website at :

www.sodimate.com

François ■