Vacuum Vapour Duct Sizing 1

[CC88]

I am looking for information on sizing vapour ducting under a vacuum. The saturated vapour pressures range from -5 psig to -12 psig. Is there an acceptable flow velocity that should be used for sizing? I use 80-120 ft/s for steam piping sizing, however, this produces excessively large vapour ducts for my application. Since the noise and condensate build-up concern of steam pipes may not apply for such low pressures, is there a higher velocity accepted?

 

[Montemayor]

ccraigen (Mechanical):

The following are Recommended Design vapor velocities for vacuum lines as used for major Chemical production facilities here in the USA:

System Maximum Allowable Desired Operating
Process Pressure Velocity, Ft/sec Velocity, Ft/sec
Torr (mm Hg) Absolute
755 - 760 300 275
725 - 755 250 225
685 - 725 200 175
380 - 685 150 150
0 - 380 150 150

When designing vacuum lines, the pressure drop should be the driving criteria, not the velocity.

I have produced an Excel Workbook dedicated to the practical design parameters involved in producing and maintaining a vacuum where the above is explained. If you are interested in having a copy, let me know: your email, your city, your name, and the company you work for or the university you attend. I'll be glad to send you a copy. My email is artmontemayor37@hotsheet.com



Art Montemayor
Spring, TX

 

[pmureiko]

I second Montemayor on pressure drop being the concern for sizing vacuum lines. Every mm Hg pressure drop places a large load on your vacuum system. The relative difference between 15 mmHg and 10 is much larger than 255 and 250 Hg.

 

[quark]

I think the pressure drop plays a vital role when we are applying a constant vacuum to excavate continuous generation of vapors. If we have to empty out air from a vessel, final vacuum will be acheived irrespective of the calculated pressure drop but this process will take more time. (I assume leakproof vessel here)

Art!

Presently I am using the method given in a book Applied Process Design for Chemical and Petrochemical Plants. I don't know the author's name because I have a xerox copy of this. Can you let me know your method of doing this calculation?

By the way if you know the author's name of the book I stated, please enlighten me.

Regards,

 

[Montemayor]

quark:

You are probably referring to Volume I (of 3 volumes) by Ernest E. Ludwig. Ernie Ludwig graduated from the University of Texas and was a distinguished engineer in the Texas Gulf Coast (where a lot of the basic chemical engineering technology rules were written). He led the Dow Chemical Plant in Freeport, TX in many process and management engineering roles during his career there and later went on to manage a Chemical Complex in Odessa, TX. He is a respected and knowledgeable expert in this part of the world. He was retired and still lives in Baton Rouge, LA I believe. His Opus, "Applied Process Design for Chemical and Petrochemical Plants" is one that I feel any self-respecting chemical engineer should read and study. I bought my copy in 1970 (US$61.85 for the 3 volumes) and still refer to it from time to time. He has dealt with process design engineering like no other author that I have read in the last 47 years. I always recommend his work(s) to young engineers. Basic design can never get better; it just gets better explained and taught.

I didn't do any calculations for arriving at the recommended superficial vapor velocities. I merely cited existing recommendations used by major chemical companies here in the Texas Gulf Coast. As you state, these velocities are for constant vacuum while evacuating a continuous stream of non-condensed vapors.

Your comment about final vacuum being achieved irrespective of the calculated pressure drop but taking more time is very true. However, you left out the part that pmureiko is stressing: it will take more horsepower or workrate. The lower the vacuum, the higher the vapor specific volume and, consequently, the higher the superficial vapor velocity - with a tendency for larger pressure drop. I think you will find that the horsepower curve starts to climb rather quickly as you get to higher pressure drops in a vacuum level. If the time factor is not a concern, then I would not worry about the pressure drop - as you say, it will come eventually.


Art Montemayor
Spring, TX

 

[quark]

Art!

Thank you very much both for the concept and the author's name(and you deserve a star). As you told, I also noticed that the piping section (for which I have a copy)in the above said book is brilliant.

If fully agree with the last para in your post. Concepts which we develop during initial days of our practical experience are hard to crack nuts and I feel happy when I come to know they are correct, by the authority of engineers like you(luckily I didn't miss anything in my work sofar)

Thanks once again,