Air through a cylinder flow problem 5

[flycast]

I am a non engineer and need some help. I have a common problem that I face on a regular basis. I thermoform and need to pull air out of pockets that trap air in tools. The problem is how many holes of what diameter do I need to evacuate a certain size pocket?

The holes are typically 0.016" diameter holes drilled 0.050" deep but diameter can change. At that point they widen out to 0.060 - 0.1" diameter holes. The wider holes travel about 0.5". The vacuum (on the wider hole diameter side) is about 25 - 28" Hg. The ambient pressure (14.6 psi) is on the small hole side, the vacuum is on the large hole side. I am looking to evacuate the air is 0.25 - 0.5 seconds.

I have looked for and found some calculators but unfortunately, they ask for quite a few parameters like viscosity that I do not know the answer to. Can anybody point me to a simple way that I could use to get this info?

 

[jmiles]

Hi Flycast,

Im not very familiar with thermoforming (read not at all) so im not quite sure what it is you are trying to do, but im going to give you a few pointers that might help you out without going into the calculations too much.

For reference though you should eb able to look up in a google search all the properties of air that you need, just keep in mind there is a difference between dynamic and kinematic viscosity.

So as i understand it you hav e air trapped in a pocket, so you drill a hole and attach a vacuum to pull the air out?

now what you need to know is how big the holes ahve to be to pull the air out.

The answer to this is variable, in short the answer is not very big.... BUT, that depends on how fast you want to pull the air out. getting the air out through a hole is basically dependant upon the differential pressure betweent he hole and the vacuum, if there is any differential pressure ebtween the two the air will flow out. what happens is the air will flow faster and faster until it is travellign fast enough that the frictional losses of the air through the cylinder equal the difference in pressure between the pocket and the vacuum.

what this also means that is the smaller the cylinder the slower the air will leave. but it will keep comming out.

now you wont get all the air out cause tis just not practical to form a complete vacuum, but you will be able to get alot of it out depending on how strong your vacuum is.

I hope this helps somewhat, though you should keep in mind that in a problem such as this your typical unknowns like roughness of the cylinders and such can ahve a fairly alrge impact ont he answer if your looking for numbers.

drilling as big a hole as your process allows will give you the best ability to pull the air out.

 

[zdas04]

Flow through a conduit as small as your drilled holes can be a tough problem because the ratio of flow area to surface area of the pipe is so small. This gives friction values that are much higher than most equations would predict. On the other hand, since your flow length is very short you may be able to treat it as a nozzle instead of a pipe.

I would tend to model this as a critical flow nozzle and ignore the pipe-flow portion. This approach uses the speed of sound at the upstream side of the flow (i.e., inside the cylinder you're evacuating) to determine exit velocity and the small diameter to determine volume flow rate--downstream pressure doesn't matter as long as it is less than about 60% of upstream pressure in absolute terms. In your case the evacuation pressure is 25-28 inHg (2.3 - 0.85 psia) so you would have sonic flow down to arond 3 psia.

To use this technique you need to determine the volume of the cylinder being evacuated. Calculate the speed of sound and determine the mass flow rate out of the cylinder. Pick an arbitrary time (say 0.05 seconds) and calculate the mass that has left in that time, then recalculate the pressure in the cylinder to get a new sonic velocity. Repeat this until you reach 3 psia. Then (and this is the iffy part) double the elapsed time to guess the non-choked-flow period.

If the result is more than your allowed time then you need more or bigger holes.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[cpretty]

To understand critical flow nozzles (choked flow), O'Keefe Controls have the best descriptions available I have found. See the link below and look at their tutorial and fundamentals pages. The tables for orifie is good for skipping all calculations if you know what size hole you are dealing with.

Cheers,
Craig

 

[flycast]

Thanks for the advice, it was helpful. Here is a curiosity question...Why is the speed of sound so significant? Obviously, it's the speed of sound but why would it limit the air flow in this case?

 

[Latexman]

Sound is nothing more than a pressure wave. If the fluid is travelling at the speed of sound and the downstream pressure is lowered further, the flow will not increase because the lower pressure signal cannot be transmitted upstream since the fluid is travelling downstream at the speed of sound.

Good luck,
Latexman

 

[Latexman]

I forgot to add, this is called "choked flow".

Good luck,
Latexman

 

[zdas04]

Latexman,
That was as good a description of Choked Flow as I've ever read.

David