vacuum pump for experiments

When a vacuum pump reduces the pressure in a vessel, this reduces the boiling point of any liquids (usually water) within the vessel. Eventually the pressure in the vessel will become so low that water will boil at room temperature. At this stage all the water will evaporate and consequently the water vapour is sucked into the vacuum pump. As gases pass through the pump they are compressed, which raises the boiling point of water again, and some water condenses inside the pump. This is not desirable because it can reduce pump performance and cause corrosion, so the water needs to be expelled from the pump. The gas ballast valve allows dry air (well, hopefully dry) from the atmosphere to enter the pump chamber just before the air is expelled through the exhaust valve. Because this air isn't saturated (i.e., it isn't at 100% humidity) it can absorb extra water vapour and carry moisture out of the pump. So the gas ballast valve helps to purge the pump of water or other contaminants. Opening the gas ballast valve does however reduce the ultimate vacuum attainable.

Hope this helps,

Chris

Break a "new" light bulb for the metal. The new tungsten is still flexible and can be formed but an old one will be brittle. Also you may want the metal to glass seal that is hard to make your self (I think).

Bill K7NOM

The answer given by Christopher is completely correct.

I could add the following though:

The idea is that pumps have a hard time pumping any 'condensible' vapor (which includes water, but also some solvents) so aside from contaminating the oil, the pressure the pump will reach is limited when pumping these vapors.

There is another port on the pump stator that communicates with the gas volume as it is being compressed and swept around by the vanes in the rotor - and this is connected to a small fixed orifice which can be opened up to room air. Because the pressure in the swept volume is still below atmosphere it will pull air in and this dilutes the condensible vapor and prevents it from condensing to liquid, and allows the vanes to sweep it around. The extra air charge in the swept volume will open the exhaust reed valve and will be expelled.

Without the gas ballast charge the pressure in the swept volume will drop very low as the rotor comes around (vapor has gone to liquid as the pressure comes up) and the exhaust reed valve will not burp.

As the rotor continues around to the intake port the pressure will rise and the vapor will still be there, along with the next slug of air/vapor. So without the gas ballast valve the pump becomes a machine that sucks only the condensible vapor from the chamber and dumps it into the oil.

Jim

And probably in better condition as it was used recently and when it was used it probably worked. Unuse is worse for a pump than use. Every pump I got from a school was is poor condition because they never use them enough or change the oil.

I've never seen a HVAC pump at the pawn shop that had dirty looking oil in it. They were all working just fine before getting lifted. Stuff at schools sits around and rots forever.

Getting an oily smoke/mist out of the exhaust when it's pumping down with a covered up inlet is a good sign it's sort of working.

it probably worked. Unuse is worse for a pump than use. Every pump I got from a school was is poor condition because they never use them enough or change the oil.

They were all working just fine before getting lifted. Stuff at schools sits around and rots forever.

Obviously there's a difference of opinion here. I personally don't see that there's much harm that can come to a pump if it's left unused, provided that it doesn't contain water and that it isn't left with the inlet open so that it can fill with dirt. It's just like leaving an engine filled with oil for years; I don't think it will do any irreparable damage. One pump I had from a laboratory which had been standing for a year or two (and DID have some water in it) was a bit stiff at first but after an oil change and a few rotations it was fine. When I dismantled it there wasn't any appreciable corrosion either, just a very slight discoloration where the rotor had stopped. I think abrasive dirt is probably the biggest cause of pump damage. But in general these belt-driven pumps are very rugged and reliable.

Chris

You missed the most important one: attempt to determine if the pump has been used with any toxic substances in its former life. This is hard to do, but some common tip-offs include:

Off-board oil filtration devices

Inert gas feed for purge or ballast

Residue around the intake or exhaust ports

Former owners include medical, or laser users

By all means, *ask* what the pump was used for, and ask who owned it before the present owner, if that information is available. Any fishy answers would be grounds for me to pass on the item.

Jim

There is enough oil left in the sump for lots of operation, and I suppose you could add some once in a while if you really wanted. I have had mine for about 6 or 8 years of occasional operation without a problem. If it ever goes bad on me, I will simply scrounge another.

Vaughn

Good point Jim. I should have remembered that one!

Chris

If you know which soldered off tubes go into the sump and the ullage space, you can install a simple vinyl-tubing sight glass gauge to monitor the oil level in the hermetic can - the problem being to determine the proper oil level to begin with.

I suppose if you call the manufacturer with the compressor model number from the data plate, they can tell you how many fluid ounces of oil, and what weight and type - mineral, POE or PAG synthetic. Do NOT mix or change the oil, the oil seals inside are different to be compatible with the oil. And you can drain and fill it to make the initial 'full' level mark.

Oh, and put an oil separation filter on the compressed air output to trap the excess oil - in refrigeration they don't care about a little bit of oil entrainment in the output, because it's a sealed system and the oil will be back soon...

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Thanks!

I managed to get the welch 1400 pump off ebay for $130. I picked it up last night and it looks good. The seller stated that his father owned it for AC work but stated he never saw him use it. The inlet had AC fittings on it (and was capped) so it appears it was used for AC work.

I have no way to test it, other than putting my finger over the intake and observing that it does suck. The noise changes considerably as the vacuum is built up. Opening the gas ballast also increases the noise a lot when the input is capped.

This pump should work much better than a converted AC compressor and was a turn-key operation for 130. can't beat that!

I will probably pick up a cheap automotive vacuum gauge to get a rough idea if its working well.

thanks for all the help chuck

The ultimate pressure you can achieve is very much affected by the oil in the vacuum pump and I'm not convinced that AC oil is good vacuum pump oil.

I think the primary factor with AC oil is that it has to be compatable with the Freon (as well as the seals). In the AC, I doubt very much that they care about the vapor pressure of the oil which is critical in vacuum operations.

Supporting data from one of the laser web sites indicate that they change the oil when they use an AC compressor as a vacuum pump.

Its a moot point for me, because I just bought a welch 1400 two stage pump for 127 off ebay. While that is more than an AC compressor, its a turn key operation and I believe this vacuum pump will perform way better than a converted AC compressor.

chuck

A bourdon tube gage won't really tell you what's going on, see if you can hunt up a thermocouple gage. These read down to about one micron and will tell you if the pump is working up to what it should be doing.

Jim

I have read a little about these on one of the laser web sites. Any recomendations where to get a good one ? chuck

Good luck with the experiments. I'd be interested to see some pictures sometime!

Chris