Originally Posted by ravindrar

Removing air from floppy (low density) powders (like skim milk powder) is difficult task. When vacuum pump sucks the air from the powder in the bag this air also carries lot of powder with it resulting in powder spilling out of the bag in vacuum chamber. May in one suggest some solution to this problem.

Ravindra

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Ravindra,

Powder is carried over because the vacuum being created causes the plastic bag to collapse and bag's open area becomes so small that the air velocity through it increases above the saltation velocity. Instead of bags, use of rigid containers will solve this problem.

Regards,

Amrit Agarwal

Pneumatic Conveying Consulting

email olypcc@aol.com ■

Why not just squeeze the bags between a couple of rollers? Too easy of course? ■

Dear Ravindra,

Assuming that you want to pack a powder in a bag, like vacuum packed coffee powder, the first step would be to contact manufacturers of these kind of packing machines.

My idea would be to fill the bags under vacuum conditions.

Then the pressure of the air between the particles is minimal. Then seal the bag.

After bringing the sealed bag into the atmospheric pressure, the powder is vacuum packed, which is also noticed by the ridged structure of the powder because of the high internal friction of the packed powder.

The high friction is caused by the forces, exerted on the powder by the atmosphere, which are not counter acted by the low air pressure between the particles.

This phenomena is also observed in pneumatic cement unloaders.

When the filter receiver is filled with cement under vacuum and when full, the atmospheric air is entering the vacuum tank, the cement packed like hard sand and will not flow.

Aeration is needed to re-fluidize the cement before it start flowing out of the tank.

Trying to remove the air from the powder by just sucking off the air above the powder will start an air flow from the voids between the particles to the vacuuming point.

This air flow will convey particles and cause spill over.

Have a nice day ■

Fine powders are difficult to de-aerate because the fine and decreasing void channels progressively resist the passage of air to escape and that air leaving the top layers of a bed is replaced by that rising from the lower layers. In a continuos system the problem is aggrevated by the velocity of flow detracting from that of the rising air. This is especially relevant in conical or pyramid shaped hoppers of funnel flow design where a narrow flow channel attains a high velocity. A plane flow hopper of mass flow design offers the optimum de-aeration prospects by providing a large cross sectional area of flow and enhanced residence time if discharged by a screw feeder that extracts uniformly over the total outlet area; no mean design feat! and is not provided by a standard feeder design. This can also be supplemented by the provision of preferential air escape routes, such as reverse sedimentation plates, tuned rods vibration at their natural frequency to create vertical 'holes' in the bed or local exit routes for the air.

Many things can be Done to accelerate de-aeration, but not work miracles as the process is essentially time related. More information on the topic is available from lyn@ajax.co.uk ■

Hi there,

maybe a VACUPRESS powder compactor (denser) could be an issue.

Just have a look under the following link:

http://www.grenzebach.com/index.php/...hnik/vacupress

Regards

Holger Höck

Grenzebach BSH

Rudolf-Grenzebach-Straße 1

36251 Bad Hersfeld

GERMANY

+496621-81 3248

holger.hoeck@grenzebach.com ■