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Author: Subject: recirulating vacuum aspirator troubleshooting
burbble
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recirulating vacuum aspirator troubleshooting

Hello!

My latest hobby project is attempting to make a vacuum evaporator. This is my first foray into anything like this, and I'm running into a little trouble with the vacuum aspirator part. I was hoping maybe someone here had some ideas and could help a noob out

First, some background. My ultimate goal is to do vacuum evaporation of foods. This means a good amount of water will be getting pulled out, so the venturi aspirator route seemed like the simplest way to deal with that (compared to building a cold trap, for example). Since the pump could be running for long periods of time, I'd also like to avoid wasting a ton of water.

Now here's my setup so far:
- A bucket with a couple gallons of tap water, about 55°F/13°C.
- A diaphragm pump, a Remco 5537. This is the sort of pump used to provide water pressure for RVs. I figured this would provide a decent approximation of household water service.
- Pump inlet connected to a 1/2" rubber hose, suctioning from the bucket.
- Pump outlet connected to a venturi style aspirator pump, Humboldt H-12110.
- Aspirator output connected to 3/8" vinyl tubing, returning water to the bucket.
- Aspirator vacuum line connected to an 8 gallon vacuum chamber with 5/16" silicone vacuum tubing.

The problem is that the vacuum flow rate seems to be abysmally slow. After running the pump for 15 minutes, the chamber only got down to about 1.2 inHg of vacuum.
The good news is that the chamber is holding this very small amount of vacuum, which suggests the piping is not leaking. The bad news is that I'm not really sure why it's pulling such a weak vacuum.

From what I've read online and on this forum, I expected an 8 gallon chamber to be evacuated within a couple minutes, not a couple hours.
Does anyone notice any obvious red flags in my setup? Pump type, tubing size, water temperature? Or are my expectations just wrong?

Oh, this is the water pump's spec:
 Code: PSI GPM Amps 0 5.11 8.00 10 4.75 9.50 20 4.30 11.00 30 3.85 13.00 40 3.40 15.00 50 3.00 17.00 60 2.55 18.00
A quart jar and a stopwatch indicates about 3GPM of flow from the water pump, but I'll need to check the amperage tomorrow to get a more accurate measurement.

Twospoons
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Does the pump have a reservoir? Diaphragm pumps would usually have a pulsing flow, so you'd need a reservoir for smooth flow. Pulsing flow in a venturi would possibly allow loss of vacuum between flow pulses.

[Edited on 12-1-2017 by Twospoons]

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NeonPulse
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I found that having the Venturi pump water outlet submerged under the waters surface in the Reservior substantially increased the vacuum pulled in my setup. Another point to increase the vacuum would be to use ice cold water or better still a more viscous working fluid like propylene glycol or ethylene glycol. I also found that having the aspirator perfectly vertical helps a lot. Can you post a pic of your setup?

Nurdrage on YouTube actually has a pretty good video of his aspirator setup and mine is pretty much identical and can pull quite a decent vacuum. Search for the. Ideo, it should be helpful.

[Edited on 12-1-2017 by NeonPulse]

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Dr.Bob
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Warm water will lower the vacuum, as it has a higher vapor pressure. Also, most aspirators have a check valve, if it is dirty, sticking, or just crappy, the thing will never pull well. I use aspirators often, and with a strong water flow, like from a faucet, it should pull 20". Try testing the aspirator on a strong faucet and comparing that to the water pump.
Maroboduus
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Also be sure the pump isn't sucking a lot of bubbles. This is hard to avoid with a small bucket as the jet of water coming out of the aspirator can make things very turbulent and somewhat frothy.

A larger bucket, or putting some sort of screen or cloth around the aspirator output may help. (They actually make an aspirator attachment that's full of screens and fits over the output end so as to reduce turbulent flow and splashing. I use one of these, but I have seen others get equally good results with some home built gear. The 'lint catcher' screens that go over the output hoses on washing machines seem to be popular for this.)

Your very low suction rate indicates something out of the ordinary is wrong here. It SHOULD evacuate that container in a few minutes. (My aspirator evacuates a 1000ml flask in a few seconds) . And you should be able to get a pretty good vacuum if it's working right.

One thing strikes me on re-reading your description: I have never tried putting a hose on the aspirator output, and have no idea if this could be a problem. I just put the splash arrestor on the end of the aspirator tube and the water just sprays out of that into the water reservoir.

Sorry if I'm being a little vague about the splash arrestor, but it has no brand name on it, and was not acquired in a way that lends itself to going back later and asking questions. It's about 2 inches long and 1 1/2 inches wide and just slips on the aspirator output line. It is black rubber.

burbble
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Thanks for all the ideas! I tried again tonight with a few changes, with slightly better (but still inadequate) results.

First, I tested it without the output hose attached. The vacuum flow rate was noticeably better, the needle on the gauge began visibly moving. Clearly the tubing was too restrictive. I cut it down to just a short piece, leaving just enough to direct it into the bucket, and that no longer seems to be a bottleneck.

I then dumped ice in the water and it got down to around 40°F. Oddly, this made it worse! Water started coming into the vacuum line, towards the vacuum chamber. Meaning the vacuum in the chamber was suddenly stronger than what the aspirator was pulling. Wha?? I understand that the highest possible vacuum using a venturi aspirator is dependent on the water temperature--but I can't explain this at all.

I let some air into the chamber, and tried again. The water got sucked back out of the vacuum line, and after about 10 minutes the chamber got down to around 9.5inHg of vacuum. At this point, the output water was only intermittently cloudy, alternating in rhythm with the pump's vibrations. This lends credence to the theory that the pulsing flow of the diaphragm is interfering with the venturi tube.

The point about bubbles going into the inlet hose is a good one. I have been carefully angling the output hose to avoid shooting air bubbles into the inlet. The pump does make a distinctly different noise when air goes in. I like the concept of a splash arrestor, I'll have to look more into that.

Changing the orientation of the aspirator doesn't appear to have any apparent affect on it. It does have a built-in check valve, which I don't think works particularly well.

I also measured the output flow rate more accurately, and got 2.58GPM. The pump has a 60PSI cutoff switch--and it isn't turning off. The pressure going into the aspirator must be less than that.

On my to-do list for tomorrow is hooking up the aspirator to a faucet and see what the vacuum pull is like with mains pressure. (Need to dig up some pipe adapters first.) Hopefully that will tell me whether the problem is the pump or the aspirator.

Oh, I also I watched NurdRage's video on his setup and noticed a few interesting things:
- He's using a diaphragm pump with about 1/3 of the power of the one I have, but it seems to be rated at twice the maximum pressure.
- The internal design of his venturi device is completely different from mine. I don't know if this matters. His consists of a cone with a small orifice, nested into a tube with a larger hole in the end. Mine is a single tube with a spiral-shaped piece of metal inside.
- The amount of visible air in the output water coming from my aspirator doesn't begin compare to what he showed in the video. It's like fish pond vs soda fountain. Clearly my setup is not working properly

More updates tomorrow. Attached a couple pictures for those interested.

Maroboduus
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Just a guess here, but I suspect having the outlet hose on the aspirator absolutely straight might make a bit of difference too.

I'd try putting a short hose between the pump and the aspirator so the aspirator can be positioned independently of the pump.

Alternately you could try screwing the pump base to a vertical surface with the outlet (and the aspirator) oriented straight down.

Some hose between the pump and aspirator might help (a little) with the pulse problem mentioned above.

[Edited on 13-1-2017 by Maroboduus]
burbble
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I tested the aspirator connected directly to the 3/4" water main. Pulled a 20inHg vacuum in just a few minutes. Could have gone further, just didn't want to waste the water.

Water output was 4.1GPM. So...clearly I need a more powerful pump to drive this thing!
Dr.Bob
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Most aspirators are designed for vertical (faucet) mounting, so the valve may not work well sideways, not sure, but I agree with testing that. I also agree with putting some distance between the pump and aspirator, rubber/plastic tubing makes a good water "capacitor" to even out the flow. Some aspirators are designed for more flow at a lower final vacuum, that might be part of the case. But 20 in Hg is about typical for a water aspirator, so it is OK.
Corrosive Joeseph
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Here's a design has been doing the rounds for years.

Interesting if anyone hasn't seen it before -

https://erowid.org/archive/rhodium/chemistry/equipment/aspir...

Click the links at the bottom of the page for variations of it.

Nice project by the way..............

/CJ

[Edited on 16-1-2017 by Corrosive Joeseph]

burbble
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Yeah I actually tried it with the pump sideways, so the aspirator was in the orientation it would be in a faucet. Ran it for a few minutes that way, but didn't make a noticeable difference.

Thanks CJ, great link. I'll probably look for a 1/2HP jet pump as they suggest. I like the design with the bucket too, that's pretty slick.
PirateDocBrown
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Here's the pump I'm using:

https://www.amazon.com/gp/product/B01IXCK5XI/ref=oh_aui_deta...

I got the aspirator surplus from a local university. Opening it up, the check valve seems sound.

I'm not getting any appreciable vacuum at all, but I'm going to try a few changes.

First, I initially used a 6A car battery charger as a power supply, but have now modified an ATX as a lab power source.

I can, of course, rearrange the hoses and positions to see if I can get improvement that way, too.

I'm just looking to get enough to do vacuum filtration.

[Edited on 1/16/17 by PirateDocBrown]
Corrosive Joeseph
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More assorted nuggets of information -

'Help with Water Aspirator Design'

/CJ

Melgar
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Submersing the aspirator probably helps because the inertia of the water in the tank is dampening out the pulsing. If you get another pump, definitely go for a submersible squirrel-cage type one. Or just skip the water pump and just get a diaphragm-type vacuum pump. There's one on eBay for about \$15 that can get down to about 3 psi absolute.

 Sciencemadness Discussion Board » Fundamentals » Reagents and Apparatus Acquisition » recirulating vacuum aspirator troubleshooting Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication   » References Non-chemistry   » Forum Matters   » Legal and Societal Issues   » Whimsy   » Detritus   » The Moderators' Lounge