Sizing process piping for vacuum service

Hi everyone,

When sizing process piping for vacuum service (Knudsen number < 0.01), do you use the Darcy-Weisbach (D-W) equation with all it’s caveats for compressible flow or do you have specialty references for conductance values and size according to some other technique? I’m usually working with sch.40 pipe (carbon or stainless steel) between 0.5" to 3" and tubing (copper or stainless steel) from 0.25" to 2".

I’ve read/heard that the D-W technique holds in the case of Knudsen number <0.01 because the flow is still within the continuous region as opposed to molecular flow. I’ve worked with high vacuum applications before, but it’s always been some variant of evacuating a fixed volume and not in a vacuum piping system application.

Yes, I use the Darcy-Weisbach equation for Kn < 0.01.

You are well into the viscous flow regime, Kn < 0.01. Kn > 1 is where molecular flow starts, and is best handled with conductance concepts. And, we all avoid the transition region between the two like the plague when 1 > Kn > 0.01. I have no idea what to do there. Use both techniques and choose the larger diameter, I guess.

Thanks Latexman. Do you know of any references that give guidelines for vacuum service piping design? My fledgling technical library has some material on vacuum pumps at the moment but not piping systems. I have the 7th edition of Nayyar’s Piping Handbook and that has a section on vacuum piping systems that is geared towards laboratory systems, so I haven’t leaned on it much to learn more for larger systems.

Yes, I’ll climb the stairs to my office tomorrow morning and give you what I have. What vacuum or pressure level are you designing for?

Thanks for all the help! I have some scope that’s at 7 psia and some scope between 1 psia to 3 psia.

Bookwise, I use Process Vacuum System Design & Operation by James L. Ryans and Daniel L. Roper. I think it is an excellent reference and the few reviews I’ve seen on line say the same. Here’s a .pdf of the table of contents:

CONTENTS.pdf (580.0 KB)

A used book is well worth $40 on Amazon, IMHO.

I also have an old company design manual which has equations for preliminary design specifically for vacuum service. Here are some snips:


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Wow, thanks for this information! I recognize the author names on other vacuum related articles I’ve been finding - I’m surprised their book was never in the bibliography. $40 for a specialty technical book - feels like stealing!

Your welcome. The company design manual info. is 10-15 years old. It shouldn’t really matter much, but just beware.

IIRC, their book came out in 1986. I thought it was excellent, so I’m a little surprised they or others haven’t had a 2nd edition.