Sciencemadness Discussion Board - Aspirator/Water Jet Query?

Aspirator/Water Jet Query?

CuriousOnlooker - 6-8-2025 at 03:28

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?

Fulmen - 6-8-2025 at 07:23

I have one of these: https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2F...

It uses appr. 220l/h (600ml in 10s)

MrDoctor - 6-8-2025 at 12:49

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

CuriousOnlooker - 6-8-2025 at 18:22

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)

Twospoons - 6-8-2025 at 19:42

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]

MrDoctor - 6-8-2025 at 22:07

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.

CuriousOnlooker - 6-8-2025 at 23:13

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.
Screenshot 2025-08-07 164738.png - 57kB

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.

jackchem2001 - 7-8-2025 at 00:53

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)

teodor - 7-8-2025 at 02:25

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]

MrDoctor - 7-8-2025 at 03:00

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.

Thanks for all the input from everyone.

CuriousOnlooker - 7-8-2025 at 05:02

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.

Sulaiman - 7-8-2025 at 05:04

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 60^oC
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

Dr.Bob - 7-8-2025 at 17:52

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.

MrDoctor - 7-8-2025 at 18:04

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

Twospoons - 7-8-2025 at 18:21

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.

MrDoctor - 7-8-2025 at 18:40

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