VACUUM CONDUCTANCE 1

[wuddog]

I have been tasked with designing a vacuum pipeline that will be connected to an existing vacuum pump. This is the first time I've delved into vacuum design. I've read about the term "conductance" and understand it to be akin to resistance. However, all the info I've uncovered merely addresses the conductance of long pipe. It does not address elbows, valves, and other pipe obstructions. Does anyone have a source that more completely addresses vacuum conductance? OR can anyone advise how one calculates and includes the conductance of the aforementioned obstructions? I know how to calculate overall conductance in series and parallel once all the various conductance values are calculated. I just can't calculate all the individual values.
Thanks for the help.

 

[TedLinsle]

It is an accepted engineering practice to add 50% to the measured length to calculate the equivalent length. Alternatively one can add up fittings individual equivalent lengths. There's a great chart for this--"Table 11-2.-Representative Equivalent Length in Pipe Diameters (L/D) of Various Valves and Fittings"-- posted currently at the following address:

http://www.tpub.com/utilities/95.htm

 

[wuddog]

Thanks Ted. My only question about this is, "Do effective lengths for positive pressure lines apply to negative pressure?"
My line is going to be around 5 torr (0.1 PSI). I believe things get a little different at these low pressures.

 

[TedLinsle]

I'm used to vac systems for labs, hospitals, shop air, etc. Guess I can't be much help here.

 

[quark]

Yes, they do. In any case, this is just gas flow and you can use the same methodology that you use for pressure piping. You should be careful to use the relevant emperical expressions if you are using other than Darcy's equation.
Crane's TP410 gives you various methods to calculate pressure drop in gas flow though it doesn't specifically deal with vacuum applications.

Earnst Ludwig's Process Design for Chemical and Petrochemical Plants deal with vacuum piping design with worked out example.

When you deal with systems that deal with constant generation of gases, for ex. distillation systems, the design should base upon the the required vacuum in the equipment and the capacity of the vacuum pump. The difference is the maximum resistance that your selected pipe size should offer.

However, when there is no constant generation of gases and only if you have to evacuate a vessel, the maximum vacuum permissible by the pump can be acheived, in theory, irrespective of pipe size. But the fastness with which you can acheive this depends upon your pipe size.

 

[handleman]

There is a pressure below which, depending on pipe size, the normal equations of gas/fluid flow break down. Your stated pressure (5 torr) is probably still in the pressure range (rough vacuum) where you can expect normal viscous/continuum flow. However, you probably want to at least be aware that you're moving on down towards medium vacuum (1 - 0.001 torr), which experiences Knudsen flow (transition between viscous and molecular flow). Down below .001 torr you get high vacuum, which gives molecular flow.

If you want to research this area further, a great resource is the 2003/2004 Leybold Vaccumm Products and Reference Book (catalog, really). It has a 160 or so page section entitled "Fundamentals of Vacuum Technology". The newest version of the catalog omits this section, but I've been told it's available as a separate booklet.

Also, any technical sales person from a high vacuum pump company (Alcatel, Leybold, Ulvac, etc) should be able to help you out. My personal experience has mostly been with Leybold. They have been extremely helpful to me in the past.

Good luck!