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jmneissa
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True Vacuum Pump vs. Water Aspirator
So today I got into an argument with my chemistry friend over vacuums in the lab. He has always been a supported of the water aspirator citing it is
cheap and handles corrosive gases which a actual lab vacuum pump cannot without multiple traps. I have always thought that with places such as ebay
you can get a great vacuum pump for under $200. A vacuum pump however can pull a better vacuum and can be used for more advanced vacuum applications.
I also replied back to him that while an aspirator does not need traps the corrosive gasses especially NO2 will slowly eat away at the nozzle. With
all this the aspirator will eventually have to be replaced
As such one can say the cost over the long run could be quite similar. As such I was wondering what you guys thought was the better vacuum device, the
true lab vacuum pump or the water aspirator?
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aonomus
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To provide another consideration: an air aspirator gives good enough suction to do filtrations and HPLC solvent filtrations, and probably some other
vacuum applications. It does however avoid the need for expensive usage of water continuously (eg: leaving a sample on a rotovap for a few hours).
Any distillation that really requires a vacuum pump you would really need to consider a trap, if not to prevent your own exposure to whatever is being
released by a reaction (such as NO2).
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not_important
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You can get heavy duty plastic aspirators, which are not bothered by most acid vapours, intended for serious low vacuum work. I have several, one is
a commercial setup that uses a recirculating pump to drive dual aspirators for two separate vacuum ports. It has pressure gauges on each port, and
connectors that allow the addition of some sort of cooler to the water loop; a similar model up for sale at the same time had a 'radiator' with water
misting as the cooler, the water consumption is non-zero but much lower than an open loop aspirator. It's done over 15 years of service without
problems, and was bought used.
I'd pick the aspirator for filtration, distillation of acids and exhausting acidic gases, and so on. You can make your own recirculation system,
people have used an automotive water pump and electric motor. Toss some marble chips (CaCO3) in the water tank and clean up acidic gases on the fly.
In the winter toss snow in the water tank to improve the attainable vacuum.
If you need to do distillations at below 30 mmHg then you'll want a better pump, but I'd still get an aspirator for handling the less demanding stuff.
The savings on traps, be it absorbents and power needed to regenerate them or LN2, will make you happy.
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peach
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If you're mainly distilling solvents, I go for the pump. The less messing around possible the better in my opinion. If you're having problems with
solvent getting to the pump, bleed air into the system to raise the pressure and condensation temperature, use atmospheric distillation, buy a better
condenser, run it cooler or reduce the rate of distillation to effect better condensation.
Running potentially corrosive vapours through a normal rotary pump is a bad idea and then it'd probably be a good idea to use the aspirator, as
chemically resistant diaphragm pumps are expensive and don't pull such a good vacuum, and resistant rotaries cost a lot. But ask
yourself how often you'll be doing that. You can also do this using a rotary pump if you put a wash bottle between it and the pump, loaded with KOH
for example if you're expecting acidic vapours.
The inside of a rotary pump gets hot during operation. So traces of low boiling point solvents getting into the oil will boil off through the exhaust.
Not necessarily great for your health, but people routinely use litres of DCM as paintstripper around the house, run nail parlors with acetone
everywhere and so on. And you can always attach a hose to the exhaust and stick it out a window, depending on what the vapour is and how much of it
there is.
In the past, I've spent far too long having to set up gear and continually watch over it. To the point that it detracts from the fun of the experiment
and makes it uninspiring to do any work. I'm all for a bit of ingenuity, but not once it starts taking more time and effort than the thing I actually
want to do. I used to enjoy playing around with pumps and gear, but there's only so long that can be entertaining.
If you're distilling things with higher boiling points, like oils and so on, you'll want a rotary pump to get the temperature down and as much oxygen
out as possible
[Edited on 24-7-2010 by peach]
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Contrabasso
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jmneissa, you have summed up your own answer! Both types of pump have their uses. An aspirator/filter pump is good for corrosives and only pulls a
poor vacuum. A motor driven pump costs more, needs more care and servicing, and doesn't handle solvents or corrosives well. However a motor driven
pump will pull down to a much better vacuum.
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peach
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Aspirators and dumping the product to waste is inherently wasteful and lazy.
Ideally, you shouldn't have vapours reaching the pump in the first place. If you're using a liebig / west to rapid boil volatiles and corrosives under
vacuum, you're an idiot. There's no way they're ever going to efficiently condense with such a small surface area. Particularly if the condenser is
not far below the BP and the vapour is moving quickly.
I keep going on about wash flasks, but here's yet another example where you can use one, meaning you can buy a $200 pump and achieve the same result
as a $7000 pump, for the cost of the bottle and a little thought.
You can gas your substrates with them, scrub highly toxic fumes as they leave the glassware, capture volatiles and neutralize corrosive vapours. Not a
lot else can do so much.
Nerve agents like VX will hydrolyze on contact with water, neutralizing their lethal properties and releasing HF. Add some base and you have salt. The
most toxic things known to man reduced to toothpaste ingredients, buy a wash bottle.
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not_important
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If you put a wash bottle between the pump and the system, the attainable vacuum is limited to around the vapour pressure of the contents of the wash
bottle. The bottle also introduces a pressure differential between the pump port and the side of the bottle connected to the system being evacuated.
Thus you'll not obtain a high vacuum with such a setup, meaning you've effectively created an aspirator pump.
A recirculating water aspirator is no more wasteful than rotary or diaphragm pumps. Do as I suggested and put some marble chips in the water system
and acidic components get converted to calcium salts; you can contain the CaCO3 chips in a chamber with outlet filter to retain stuff like CaF2 and as
its not in the vacuum stream it doesn't affect the obtainable pressure.
Before mechanical pumps were common, an old school way to get low pressure with an aspirator pump was to pump the system down and heat it enough to
drive off volatiles. Then cool it with the pump running, valve it off from the pump and fill with CO2, reconnect to the pump to suck out most of the
CO2, repeat several times. After the final pumpdown, valve the system to a flask containing KOH and cooled in running water or ice. The KOH absorbs
the traces of CO2 (and H2O) and gives a fairly high vacuum, if there's no low boilers left in the product being distilled (everything in it can be
condensed at the system pressure) then the vac stays good.
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peach
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Good points, but how many distillation require a pressure that'll boil a fridge cooled wash bottle? And you can always change the solvent or absorbent
to something else.
Left alone, we'd argue it out to 10^-12 mbar and lower. But this kind of question is geared towards 100mbar to 1mbar distillation. In fact, the OP
doesn't even mention the pressure range. With something like DCM boiling like crazy at room temperature once I remove some atmosphere and oils
refusing to do anything at microns.
If you're distilling something highly corrosive and that boils at temperatures within the normally condensible range (not below zero), a wash bottle
will work.
It shouldn't be in the exhaust in the first place. Buy a better condenser (coils / double coils etc), run the plate slower or use colder coolant.
If it's getting to the vacuum pump, your condenser is probably pants.
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zed
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It's a excellent idea to have both. You need 'em both.
If you are creative, you can do a lot of unexpected things with an aspirator.
One possible problem? A lab can be very "thirsty". Lack of adequate water supply can become a serious issue. If you are using conventional
condenser cooling for a reflux or distillation, and an aspirator to lower pressure or trap corrosive gases. Lack of adequate water flow, or changes
in rate of flow, can drive you crazy.
Remote faucets being turned off and on, the automatic sprinkler system activating or deactivating, or somebody flushing a toilet, can throw your
carefully adjusted waterflow completely out of kilter. Water supply hoses can pop off of your condensers, spraying water where it shouldn't be going.
Additionally, adequate water flow through your aspirator could fail....Allowing those dangerous gases you have been carefully evacuating....... to
escape into yer breathin' air.
I'm starting to see the wisdom of recirculating cooling water, via a special pump arrangement.
[Edited on 24-7-2010 by zed]
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jmneissa
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All great points, however, with the costs very similar I would choose a vacuum pump here is my reasoning:
Firstly, you can achieve a greater vacuum allowing distillation of higher boiling point oils. It would be necessary though to attach a wash bottle
however a rudimentary one can be fashioned for next to nothing. Secondly, while water is cheap on average $1.50 for 1000 gallons for long
distillations this while not being expensive is an extreme waste of water and the potential for error increases as Zed has stated. I would think that
in the long run most chemists who will slowly advance to more challenging topic requiring more advanced synthesis's will appreciate the benefits of
the vacuum pump as it will allow them to do more. However, this is just my opinion and everyone is allowed to have their own.
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aonomus
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Don't forget that wash bottles = suckback. Not a worry if you are distilling to drive off something, but a terrible worry if you are trying to recover
your product as the distillate, and then your wash bottle gets sucked into your product. Its happened to me before, and it sucks (literally).
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peach
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I know precisely how much it sucks. When stupidly distilling sulphuric battery acid using only wash heads and no true condenser, I managed to have the
steaming hot water suck back into the 380C concentrated acid. It went ape shit, to say the least. I had purposefully thought before hand, I should
probably leave a keck clip off here. Luckily I did, as the lid simply blew off and shot vapour out. If I hadn't, it may have exploded instead. I was
sat beside it. Balls naked. I took the hint.
Suck back isn't an issue if you stick another flask in the way.
One major pain with wash heads, gas handling and tubing is controlling moisture and condensate in them yes. Often, it'll activate things that
otherwise wouldn't corrode the tubing, or syphon where it shouldn't be, perhaps dangerously so. It does require care and it's one of those things
you'll only really appreciate once you've had to deal with it's many exciting personalities first hand.
This is why I'm getting some Tygon 2075. I'm using black neoprene at the moment and I can't see if it's clean or if there's something trapped in it
and making it's way to somewhere bad.
It's funny you mention flushing the toilet, I was just imagining that. Sat there watching the temperature going up and down and wondering wtf is going
on.
I read a post by some guy who was using a recirculating aspirator. He'd found the motor would heat the water fairly quickly if it was a submersible,
so recommended filling it with fizzy drinks bottles full of ice; or use an external pump. Another problem was bubbles from the aspirator or return
hose splashing getting sucked back into the pump and going back through, which caused the vacuum to bump. He solved that by having the pump over the
other side of the tank I think, and a I seem to remember him suggesting putting a little plastic shield in the way to deflect any remaining bubbles
away from the pump intake.
Considering the wasteful nature of aspirators and the idea of flushing gases away as opposed to absorbing them before the pump, I agree with jmn, a
rotary is the better idea.
If you don't need a stable pressure to get a clean distillate, if you're just ripping a solvent off, fridge vacuum pumps will go down to around
30-50mbar, which is the same as an aspirator. They're free and don't require any water. I've beaten the crap out of them and boiled off solvents like
DCM without even bothering to condense them (putting them straight through the pump). I've left them running for 12h with no cooling. They're
troopers.
A typical fridge pump;
Two of them in series;
A beat up Edwards, which I'm refurbishing (as soon as someone else does their job);
SHIVER ME TIMBARS! IT BE A SCOTLAND YARD RAIDIN' PARTY HERE TAR HORSEWAGGLE ME TRESHAR! Oh no, wait... it's the idiot child kitty, fugicle. One my
latest bits o' plunderin', fresh outta Davy Jones' locker (the box);
I love this tiny pump, it's for keeps, fo showa nigga'. It'll theoretically do 0.001mbar at 1.3 CFM, but it's actually around 0.06mbar (43 microns, 6
pascals, 0.00043 psi, 0.000056atm, 0.042 torr). Considering most solvents want boiling between 100 and 50mbar, that's not really an issue. Note the
highly technical connection method used by the previous owner, a bit of loose pipe stuck down the inlet;
VIDEO OF THE PUMP RUNNING
I've also owned one of these 10 CFM monsters. Very nice for a gear whore, but far too big, far too heavy (it was difficult to pick up & about two
foot long) and almost entirely unnecessary for anything other than resin degassing;
[Edited on 26-7-2010 by peach]
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Panache
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Quote: Originally posted by peach  | I know precisely how much it sucks. When stupidly distilling sulphuric battery acid using only wash heads and no true condenser, I managed to have the
steaming hot water suck back into the 380C concentrated acid. It went ape shit, to say the least. I had purposefully thought before hand, I should
probably leave a keck clip off here. Luckily I did, as the lid simply blew off and shot vapour out. If I hadn't, it may have exploded instead. I was
sat beside it. Balls naked. I took the hint.
I read a post by some guy who was using a recirculating aspirator. He'd found the motor would heat the water fairly quickly if it was a submersible,
so recommended filling it with fizzy drinks bottles full of ice; or use an external pump. Another problem was bubbles from the aspirator or return
hose splashing getting sucked back into the pump and going back through, which caused the vacuum to bump. He solved that by having the pump over the
other side of the tank I think, and a I seem to remember him suggesting putting a little plastic shield in the way to deflect any remaining bubbles
away from the pump intake.
[Edited on 26-7-2010 by peach] |
Thats nothing, i was distilling 12 pounds of potassium under pressure using a couple of old beer bottles and an oxy actelyene torch. It was taking
awhile so i moved the setup next to the spa bath and sat there naked when the whole shebang fell into the spa wit me!! Tell you what potassium at
1000C doesn't get along with water very well and luckily i had my resist injury key on otherwise i would have been toast.
I collect glass aspirators, this one here is an interesting design i have come across, i had to connect to understand it's function. Although the
aspirator itself is rather primitive the bulbous part on the bottom serves to exclude all gases as they port out the outlet at the top while your
liquid goes out the bottom. The photo isn't great but the outlet at the base of the tube running into the bulbous part deflects the outcoming water
and gases by 90degree's in two directions.
I had always wondered as to the point of glass aspirators, obviously for corrosives they are good but they also allow for interesting designs to be
readily made, something not quite possible with other material.
Another benefit is you can see the entire thing running giving you a sense of your system and if there are any blockages.
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benzylchloride1
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I have a Precision Scientific Model DD 20 vacuum pump that I obtained off of Ebay. The pump specifications say that it should obtain a vacuum of 5
micron, the pump has a capacity of 20 l/m. I run vacuum distillations quite often and via extrapolation using a nomograph, the pump is pulling a
vacuum of about 5 mmHg. I changed the oil, the original pump oil was full of water, very bad practice of the previous owner. Does this pump need to be
rebuilt? Does anyone know where to get parts for it cheaply? A dry-ice acetone trap is the minimum that you should use for protecting your vacuum
pump. I was doing a distillation of allyltributyl tin that was extracted with toluene, all of the residual toluene that was not removed with my rotary
evaporator ended up in the trap. I use a Buchi recirculating water aspirator to run my rotary evaporator, it works well until the water in the
aspirator heats up.
Amateur NMR spectroscopist
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peach
Bon Vivant
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| Quote: Originally posted by Panache |
Thats nothing, i was distilling 12 pounds of potassium under pressure using a couple of old beer bottles and an oxy actelyene torch. It was taking
awhile so i moved the setup next to the spa bath and sat there naked when the whole shebang fell into the spa wit me!! Tell you what potassium at
1000C doesn't get along with water very well and luckily i had my resist injury key on otherwise i would have been toast. |
One of the journalists from the esteemed online Nature podcast radio show produced at Cambridge university each week also produces a show called "The
Naked Scientist".
I can gaurantee they're not naked.
It's us home chemists who throw caution and nay saying to the wind, gently bouncing our bean bags off the side of a dangerous distillation flask and
laughing in the face of criticism.
I have noticed a strong trend on chemistry forums towards people often being more naked the more practical chemistry they perform.
That aspirator may even be for direct connection back to the pump inlet, with the bulb being filled to act as a reservoir. Personally, I smash
glassware like an angry house wife, so there's no way I'd buy that other than put it in a display cabinet or mount it in something less breakable.
| Quote: Originally posted by benzylchloride1 | | I have a Precision Scientific Model DD 20 vacuum pump that I obtained off of Ebay. The pump specifications say that it should obtain a vacuum of 5
micron, the pump has a capacity of 20 l/m. I run vacuum distillations quite often and via extrapolation using a nomograph, the pump is pulling a
vacuum of about 5 mmHg. I changed the oil, the original pump oil was full of water, very bad practice of the previous owner. Does this pump need to be
rebuilt? Does anyone know where to get parts for it cheaply? |
For those who don't want to do the conversion, 5mmHg is around 5000 microns; a micron is umHg, which is 1000 times less.
If it's drifted up that high, I suspect the vanes are damaged or you have a seal problem.
I don't know where to get parts for that specific pump, but you could probably substitute normal O-rings and such if you're not in the mood for
searching around or paying a premium.
E.g. you may notice the base on that Edwards pump is missing. So far, I've been quoted over £60 (probably without VAT and postage) for the
replacement part, which is a stamped piece of metal with a few holes in it.
A lot of vacuum pumps are actually rated on their water carrying capacity per hour. The gas ballast is there specifically to lower the compression in
the secondary stage and reduce liquefaction of moisture or solvent in the gas stream.
| Quote: Originally posted by benzylchloride1 | | A dry-ice acetone trap is the minimum that you should use for protecting your vacuum pump. I was doing a distillation of allyltributyl tin that was
extracted with toluene, all of the residual toluene that was not removed with my rotary evaporator ended up in the trap. I use a Buchi recirculating
water aspirator to run my rotary evaporator, it works well until the water in the aspirator heats up. |
You could also just close up the vacuum port to reduce the CFM drawing capacity or bleed gas into the system to raise the BP point above zero, so it'd
liquefy at the condenser.
Assuming you were using your aspirator to run this solvent removal, your toluene should have been boiling around 18C, so a coil condenser at 0C would
capture most of that I expect.
If you were using your rotary, it'll have been below 0C. Whether it was necessary that it be so I'm not sure, as I don't know the specifics of the
material it was acting as a solvent for. But it is easy to overdo it with the vacuum around volatiles.
[Edited on 26-7-2010 by peach]
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zed
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You probably know more about vacuum pumps than I do, but sealing the pump by means of some sort of lacquer may be helpful.
Little micro-leaks along seams, around vacuum fittings, and under screws and bolts, can make a significant difference in ultimate vacuum achieved.
Pull as hard a vacuum as you can, and go at the pump with lacquer or similar material, and a paintbrush.
The lacquer gets pulled into the leaking spots and fills them, the Vacuum dries it almost instantly. Within a short time, you either have a better
vacuum, or you know the problem is elsewhere.
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peach
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I guess that'd work for some leaks, but there are a stack of o-rings and seals inside the pump it's self, which is usually a head sitting inside an
oil bath. The bit you see if the outermost casing. PTFE tape works well for leaky threads. High temperature silicon can be used to - carefully - make
new seals if they're an odd shape.
5000 microns is 6 -> 7mBar
That's fine for distilling most things. An aspirator will only do around 35mbar, so you're already about six times below that.
Going too low is actually a problem with a lot of volatiles, as has been hinted at, because they won't condense without dry ice / acetone flasks
around.
Need to start talking in pascals really, and switch mph to kph (that last one is proving incredibly difficult to effect for some reason).
[Edited on 27-7-2010 by peach]
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zed
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I was responding to BenzylChloride1, regarding a problem I have also encountered.
I had a brand new "Nelson" that wasn't cutting it. It pulled a pretty good vacuum, but it wasn't quite good enough.
The high boiler I was attempting to distill, just wouldn't come over at a reasonable temperature, under the vacuum I was generating.
There was nothing wrong with the pump, and after sealing, it performed admirably for about 10 years that I know of. The pump was still working even
then, but I lost track of it.......As, it was sold away, to someone that I didn't know.
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jmneissa
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Yeah I use a Welch Directorr 3 Pump which has worked out great for me since I bought it a few months ago. What I have found though is make sure you
are utilizing a your gas balast and have it set to the high throughput setting. Without this I was finding it very hard to pull a significant vacuum
during a distillation or filtration. One thing I would mention though is that you really out to have more then just a dry ice acetone bath. My rule is
that the trap has to be colder than the condenser e.g. for a toluene distillation I would use a dry ice acetone bath for the condenser and a liquid
nitrogen bath for the trap along with particulate filters to protect the pump.
With this I inspect my pump regulary and use a high quality oil made for the pump and have never had a pump fail. A note on ebay however, when you
receive your pump, if you feel comfortable doing so I would strongly recommend disassembling your pump thoroughly inspecting every part and changing
all damaged parts and replacing oil and seals. I understand this can be expensive sometimes a few hundred dollars but it is so worth it in the end to
know that you have a reliable working pump that will not fail on you during an important distillation.
[Edited on 28-7-2010 by jmneissa]
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peach
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You'd probably save yourself a lot of money if you simply bled some gas into the system to raise the BP to a level that's easier to deal with. Unless
you need subzero temperatures with regards to the substrate in the solvent, there's no need to have them during the solvent removal.
I'm sure abusing a rotary isn't a good idea but, as I say, I've had fridge pumps running for 12h removing DCM without any form of condenser or trap,
and they'll still pull the same vacuum afterwards. So I am skeptical that trace quantities of toluene making it into a rotary are going to do all that
much.
Opening the gas ballast on my rotary reduces the vacuum. It bleeds atmosphere into the secondary stage and reduces the compression ratio.
[Edited on 29-7-2010 by peach]
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aonomus
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@peach: what do you do for oil in your fridge compressor, both in regards to oiling it, and retaining oil from the output?
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peach
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Zero.
I know the oil is supposed to be an issue. If I leave it running for 12h there will be a few drops on the outlet. But I don't even have the loop left
on the outlet, I cut it right the way back at the pump because the tubing has a wider OD there, which just happens to ideal for pushing the vacuum
hose onto.
I have seen guys adding ping pong ball oil traps and such to the outlets of these pumps and suggesting they'll fail instantly without.
I expect they probably would fail if they were left running for years with the outlet open like that. BUT... I have had one for literally a decade. My
neighbour pulled it out a fridge and made it into a little airbrush compressor for us. I've had them get knocked over on their sides and a tablespoon
or so of oil has run out. It still worked, a decade on.
The oil running out is far less of a problem than it is for rotarys, that'll empty mugs of it out if knocked on their side or shipped upside down.
These pumps have tiny reed valves on the outlet, that are like the reeds in a harmonica. They'll only open when there's a significant positive
pressure gradient in the outlet direction.
It might be worthwhile to cut the top off one and try removing, or spiking open, those reeds. I think they're there to ensure refrigerant doesn't
backflow in their normal day job. But, I'm less sure if they actually help it perform as a vacuum pump. It's possible they reduce the flow rate and
they almost certainly reduce the ultimate vacuum level by placing resistance on the outlet path. They may also be responsible for the vacuum
'bumping', which is about the fridge compressors only downfall. What happens in my experience is that the pressure will fall, the pump sounds like
it's cut out, the pressure rises by about 10-20mBar over a minute or two and the pump starts again. Not an issue for solvent stripping, but it is one
for bumpy distillations or if you're after a pure fraction and temperature. I can't imagine why the piston it's self would do that, so my vote goes
for the reed valves.
Oil streaming out is only really a problem when a pump (including a rotary) is moving CFMs. When I switch my rotarys on, there'll be a nasty plume of
it from the exhaust. As soon as the inlet is restricted and a vacuum forms, the CFM drops to essentially zero and the misting disappears altogether.
These fridge pumps have very low CFMs indeed, so misting isn't really an issue even when they're freely pumping atmosphere (although, I would add an
oil filter if I was using them in the opposite direction, for an airbrush as an example).
Couple that with the fact you can go down to the dump each day and collect as many of these as you can fit in the boot of the car, for free, and the
oil is not only not an issue, neither is using them aggressively and carelessly; which has still yet to kill one in my experience.
If I had an angle grinder to hand, I'd try the reed valve trick. If that works, you could have an extremely serviceable pump for the time it takes to
chop the top off the casing.
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mycotheologist
Hazard to Others
Posts: 154
Registered: 16-3-2012
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Mood: No Mood
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Wasting water is a significant disadvantage of water aspirators. I pride myself on using an aquarium pump in a bucket of water rather than a tap to
feed water into my condensers. With a decent pump, you could do the same with water aspirators.
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elementcollector1
International Hazard
Posts: 2684
Registered: 28-12-2011
Location: The Known Universe
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Mood: Molten
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The waste of water is going to be an issue for me if I buy an aspirator (the only pump I have is the garden hose), so I think I'll go with a hand
pump...
Elements Collected:52/87
Latest Acquired: Cl
Next in Line: Nd
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elementcollector1
International Hazard
Posts: 2684
Registered: 28-12-2011
Location: The Known Universe
Member Is Offline
Mood: Molten
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Ahaha. Nostalgia.
Going to buy this vacuum pump (http://www.nationaltoolwarehouse.com/18-CFM-Vacuum-Pump-P181...), but now I'm worried about having too much vacuum, and thus being unable to
distill things like nitric acid and such. For those of you who are wondering, I do have a usable aspirator, but it wastes so much water it's not even
worth it. Plus, this thing's only $90, and pulls down to something like 0.08 torr. What I'm wondering is, is there a reliable way to decrease the
vacuum being pulled, so that the volatile boils but can also be condensed without the use of dry ice and solvent?
Elements Collected:52/87
Latest Acquired: Cl
Next in Line: Nd
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