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brengine (Mechanical) (OP)
8 Dec 04 22:06
I am looking for tabulated values of friction losses for air in a vacuum? I have seen them for water under compression (i.e. head loss thru a reducer, elbow, fitting, etc...) and was hoping there was something out there for vacuum as well.

My end objective is to make sure the vacuum source and accumulator (for our pick-and-place mechanism with 200 suction cups) get sized properly.

Thanks,
Ken
sailoday28 (Mechanical)
8 Dec 04 22:31
What is the tabulation to consist of?
For example
For a given pipe ID, head loss/length at a specified flow (SCFM)
Montemayor (Chemical)
9 Dec 04 12:12
Ken:

First of all, lets agree on a fact: there is no friction loss in a vacuum; this is so because there is no matter existing in a vacuum.  So the statement of losses for air in a vacuum is illogical.   What you must be referring to is a partial vacuum.   Now, that is a situation where you have partial matter involved and that is what is required for identification in order to arrive at a pressure drop.

If you have the required basic data identified: fluid, flowrate, density, temperature, viscosity, diameter (of duct, pipe), etc., etc., then you should be able to simply apply the conventional fluid flow equations in combination with the expected friction factor.  However, be aware that the usual inaccuracies and errors in the relationships are now applied over a very narrow range: approximately 14 psi.   Therefore, if you are looking for calculations that will reveal friction losses in the range of fractions of a psi, I wouldn't trust them.  I can't imagine what you application is that would require such precise calculations and you haven't told us.  Therefore, I can only assume that you need to estimate the friction losses and these will always be well within the range of no greater than approximately a psi.   If that is the case, why would you require to have the calculation?

Could you please be more explicit with what your application is and what your basic data looks like?   Air in Denver Colorado, Mexico City, La Paz Bolivia and such sites is at partial vacuum (a partial vacuum defined as pressure less than an atmosphere, 14.696 psia); a vacuum (0.000 psia) doesn't exist in real, empirical industrial applications.   So, if you are just circulating air through a pipe in Denver, you have partial vacuum conditions exposed to a friction drop for the air.  This could be handled with the Darcy-Weisbach equation.

There are published air tables for such applications; I'm not at home at present so I can't cite the sources.  But I'm sure other Forum members will mention them.

Art Montemayor
Spring, TX

brengine (Mechanical) (OP)
9 Dec 04 12:38
Montemayor,

I'm in a little over my head here, but learning as I go.

The circuit could be considered this:
-Vacuum source
-then 25’ long hose
-then to an accumulator (or main vacuum manifold if accumulator not needed)
-then have 16 hoses out of it, 18” long, each going to an individual on-off valve
-then have one hose, 12” long, from each valve to one manifold
-each manifold will be 25” long and have 14 suction cups screwed into it, picking up one part per suction cup for part/material handing/transporting.

We have a worst case number of missing parts, that yields a "leaking" flow rate. I have been trying to size a vacuum pump that can pull down a vacuum (from the on-off valves thru the suction cups) in a fast enough evacuation time to meet our over all cycle time of 5.5 seconds. I have been told that an accumulator should provide enough reserve to compensate for any losses in the line.

I'm not sure what I am doing and have been relying on venders to adequatly quote me any components that I'll need, but they have not been as quick at responding to me as I would like plus I would just like to have a little more confidence in there findings other than having to tell my boss that this other guy said it's good enough and I have no idea myself.

I know that this is a pretty in-depth question (and I've probably still left other important items out), guess I was just looking for an existing source that I could read/research to help me in the calculations (for this and future applications).

Thanks for your input,
Ken
pennpoint (Mechanical)
9 Dec 04 13:23
KenBolen
Step back and take a look at this site.
Instead of building a vacuum system use compressed air (you may already have available).
 www.piab.com/us/index
Once the product is picked up by the suction cups you may need a means of releasing it (blow off)hince compressed air.
Point of use vacuum effectors can be less troublsome than a single large vacuum pump/reservoir/piping sytem.

Good Luck
pennpoint
sailoday28 (Mechanical)
9 Dec 04 20:39
Montemayor (Chemical) Dec 9, 2004  STATES

"First of all, lets agree on a fact: there is no friction loss in a vacuum; this is so because there is no matter existing in a vacuum."
I hate to haggle, but a vacuum can be perfect or partial.

quark (Mechanical)
10 Dec 04 6:09
I fully agree with Mr Montemayor about pressure drops in vacuum systems. In other way, frictional drop in pipeline will restrict you from acheiving the desired vacuum at the user end. So the final vacuum at the pump discharge should be equal to the sum of desired vacuum at the user point and the pressure drop in the piping in gauge terms.

Applied Process Design for Chemical and Petrochemical Plants by Ernst Ludwig deals with pressure drop in vacuum systems in detail. The reference cited is Standards for Steam Jet Vacuum Systems of Heat Exchange Institute.

http://www.heatexchange.org/pub/default.htm

You can use ejectors driven by compressed air as suggested by pennpoint in conjunction with a back pressure sensor or proximity sensor to check the presence of parts but just check the economics. Compressed air is generally costlier.

Regards,
 

katmar (Chemical)
10 Dec 04 7:22
Hi Ken,

This sounds like an interesting application and I would be happy to do some calcs for you if you give a bit more information.

Please check the lengths you gave in your post. These short hoses would result in masses of cups in a very small space - is it really like that?  Also what are the ID's of the hoses? What is the ID of the nozzle where the hose is attached to the cup? Do you have a Cv for the valve?

You have been advised that the accumulator will provide a reserve of (partial!) vacuum during the cycling of the machine. This makes sense, but can you give a bit more info - do all 200 cups cycle simulteously?  What is the 5.5 seconds?  What we need to know is how long is the valve open, and then how long is it closed, in each cycle.

Have you got a size for the accumulator?  Do you have a curve for your (partial) vacuum pump?

We also need to know what pressure (which will be less than atmospheric i.e. a partial vacuum) is required in the cup to hold your parts. Is this defined by the supplier?  If you can give the mass of the part and the area of the cup we could estimate the required pressure.

And just to make certain that you are nor working in one of those high altitude cities listed by Art, what is the altitude of your plant?  This is actually very important. I worked in such a city for several years and the lower atmospheric pressure caused problems in so many ways. Even something as trivial as boiling an egg is affected because of the different boiling point of water. And it wreaks havoc when you come to chemical engineering calculations.

regards
Katmar
brengine (Mechanical) (OP)
13 Dec 04 22:07
Thanks all for your input. We have broken our system into 16 rows of 14 suction cups each, and are going to use an ejector on each row. For right now we are relying on the Performance and Evacuation Curves to answer our questions, and going to go with that.

Ken

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