Vapor collapse in Condensers 1

[pianoman1]

I have a client that has an old shell and tube condenser that is not rated for full vacuum on the shell side. Based on the operating conditions, I've determined that the vapor mass could condense in about 1 second if the shell side were closed in and the cooling water left running, requiring almost 1,000,000 CFH air flow through a vacuum vent to keep the shell from collapsing. If this were a tank, I would size a vacuum vent for these conditions, but the vent will be huge compared to the shell and I'm wondering if this is practical. I'm inclined to recommend administrative controls (per API-521) until they can replace the unit with one that is rated for full vacuum.

Any feedback on this issue? Never bumped into a condenser that was not rated for full vacuum.

Thanks.

 

[georgeverghese]

If this HX is small dia, maybe the pressure vessel can inherently withstand FV without vacuum reinforcement - I'd check with a mechanical engineer if this is the case. If it indeed cannot handle FV, locking open one or more the shellside valves may be acceptable if the shellside fluid is not flammable or toxic.
Presume you've computed this 1sec based on the (mass blocked in x latent heat of vaporisation) / (exchanger overall U x del T)?

 

[Latexman]

Just because a vessel has no vacuum rating, does not mean it cannot withstand FV. It may mean it was not important in the designer's eye when it was specified. Have it rated.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[pianoman1]

Thanks for the responses. Actually I had the original manufacturer check the shell rating, they came back and said it was good for 3 psig vacuum only. And yes, I used the latent heat of vaporization of the vapor to get to the timing. It makes sense when you cosider how little mass is in the shell with all the tubes taking up half the space. We're going to look at administrative controls and/or a big vacuum vent if replacement is delayed.

 

[georgeverghese]

If you've got a multicomponent mix on the shellside, it may not be as simple as this - if it is, as the vapor mix gets leaner and leaner, required dewpoint temp will drop a lot as shellside pressure drops also.