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Author: Subject: Can't get water aspirator pump working
thors.lab
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[*] posted on 25-12-2020 at 22:21
Can't get water aspirator pump working


Hi,

So I've been looking into solutions for doing vacuum distillations, vacuum filtrations, drying compounds under vacuum, and vacuum chromatography.

A couple years ago I got a water aspirator, something like this

Then, I got a 12V water pump, like this.

It did not work. It could barely pull a vacuum most of the time. It was just enough for the most basic filtrations, but anything with fine particulate was totally off the table. Certainly the vacuum wasn't even close to good enough for vac distillation or for drying anything.

Now, I'm wondering if this is a fault of the water aspirator or of the pump. I'm thinking the pump. It's kind of hard for me to imagine that a very basic well known bit of physics would just not work for me. Can anybody recommend a pump that would put out enough water pressure to get reasonable vacuums with this sort of thing?

Altneratively, can anybody recommend a sub $100 pump that would work for vac distillations? Now, on amazon we got tons of rotary vane pumps that can pull good vacuums for only $60. At first I figured, no big deal. If vapors destroy this eventually, it's only $60. But posts on here are making it seem like a single distillation is enough to destroy the pump. How long would a pump like this reasonably last, distilling solvents, maybe some acid vapors, etc?

Thanks,

Edit: Actually the exact pump I got was this. I assumed that 1.2gpm and 80psi would be enough to get a good vacuum, but it just wasn't.

[Edited on 26-12-2020 by thors.lab]

[Edited on 26-12-2020 by thors.lab]




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valeg96
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[*] posted on 26-12-2020 at 00:09


Water "aspirators" are meant to be used with water tap pressure. Whether they are glass, plastic or steel they require a running tap, usually completely open.

Your 12 V pump does not create enough pressure for the water aspirator to work. Either attach the aspirator to the tap or buy a rotary pump and use it for filtrations only and with vapour filters (e.g. a column packed with Na2CO3 to stop acid vapours). Unless you have a decent pressure regulator, rotary pumps are quite too powerful to use in a distillation, and unless you have a good trap (read: liquid nitrogen) they will chug a lot of solvent. If you don't have any of these, the most you can do is get a couple liters of pump oil and change it three times a year or so. Also, most non-laboratory rotary pumps do not have an air outlet and an oil filter, so after a couple of minutes running (they normally heat up to 80-90°C) they will emit a fine mist of oil and whatever chemicals they sucked up.

Oh, also. I used my small 100€ rotary pump for 6 years for all kinds of filtrations (no distillations) and it still works, albeit a bit rusted inside. I guess it's a matter of luck, frequent oil changes, and careful use.

[Edited on 26-12-2020 by valeg96]





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egret
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[*] posted on 26-12-2020 at 05:49



Quote:

Then, I got a 12V water pump, like this.



The real flow provided by cheap 12V pump is much lower than announced. I had a similar problem. The solution is a second, the same pump connected parallel.
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Jome
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[*] posted on 26-12-2020 at 07:56


I've actually built my own venturi effect vacuum-pump setup with both that exact aspirator and the first pump. If you pump water through the pump+hose-setup, do you achieve the pumping rate written on the pump? I didn't at first, of course built everyhing wrong, used too thin hoses, too long connecting hoses, a collapsing vacuum output hose, and whatnot. Changed the connecting hoses to thicker, stiffer ones with the shortest possible lengths and so on. The measured pumping speed went from 1.8L/min to 4.2-ish L/min.

It should work, Nurdrage achieved a good vacuum with his similar-looking setup.



[Edited on 26-12-2020 by Jome]
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RustyShackleford
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[*] posted on 26-12-2020 at 10:41


Me and several others have had the same experience with similar pumps. Do not trust the ratings on any product manufactured in china, they lie about everything. the pumps simply do not have the flowrate capability the aspirator requires, especially not through the small oriface.
You could try putting 2 of those pumps in pararell, but it still might not work.
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Sulaiman
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[*] posted on 26-12-2020 at 17:10


Many small water pumps (eg ac powered centrifugal pond pumps)
operate on a near straight line on a graph of pressure vs. flowrate,
eg a 6 lpm 2 bar pump can give:
6 lpm @ +0 bar
3 lpm @ +1 bar
0 lpm @ +2 bar

So if you want a water flow of 6lph @ +2 bar
you need a 12 lph +4 bar pump.

In any case, check the flowrate vs. pressure for your pump before buying.
I
Not all pumps operate on a straight line,
many have a more efficient curve.
So doubling both the pressure and flowrate is a fairly safe rule of thumb,
but some pumps do better.
...................
Putting a second pump in parallel should double the flowrate - at the same pressure.
Mathematically, four pumps could be required in a 2x2 series-parallel arrangement to achieve what you originally wanted,
but it would be easier and cheaper to buy a single larger pump.
(after checking its specifications)
.............................
I have bought many pumps from China and mostly they perform near to specifications.
(unlike 18650 batteries etc.)




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Jome
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[*] posted on 27-12-2020 at 00:40


If I hook my vaccum hose up to a reversed measuring cylinder in a waterbath to measure suction speed by raising the water pillar inside, how fast should it be to be considered decent? What is the performance of rotary pumps when tested like that?
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wg48temp9
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[*] posted on 27-12-2020 at 10:40


Quote: Originally posted by Jome  
If I hook my vaccum hose up to a reversed measuring cylinder in a waterbath to measure suction speed by raising the water pillar inside, how fast should it be to be considered decent? What is the performance of rotary pumps when tested like that?


A typical small rotary vane vacuum pump has a free air displacement of about 2CFM (57lpm) while the OP's Humboldt aspirator pump has a pumping rate of 1.5lpm(assumed free air) displacement.

You appear to be asking for a qualitative answer (decent) which will depend on what you want to do with the pump. In the case vacuum filtration do you really care if it takes one minute or ten seconds to filter half a liter of fluid.

Your test is almost at ambient pressure so it corresponds to the free displacement of a pump that is not the usual condition of vacuum filtration.

A useful index for an aspirator pump is what is the achievable vacuum for a given water flow rate. Unfortunately that is usually not specified. Most of the videos and posts in here on the achievable vacuum of an aspirator do not say what the water flow rate was although its easily measured by measuring the time to fill a container of know volume.

As so many people use the Humboldt aspirator pump I am surprised that no one (that I could find) has measured the flow rate. It wold be particularly useful when first testing a pump aspirator combination and for selection of the water pump.

PS: One youtuber tested his diy aspirator vacuum pump and achieved about a vacuum 0.1bar (absolute) with a water flow of 2l/min. It looks similar in nozzle diameter and choke diameter to a Humboldt pump. See https://www.youtube.com/watch?v=_6PNs9TQ44g

OP@ check your water flow rate.



[Edited on 12/27/2020 by wg48temp9]




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[*] posted on 27-12-2020 at 13:22


Rotary vane, always
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[*] posted on 27-12-2020 at 15:25


I can get a bare vacuum with my mains which gives 5bar and 10-15C minimum temp water. Ejector puts through about 5-6 liters per minute. Don't remember exact vacuum, but one reading I remember determining was 60mbar.
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