CuriousOnlooker
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Aspirator/Water Jet Query?
I am considering getting a water jet/aspirator.
I don't anticipate the need for a particularly low vacuum, just low enough to facilitate simple distillations such as HNO3, or general
reduction of distillation temperature.
To avoid sending ridiculous amounts of water down the drain I'm looking at using a pump to recirculate the water, the same as with distillation
cooling water but at a much higher flow rate.
A quick browse around shows a vast range of supposed throughput, from 480 litres/hour to 8,000 litres/hour.
This is more of a wish-list item that I don't strictly need, but would like to have. Also, I don't want a long-term solution as I'm hoping to clear
all of my materials in the coming months, so there is a compromise with longevity and price.
For those who have used such an arrangement, what flow rate should I be looking for in a pump as a general purpose utility?
Condemnant quo non intellegunt.
Never fire a warning shot. It is a waste of ammunition. ~ Hunter S. Thompson
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Fulmen
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I have one of these: https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2F...
It uses appr. 220l/h (600ml in 10s)
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MrDoctor
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pressure is important, being able to build up 100PSI or more. pretty much you want to be able to match your mains flow and pressure.
i have a glass aspirator and a metal one, both pass -700mmHg, the glass one with my 12v 100W pump which claims to do up to 80PSI (thats its peak),
maxes out the glass aspirator but the metal one only gets to about -500mmHg, it has a larger diameter jet.
now, instead of a 12v device you could also cheat and just grab yourself scaled to mains, 200W+ and essentially is a small swimming pool pump or
something designed for supplying water to your house from a well or tank, or perhaps the pump in a caravan for showering. actually, if you get a
non-piston pump its going to essentially be perfect anyway, since you wont have to worry about solvents attacking anything as it just uses an impellor
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CuriousOnlooker
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Thanks Fulmen and MrDoctor. I had my eye on a mains powered submersible, listed as 400W, 8,200 litres/hour, 5m head, simply recirculate through a 20L
polypropylene bucket. A limiting factor is that I'm in an apartment, which makes some things I do challenging (Bang? What bang? I didn't hear
anything)
Condemnant quo non intellegunt.
Never fire a warning shot. It is a waste of ammunition. ~ Hunter S. Thompson
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Twospoons
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Dont get too powerful a pump either - all that excess power is going to add unnecessary heat to your water loop and reduce the performance of the
aspirator. Pick your aspirator first, then match a pump to it. Remember also that pump pressure vs flow follows a curve - that 5m head figure you
quoted is the backpressure at which flow drops to zero, and conversely the max flow quoted is at zero pressure.
[Edited on 7-8-2025 by Twospoons]
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MrDoctor
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5M head gives a terrible pressure if im understanding it right as the static pressure under low flow. if you google "5A 12VDC self priming pump" thats
what i have, the one where it has the red protective caps on the inlet and outlet.
getting a 100-150W (9-12A) model of that will probably do the trick for the cheap common metal aspirators from ali and on amazon, they dont do high
flow rates but, the pressure will soar with minor resistance. based on an example i looked up since i cant find mine right now, something like 5l/min
and 130PSI/0.9MPa would probably be able to max out most. i cant imagine mine i have hooked up to a garden hose in the shed right now though passes
much more than 10-15L per minute at most.
the thing is, if its mains powered, it might struggle to build up the appropriate amount of pressure, since its going to be using an impellor rather
than pistons, that design also tends to struggle with pressure at low flow, and it takes like 700-1400W just for that pressure to scale up, sometimes.
A convenient tradeoff though is that you can put a tap/regulator on the output and cut it down no problem.
for a given resistance, just like with ohms law, water obeys the same rule, flow = pressure/resistance, and pressure = flow x resistance. for a given
flow rate, the pressure difference on one side vs the other is unique, its a non intersecting curve. it also means that high volume flow when blocked
doesnt neccesarily translate to high pressure buildup.
if you already have your aspirator, try connecting it to a garden hose and see what that does.
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CuriousOnlooker
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Quote: Originally posted by MrDoctor  | [snip] .. if you google "5A 12VDC self priming pump" thats what i have, the one where it has the red protective caps on the inlet and outlet. [snip]
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Not sure which one you mean Doctor -- what you see may be different to my search results, maybe because I use a VPN, but these are the first four in
the search results from that query.
   
Can you capture the screen portion of the one you were referring to?
I as looking at the metal aspirators in Ali. There are these two, at least one of which shows what's inside the things.
 
Both of these day: 1.5L/minute, max. vacuum 420mm Hg, and with a ball to prevent backflow.
More pointers in the right direction gladly sought.
Condemnant quo non intellegunt.
Never fire a warning shot. It is a waste of ammunition. ~ Hunter S. Thompson
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jackchem2001
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One thing nobody mentions about pumps is the noise. I have to wear earplugs whenever I use my recirculating aspirator setup (but it actually works
pretty good for what I need)
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teodor
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As far as I remember a bit of aspirator theory the vacuum is more dependent on the water temperature than on the flow (after some minimal necessarily
flow is provided). This is because the typical vacuum which aspirator can create with a typical tap flow allows to boil the water inside the
aspirator. So increasing the flow will not help after this point.
More power is in your pump more heat will be transfered to the water rising the minimal pressure you can achieve. I can tell you that when I
investigated my lab reports I noticed that the vacuum pressure in winter always at least twice is lower than that in summer. I use a tap but also was
thinking for switching to the pump. So I also was interested what other people can recommend as a flow because my idea was to buy a dual-purpose pump
- for watering my plants in a garden and for the aspirator.
Reading this thread I've got an idea how to proceed.
1. Select an aspirator and attach it to a tap.
2. Measure the pressure dependency on water flow. Find the point when increasing the flow doesn't change the vacuum much.
3. Measure the flow by measurement the time some defined value is filled with water.
4. As other people pointed the real flow is dependent on the resistance. You can try to guess or measure the pressure of the water in your tap and
look into the graph in the pump documentation, usually the graph flow vs height of water is provided. The heigth of water is the pressure.
5. Multiply the flow you found on the step 4 by some "safe" coefficient, buy the pump, try. If the flow is not enough return the pump / exchange with
another one.
But also you have to think how to cool the water inside the water loop because pump and room will heat it.
After considering all those issues I think buying a cheap oil pump could be not so bad solution for most of vacuum tasks (filtration etc).
P.S. Strictly speaking after some flow rate the vacuum becomes more dependent on the water vapours pressure which is the function of water
temperature. You can't get less than the water vapours pressure. It's not necessarily the water is already boiling. This vapour pressure is exactly
why oil is usually used in a closed-loop system.
[Edited on 7-8-2025 by teodor]
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MrDoctor
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| Quote: Originally posted by CuriousOnlooker | | Quote: Originally posted by MrDoctor | [snip] .. if you google "5A 12VDC self priming pump" thats what i have, the one where it has the red protective caps on the inlet and outlet. [snip]
|
Not sure which one you mean Doctor -- what you see may be different to my search results, maybe because I use a VPN, but these are the first four in
the search results from that query.
Can you capture the screen portion of the one you were referring to?
I as looking at the metal aspirators in Ali. There are these two, at least one of which shows what's inside the things.
Both of these day: 1.5L/minute, max. vacuum 420mm Hg, and with a ball to prevent backflow.
More pointers in the right direction gladly sought. |
I was reffering moreso to the type than any specific model, the only real difference between these is the power rating of the motor. from experience,
the 60W 12v 5A isnt strong enough for the metal aspirator but it exceeds the requirements for a glass aspirator, its just super common and easy to
find as an example. all those metal ones on ali are all the same, the anti backflow is probably what that -400mmHg is about, the check valves in
aspirators are generally useless, youll want to do some research into the practices employed to prevent water creeping up the lines when your setup
doesnt leak enough or produce any gas/vapor to do it and you dont have a regulator that bleeds as needed to maintain vacuum.
as for being 1.5L per minute max, thats funny. im pretty sure at that flow rate they will struggle to work, period. dont worry about what the
aspirator says.
One thing to be mindful of though, from ali, all stainless steel aspirators are defects from deschem, or selling deschem stock. In a sense, they are
fine, but also, there is barely 1 loop of threading on the vacuum inlet, and the o-ring is structural, coated in superglue, it works as long as you
dont strain it. but frankly i didnt find much difference in functionality between my aluminum one and the stainless one, and i treated the aluminum
one like the disposable $15 chunk of metal it is, it constantly had water in it, as did the vacuum line which was copper, there was blue, nothing
really happened over the years.
Get a self priming pump of that type as you have provided here, of 90-150W+, with over 100PSI of pressure, and ideally a rating of over 4/min, or go
way overkill, and also grab a cheap little DC speed controller if it blows up the hoses or whatever. the 90W ones are only like $20-30.
Another thing. there is a very obvious problem in the matter of actually connecting a hose to these aspirators, i can only assume that these normally
plug into some massive rubber block or some sort of clamp-mechanism and not sliding a hose on it. personally i think a better solution than using
reducers and hosing and clamps, is just epoxy or weld/solder a reducer on there, or if you can, tap a BSP thread and screw a reducer in.
the aspirator itself is the one thing you cant damage with pressure, so push it as hard as you are able.
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CuriousOnlooker
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Thanks for all the input from everyone.
I realise that connecting the pump input/output to appropriate hosing will likely to require some creative solution(s), although I hadn't considered
the noise factor so I'll look at making some little sound-proofing container for the pump to reduce the noise.
Thanks again.
Condemnant quo non intellegunt.
Never fire a warning shot. It is a waste of ammunition. ~ Hunter S. Thompson
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Sulaiman
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a pump like this 12V -85kPa unit may suffice
(instead of the complexity envisioned.)
https://www.ebay.co.uk/itm/387689637858
eg Az. nitric would distill over at about 60oC
and similar pumps are more than adequate for vacuum filtrations.
Yes the fumes will slowly kill it, but
. your needs seem short term
. fumes can be pumped away rather than dissolved into your recirculating water or released into your atmosphere.
. much less fuss than a water recirculating system
. cheaper
-85kPa is not ideal for reduced pressure distillations
my -80kPa pump ran for years but
ended up useful only for vacuum filtrations at -70kPa
CAUTION : Hobby Chemist, not Professional or even Amateur
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Dr.Bob
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You will need about 4g/min to get a good vacuum, and if you put some ice in the water, it will keep the water cool and help some also. I have used
aspirators and pumps, and for most simple organics like hexane, ethyl acetate, DCM and ether, a water aspirator will create enough of a vacuum for a
rotovap to work well. You do need to cool the condenser water to ice temp however, if you want to get a good solvent evaporation with an aspirator.
For filtrations a lower flow may work, but it depends on the scale, normal sized buchners are pretty easy with a simple aspirator, but if you go to
glass frits or large buchners (4+ inches), a better vacuum is easier to use.
Keep in mind that you can often find cheap vacuum pumps for HVAC work at places like Northern Tool or Horror Frought Tools (Harbor Freight...), some
are as cheap as $100 for a simple one. That might be cheaper than some complex water pump systems.
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MrDoctor
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| Quote: Originally posted by CuriousOnlooker | I realise that connecting the pump input/output to appropriate hosing will likely to require some creative solution(s), although I hadn't considered
the noise factor so I'll look at making some little sound-proofing container for the pump to reduce the noise.
Thanks again. |
noise isnt much of an issue for me, using those exact aspirators. i have a tube from the water jet outlet, into a bottle, submerged, its no louder
than a garden hose on jet-setting. And im literally 2 feet from that. these aspirators need to have the output submerged in water to form the seal
properly, though a small sort of bulb or suitable geometry to force a hydro-lock probably is placed on other designs so they just spray water out
directly and work fine that way with no regard for how or where the stream goes, that could be noisy and possibly difficult to manage without adding
resistance to the process. But i wouldnt worry about these cheap china aspirators.
For vacuum filtrations id recommend getting a very good sealing buchner and vacuum-safe flask, and use a hand pump, or just a cheap disposable little
diaphragm pump.
While on the subject of pumps, iirc someone once said that you can make fridge compressors get crazy low pressures if you add another pump in series
sucking on its output, assuming one of those little -85kPa ones has a higher volume flow than the fridge compressor, would the two of them add up to
something similar to a 2-stage HVAC pump? i do recognize they use a totally different mechanism, im just curious because i dug up my old project one
and it pulls like -710mmHg just by itself, i was considering though using it in conjunction with my aspirator at some point to see what kind of silly
low pressures could be reached
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Twospoons
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Fridge compressors rely on oil in the refrigerant loop for lubrication. For use as a vacuum pump you would need to trap and condense any oil mist
leaving the outlet and return it to the inlet.
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MrDoctor
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a 2nd pump sucking on the outlet helps with that, since the sort of filtration it takes to catch and condense everything also adds appreciable load on
the pump decreasing the vacuum. a capillary of sorts can be used to return the oil such as that it gets sucked into the inlet but not so quickly that
it creates a problematic vacuum-leak, or depending on case pressure, just build up and slowly drip into the case when its turned off. Fortunately,
iirc, misting gets much less problematic the thinner the air is, or the less compressing occurs
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