OT(?) vacuum pump

Why not just buy a any name brand two stage refrigeration pump? Its just a roughing pump. Are you worried about lubricant contamination from the pump?
Am I missing something? The Robinair 15800 is only about $250 bucks so its worth a shot I think. Refurbed 15600s can be found for under $200 (plus shipping).

I've only used "expensive" lab pump, keep in mind they're never cheap unless they're used or remanufactured- which is actually a good way to get one.
It all comes down to what pumping capacity do you need, what vapors (and how much moisture) are you pumping etc.
All the "expensive" pumps are designed to run nonstop forever, or until you mess up the oil. The HVAC pumps you see at the pawn shop are not built or rated for this sort of duty, and the price shows this.
Keep in mind that tossing away $250 pumps may still be cheaper than sending in a good pump for rebuild if you mistreat it. The worst thing you can do is let water condense in your oil, then shut the pump off, letting everything rust and corrode. I've not asked them, but it looks like the commercial HVAC people just replace their pump oil after any big job. This is based off the number of pump oil containers in the trash after the guys leave.

Ahh OK that makes sense. (There were always several gurgling away 24/7 back in the lab in grad school.) Yeah this will be more limited duty. But I guess I could picture leaving it on over night. That's what the ballast value is for... I think? (not a vacuum expert) I'ts kinda too bad 'cause pumping on water is fun. Thanks for the advice.
George H.

Excuse my ignorance, but is the refrigeration pump different from the HVAC pump I posted? Ohh... it'll be for continuous duty. Well not at the moment. Nope I just didn't look at the Robinair 15800.
Thanks I'll look into refrigeration pumps.
George H.

Those mechanical vacuum pumps are pretty amazing in that all they can do to update them is change the paint jobs every decade or so.
Yup. The gas ballast is just a valve that lets a pump suck in air with whatever you're pumping. The idea is as long as the pump is still pumping air along with water vapor from whatever you're evacuating the water vapor won't condense into water droplets in the oil leaving you emulsified oil and water. Constantly sucking on some fresh air also keeps the pump running hot so the water in the oil will evaporate. Once the water load is gone, you close the gas ballast and the pump does it's stuff to "full" vacuum.
The problem with gas ballasts is they cause the pumps to emit a nasty fog from the outlet of super fine oil droplets that will pass though nearly any sort of filter. They do make mist eliminators to prevent this awful fog from covering everything near the pump. You can also just pipe the stuff outside and call it "water vapor" if anybody asks.
Since it's sort of impossible to not pump water vapor, pretty much all the pump makers will have the water pumping capacity as part of the spec sheet.

Take a look at Harbor Freight. They have 2 vacuum pumps. I haven't used either, so this is no reccomendation, merely the hope you look, and report your findings here.
Hul
snipped-for-privacy@gmail.com wrote:

Yeah, I've gotta pipe the output outside. (the lab space also has optics... the last thing I need is oil vapor.) I don't think this is on the spec sheet, but I'd give up a lot to have a quite pump. That may be an advantage for the smaller sizes.
George H.

I'm sure... If you go to amazon, there are ~100's vacuum pumps. The Chinese stuff is all cheaper. I've no time to survey the options.
George H.

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George, I've used Robinaire pumps for decades. I've never had a bad one out of the box, and they work for ten years at least in twice-weekly service, if you change the oil on every job (which is _required_ for warrantee).
Those folks who got 'burned' probably bought from questionable vendors who were selling refurbs as new, or pumps known to have been dropped from pallets, etc. It doesn't take much impact to break a vane.
They work great, and if you get a bad one, the vendor should replace it-- no questions asked.
LLoyd

Thanks Lloyd, If it's not too much trouble, can you tell me why you have to change the oil so often? I've only used vacuum pumps in a research settin g. And there the only time I recall changing the oil was when I rebuilt an old pump, or when I burned up the oil in a diffusion pump.
George H.

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I use them for refrigeration jobs. Refrigerant, water, and 'gunk' from systems will contaminate the oil. It's just good practice to change the oil after any job where you've been pulling down an 'opened' system.
Just CYA. The oil is sure not expensive compared to a new pump, and the job charge is set to cover it.
You could probably get away with almost never changing it if you were pulling dry gasses. Water... eh! It emulsifies _some_ with the oil, even when you open the bias valve.
Lloyd

OK just to cap this thread. I bought an old Sargent Welch Duo seal 1400 on ebay last night. It was so cute I just had to buy it and take it home :^) ~$200 delivered.
Thanks for the input.
George H.

That's a nice pump. I used to get parts for my Precision Scientific pumps from the factory when they were made in Chicago. The Welch ones are still made just outside the city.

Vacuum pumps pull the moisture out of the HVAC system and into the pump, which is filled with oil. Moisture is a contaminant, so change the oil every time. With our old system pumps for automotive use, we changed oil once or twice a year, but they kept working despite the abuse. The Boss said "It's too expensive to change oil each time."

I change mine after each use, but the oil gets collected and I let most of the water settle out. Then it goes through a vacuum distillation system to remove the rest.

Would it help to pump down with a carbon vane pump before switching to the oil pump?
Gast recommends running air through the pump for 5 minutes to dry it.
jsw