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