Running an air compressor as a vacuum source?

[Mesa]

Hi, I found these forums through a completely unrelated google search, however I figured it would probably be the best place to get decent answers to an idea I've had for a while. Apologies if this post is a bit long winded, but given the application I want to use it for is probably unfamiliar to most of you, I want to make as clear of a picture as possible.

I do a fair bit of amateur/home chemistry, and as such, have been looking around for a decent way to rig up a vacuum apparatus for distillations(usually pretty standard, yet extremely expensive lab vac pumps are used.)
I own a 2.5hp 40l air compressor that is rarely used these days. The intake filter is threaded, and recently I came across a fitting for a drain cleaner which was the exact same sizing, and terminated in a barb that also happened to be the same size as my vacuum adapter(http://www.erowid.org/archive/rhodium/chemistry/equipment/pictures/zubrick098.gif
first google images result that gives an accurate depiction, far right of the pic.)

Given this, is there any particular reason why it would be unwise to use it as a vacuum pump? I can use an inline filter to prevent any nasties from getting into the compressor but I'm not sure if there are any other potential hazards here. Any input would be appreciated.

 

[A_Pmech]

While your compressor might be usable as a vacuum pump, it would not be desirable.

Gast dry rotary vane vacuum pumps in the 1-4 CFM range are sub $150 New Takeoff and NOS. They will evacuate to 28" Hg more or less.

http://www.ebay.com/itm/Gast-Vacuum.../301008689789?pt=BI_Pumps&hash=item4615841a7d

 

[slo_nlo]

How much vacuum are you needing to pull?

I wouldn't personally restrict the intake on the compressor like that. Chances are it won't like it and will cause you problems. It's not designed that way.

However, you aren't out of luck. I've seen people use compressors to pull a vacuum for doing vacuum bagging projects with carbon fiber, and stuff like that. What they do is use an ejector. Basically, a venturi with a third ****** located at the point of lowest pressure, so when you push compressed air through the venturi, it will cause the 3rd port to pull a vacuum on whatever is connected to it.

The picture attached is a good example. Where it says "Motive fluid" is where you would push the compress air in. "Outlet" is where the compressed air exits (just to atmosphere). You would connect the port labelled "Inlet gas" to whatever it is you are trying to pull a vacuum on.

You would need to run a vacuum gauge to see how much vacuum you're able to pull. It can be varied by changing the flow rate of the compressed air, the pressure of the compressed air, or making physical changes to the ejector itself (the geometry of the venturi, etc.).

Might not be what you're after, or may not meet your requirements, but it's better than restricting the intake of your compressor and stressing parts that normally aren't used to fighting for air that is usually at ambient pressure.

 

[Chuck122]

http://www.scilabware.com/Liquid-Handling/Pumps/Water-jet-pump/p-82-83-356/
This is what you need. It is connected to your water supply and running water creates the vacuum with ventury effect

 

[slo_nlo]

Mind if I ask what it is you're distilling under vacuum? Are you trying to keep a boiling point below a specific temperature, or just lower the heat input that is required?

 

[doan]

I bought one of these for $12 at Harbor Freight to pull down an AC system.

Not sure how much vacuum you need, but this might be a cheap solution.

 

[Fixnair]

An air compressor makes an excellent vacuum pump. But you need a filter on the inlet. While it will not pull down into the micron range it will go down to 29" hg. The air powered vacuum device will not go down into the micron range either and requires a great deal of air to operate.
Nothing bad will happen to your compressor though other than a little oil coming up past the rings. You can minimize this by connecting the suction line to the crankcase to equalize the pressure above & below the piston rings. You will need a sealed crankcase for this to work. Sometimes you may even need to reverse the shaft seal on the crankshaft so it seals the air out.
Go ahead give it a try. How much vacuum do you need?

 

[larry_g]

If you have air then you could use a 'venturi vacuum generator' One example is here http://www.vaccon.com/hvp-home.aspx Though there are many players in the field.

lg
no neat sig line

 

[Mesa]

Thanks for the posts so far, while I'm still without a definitive answer, the posts have given me quite a few new angles to think about.

I'll try to answer the pertinent points without turning this into an essay:

How much vacuum are you needing to pull?

Non-specific amounts, I have quite a few projects that have been put on hold due to requiring vacuum distillation. Some of them are organic compounds that are likely to decompose if I try to distill them at atmospheric pressure, some require temperatures far higher than I can generate with my current equiptment(Vacuum = lower temp requirements.)

I wouldn't personally restrict the intake on the compressor like that. Chances are it won't like it and will cause you problems. It's not designed that way.

I had thought of this as well, however when I first removed the air filter, I realized the intake hole behind it wasn't that much bigger than the diameter of the tubing I was planning to use anyway, I don't think there will be that much more of a bottleneck, however I'm not exactly confident in that assessment.

http://www.scilabware.com/Liquid-Han...p/p-82-83-356/
This is what you need. It is connected to your water supply and running water creates the vacuum with ventury effect

Yeah, I have looked into water aspirators such as this for a vacuum source previously. However it requires a constant flow of water in fairly high volumes to be effective. Running it from the tap will work, but given the issues we have over here with water shortages(I live in Australia) I don't think it would be feasable to do so. The water pump's I've seen that would be suitable(fairly high head and LPH/GPH specs,) cost somewhere in the vicinity of $200. I can afford to get one but I want to look into alternatives before forking that much out.

Mind if I ask what it is you're distilling under vacuum? Are you trying to keep a boiling point below a specific temperature, or just lower the heat input that is required?

The latter. Vacuum distillations in most labs have a pretty varied range of pressure over the course of the procedure. It's difficult to keep the vacuum constant when you are producing a gas/boiling a liquid. I would still want to know what I am pulling, but the accuracy of the gauge isn't of utmost importance.

An air compressor makes an excellent vacuum pump. But you need a filter on the inlet.

When I initially planned this out, I decided to use a dreschell bottle with either activated carbon, or a liquid filter of some sort, in order to trap any gases/particles that could damage the compressor. The air going in should be cleaner than when its being operated with the standard paper air filter attached.

Nothing bad will happen to your compressor though other than a little oil coming up past the rings. You can minimize this by connecting the suction line to the crankcase to equalize the pressure above & below the piston rings. You will need a sealed crankcase for this to work. Sometimes you may even need to reverse the shaft seal on the crankshaft so it seals the air out.
Go ahead give it a try. How much vacuum do you need?

Unfortunately what you are describing here is past the limits of my understanding of air compressors. I will try to get in contact with a friend to get a better idea of what you mean, it does sound like a viable option though, thanks!

 

[racingtadpole]

Bio diesel?

You could use that water powered venturi in a closed circuit with a mains powered pump to provide the pressure. The 50 or so litres you'd need wont make dent in the water reserves.

 

[egnorant]

Old refrigerator motor works well. And it is quiet too!

Bruce

 

[Warrenator]

Real lab vacuum pumps are usually small, fractional horsepower motors hooked up to a pump with a trap and filter. The oil gets contaminated anyway. The refrigerated traps are best but even those let the oil get very nasty. The last one I rebuilt had all carbon parts inside I imagine because metal ones would corrode away. I would go ahead and get a ddedicated real vacuum pump like one used for evacuating air air conditioner systems and plan on changing the oil after any really corrosive distillations. An air air compressor would work but you would kill it in short order. The venturi style vacuum generators are very inefficient but if you used a pump and recirculated the water that that might work ok. Plan on only getting 27 inches of vacuum.

 

[Mesa]

There are somewhat commonplace techniques used in undergrad/home/small business labs to scrub the air of any gases or vapor which may damage a makeshift pump. They are built in to the 'proper' vac pumps designed for chem work, however for those who plan to set up a lab out of their own pocket(or on a limited budget), generally they will end up modifying an existing appliance rather than buying the pumps sold by specialist scientific suppliers(statement is based on my own experience with the community over at the sciencemadness.com forums. Perhaps its just in that community specifically that people are doing these kinds of mods rather than buying a proper unit?)
I could be completely misunderstanding your post though. I am happy to admit my understanding of the mechanical workings of air compressors/vacuum pumps is extremely limited. I'm not quite sure what is meant by a previous poster mentioning 'the oil coming up past the rings,' and similarly at the points where your post mentions possible oil contaminations.
My working assumption is that it means oil from the pump may be drawn up into the inlet tubing. As I've pointed out, any air/gas/vapor exiting the distillation setup will be scrubbed of anything that may harm the compressor.

@racingtadpole:
It wont be used for anything related to biodiesel. I've briefly looked in to home setups for biodiesel, but given my bike runs on unleaded, and is quite fuel efficient, I decided against it. I've made small qty's before, but never planned to purify through distillation as it worked well enough as is. The only other part of the process where distillation may be used is in the recovery of excess methanol from the final product. Given methanol boils at around 65*c, standard distillation would work quite well at atmospheric pressure.

@egnorant: Yeah, that is the vacuum source that seems to be most popular amongst the non-commercial chemists. I ask around friends/family every now and then to see if anyone has a broken/old fridge that they don't want anymore, but so far, no luck.

 

[monkeyspanners]

Theres lots of cheap vacuum pumps on ebay, either new chinese one marketed towards the hvac trade or secondhand models.

 

[Mesa]

To re-iterate, I do realise I could simply purchase the equipment from any one of dozens of different shops. However if there are no problems with the method I've put forth, it would cost a total of $5, substantially less than the $150+ that it would cost for a pump from ebay(including shipping. Australia is pretty expensive to have items shipped to, and there are no local resellers for anything approaching that price.)

 

[A_Pmech]

You have to balance that with the possibility of destroying your air compressor. Without a thorough knowledge of the machine, it is impossible to say whether it would make a good vacuum pump or not. The Quincy QR-25's make good vacuum pumps, but the average homeowner oilless would probably have a high-speed come apart. On an oil lubricated compressor, extended run times at high vacuum levels without balancing the case pressure would probably cause it to hydro lock.

In addition to the possibility of a mechanical failure, there is the possibility of a scrubber failure allowing corrosive gasses to enter the pump.

All in all, if you're serious about chemistry, I would buy or build an appropriate vacuum pump. As another poster stated, old fridge and freezer compressors make excellent low volume vacuum pumps in a pinch.

To re-iterate, I do realise I could simply purchase the equipment from any one of dozens of different shops. However if there are no problems with the method I've put forth, it would cost a total of $5, substantially less than the $150+ that it would cost for a pump from ebay(including shipping. Australia is pretty expensive to have items shipped to, and there are no local resellers for anything approaching that price.)