Regeneration of LN2-Cooled Sorption Pumps

Sorption pumps, such as the Varian Vacsorb, are capture pumps. They pump by condensing gasses onto a cold surface. As with all capture pumps, the eventually reach their capacity. At this point, the gasses captured must be expelled.

While cold, capture pumps can pump a large quantity of gas and maintain inlet pressures of less than 10E-4 Torr depending on design. When these pumps warm, the captured gas can create a dangerous build up of pressure in the pump. Recall PV=NRT? Consequently, all capture pumps (including our own Linear Accelerator) have pressure relief devices to prevent injury to personel and damage to the pump.

Regeneration is the process of ridding a capture pump of its accumulated gas in a controlled manner. The regeneration of cryopumps is covered elsewhere on this web site. Information regarding the regeneration of LN2 cooled sorption pumps follows.

General Procedure for LN2-Cooled Sorption Pump Regeneration

The basic idea behind regeneration is to raise the pump well above it's operating temperature to drive off adsorbed gasses. Ideally, one should exceed the boiling point of liquid-gas phase change temperature for each gas pumped. In practice, water vapor is the last gas to be driven off during regeneration, so exceeding the boiling point of water is desirable. Moreover, the longer one keeps the pump hot, the better the regeneration. Typically, overnight is ideal, and anything beyond that is of minimal gain.

Regeneration is generally initiated with the pump at ambient temperature, however one can start the heater with the pump at LN2 temperature if the LN2 dewar is of the stainless type, or with the plastic dewar removed. Turning the heater on when the pump is cold is ok. I have often done it with no problem. However, please note that the reverse procedure it not acceptable, that is, you MAY NOT add LN2 to the dewar when the pump is hot.

At this point you may walk away if you like. If the pump has a large amount of gas in it, it should expell the rubber stopper that is the pressure relief device, allowing the gasses inside to escape. If the pump is not full, you may have to do this yourself. Wait until the pump is warm to the touch (carefull - it gets quite hot) and then just pull the plug. IF you are starting from ambient temperature and can vent your pump to nitrogen, do that. Then just pull the plug, turn on the heater, and walk away. NEVER pull the plug when the pump is cold. You'll just pump more gas into the pump before it warms. This gas must then later be expelled. Doing this could also cause contamination of the sieve material, which means that I have to change it out, which means that your experiment will be delayed and I'll be grouchy with you.

Once the pump has been hot with the plug pulled for at least an hour, you may re-install the plug and turn off the heater. The pump MUST be allowed to cool to ambient before adding liquid nitrogen.

Regeneration of sorption pumps after use is a courtesy that you extend to others. It's easy to forget. Consequently, it should probably be part of your pre-experiment planning.

One more thought: Often, I've had students feel that they needed to regenerate a sorption pump when in fact, they simply did not pre-cool long enough. A small sorption pump should be filled at least 20 minutes prior to use. Something on the order of an hour is much better. You can use the boil as a guide. When a pump is fully cold, the LN2 surface is relatively calm.

As soon as I get my computer issues resolved, I'll get the manual for these pumps posted. Also, if you want to see what's inside one, stop by my office - I have one cut open and use it as a pencil holder...

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Last updated: 14 Dec 2006
Send any comments to: pbarber@mailer.fsu.edu