Sciencemadness Discussion Board

Vacuum from a fridge compressor

Contrabasso - 14-8-2010 at 09:13

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!

entropy51 - 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.

peach - 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]

Contrabasso - 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

bbartlog - 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]

Ramiel - 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?

peach - 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]

Eclectic - 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]

peach - 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]

entropy51 - 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.

peach - 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.


peach
Posts: 340
Registered: 14-11-2008

----------------------------------

entropy51
Posts: 1081
Registered: 30-5-2009

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]

entropy51 - 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

peach - 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.

Contrabasso - 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?

peach - 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]

aonomus - 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?

peach - 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]

entropy51 - 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.

peach - 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]

bbartlog - 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).

peach - 20-8-2010 at 14:59

You're right.

peach - 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. ;)

watson.fawkes - 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.

aonomus - 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.

peach - 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]

dann2 - 24-8-2010 at 04:42

Hello,

This I believe,

In the UK (and Europe?) most if not all domestic refrigerator pumps are piston pumps unlike the USA
where they are all rotary pumps (stater + impellor).
The USA pumps do a better job at pulling a vacuum, thats not to say that the piston pumps will not pull a vacuum. The vacuum will not be as good.

Dann2

[Edited on 24-8-2010 by dann2]

peach - 24-8-2010 at 05:04

Have you cracked them open to have a check of that?

From the schematics of pumps I'd seen, I'd been expecting to see the piston vibrating inside a solenoid, yet it was on the end of a camshaft stuck to the top of the rotor.

Quite a lot of them are coming from the same people as well, like HSH, who all seem to use the camshaft / piston layout.

I would be interested to hear some numbers on them there'z USA pumps, as I have checked multiple UK ones with both a mechanical and digital filament gauge, so I'm very sure of the numbers the ones around here can do.

Contrabasso - 24-8-2010 at 10:17

Whatever the nature of the pump I bought from the fridge breakers, I'm very satisfied with the vacuum it pulls for reduced pressure distillation.

peach - 24-8-2010 at 21:27

Glad you've got one you like contra. Especially for £25, versus £2.5k

Some of them do this odd trick, particularly once they've been left on for a few hours under vacuum, where the pressure goes into a cycle of floating up and down a bit every few minutes. The first few times you try fractional distillation with it, keep an eye on the thermometer and look for any signs of the solution bumping, or trying to (e.g. bubbles disappear a little, then return with vigor).

aonomus - 26-8-2010 at 16:31

I just got ahold of a LG QA series rotary compressor, in the only online documentation I could find, it detailed a few issues.

1. Duty cycle: recommended 6 shutdowns/hour, each lasting 3 minutes/shutdown
2. Exposure to atmosphere: minimize it, the manual sounded like its lubricated with n-BuLi, and the universe would implode if you didn't keep it under N2.

Also:
- Has anyone found it necessary to add a run capacitor to their compressors? I would suspect the load is lesser on a vacuum pump vs a compressor pump
- What about duty cycle? Without cooling gas (ie: refrigerant), won't the motor windings get much hotter? I tested the pump for a few minutes, outer casing hit 80degC.
- I managed to achieve <29" Hg vacuum with poor sealing on the vacuum fittings

[Edited on 27-8-2010 by aonomus]

[Edited on 27-8-2010 by aonomus]

peach - 26-8-2010 at 21:00

Awwww gawd, not "hg.... [opens google]

Right, that sounds about right. I've usually seen them around that, 30mBar. One of them I could never get to below 100mBar, but the others are closer to 30.

When I was taking them to bit recently, I tried emptying the oil out and squirting some BOC Edward Ultragrade 19 in instead, and managed a constant 15mBar, but the pump was running warmer with the thicker oil.

I supposed one could try even thicker oil and simply fan cooling the pump. Given how well they're sealed (welded), provided care was taken around the electrics, the entire thing could go in a water bath. Never tried it however, and seems like quite a lot of effort for no real difference in it's performance with regards to chemistry. It'll never get low enough for analytical gear, and it's not a massive change in temperatures for distillation etc.

At their normal 30 odd mBar, that's 97% of the atmosphere left. Changing the oil squeezed another 2% out. About 1/50th better.

About them getting hot, I doubt they're designed to be left hot for so long, but they can manage 12h runs without burning out (with their thermal cut off cut out), and keep doing it. I've not had one go from overheating. If you live somewhere where it's 30C everyday, lucky you, but the pump may go quicker. 20C is about the warmest it gets here most of the time.

But, if a lot of volatile solvent goes through, a hundred ml's or so of something like DCM, it seems to rinse the oil cushion off the piston and it temporarily seizes.

Some possible solutions;

a.) Use some ice in the water for the condenser to stop the solvent first
b.) Leave the pump running for a while, open to the atmosphere, when you're finished, to replace the oil cushion
c.) Leave it upside down for a while to encourage the oil back into the piston
d.) Try a bigger starting capacitor, to overcome the higher resistance of the oil less piston (that may burn out the windings, but it's also likely the windings are a lot heavier than they need to be, for duration, and the capacitor already on there is the smallest they can get away with for cost reasons)
e.) Cut a hole in the top and manually flick the camshaft round to recoat the piston

The only time I've had one die from solvent abuse was after being incredibly lazy and removing a lot of DCM without a condenser, then immediately turning the pump off.

aonomus - 26-8-2010 at 21:38

The compressor is a taller cylinder, not a pancake type. Rotary vane, for R22, new old stock. The 'manual' states a cutout at around 120degC, and I'm sure that it would reach that point without much trouble. I won't be doing too many reduced pressure distillations, or pulling too much solvent through the pump. My main concern would be the temperature, and the flash point of solvents (hot surface in contact with solvent vapor at the exit port means bad things.

I'm thinking of making a bunch of aluminum plates and clamping them on as makeshift heatsinks, anything to help wick away more heat from the casing including fan-cooling. I mainly wanted more confirmation that running it so hot wouldn't cause immediate seizing or failure. I did have the pump trip the power bar and had to move a little bit upstream to get power straight from the outlet. Batteries on my clamp meter were dead so I couldn't measure the current usage either.

Finally, it would appear that the start capacitor is internal and integral, though 3 terminals exist for start, run, and common, and no run capacitor exists. I'll have to pick one up next time I'm at the surplus place, but the motor runs fine without it, and maybe find a better vacuum gauge.

I'll probably bolt the compressor down onto a piece of plywood with a metal sheet to help reflect heat away from the wood, and add on a cooling fan, etc.

Any solutions for a makeshift oil-coalescing trap? I was originally thinking one made of steel wool or another high-surface area material for the hot mist to condense and drip down onto, though that might never allow droplets to merge and settle for recovery.

peach - 26-8-2010 at 23:47

If it cuts out at 120C, it's going to be fine.

Flashpoint is the point at which a solvent forms a flammable mixture with the air and will ignite when a spark or flame is around. Autoignition is the point at which it'll spontaneously burst into flames when mixed with air.

Don't over complicate and end up out of time, like a fly hit with the wasp killer, start by silicon / epoxy fixing a big, cheap computer fan to the top.

It tripped the breaker? If it was going to trip it, I'd expect it to go when you flicked it on. That may be due to a lack of starting capacitor or it may just need running in to lower the starting friction. If it went half an hour after switching it on, that would be a bit odd to me.

If it's like the ones here in Europe, inside the black shell is full of oil, gases under odd pressures and gets hot. The capacitors are always on the outside, or not there.

Oil leakage won't be much of an issue, if one at all. First of all, if it still as it on there, leave the looped bit of copper on the outlet side. The air coming out is barely warm on the ones I have. The steel wool idea should work. For the ones I've used, I've seen about one or two drops come out over 12h, and no visible mist whatsoever. With your's being a rotary, I'm not sure how those are designed inside for fridges. If it has an oil pump spraying back into the rotor, mist may be a significant issue (report back on that, I'd like to know).

[Edited on 27-8-2010 by peach]

aonomus - 27-8-2010 at 02:49

Sorry for the 2-3am vaugeness/verbal ambiguity. The compressor tripped the thermal breaker on the *power bar*, not the upstream 15A panel breaker. I'm a little unsure as to whether the thermal cutout is integral with the unit, or if its a external clamp on device. I have some big microwave oven fans I can mount to really put some air onto this thing (at the cost of noise), we shall see just how effective that is (I'm going to tape on a thermocouple and take readings without and with the fan(s)).

As for the oil mist, when the pump shell is cold, no mist emerges, but once it warms up, like a real rotary pump, it really starts spewing the oil as a mist. The pump wouldn't get that hot in normal operation anyway, so the real question is whether or not I can capture the oil loss on the output side and occasionally return it to the input. As far as I know from the drawings I've seen, LG has an inverted rotary design (rotary on bottom, motor above), meaning the rotary compartment is submerged under oil at all times, and any oil that is lost to the output tubing draws in an equal amount from the internal reservoir.

I really want to make this work as there is a whole pile of these compressors at the surplus store, stringing together 1+2 for a 2 stage system would be an interesting experiment to see what ultimate vacuum I can pull. They are for R22, so they will likely never see use in refrigeration ever again.

watson.fawkes - 27-8-2010 at 05:17

Quote: Originally posted by entropy51  
There is a lot of useful vacuum information for amateurs on the Bell Jar website where that link is located.
Also on that site is an article called Vacuum on the Cheap, which talks about the switch, by and large, from rotary to piston pumps for refrigeration. Pretty much everything in that short article addresses something that's been raised here.

Changing focus ...
Quote: Originally posted by peach  
At their normal 30 odd mBar, that's 97% of the atmosphere left. Changing the oil squeezed another 2% out. About 1/50th better.
That's right if you're focused on the pressure against evaporation, as is the case for solvent stripping. If you are trying to evacuate a chamber, it's three times better. I mention this because the articles in The Bell Jar are focused around evacuation, not lowering ambient pressure.

peach - 27-8-2010 at 07:26

Quote: Originally posted by watson.fawkes  
That's right if you're focused on the pressure against evaporation, as is the case for solvent stripping. If you are trying to evacuate a chamber, it's three times better. I mention this because the articles in The Bell Jar are focused around evacuation, not lowering ambient pressure.


Because the evaporation in a flask is only concerned about one dimension, up?

Interesting page, I think I may have seen it before. I have also noticed a lower pressure with two of the piston kind in series, somewhere around 15 to 10mBar.

[Edited on 27-8-2010 by peach]

watson.fawkes - 27-8-2010 at 10:14

Quote: Originally posted by watson.fawkes  
That's right if you're focused on the pressure against evaporation, as is the case for solvent stripping. If you are trying to evacuate a chamber, it's three times better. I mention this because the articles in The Bell Jar are focused around evacuation, not lowering ambient pressure.
Quote: Originally posted by peach  
Because the evaporation in a flask is only concerned about one dimension, up?
More or less, that's right. In particular, it's the interaction at the liquid (solvent) -- gas (atmosphere) boundary. That's a two-dimensional boundary, so the orthogonal complement is one dimension. You can call it "up" if you want. The important thing is that the pressure near the boundary changes the rate at which solvent evaporates. The pressure near the boundary doesn't change the partial pressure of the solvent at the boundary. What it does change is the rate at which gas molecule near the boundary diffuse away from the boundary, which enables new liquid molecules to hop the boundary and go from liquid to gas phase.

aonomus - 27-8-2010 at 11:47

I found this interesting video on youtube with a high speed camera shot of the vane/rotor in a rotary compressor. This might explain why we see fluctuations in pressure that make it unusable for vacuum distillations.

Edit: the link would sure be useful... derp.

http://www.youtube.com/watch?v=w_hpMZY7HbM

[Edited on 27-8-2010 by aonomus]

peach - 27-8-2010 at 16:14

Quote: Originally posted by watson.fawkes  
More or less, that's right. In particular, it's the interaction at the liquid (solvent) -- gas (atmosphere) boundary. That's a two-dimensional boundary, so the orthogonal complement is one dimension. You can call it "up" if you want. The important thing is that the pressure near the boundary changes the rate at which solvent evaporates. The pressure near the boundary doesn't change the partial pressure of the solvent at the boundary. What it does change is the rate at which gas molecule near the boundary diffuse away from the boundary, which enables new liquid molecules to hop the boundary and go from liquid to gas phase.


You're a fountain of knowledge Watson, indubitably.

Quote: Originally posted by aonomus  
I found this interesting video on youtube with a high speed camera shot of the vane/rotor in a rotary compressor. This might explain why we see fluctuations in pressure that make it unusable for vacuum distillations.

Edit: the link would sure be useful... derp.

http://www.youtube.com/watch?v=w_hpMZY7HbM

[Edited on 27-8-2010 by aonomus]


That's a neat video.

I've also seen pressure fluctuations on the piston kind. I don't know how they float around on those rotary fridge pumps but, on the piston kind, it's a very slow oscillation, minutes long; even though the piston is moving a lot quicker than that.

It would help if my vacuum gauge would work as a datalogger. It's brand new but, as normal, I had it in bits to look at it yesterday. If I can find a suitable point to remove the readings from, I may be able to get it datalogging and could plot graphs of the pressure. My dream is to have a cheap datalogger for the still head, maybe another thermometer, pH, pressure and perhaps flow rates, all in one box and one application that will work on old computers for the lab.

These fridge rotaries are essentially backwards to the way I've seen dual stage rotaries designed, where the vanes are usually inside the rotor and pushed out against the walls of the chamber. In BOC Edwards', there's squishy springs between the vanes. In Alcatel pumps (the one's with the orange heads), I've seen them using nothing other than the centrifugal force of the rotor spinning to flick them out.

He's got a video on there of pumping water through them. Wonder how long they last doing that and what pressures you can get.

[Edited on 28-8-2010 by peach]

aonomus - 27-8-2010 at 16:58

Wouldn't be too hard to set up a datalogger with a computer, it just wouldn't be intrinsically safe. Arduino + AD595 + K type thermocouple in the still head, and some pressure transducer by Omega by the vacuum port on the receiver adapter. Too bad the pressure transducer is a $200+ part.

densest - 27-8-2010 at 22:48

It's pretty easy to isolate transducers especially after signal conditioning. Analog Devices & Texas Instruments/Burr-Brown sell inexpensive isolation amplifiers with pretty good accuracy.

Arduinos can work well, though you still have to isolate the RS232 or USB or whatever.

Pressure transducers don't have to cost $200 if all you need is 0 to -1 atm with 3 digit precision. If you need ion gauges or such, they're more costly. A Dwyer 4-20 mA vacuum transducer costs $114 from Grainger, and that's retail. *waves hands* automobiles need vacuum sensors for the control computers. I'm sure they're as inexpensive (cheep cheep cheep) as possible!

If you need ion gauge sensitivity that's another matter.

peach - 28-8-2010 at 03:00

Isolation isn't hard at all, as densest says.

I'm currently talking to one of the manufacturers of a vacuum sensor about the price of them now. And another guy on sparkfun who's interested in building a little, cheap interface box for them.

My vacuum gauge has a hot filament in it, it's digital, displays about 9 different units, reads to 0.001mBar and cost £85, so there's a cheaper source than Edwards out there.

I've seen the Arduino's on sparkfun, but never had a go with one. I'll have to investigate.

watson.fawkes - 28-8-2010 at 05:27

Quote: Originally posted by densest  
It's pretty easy to isolate transducers especially after signal conditioning. Analog Devices & Texas Instruments/Burr-Brown sell inexpensive isolation amplifiers with pretty good accuracy.
These days, it's cheap to isolate after digitization. AD 595 to condition the thermocouple itself, a small AVR to digitize it, and then an optocoupler pair on the I2C bus.

aonomus - 28-8-2010 at 06:35

Did a little more searching about automotive vacuum sensors. Manifold absolute pressure (MAP) sensors are the ones to get. They give a analog voltage output proportional to the absolute pressure on the port, both above and below atmospheric. A ADC sensitive down to mV should be fine for using as a pressure meter.... thanks for the tip densest. I'll start looking for cheap MAP sensors.

Note: don't confuse MAP with MAF sensor, some cars use one or the other

densest - 28-8-2010 at 16:01

Hmmmm... From www.mouser.com, Freescale Semiconductor MPX2102AS is $7 (plus shipping). Output is analog, 40mV from vacuum to nominal 14.7PSI. An inexpensive zero-drift op amp will put that up to 0-2.5 or whatever the ADC wants. That particular one isn't available until Oct. 20, though. Other similar ones should work as well.

Auto sensors are fairly expensive new (super-wide temp range, etc.) but should be practically free from junk cars - once you know what they look like!




[Edited on 29-8-2010 by densest]

peach - 29-8-2010 at 01:17

Here's some more, new in the box...

I'm not too sure about how accurate these are going to be once they hit fridge pump levels and below.

I'm also absolutely sure I'm no smarty pants when it comes to modding cars, but the vacuum lines for turbos and things typically aren't like the thick ones found in labs; had a quick google but couldn't find a torr / mbar for the manifolds themselves.

The vacumm to ... rating on these likely means, they won't break under a full vacuum. But they'll probably go fairly squiffy as the vacuum starts getting harder, or the guys using turbo molecular and ion pumps could use them and save a few hundred / thousand.

Provided they work down to fridge pump levels, or a bit below, they'd be good for those users and the aspirator guys. Scraping the bottom of the barrel at 0.1mbar and below isn't really that essential for distillations, and is more of a problem for me a lot of the time (which is why I'll purposefully use fridge pumps over a rotary sometimes, and save the rotary for things like driving off the very last few drops of volatiles).

I asked some of the guys on eBay who sell silicone vacuum hose about using it at 10 and below mBar values, hoping it'd be more flexible and easier to use than the thick orange rubber I have now, but they said it'd probably collapse.

[Edited on 29-8-2010 by peach]

watson.fawkes - 29-8-2010 at 07:17

Quote: Originally posted by densest  
Hmmmm... From www.mouser.com, Freescale Semiconductor MPX2102AS is $7 (plus shipping). Output is analog, 40mV from vacuum to nominal 14.7PSI
The odd thing about these is that their maximum pressure rating is specified at slightly than atmospheric pressure, not more. I'm assuming this isn't a problem for device lifetime, but it is a little funny to me.

I couldn't determine what the packaging material is made of, but it is injection molded; there's a maximum dimension for flash listed. I'd guess the package material is a weak link in using one of these for solvent stripping, as there's certainly some solvents that will degrade the package quickly. You could protect it with a U-trap, but then you'd need measure the manometer you've created.

The other packaging material are a "stainless steel metal cover" and a "silicone gel die coat", neither of which seem potentially as problematic as the plastic case.

Jimbo Jones - 10-4-2011 at 07:52

http://www.instructables.com/id/convert-a-tire-inflator-type...

Sounds interesting. Any input from someone with more experience in this field?

deathmetals - 10-4-2011 at 08:38

Quote: Originally posted by Jimbo Jones  
http://www.instructables.com/id/convert-a-tire-inflator-type...

Sounds interesting. Any input from someone with more experience in this field?


I worked with those at my internship.
We did an indurence test with them they can fly appart with in a month. After 10min an hour 9hours a day. but some work longer a lot
You probably will need to change the oil its that chinese grimy black oil. It will work better with new oil.
I also found difrent engines with in the same batch we ordered.
They where in the same casing tough
The test where done bu letting the compressor go to 3 bar (if i rember correctly) and back again.
Also found that some cant get to above 10 bar or the manometer was broken
Never used them to pull vacumes tough.

Tldr: the quality varies alot but if u can get a good one it may be very useful.

Edit: if they fly apart, they fly apart hard! Plz build. Somekind of protection or casing wile you run these things. a part went about 0.75 cm in to some building wood.

[Edited on 10-4-2011 by deathmetals]

Jimbo Jones - 11-4-2011 at 03:29

I plan to buy some of the powerful brands and give it a try. Maybe some tuning will fix the things, but before I get the air compressor these are just some wild dreams!