I do my best to avoid becoming an armchair chemist, so today I did a miniproject while I wait for chem supplies to arrive. So far, I've used fridge
compressors to pull a vacuum, but now stage I is dying and stage II is also coughing up a lung, so I'm building a low-vacuum pump from stuff I had
lying around (plus a few dollars worth of screws and brackets). Video of it operating so far: link. The silvery stack of shims under the piston rod are temporary crosshead guides. Here are more pics:
With vacuum receiver
With vacuum filtration
Close up of crosshead bearing
Close up of improvised crank mechanism
My old system mabuse_ - 28-6-2012 at 02:38
This is cool, I like it
There must be some kind of valve to keep the vacuum during the exhaust cycle of the piston and eject the sucked up air somewhere? How do you implement
that?
I just tried an old vacuum cleaner for my buchner setup, it has three power settings and even the smallest works really well. Cumbersome and loud, but
if you have this thing around anyway, why not...Pyro - 28-6-2012 at 07:05
you could have 2 of those working in unison, just put another square of wood on top of that one, and have the same setup, but so that syringe 1 is
open while syringe 2 is closed.
totally awesome idea!Hexavalent - 28-6-2012 at 07:48
Very nice idea and very inventive!
How strong is the vacuum generated here?bbartlog - 28-6-2012 at 09:16
Neat! Like mabuse I wonder where you've put the exhaust valve. Also, I am not so sure that a hand drill will have good characteristics as an input
motor here... it looks like you'd want something with higher torque and lower RPM. If you actually apply enough torque to drive this thing at typical
drill speeds (even as little as 60RPM) I expect you will have problems with heat from friction in your piston.DoctorOfPhilosophy - 28-6-2012 at 10:50
Thanks everyone!
The projects not quite finished yet - to direct the airflow, the crank web will be shaped into a cam and the cam follower will be pulling on a spring
loaded wire (much like a bicycle brake) that opens and closes the blue valve on the front.
I'd worry about using a vacuum with chemicals, isn't the room gonna fill up with contamination?
There is a lot of ways to configure multiple syringes for more steady gas flow, but the valving mechanisms start getting complicated. I wonder if one
way valves would be too leaky... The syringe I used was lying around so I thought I'd do something with it.
To power it I used a massive Bosch concrete drill, so for a small design there's enough torque, and I can adjust the speed to a reasonable value. For
a larger design though (I hope to make entirely of PTFE!) yeah, it'd need some gears (unfortunately I can't think of any other electric motors in the
house I can hook up easily.)
I do expect it to meltdown but I'm not sure at what speed. I'll put some vacuum grease on the piston and see how fast I can drive it before I need to
replace the syringe. And I'll be sure to post the vacuum quality once I have it!bahamuth - 28-6-2012 at 12:18
You are very inventive and handy, I give you that much. But that thing will not last more than 50 strokes even at 0.5 cycles a second...
There is a complete thread about fridge compressors as vacuum pumps, would bet my money on that instead of using ones time to test
that thing.
Only thing I can see that would help that thing is to feed it silicone oil through the cylinder liner at low dead center (piston max out) in such a
way that some oil greases the i front of the rubber piston too, and subsequent removed by passing exhaust into a oil reservoir and out.
Another thing is that the syringe piston rubber swells several times its original size by virtual any solvent, and also is prone to seize after short
term inactivity.
Additionally, the redirecting valve needs to be closed long before the return of the piston from low dead center as to not let any gas into the
vacumized system and this will induce a high strain on the piston and cylinder creating alot of heat. Watercooling would aid in that but then again
why reinvent the wheel.
Piston/cylinder pumps of PFTE is a bad idea, as it has a insane friction coefficient and isn't really a solid, more like a sintered solid. To get a
PFTE piston pump vacuum tight one would get a meltdown very fast.
Unless, you use high viscosity oil to create the seal, and as above recycle it.
I'm sorry if I offend you but I can draw parallels between this and weed smokers building intricate bongs of anything. You want vacuum, buy a membrane
pump or build one yourself, and believe me it is cheaper to buy one, even if you have to guard ebay like a vulture for months. Or an aspirator pump,
if water supply is an issue or costly, add a reservoir and a 4-6 bar water pump.
Or just harvest a fridge...
Any which options you choose you will need to clean the input gas, to remove anyvapors of particles from entering your pump. This is often
task-at-hand determined and there is a lot of threads about that too. Membrane pumps by the way is superior in that they can cope with particles and
small amount of fluids without virtually any wear or explosion from not being able to compress the liquid as it reaches top dead center since the
membranes usually is slightly elastic or will just rupture and replacing one is easy.
Hope you take my critique as an aid and not as me trying to put you down.DoctorOfPhilosophy - 28-6-2012 at 19:29
Thank you, and no offense taken, I've also been criticized of being too dismissive. having said that, the video link above shows that the pump can in
fact last for about 20 seconds at ~60RPM. Keep in mind the piston isn't even lubricated in the video, and it only heated up mildly. You might argue
that there is less airflow pumping a vacuum, but when the piston is at bottom dead center and the valve vents to atmosphere, air flows in before it's
pushed out. This provides some measure of cooling. The pump can operate at a continuum of RPMs from 0 -> [some large number]. Somewhere along the
way it's gonna fail. The question in my mind is how good of a vacuum can it pull at this max speed. And I'm confident it's going to give something,
however little.
The videos on the "let this signal the end of the fridge pump questions!" thread aren't working. Plus I've been using one for a while (as I mentioned
in the first post) and despite taking the best care of them they're dying. Plus I have no way of checking the oil level, servicing them, or seeing if
they contaminated inside.
I'll definitely play around with my high vacuum grease but I'd imagine applying before running the pump should be enough to keep it oiled. The rubber
plunger is not ideal, but I'm hoping to get a polyethylene-only (yes I've seen them on sale) plunger and compare the vacuum quality. I don't see why
the valve needs to be switched before bottom dead center, it seems that at top dead center the time is the most crucial, and even then, it has to be
exactly at top dead center and not a bit off either way to avoid backflow.
PTFE insane friction coefficient? huh? According to Dupont, PTFE has a friction coefficient of 0.05-0.1 http://www2.dupont.com/Teflon_Industrial/en_US/tech_info/tec... Apparently that's the third lowest of known materials. You have a point about it
being sintered though, I read up and PTFE does have a high gas permeability. Nevermind that idea then.
Anyway as I pulled from BromicAcid's signature, H. St. C. Deville once said "There is no need to argue if an experiment can be made". It's not a
heavily invested project, so in the worst case scenario, I will have proven your point!
I appreciate the analysis though SulfurApothecary - 28-6-2012 at 21:04
I like it very much, I think it is ingenious.bahamuth - 28-6-2012 at 21:56
That is completly correct, but a truth with modifications. The friction coefficient mostly has to do what you measure it against, And due to the
graininess of PFTE one should not use it on both parts of a moving machinery, like glide bearing and axle, piston and cylinder and so forth. This can
be tested with a PFTE rod in a drill against a PFTE plate or solid, it will disintergrate very fast.
And I can not figure out for the life of me why I didn't catch that you said you had been using a fridge compressor, and with a picture
A tip, if you want to pursue this you should devise a membrane valve (reed valve) system for input/exhaust gas, those are superior in opening and
closing and are "self" run so to speak. Also I think the all PE syringes (seen those too) are a better option as you stated.
And as the rest I would like to see the final result, even if it doesn't work, as learing by failure is half of the fun
