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[*] posted on 14-8-2010 at 09:13
Vacuum from a fridge compressor


Looking at a vacuum pump with an occasional need for pressure too, I was looking at acquiring a monoblock fridge compressor cheaply.
Would it work? would I get some reduced pressure (for distillation not thermionics!). I can find real vacuum pumps from £100 to a lot more, I can find fridge pumps from £25 new. As ever once it works cost is all!
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entropy51
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[*] posted on 14-8-2010 at 09:24


Of course they will work! How well they work depends on what you want to use them for. I've been using one for over 40 years to pump down my vacuum dessicator. I've used it a few times for small vacuum distillations. Mine produces a vacuum better than a water aspirator, but not nearly so good as my Precision Scientific pump. I doubt you can run them continuously for a long time without overheating however.

There is a lot of useful vacuum information for amateurs on the Bell Jar website where that link is located.
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[*] posted on 14-8-2010 at 09:54


Quote: Originally posted by entropy51  
Of course they will work! How well they work depends on what you want to use them for. I've been using one for over 40 years to pump down my vacuum dessicator. I've used it a few times for small vacuum distillations. Mine produces a vacuum better than a water aspirator, but not nearly so good as my Precision Scientific pump. I doubt you can run them continuously for a long time without overheating however.

There is a lot of useful vacuum information for amateurs on the Bell Jar website where that link is located.


In a curious twist to current events, I (almost) entirely agree with entropy.

They are fine for that, and they do pull a harder vacuum than the theoretical maximum of a water jet aspirator.

I disagree, somewhat, on the duration side of things. I've had mine running for 12h straight with no problems. Stick a computer fan to it and you're even less likely to have issues.

It's funny you should just post this. Last week I decided to go on a super-undercover-expos'ay of fridge compressors, and made an hour long video. In which, I compare them to my rotary, a vacuum cleaner, how hard I suck (with my mouth [awaits joke from entropy]), theoretical aspirator pressures, saw the top off two of them and take them to bits and have a talk about cheif's ideas regarding liquefaction of gases with them. I also look at changing the pump oil to decrease the pressures they can achieve and discussed things like contamination, demonstrate how they can be used for sintered filtration, degassing and so on.

I tried to compile it (as it's a collection of lots of clips), but the result of the last attempt made me sound like a chipmunk, because the audio had doubled in speed and gone out of sync with the video.

I'm using ubuntu, and will happily admit that I have zero interest or skill in video editing. I just want the "push the button, get the fucker to work" option with regards to that.

I've been trying to compile them with avimerge, but that wasn't happy. Despite downloading multiple codec packs, I also can't get VLC (which will usually play anything, even betamax) to compress and encode the gigantic file down to something youtube will accept.

So, if anyone has more experience with that, PM me on suggestions and I'll get it uploaded as soon as it's done.

I'll have a look around and see if I can sort it in the mean time.

I made the video primarily due to chief's speculations, but also the number of times this question comes up on every chemistry, physics, electronics or engineering forum I visit.

I'll post again once it's up, with a link.

For now;

Vacuum filtration = Yes, absolutely super
Solvent 'ripping' = Yes (better than rotaries in a number of ways)
Degassing = Yes, to some extent
Drying = Majority, fine. One or two minor issues (not a problem 99% of the time)
Fractional vacuum distillation = Not really to Not at all
Starting up analytical gear = No
Molecular beam epitaxial two dimensional quantum layer deposition of superconductors = mmmmm.... nah

I have owned numerous high end rotary pumps, that would cost thousands new. I just finished refitting a BOC Edwards, which I'll post some photos up of. I still choose the fridge pump over the rotary for some vacuum work, in the real world and with the rotary already sat on the bench, hooked up. I'm also from the UK, so what I'm saying may actually be even more understandable to you. If you can wait for the video, you might be able to decide what you're after with, hopefully (for the sake of my neighbour's angle grinder discs), a little more information in mind.

[Edited on 14-8-2010 by peach]




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[*] posted on 14-8-2010 at 10:35


Item number: 220652728724 While it lasts on ebay (aug 2010) -what do you think? There seem to be several
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[*] posted on 14-8-2010 at 15:44


I found peach's post quite useful. I have a couple of refrigerators in dubious condition and have to decide whether it's worth the effort to salvage and evaluate the vacuum pumps. Getting an overview of the tasks they can feasibly be used for in a lab helps a lot. If the extra detail / color in the posts is a problem for you to get through, I suggest http://www.ewrd.com/ewrd/index.asp.




[Edited on 15-8-2010 by Ramiel]
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[*] posted on 15-8-2010 at 03:45


I'm tired of this s***posting and forgot about moderation; flipped out and deleted half the posts in this thread.
I feel apprehensive because everyone here is more mature and knowledgable than me.
Blame it on a rush of blood to the head.

Are there concerns with contaminating the pump with solvents/corrosive material?




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[*] posted on 15-8-2010 at 04:41


Quote: Originally posted by Ramiel  
I'm tired of this s***posting and forgot about moderation; flipped out and deleted half the posts in this thread.
I feel apprehensive because everyone here is more mature and knowledgable than me.
Blame it on a rush of blood to the head.

Are there concerns with contaminating the pump with solvents/corrosive material?


I discuss that a fair bit in the video.

Whilst I still attempt, with my very beginner skills, to piece it together and compress it, I'll try to mention them in short.

The pumps will cease to function provided they take a severe battering from the solvent.

By that, I'll quantify it, I have run 100ml plus of DCM through them without even bothering to condense it.

There are 'tricks' to avoid them jamming. Firstly, as with a rotary, don't finish the work then switch it off. By the time you're done, the pump will probably be hot coffee temperature anyway, just leave it running for a few hours (using next to no electricity) and the solvent will tend to boil off on it's own due to the heat. DCM boils at 47C for example, and the pump will be above that after an hour or two of running; one benefit to not using a fan.

Also, when I cut mine open, I couldn't actually find any rubber seals. The piston doesn't have one. Neither does there appear to be a bearing that can seize. What I think happens when they 'die', is that the oil cushion around the piston has been rinsed out. So there's too much starting friction for the motor to flick it on again. If you made a hole in the top that you could reseal with a bung, you could probably fix that (if it happens) by sticking a screwdriver in and flicking the cam shaft, then leaving it running for a while (you can easily reach it from a hole in the top). A bigger starting cap might even overcome that problem.

I have managed to catch ethyl acetate on one of these with a coil condenser running at room temperature. If you dry ice trap it (even with a DIY testube / jar method), there won't be any problems. Your freezer will do down to around -15 to -20C without the thermostat knocked out (then it'll do -30 or so according to chief). Meaning, you may not even need the dry ice if you put the trap in the freezer.

Their pressure of 15 - 35mBar is also good with regards to this. People always seem to go straight for a rotary and stick the solvent directly on it. The liquefaction temperature rockets down to near or below zero, meaning it needs dry ice trapping to stop the solvent.

But where's the logic there? Is the question I find myself often asking. If your reaction and product isn't incredibly heat sensitive, why does the solvent need to come off at 0.1mBar and dry ice temperatures?

At around 100mBar, most solvents will be starting to go at room temperature. 15 - 35mBar, almost all of the lighter ones will, or slightly above RT. And a fridge pump will move them fast at those pressures; since the pump rate is effectively zero when condensing the gas anyway (meaning CFM means nothing at that point). So, why spend £2.5k on a new BOC and then another hundred on the trap, then deal with the ice issues (which are big for people in the UK) to make things harder on yourself when you don't need to?

This why a lot of lab vacuum gauges read in the 100mBar range.

There are other benefits to them that I'll mention in the video, once it's compiled (hopefully the magic fairy will do it for me if I just ignore it, but it's unlikely). I may have to boot... windows.

Essentially, if you can't put it through a rotary (which I've taken apart as well), you can't put it through a fridge pump. But, I think fridge pumps are actually a little more ruggedly built (zero rubber seals, just the oil cushion).

[Edited on 15-8-2010 by peach]




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[*] posted on 15-8-2010 at 12:03


Regulating pressure by bleeding air into the vacuum side would likely reduce build up of solvents in the pump oil.

And also help cool the motor.




[Edited on 8-16-2010 by Eclectic]
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[*] posted on 15-8-2010 at 15:05


Yep, a ballast valve in effect.

Rather than attempting to ballast them out, find the boiling point of the solvent, get a nomograph handy and set the atmospheric BP. Set the new boiling point to around 10 - 20C. The lowest most people can cool their condenser is around there (0-10C), and you need around 10-20C difference in temperature to condense effectively, depending on the condenser style and the specific heat capacity of the material going through.

Ideally, you then want your vacuum set around the level the nomograph suggests.

E.g. Water ATM BP = 100C. My condenser is running at 23C usually. The nomograph says the lowest the pressure can be, for liquefaction to still occur (theoretically) is now 69mBar (with a theoretically perfect aspirator managing around 37.5mBar IIRC). Going any lower, without changing the condenser setup, is pointless (loosing solvent to waste, damaging the pump).

Why bother applying the vacuum in the first place? 69mBar (whilst seeming terrible) is still 14-15 times lower than atmospheric pressure and the solvent is still being remove significantly quicker and cooler than before, for free.

Also, very easy to do, takes seconds with a thermometer in the condenser's water loop and a ruler on the nomograph.

Got the video compiled, discovered youtube no longer allow them over 15 minutes without approval and their adverts on them. So, change of site in order. Should be sorted by tomorrow.

VOTE NOMOGRAPHS FOR BEST BITS OF PAPER OF THE YEAR!

John

[Edited on 15-8-2010 by peach]




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[*] posted on 15-8-2010 at 16:14


Quote: Originally posted by peach  
Yep, a ballast valve in effect.
Not really. Ballast has a specific meaning. It ain't at the pump inlet.

Quote: Originally posted by peach  
Got the video compiled, discovered youtube no longer allow them over 15 minutes without approval and their adverts on them.
Apparently even the vulgar Youtube does not embrace verbosity.

Signal to noise, dude.
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[*] posted on 19-8-2010 at 16:43


Quote: Originally posted by entropy51  
Not really. Ballast has a specific meaning. It ain't at the pump inlet.


Quote:

Means through which air or another non-condensing gas is admitted into a vacuum pump are referred to as gas ballast.


Pfeiffer vacuum, 2.1.6 Gas ballast

Quote: Originally posted by entropy51  

Signal to noise, dude.


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entropy51
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I've reported that post in addition to the last one. If you'd left it at talking about the ballast, that would have been okay. Telling me I'm wrong, incorrectly, and then making comments that are amusingly ironic coming from you with regards to my verbosity are the reasoning behind reporting this one as well.

With regards to what I have to say, I will report every reply you make containing this pointless, personally directed rubbish from now on; as per the example of cleaning glassware, where you've clearly put effort into copy and pasting my replies between threads for the sake of absolutely nothing other than causing trouble.

[Edited on 20-8-2010 by peach]




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[*] posted on 19-8-2010 at 17:04


What part of "gas ballast is not at the pump inlet" is giving you trouble? Anyone who's ever rebuilt a pump knows better, son.

Not one of my posts includes a silly pic of gloves lying in the grass, now does it?

Is that signal, or is it noise??:P
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[*] posted on 19-8-2010 at 18:58


Ballast, as per the people who design these for physics labs, refers to gas that's being purposefully added, doesn't condense and that flows through the pump. There is no mention of the position it enters the system.

Dual stages port it into the second stage.

Single stages have no second stage to port it into, but do feature ballasts.

A fridge compressor is a single stage pump, there is no place to ballast them but at or before the inlet.

I have owned four Edwards & Alcatel vacuum pumps, taken them to bits, put them back together and had them running better than before.

I posted a photo of the gloves I use in response to someone mentioning gloves.

Reported.




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[*] posted on 20-8-2010 at 11:09


Thanks all for your input. The local fridge breaker found me a working tested pump and fitted the lead and plug for the price of a coffee! If it ever dies, i'll just go back for another one.

Do they need periodic oil top up?
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[*] posted on 20-8-2010 at 12:22


Not really.

They're not like rotaries in that sense. A modern rotary has an oil pump built into the rotor that sprays oil back into the chambers, which then blows out the exhaust as a mist at atmospheric. If you tip a rotary on it's side, it'll dump all of the oil out almost straight away.

Fridge pumps don't do that. The oil on the piston is just a thin cushion, trapped by a groove ground into the circumference of the piston. There are also vane valves on the input and exit of the chamber, so I can turn a fridge pump upside down and nothing will come out. Some of them will drip if left on their sides for a long time (as it creeps round the valves). Sometimes you'll see a drip or two at the exhaust if they're left running for 6-12h as well, but there's around 150 - 200ml of oil in them.

Neither a fridge pump or lab rotary should produce much oil mist when running under vacuum, it's when they're open to atmosphere that the mist blows out. The gas going through is literally blowing the oil out of a rotary, quickly. With no flow rate (under vacuum), there's no gas to blow through, so no mist. A rotary will start misting again if the ballast is opened, as gas is flowing through.

There is a very big difference in misting rates between a fridge pump and a rotary. The rotary will make the room look like a speakeasy in a few minutes. A fridge pump can be running for 12h and you won't see anything.

Fridge pumps, the only people who need to worry about oil escape are those using them to power an airbrush or spray gun, where minute traces of oil or moisture in the air will ruin the paint's finish. They also have it worse off, because compressing the gas (rather than pulling a vacuum) tends to make those things drop out of it, as liquid; creating spatter at the nozzles.

The only time you'll have serious issues with a fridge pump and oil is if you switch it off whilst it's connected to something under vacuum, which will suck the oil out of the pump and into whatever it's connected to; over about a minute or so. Undo the vacuum first (at the glass), then switch it off. That also helps prevent little bits of shit in the vacuum hose spraying back into the flask, which will happen if you disconnect the pump away from the glass; this is partly why vacuum adapters for glassware have the tap on the glass and don't have you disconnect it three foot away.

Edit: I have been wondering if one could easily modify the internals of one of these pumps for other jobs. The piston speed is considerably slower than a rotaries blades, meaning a thin vacuum grease may work as a better seal (doesn't for anything moving quickly). Alternatively, perhaps they could be used for some bastardized approach to chromatography, with them being able to develop 500psi+.

[Edited on 20-8-2010 by peach]




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[*] posted on 20-8-2010 at 13:22


I have been wondering about building a 'standby' vacuum system using a small water tank, a vacuum pressure switch, and a contactor. It would make simply opening up the vacuum tap real easy instead of stopping work to turn on/off a pump. Is it reasonable to pump down a small water storage tank to whatever pressures are achievable by a fridge compressor?
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[*] posted on 20-8-2010 at 13:39


[deleted]This reply is incorrect and genuinely 'noise', so I'm deleting it. Entropy is correct about pressure vessels potentially imploding under vacuum.

[Edited on 21-8-2010 by peach]




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[*] posted on 20-8-2010 at 14:19


Quote: Originally posted by peach  
Even if you connect a turbo molecular or ion pump to the water storage tank, the maximum pressure it'll ever feel on it's walls is about 14psi, since that's all the pressure the atmosphere can exert is. If it can withstand that (e.g. the 3bars / 30psi the water mains usually runs at), it can withstand a theoretically perfect vacuum.
As JohnWW has pointed out in another thread, the physics of internal and external pressure forces on a tank are different. A tank designed for even a high internal pressure may often collapse if internally evacuated. I suspect a Google would disgorge this fact; it is well known to engineers. Trust me, I once saw such a tank collapse under vacuum. It was quite impressive.
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[*] posted on 20-8-2010 at 14:45


[deleted]This reply is incorrect and genuinely 'noise', so I'm deleting it. Entropy is correct about pressure vessels potentially imploding under vacuum.

[Edited on 21-8-2010 by peach]




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[*] posted on 20-8-2010 at 14:56


A tank designed to withstand internal pressure can have extremely poor resistance to external pressure. Look at a two liter soda bottle. It takes roughly 5-6 bar internal pressure to rupture such a vessel. It can't withstand a tenth of that in external pressure without collapsing. The situations are simply not symmetrical, as the resistance to internal pressure depends on material tensile strength while the resistance to external pressure depends on stiffness and/or compressive strength (broadly speaking).
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[*] posted on 20-8-2010 at 14:59


You're right.



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[*] posted on 20-8-2010 at 15:23


Quote: Originally posted by entropy51  
Quote: Originally posted by peach  
You're right.
Age and experience trumps youthful enthusiasm every time. ;)


Modesty trumps arrogance, like when discussing a ballast. ;)




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[*] posted on 20-8-2010 at 15:27


Quote: Originally posted by peach  
Can you supply a description of this tank; material, form, use, dimensions, rated pressure, when it was last checked and so on.
Years ago, a local brew pub ran an ad with a collapsed tank, one of the big shiny all-stainless brew tanks, together with their hapless employee who had mistakenly closed a steam valve at the wrong time. I forget how they spun it, but the photo was excellent.
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[*] posted on 20-8-2010 at 16:17


An air compressor tank such as http://www.princessauto.com/tools/compressor-componentry/tan...

My likely use would be to hold a small amount of vacuum so I could start filtering right away without stopping to turn on the pump (or off). I'm used to house-vac for simple things like that, its more of an item for convenience than anything.
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[*] posted on 20-8-2010 at 16:39


Even a fridge compressor will start a filtration within 10-30s, unless it's a 5l flask and the filter is clogged before starting.

It takes between 30 - 60 seconds for a fridge pump to reach 35 - 10mBar (atmospheric being 1013.5 and an aspirator working around 37)

If clogging is an issue, trying warming the stem and filter disc (sinter) with a heat gun. Then try stirring (doesn't work so well with papers).

If the particle size will clog the pores, the vacuum develops in the flask, pushing the BP down, which clogs the filter yet more with solid solvent, snowballing the problem.

[Edited on 21-8-2010 by peach]




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