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Author: Subject: Everything you wanted to know about Aspirator "pumps" but were afraid to ask
jack44556677
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[*] posted on 15-3-2019 at 03:08
Everything you wanted to know about Aspirator "pumps" but were afraid to ask


Hey All!

I've been lurking for a little while now, and am excited to greet you all in my first tome/post! Salutations!

Due to my lingering childhood fascination with aspirators, I'd like to start (and ideally finish) THE "sticky-able" reference thread for all things aspirator. I'm unfortunately much too green to accomplish this, and am hoping some of the more knowledgeable here might be interested in helping me.

It is my (lofty) intention to refine this thread with compiled answers from here and elsewhere until most all reasonable questions find an intelligible answer here.

In general, there is a critical lack of understanding and data surrounding aspirators that make it impossible for anyone to make a well informed decision about what vacuum generation apparatus to use. Hopefully this thread will provide (ultimately) people with the metrics they require to understand and compare the options critically, prior to purchase.

Thread Goals :
  1. List and Answer Open Questions
  2. Establish reference table of aspirator performance based on, ideally, 3rd party or manufacturing testing / datasheets or our own testing / measurement / experience.
  3. If possible, provide the mathematical framework for a "simple" estimation of vacuum generated by an aspirator based on water pressure, temperature, flowrate, flow area, and physical dimensions. I've made some progress on this, but I don't think I have things quite right. None of the Bernoulli's formula variants I've seen factor in the temperature of the water (though maybe this would be done with viscosity parameters), so that would be extra points I should think. I may have to make another thread for this one alone...
  4. Based on #2, hopefully come to some conclusion on what the best / most efficient model aspirator (and material used in construction) is.


Open Questions :

Q1. Are virtually all aspirator models above a certain PSI / flowrate able to reach the maximum vacuum as defined by the vapor pressure of the water (at the temperature it flows through the aspirator)? Based on testing conducted by a 3rd party chemist on Humboldt aspirators I have as well as anecdotes on this site, this seems likely... Can anyone confirm this? There seem to be quite a few under the mistaken impression that -25 inHg is "the best you're gonna get" from a tap, so it would be nice to set them straight and or be able to provide real metrics for what the performance of your tap and/or aspirator needs to be to achieve vapor pressure.

A1. TBD

Q2. Of all the variables (ideally in the equation specified in #3 of thread goals), which one makes the most impact / difference? e.g. If I have 60 PSI will it not matter much that I only have 1gpm? etc...

A2. TBD

Q3. This is the main reason I made this thread, so bear with me : https://en.wikipedia.org/wiki/Aspirator_(pump) - mentions that "Steam ejectors with two, three, four, five and six stages may be used to produce vacuums down to 2.5 kPa, 300 Pa, 40 Pa, 4 Pa, and 0.4 Pa, respectively". I got lucky and found a copy of the book referenced and based on my understanding of what I read, it seems that the reason multiple steam aspirators are used in "stages" is because pumping steam against regular pressure requires significant steam flow to accomplish, and the amount of steam you can push through a steam aspirator (usefully) is functionally limited by the steams tendency to become water if put under enough pressure. This water causes a "choking" effect in the steam aspirator operation, and because when pumping against a lower pressure than atmospheric (second stage and beyond) induces / requires less input steam flow, the subsequent stages don't have this problem as much even when you jack the pressure up. Another resource (lost the url) about general water aspirator function said that combining water aspirators in stages could only be used to increase vacuum flow (lpm/cfm) and will have no effect on maximum vacuum. Is this right? I am sure this statement assumes you have maxed out at the vapor pressure of water in the first stage, which you ought... but in the case where water pressure was inadequate (or flow rate, or pressure, or aspirator), could setting up water aspirators in stages be beneficial?
A3. TBD

Q4. https://en.wikipedia.org/wiki/Aspirator_(pump) also says "Condensers may be used between stages to reduce the load on the later stages." It is specifically talking about steam aspirators, but both here and elsewhere I have seen mentions that cold traps and other condensers can be used, even only single-stage water aspirators, to generate vacuum below the vapor pressure of water, and generally lessen the load on the vacuum source. Can someone confirm this who has experience or documentation on it? How significant is the vacuum increase? Can you really go beyond the vapor pressure of water in a single-stage water aspirator setup?
A4. TBD

Q5. The book I mentioned in Q3 it makes clear that the steam aspirators are all custom built / designed for their specific, usually industrial, purposes. As such, the first stage is designed to handle a specific flow of steam at a specific pressure, and the subsequent stages (if present) are also custom designed for optimal function. The implication in the book is that if you tried to use the exact same aspirator for the first and second stage, it wouldn't work (well? / at all?). How much does media affect aspirator design? All of the designs for steam aspirators in the book (1966) look exactly like the standard agricultural venturis (perhaps only differing in scale). Many of my aspirators have "Airejector" stamped on them, and though they are only ever sold as water aspirators, I HAVE seen other sites selling the same model (looks identical from the outside, and matches scale too!) for use with compressed air. Venturis are so crazy simple, so it seems reasonable that the media doesn't matter much if you have a working aspirator design, but obviously though they appear simple they are quite difficult to understand. Can/should I use my water aspirator with air input, or steam? I intend to test this one at my earliest convenience, but currently don't have any pumps of any kind (other than aspirator! only figuratively called a "pump" as the actual "pumping" is still being done by a traditional pump... it is more like a pump media converter).
A5. TBD

Q6. There are many anecdotes supporting glass aspirators as the best / most efficient... Is this really true? Their designs are much more intricate than the metal ones, does anyone know where their designs are coming from and why they are so massively different than their metal or plastic counterparts?
A6. TBD

[Edited on 15-3-2019 by jack44556677]

[Edited on 15-3-2019 by jack44556677]

[Edited on 15-3-2019 by jack44556677]

[Edited on 16-3-2019 by jack44556677]
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Sulaiman
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[*] posted on 15-3-2019 at 05:45


With almost all pump types there are TWO parameters that interest the user
- flow-rate, and pressure (difference or absolute)
- both need to be considered to make (or characterise) a practical pump.

I do not use an aspirator, mainly for environmental/safety reasons.
They either require a significant continuous flow of water from source to sewer,
(in my case un-metered, but that's no excuse for waste)
or
the electrical power consumed for water recirculation and cooling is more than that of an equivalent mechanical vacuum pump.
and
with a mechanical pump you must (or at least should) scrub your exhaust gasses,
an aspirator enables/encourages dumping of toxic and/or corrosive gasses into the sewer system.

I do see the academic interest in aspirator design,
(one of my current projects is a precision pendulum clock)
but I'm sure that you will just be re-inventing the wheel - without tyres.

P.S. if it is only the Bernoulli/Venturi effect that needs to be modeled,
why does the length of the water out pipe matter? :D

[Edited on 15-3-2019 by Sulaiman]




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[*] posted on 15-3-2019 at 06:07


Steam ejectors used to be a very common way of producing vacuum in the process industries (if you're looking for more detailed info on their design and operation try Perry (Chemical Engineers Handbook) which has a whole section on them, but lots of chemical engineering and fluid mechanics books will also cover them.) The reason they were so common was that they had no moving parts (so less maintenance), hp steam was onhand throughout the site and anything you could do to avoid electricity on a lot of chemical sites was desirable from a safety point of view - hence why so much of the old control systems were air based. Life (and technology) has moved on, vacuum pumps have become cheaper, sealing materials more durable etc and steam ejectors no less inefficient! Similarly in undergraduate labs and schools water powered aspirators avoided mixing electricity and fluids (most of the heating was gas bunsens etc) and pumps were an expensive investment.
I'm with Sulaiman if you can afford a vacuum pump (and a decent one is only ~£40 these days new) then avoid the waste and environmental issues and get yourself a better and more reliable vacuum :)
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[*] posted on 15-3-2019 at 06:21


i've never heard of any amateurs using steam vacuum aspirators, a water one already needs some kind of setup (to be portable and eliminate water waste), steam is way harder, and if you really need to go as low as a few pascals, a rotary vane vacuum pump can be bought now from ebay for less than $60.
@Sulaiman The reason for the importance of the exhaust pipe is to for its function as a lock. i once triet my aspirator with an exhaust tube a bit too wide (transparent vinyl tubing) and i couldn't make any vacuum even with water full blast. i could see the exiting water forming a jet inside the tube, but not touching the sides, but as i put my finger on the tube opening to make some resistance, the tube filled with water and once i removed the finger the water remained adeherent to the walls of the tubing, the vacuum was now on. i had succes forming a vaccum even with a short piece of tubing by plunging the end a few cm under water.





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[*] posted on 15-3-2019 at 08:49


Quote: Originally posted by Ubya  
... @Sulaiman The reason for the importance of the exhaust pipe is to for its function as a lock
... i had succes forming a vaccum even with a short piece of tubing by plunging the end a few cm under water.


Thanks ... how counter-intuitive is that.
I suppose I'll have to make an aspirator one day - just to play.

EDIT : my brain just melted...
if there is a column of water in the water outlet tube,
why does the vacuum above it not suck up the water ?

[Edited on 15-3-2019 by Sulaiman]




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[*] posted on 15-3-2019 at 09:57


Quote: Originally posted by Sulaiman  


EDIT : my brain just melted...
if there is a column of water in the water outlet tube,
why does the vacuum above it not suck up the water ?

[Edited on 15-3-2019 by Sulaiman]


probably momentum given by the speed of the jet or the 100psi water stream from the tap is enough to resist to the vacuum pulling the column back





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