Rotary valves are good for 1 bar or 100 kilopascals and are easily available in the market. So there is no need to stack 2 together. ■

Rotary locks and pneumatic conveying !

Be very careful.

Conveying air can leak through the gap between the rotor and the housing.

(Air loss through the pockets cannot be prevented but can be calculated)

This can have 3 effects

1)In case of very abrasive material, very fast wearing out of the gap will uccur.

2)Leak air is taken from the conveying air, thereby influencing the pneumatic conveying.( leakage is pressure related)

3)If the leak air is also influencing the filling factor of the locks (volumetric efficiency), the pneumatic conveying can (and probably will) become unstable and starts oscillating.

In case you choose for a stack of 2 rotary locks in series, there has to be installed 2 spring relief valves with piping to control (or divide) the conveying pressure over the 2 locks.

The 2 rotary locks might have to be of different size, because of the different counter-air volumes.

The intermediate space has to be constructed for the conveying pressure, to be save.

Be certain to cover all possible consequences of such a design

successs ■

Cacing

Just to elaborate on the replies from the others, rotary airlocks are available for differentials up to 3 bar but are application limited.

Proper airlock venting is ALWAYS a primary concern, regardless of whether you are using one or two airlocks.

If you do decide to use two, I would recommend that the top one be driven to provide the rate you want and the lower one should run at a slightly higher rpm to avoid any back up.

If you have the room, you can also use a lock hopper arrangement which allows the chamber above and below a single airlock to be equalized and therefore there is no differential of pressure across the airlock. This provides the continuous discharge of material without the leakage or airlock wear.

Regards ■

Cacing

RPD makes a good point about product trapping or "clipping" as we call it. You can consider an inlet baffle or deflector plate which helps prevent this from occurring.

Rotating the lower airlock to an angle or using a "side entry" inlet are also solutions.

If you have the space to install to rotary units, you most likely have the space for a small lock hopper arrangement which may be a much better choice for your application of coarse and abrasive materials.

Regards ■

Cacing,

I agree with Dennis. One other item that you need is a good venting system for rotary valves used in high pressure applications. Reason is that poorly vented rotary valves will interfere with material flow into the valve and reduce its capacity. Suggest that you read my article "Design of Venting Systems for Rotary Valves" published in the March 2005 issue of Chemical Engineering for the design of the venting system.

Regards,

Amrit T. Agarwal

Consulting Engineer

Pneumatic Conveying Consulting

Email: polypcc@aol.com

Ph and Fax: 304 346 5125 ■

Dear mr cacing

It took me a while, but now I think it will be very difficult to get 2 rotary valves in a stack working. (Actually I think it will not work)

There are 3 effects to consider:

1)Material flow. The lower rotary lock has to have a slightly higher capacity than the upper rotary lock to prevent the build up of material in the intermediate section.

2)Air leakage through the gaps. As the leaked air through the lower gap increases in volume (expands) in the intermediate section, the pressure drop over the upper rotary lock is higher than over the lower rotary lock (d(p) # v^2 )

3)The rotary lock pockets transport not only material from top to bottom, but also air from the convey air zone to the bin feeder zone. That air expands as well. To maintain a pressure in the intermediate zone of half the convey pressure, the volume of the upper rotary lock has to be p(conv)(abs):

2 x p(conv)

------------------- x lower rotary lock volume

2 x p(conv) +1

This is contradicting with consideration No 1)

Two equally sized rotary locks will end up with almost the conveying pressure in the intermediate section.

A simple solution as using 2 rotary valves to divide the pressure turns out to be rather complicated.

If you want the calculations, I can e-mail them to you.

Best regards ■

dear members,

The formal in my preveous reply should read :

2 x p(conv)

------------------- x lower rotary lock volume

p(conv) +1

sorry for the inconveniance ■

Agree

More, 1 more rotary valve dose not help to share the pressure drop because the below one will always take 3/4 of the differential pressure.

Am I right? ■

The original question of this thread was how to divide the conveying pressure of 70 kPa over 2 rotary valves.

The answer is basically; It is not necessary to use 2 rotary valves, because 1 rotary valve is capable of taking 70 kPa (or more)

Then I tried to figure out : How is the pressure divided over the 2 rotary valves.

I came to the already mentioned conclusion.

It will not work, unless special (complicated) measures are taken. (Like controllable vent valves from the interspace to the atmosphere, to control the interspace pressure with varying convey pressures)

To shengie2004:

Which part of the conveying pressure will be taken by the lower rotary valve, is depending on the layout of the installation.

It might be even worse than of the conveying pressure.

To RPD :

Venting the interspace almost to atmospheric is not really a contribution to divide the conveying pressure 50% by 50% over the 2 rotary valves.

(Which was the intention)

In case you (or any member) wants to receive the mathematical derivation, please let me know.

All for now ■

Dear Teus Tuinenburg,

Sorry the upper one takes more pressure drop than the lower one:

KV1^2=DP1

KV2^2=1-DP1

V1(1+DP1)=V2*2

->DP1=0.609Bar

DP1: Pressure drop though upper valve

V1: Air flow rate through upper valve

V2: Air flow rate through lower valve ■

Dear Shenjie,

I cannot figure out your equations.

V1(1+DP1) # V2^2

It seems that the dimensions of the left term do not correspond with the right term.

You also calculate DP1= 0.609 Bar, without knowing the conveying pressure.

Take care that V1 has a higher value than V2, depending on the conveying pressure and the interspace pressure.

Your equation seems to be based on flow resistance (f.i. through the gaps),

but, a rotary lock has also a positive displacement characteristic through the pockets

good luck ■

Dear Teus,

I just make it simpler to be understanded by supposing the conveying pressure as 1.0Bar.

And for sure it is not calculated in my way. it is easier if we measure an existing process.

Thanks ■

Dear Shenjie,

With a pneumatic conveying pressure of 1 bar(o) = 2 bar(abs),

I calculate, due to gap losses of 2 equal rotary valves:

pressure drop over upper valve = 0.5616 bar

pressure drop over lower valve = 0.4348 bar

conveyinpressure = 1 bar(o) underneath lower rotary valve

BUT,

There are 3 effects to consider:

1)Material flow. The lower rotary lock has to have a slightly higher capacity than the upper rotary lock to prevent the build up of material in the intermediate section.

2)Air leakage through the gaps. As the leaked air through the lower gap increases in volume (expands) in the intermediate section, the pressure drop over the upper rotary lock is higher than over the lower rotary lock (d(p) # v^2 )

3)The rotary lock pockets transport not only material from top to bottom, but also air from the convey air zone to the bin feeder zone. That air expands as well. To maintain a pressure in the intermediate zone of half the convey pressure, the volume of the upper rotary lock has to be p(conv)(abs):

2 x p(conv)

------------------- x lower rotary lock volume

p(conv) +1

This is contradicting with consideration No 1)

Two equally sized rotary locks will end up with almost the conveying pressure in the intermediate section. ■

You Win!! ■

dear Shenjie,

It is not a matter of winning, but agreeing.

makes the world much easier

best luck ■

dear cacing,

let me know your e-mail address.

Then I can send you the word document

"stack of 2 rotary valves.doc"

hear from you ■

May be it can help you. ■

Dear Sea-will

Can you send me a copy of picture?

thanks ■