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09-28-2011, 05:48 AM #1
Converting Air Compressor to Vacuum Pump
I am in need of a high flow vacuum pump developing approximately 27/28mm Hg. It occurred to me this morning that I have an old two hp speedaire compressor with cast iron cylinders.
Would it be a viable possibility to convert this compressor to a vaccum pump?
I'm not trying to pull ultra vacuum or anything, I just need relatively high flow. I've been trying to keep my eyes open for an old milking pump, but haven't seen one in a long time. I'm out of farm country, so that doesn't surprise me.
09-28-2011, 06:04 AM #2
You could use a compressor pump to pull a vacuum, but will not acheive the 27/28 hg level you desire. Most pumps for vacuum have the pumping unit enclosed in an oiled filled case to help with sealing and holding a vacuum. A rotary vane type would give better flow.
09-28-2011, 06:13 AM #3
ive done that before form memery it had 20 psi in a closed vacum tank i might still have some pics at home but cant be sure. We were using it to pull the air out of a silicon mixture. i dont know how high flow it would be.
09-28-2011, 06:38 AM #4
We use an old compressor in exactly the way you are describing. Have been for years, actually. Expect to lose oil at a fair rate as valve seals designed for a compressor apparently don't work well when you use the compressor as a vacuum pump. Also, you may not get the flow rate you are after. We have a couple of good sized accumulators that get pulled down when the system is at "idle".
09-28-2011, 06:47 AM #5
Possible to use your compressed air through a venturi to pull your vacuum? Good volume, but not very hard.
09-28-2011, 08:35 AM #6
I have used a shop comp. to pull a vacuum. Another alternative would be to stop by a salvage yard where old fire trucks are sitting and purchase a primer pump from one of the engines, some are 12 v. mostly vein but some are gear they work well and use oil or another oily type liquid for a seal. These pumps pull a good vacuum whrough a large volume area, as fire pumps for priming for drafting water. Probably cheap. My two bits John
09-28-2011, 08:40 AM #7
Diaphragm pump/compressor will work in that range. No oil involved...
09-28-2011, 09:16 AM #8
First, you say you want 28 mm hg vacuum. That is only a little over an inch, which you could get with a good blower. If you mean 28 inches, that is an almost meaningless statement because measuring against atmosphereic pressure doesn't work there. The barometer can go below 28 in, and a gauge measuring against it would not read 28 even if you had a perfect vacuum. I have done lots of this sort of work and one of the greatest hurtles is educating customers about vacuum measurement. At that level you need an absolute pressure gauge that reads the way scientists do. In a sense, there is no such thing as a vacuum, only low pressures. You need to get a better definition of your requirements. Some compressors work fine as vacuum pumps, but the Gardner Denver ACR I had set up for vacuum stopped dead at 25 in. The reason was that past that there wasn't enough pressure differential to open the valves. Since the exhaust valves were working against atmosphere, the 25 in reading would be the same regardless of the barometer reading. The absolute pressure in the chamber would vary with atmospheric pressure.
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09-28-2011, 09:54 AM #9
Gotta seal the compressor. That means a synthetic shaft seal installed lip out, the opp end blanked if not solid, the dip stick hole plugged or capped. and the compression relief removed and the holes plugged. The amount of vac it will pull depends on the piston to to head clearance and the excellence of the valves. maybe take a cut off the head jpint of the cylinder casting to minimize clearance.
It takes about 1/5 the HP to run a compressor as a vac pump than as a compressor. You can speed it up if you don't mind a little shorter pump life. You can't speed it up much because the valves aren't up to the increased volume at inital drawdown. Once vacuum is well estaablised you can speed the pump acordingly. I dunno. Maybe I'm getting too fancy..
I' don't know what to say about oil. Somewhere when you talk about vacuum pumps oil volatility enters the picture but I've never monkeyed with vacuum except peripheraly.
09-28-2011, 10:50 AM #10
Harbor Freight sells an air operated venturi vacuum unit that they sell for AC work. It is priced about $10. when on sale.
I was told that it didn't work that great by someone that might not know the difference. ???
09-28-2011, 10:58 AM #11
We have a Speedaire compressor head we bought from Grainger. It's used for vacuum in a clean room, and runs all day long, 5 days a week. I believe that the compressor is run in reverse, but I'm not sure.
09-28-2011, 11:00 AM #12
Most vac pumps I have seen look just like a roots type supercharger, which they use on diesels maybe ? Any oil leaks will be in the discharge ? They have one at work they use on a router and it leaks some oil, makes quite a smoke cloud when they run it.
And an air compressor does not "care" which direction it runs as far as pumping ? It is like a 2 cycle engine in that respect, the reed valves work either direction, the engine has spark timing issues in reverse, but it will run.
09-28-2011, 12:43 PM #13
Thanks for all the replies...guess it's time to take a drive and grab the compressor and see what I can do.
I meant 28in Hg, not 28mm :-) And I guess I need closer to 29. Just need to be able to boil water at a slightly elevated temperature.
Building an investment casting machine, mostly for the helluvit, though I've got a couple waxes that I carved that I have been waiting for years to cast. I'm a little afraid of the centrifugal machine since I don't have a guard around the outside of it, and I'm out of space in the shop, so no guard will be added.
Getting sick of cleaning the shop and need a break. Not feeling the greatest today.
09-28-2011, 03:22 PM #14
09-28-2011, 03:32 PM #15
A "standard atmosphere" is 29.92 (give or take) " hg ABSOLUTE pressure. Gauge pressure, of course would be "zero".
09-28-2011, 08:39 PM #16
Thanks for all the advice. I actually dug up in to my attic tonight to find my old vacuum pump. It only moves 1 CFM, but that will work for now.
I flushed a little 30 weight oil through it and it pulls to a bit more than 29in hg. I'm sure if I flush it again with the correct vacuum oil it will do what I need right now.
Hell, people make these things with defunct refrigerator compressors.
As long as gold recovers, I'll probably stop at HGR and buy a nice pump when I drive out to the refiner with all of my computer recycling junk.
09-28-2011, 08:48 PM #17
09-28-2011, 09:42 PM #18
09-29-2011, 06:12 AM #19
"...I meant 28in Hg, not 28mm :-) And I guess I need closer to 29. Just need to be able to boil water at a slightly elevated temperature.>>>"
If this is so, what you need is a pressure cooker. No pump involved.
09-29-2011, 05:10 PM #20
Depending on the volumes involved, nothing wrong in using a compressor to evacuate the majority of the air and have your deeper draw/smaller volume vac pump in parallel.