Sciencemadness Discussion Board - help with pump aspirator

help with pump aspirator

CouchHatter - 9-2-2018 at 17:24

I got a fuel transfer pump for free (Northern Industrial 680480) and thought that it would make a great source of vacuum. By recirculating fuel in a bucket and putting an aspirator in the outlet hose, I'd hoped to have an easy way to pull >20 inHg. It's strictly for diesel fuel and rated at 8 GPM, which is pretty comparable to what the average kitchen flow rate is.
However when I hooked it up, the fuel simply backs part-way up the tube intended for vacuum... Is my aspirator positioned incorrectly? I haven't ever used a water aspirator (like what you would attach to a sink) but I believed I understood and had duplicated the concept quite well. I have hooked my hand-operated aspirator to the outlet and it definitely doesn't pull any vacuum whatsoever. Apparently, I am overlooking something, probably simple. Any advice?

I have tried operating the pump with the bucket lid on and off. I haven't measured the output but it's definitely cycling a lot of fuel. The pump is in good working order.

Attached is a diagram of the pump-bucket setup.

[Edited on 10-2-2018 by CouchHatter]

NEMO-Chemistry - 9-2-2018 at 17:31

Why you using fuel? its intended to run on water, maybe the fuel is too viscous for the jet in the aspirator.

besides some thing you pull a vac on might not mix too well with a fuel.

[Edited on 10-2-2018 by NEMO-Chemistry]

Vosoryx - 9-2-2018 at 17:50

What's the cut-off pressure of your fuel pump?
If you're using a water aspirator, i think NEMO is right. But I know you can buy aspirators for ethylene glycol, so you might be able to find one for diesel.
I also seem to recall that air is not very soluble in diesel. Since aspirators rely on solubility, this might impact the vacuum it can pull.
Do you have pictures of the device?

SWIM - 9-2-2018 at 19:46

Quote: Originally posted by Vosoryx

What's the cut-off pressure of your fuel pump?
If you're using a water aspirator, i think NEMO is right. But I know you can buy aspirators for ethylene glycol, so you might be able to find one for diesel.
I also seem to recall that air is not very soluble in diesel. Since aspirators rely on solubility, this might impact the vacuum it can pull.
Do you have pictures of the device?

I thought they relied on the Bernoulli effect?

Is that an actual aspirator there, or just some pipe fittings you put together in the shape of an aspirator?

Without smooth, flowing transitions and well shaped nozzles It's a lot harder to get vacuum.

That is assuming I understand how it works, but if solubility is what makes the mare go, I'm totally lost here.

Vosoryx - 9-2-2018 at 20:24

Yeah, if it's an actual aspirator or not would also have effect.

My understanding of aspirators:

They do rely on the Bernoulli effect. (AKA Venturri effect) That's how the water gets going fast enough. Once inside the "chamber" any gases that come into contact with the water may dissolve into it, and then the water exits the aspirator. The speed is required because of the lowish solubility of some gases in H2O. If the water weren't moving out of the "chamber" at high speeds, no more gas would dissolve into it. This is why cooling the water bath with ice helps the vacuum - the cooler the liquid, the higher the solubility of the gas.
Am I wrong?

YES

Sulaiman - 10-2-2018 at 02:23

NEMO-Chemistry - 10-2-2018 at 03:27

Also you dont put a bend that close to where you want the aspirator, the water slows at bends. The drop should be bigger...

Sulaiman - 10-2-2018 at 06:00

The datasheet for the pump gives 8 feet lift max for diesel ...c 6.5 feet water ... c0.2 bar which I believe is waaay to low for a water aspirator vacuum pump.

NEMO-Chemistry - 10-2-2018 at 06:08

Try pissing down the tube, might be a better option. other than that get a water pump.

CouchHatter - 10-2-2018 at 06:51

Thanks for the advice! I didn't think diesel was THAT much more viscous than water, but hadnt accounted for the air solubility or how low temperatures would affect it. (Duh!) Yes, you're right about the 8 feet of lift being sorely inadequate. I thought it relied merely on the displacement of air, but now I see that air dissolution is a part of the same principles? Sulaiman, do you care to explain your YES?
Fuel instead of water because it was free; I wanted to see what it was capable of. I had plans to test it with a non-valuable distillation and see if it introduced contaminant into the system after I had implemented a cooling system (not pictured).
You're probably right Nemo, a better vacuum could be achieved by pissing or taking a couple deep breaths. I will be looking at other solutions.

Vosoryx - 10-2-2018 at 09:39

Quote: Originally posted by CouchHatter

Sulaiman, do you care to explain your YES?

I believe he was answering my question "Am I wrong?" about how aspirators work, albeit without telling me why. I even checked on this before I made that post, and I found resources explaining that position. It does rely on the venturi effect, as well as gas solubility.
In order for the air to be removed via the venturi effect, it has to have at least SOME solubility in the liquid.

Reboot - 10-2-2018 at 12:29

Aspirators need a fair bit of water/fluid pressure to create the vacuum. My guess would be that the transfer pump is designed to deliver relatively low pressures, which could be the problem. Your layout looks fine.

Magpie - 10-2-2018 at 13:05

An aspirator uses the venturi principle to create a vacuum. This is based on the Bernoulli equation. Look it up, please.

I don't think gas solubility has anything to do with it moving air. The air is merely entrained in the outflowing motive liquid, which could be any liquid. And any gas could be removed by the created suction.

I have used steam jets to suck liquid at work. Same principle.

Temperature comes into play because of its effect on the vapor pressure of the motive liquid. If the temperature is too high an adequate vacuum cannot be created.

Diesel should work as a motive fluid but may have vastly different performance characteristics than water, as has been discovered.

Aspirator internal geometry is critical requiring proper dimensions. I wouldn't try making my own. Why would I? I can by a perfect chrome plated brass one from Carolina Biological for around $25. With this and cold water at 60 psig at my lab tap I can suck a very good vacuum, ie, around 28.5 mm Hg.

Dr.Bob - 10-2-2018 at 14:03

Your fuel pump is not going to create enough flow and pressure. You really want water at 100 psi or so, and a good flow to create a vacuum with an aspirator. Most pumps (especially cheap ones from places like Northern) are rated based on pumping liquids with almost no resistance or back pressure or head. Thus under any real load they choke.

The diesel also may have a higher vapor pressure than water, which also hurts the vacuum. Also just a bad idea to use a combustible or flammable liquid in an aspirator which is used with any chemical process. You can find cheap ways to create a vacuum, but this is not a good one.

NEMO-Chemistry - 10-2-2018 at 14:17

Quote: Originally posted by Reboot

Aspirators need a fair bit of water/fluid pressure to create the vacuum. My guess would be that the transfer pump is designed to deliver relatively low pressures, which could be the problem. Your layout looks fine.

My maths is my weak spot, but two 90 degree bends that close to the pump and venturi is at least a loss of around 21% in available water/fuel pressure.

I am positive my figure is off, but not way way off. Pipe stuff isnt my bag, but trust me two bends on an aquarium system pump like that would be a no no.

So calcs wrong, principle absolutely spot on from messing with water and pumps in aquariums.

Magpie - 10-2-2018 at 14:21

Agree, NEMO. 90° bends are killers on flow and introduce asymmetric turbulence. Venturis love symmetric flow and high velocity.

[Edited on 10-2-2018 by Magpie]

Sulaiman - 11-2-2018 at 00:46

If you do not have free high pressure water then for vacuum filtration I can recomment this type of pump
https://www.ebay.co.uk/itm/DC-12V-24V-80Kpa-Micro-Piston-Vac...
mine runs continuously for days when required.
Occasionally i have dismantled the pump for cleaning- quick and easy.
It is not chemically compatible with all solvents but so far no problems.
I belive that you could buy several of these pumps for the cost of water pump + venturi pump.

Vosoryx : sorry - I was in a 'mood' at the time.
I'm almost certain that solubility is not required (e.g. mercury could be used as a pumping fluid)
however, most of my learning has been here, youtube and Wikipedia e.g. https://en.wikipedia.org/wiki/Aspirator_(pump)
so there may be more going on than I understand.
(There usually is

Vosoryx - 11-2-2018 at 09:29

Yeah I think you are right here.

Don't know why I thought solubility was a factor, but I guess I did.