Fusion Message Board

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Subject: Re: Sorption Pumps
Date: Jan 18, 10:58 pm
Poster: Richard Hull

On Jan 18, 10:58 pm, Richard Hull wrote:

>Can you describe the bakeout cycle, please? Do you think it's possible to bakeout with an aspirator sucking up the exhausted vapors?


The bakeout cycle, I believe, has been discussed before here, but here goes again.

The sorbant is load with bad stuff, let us say. In a good sorption pump manifold there are at least 2 pumps, sometimes three and a minimum of four valves.

The key thing is to remember that you can mess a system up if the valving is not handled well. In general, the dirtiest pump will be the one first brought on line in evacuation.

First turn on the forepump, (mechanical or aspirator.)

Next, open the valve to the first sorption pump and at the same time, operate its heaater. Once the heater reaches final operating temp (30 minutes or so) give the pump 1-2 hours at full temp. Next open the valve to the next pump and start its heater. Again, once up to temp give it one to two hours. And so on for any additional pumps. All this long time, the forepump must be left running!!!

Some people choose to physically remove the sorbant from all pumps and heat the mass in a vacuum or vented oven for just two hours. This avoids multiple valving and long forepumping periods.

I assume that you know the down side of sorption pumps in that you must forever keep the pumps under LN2 even after valve off from the main chamber. This is especially true of the first pump in the manifold. If the pump which is sealed gets warm again, the outgassing sorbant can give rise to possible dangerous pressures. The other option is to open the forepump valve on the first pump, once the second pump is finished, valved off and the third pump is brought on line. At this time the LN2 bucket can be slipped off of the first sorption pump. Warming of the valved off second and third sorption pumps to room temp is not dangerous, as they have small amounts of contained gas compared to the first.

The aspirator is OK, I suppose, but it is limited by water's vapor pressure at any given temperature and water backstreaming might be a severe problem. I would watch the pressure on such a pump, and as long as it drops, you are OK, but the moment it starts to rise, your sorbant is loaded and you must valve off that first pump and start the second.

Richard Hull