Vacuum Pump Reccomendation?

[InigoMontoya]

At the office I have a few air tanks that combine for a volume of roughly 1200 cubic feet. They are normally used for the storage of 3000 psi air (Note: When I say they're 1200 cubic feet, I don't mean that they hold 1200 scf of air; I mean that they have a combined volume of 1200 cubic feet.).

 

In any event, despite the use of air dryers, we get condensation in the tanks. This is bad (mmmkay?).

 

What I'd like to do: Periodically vent the tanks and then pull a vacuum on them to force any condensation in the tanks to boil off thereby drying the tanks out.

 

Early in the process this should be easy, but as the vacuum hardens (read: pressure reduces to < 0.5 psia) things could get dicey. More to the point, it is my understanding that pulling high concentrations of water vapor through a vacuum pump can destroy a vacuum pump. CAN destroy. Not WILL destroy. Again, it's my understanding that the difference between "can" and "will" in those two sentences is primarily depending upon the type/quality/etc. of vacuum pump.

 

I confess to knowing virtually nothing about vacuum pumps.

 

Thus, with all that said I am asking: Does anybody here know enough about vacuum pumps to recommend one for my situation/application?

Edited January 27, 2012 by InigoMontoya

[CaptainPanic]

Is it not easier to vent the tanks with dry air (or nitrogen - which is more expensive, but already dry)?

It shouldn't be hard to get an air dryer or sufficient capacity. Just blow the air through. Dry air or a vacuum should create the same concentration difference between water droplets and bulk gas phase, and assuming that the temperature is also the same, you get the same mass transfer.

 

Vacuum processes are especially useful if you want to recover your vapor - but I guess you don't shed a tear if your condensate is lost forever.

 

It might also be safer for your equipment. It seems rated for overpressure of 3000 psia, but can all bits and pieces also handle vacuum?

 

Sorry for not answering the actual question.

[Klaynos]

Have you considered using a decent desicator to run your air through before it enters the tanks?

 

We use something like this:

 

https://secure.drierite.com/catalog3/page13.cfm

 

We've experienced problems with drying desicant under vacuum in ovens as the water mixes with the vacuum oil and all hell breaks loose...

 

Your best bet if you are set on a pump is to contact the manufactures and find out what water levels they can cope with.

[ajb]

My advice is to be careful goggling "vacuum pumps". I tried it years ago as part of an undergrad project (I forget what now) and I got lots of vacuum pumps...

[InigoMontoya]

Is it not easier to vent the tanks with dry air (or nitrogen - which is more expensive, but already dry)?

It shouldn't be hard to get an air dryer or sufficient capacity. Just blow the air through. Dry air or a vacuum should create the same concentration difference between water droplets and bulk gas phase, and assuming that the temperature is also the same, you get the same mass transfer.

That's what we've done in the past. It's taken *weeks* to dry the tanks out. I suspect much of this is because we have a lot of "dead ends" in the configuration wherein it's difficult or impossible to get steady flow over certain interior surfaces.

 

Vacuum processes are especially useful if you want to recover your vapor - but I guess you don't shed a tear if your condensate is lost forever.

Nope. It's just water.

 

It might also be safer for your equipment. It seems rated for overpressure of 3000 psia, but can all bits and pieces also handle vacuum?

All piping is stainless steel. The tanks are steel. No flex lines or anything like that involved.

 

 

Have you considered using a decent desicator to run your air through before it enters the tanks?

We have had chemical-based desicators in the past (similar to your link, but larger). The maintenance requirements and fragility of the design kills us.

 

We then switched to refrigeration cycle type desicators. They're low maintenance and very robust, but they don't quite remove all the moisture.

 

Your best bet if you are set on a pump is to contact the manufactures and find out what water levels they can cope with.

Oh, I will, but I like getting 3rd party opinions. I've been burned many, many times by vendors who look you straight in the eye and then sell you something that won't work for the application you asked about. Take it to our legal department and they're like, "What? $100,000? That's not even worth our time trying to recover. Call us when somebody burns you for a few million..." Thus, I've learned to take sales reps not with a grain of salt, but with an ore cart full.

[Klaynos]

Have you considered using a chemical desicant after the refrigerator one?

 

The problem you're going to have trying to find third party comments is what you're trying to do is quite specialised, with many unknowns it's really difficult to say just how much water it'd be. As I said I have seen pumps die due to the water threwput but that was evacuating quite a small volume with quite a lot of water in it many times.

[InigoMontoya]

Have you considered using a chemical desicant after the refrigerator one?

Again, we've had desicant-based dryers in the past. The maintenance requirements and fragility of them make them show stoppers for us. While the small amounts of water that get past our dryers are annoying, they aren't show stoppers. Dryers that are down for maintenance, on the other hand, ARE show stoppers.

 

The problem you're going to have trying to find third party comments is what you're trying to do is quite specialised, with many unknowns it's really difficult to say just how much water it'd be. As I said I have seen pumps die due to the water threwput but that was evacuating quite a small volume with quite a lot of water in it many times.

Appreciate the feedback.

[Xittenn]

I think the problem is 'Rotary Vane Vacuum Pumps' come in both oil sealed and oil-less self-lubricating carbon vane. Neither of these mix well with water, as far as is logically foreseeable. I think however, water ring vacuum pumps tend to be more suited to air conditions where the air is humid. Something to look into I guess. I wish I had more to say, but our water problems never interfered with our vacuum pumps, as they were separate systems.

 

One thing though, is it really a wise idea to subject a compressor tank to below atmospheric pressures???????? I would be careful about that personally. IMHO!

 

 

Are most tanks welded inside as well? I'm not confident they are unless it was especially noted.

Edited January 28, 2012 by Xittenn

[InigoMontoya]

Well, the good news on that front is that I have one tank that is not used to experiment with. If I crush the tank, no loss. If it survives... Hey, 8 more just like it that I actually use.