Pneumatic Transfer Question 1

[schnipp]

We convey dry sugar throughout our plant, from dryer to classifiers, bins to packaging lines, etc.

We use positive (push) and negative (pull) systems with varying degrees of seccess.

The problem we have is that in the positive system, we develop a lot of blow by past the feeding airlock which sends air up into the hopper and causes the product to 'hang up'.

Does the velocity pan have anything to do with the amount of blow by around the airlock? Is there some baffling or certain design criteria that can overcome this?

(We term the velocity pan to mean the piping connection directly below the airlock where the air stream and the product first come in contact)

Kurt.

 

[JimCasey]

Blowby around the airlock is almost always caused by wear. Increases clearance between the rotor and housing and the air leaks through.

Inexpensive airlocks have 6 blades. Top-end airlocks have 10. Different materials, purged bearings, other features may make the airlocks provide better service for you.

Some manufacturers make airlocks with adjustable blades, but that is more show-biz than effective: the blades don't wear straight across so adjusting won't correct for wear.

A more useful modification for Sugar is having relieved tips on the blades, so there is reduced area for sugar to be trapped and sheared between the tip of the rotor and the housing. With plain square-end tips, Friction can cause the sugar to melt with multiple adverse effects: Higher power required, caramelizing of the sugar in contact with the tip and the housing, and "cotton Candy" being drawn, making birds-nests in the downstream equipment.

In a blown system as you describe, if somebody cranks up the blower pressure, Isentropic compression can result in very high instantaneous temperatures. The sugar granules soften and shear in elbows, making more cotton candy, birds nests, and headaches for you..

Still- you complain about blowback. I have had very good response from Premier Pneumatics. They have an elaborate flow lab and can model your system to demonstrate the solutions to your problem(s). (No, I do not work for them)
Here is their website.

http://www.premierpneumatics.com

 

[schnipp]

Thanks, JimCasey.

I'm not sure exactly how many blades we have on ours (8 off of the top of my head). I'll check on that in the morning. We're actually trainsporting dextrose, not exactly sugar (but a lot of the same difficult properties). We do have the releived tips in at least some of the units.

Our blower outlet pressure is more of a function of how plugged the line is between the feeder and receiver. We don't adjust the pressure for production reasons.

An issue I found out about today is taht we run the airlocks at ~4 to 6 rpm. We may try a smaller unit with higher speed to stop deflection and unusual wear patterns.

 

[JimCasey]

>>An issue I found out about today is that we run the airlocks at ~4 to 6 rpm. We may try a smaller unit with higher speed to stop deflection and unusual wear patterns. <<

4-6 rpm is pretty slow. An upper limit is around 20 rpm. Overspeeding the airlock causes enough centrifugal force to keep the product from entering. Start with a really slow speed and the flow through the airlock will be low. Increase the speed of the airlock and the flow increases. Keep increasing the speed and the flow begins to decrease. But the power consumption, melting of the product, and general dissatisfaction get worse.

If the airlock is too big it has larger leak paths and that could explain a lot.