Octavian
Harmless
Posts: 19
Registered: 25-5-2015
Member Is Offline
Mood: No Mood
|
|
Help: Poor rotary vane pump performance.
I recently upgraded from an aspirator to single stage rotary vane vacuum pump. Now the pump in question is rated as 3CFM and a max vacuum of 5Pa
(0.037 Torr). However when I hooked my gauge up to it I found it would only pull 70 torr, it DID go down to 60 torr for a brief period after being run
non-stop for over an hour. I tried running slightly less oil, tried running more oil, tried draining and refilling with new oil (although the oil in
it to start with was new).
After all of that I ended up returning it and having it replaced, the replacement come missing screws and looking like a dud right from the box, but I
tried it anyway. Filled it with the supplied oil, let it run for a bit, topped it up with an extra 20ml or so to get it to the ideal level on the
sight glass, switched it off while I got my gauge and set it up, run the pump for a good 20 minutes to warm up then connected the gauge - once again
maximum vacuum it would pull is 70 torr.
This is my first time using a real pump so I can't comment on the quality of the supplied oil and make any assumptions as to whether that could be the
issue. Now I have to send this pump back too ($20 each time) and I'm wondering if anyone has any insight or suggestions. Should I even bother
considering another replacement? I'm not really expecting one of these to pull sub-1-torr like they're rated considering it's a $150 pump but it seems
that if a rotary vane can't pull at least 5 torr or lower when brand new with perfectly clean oil it's rather useless.
Attached is a picture of an almost identical pump for reference. Any and all help/suggestions are very much appreciated 
|
|
|
Heavy Walter
Hazard to Others
Posts: 127
Registered: 17-12-2015
Location: Argentina
Member Is Offline
Mood: No Mood
|
|
Hi Octavian
Please comment in the setup you use to measure the vacuum level.
No leaks? New or used gauge? Calibrated or uncalibrated gauge?
|
|
|
BromicAcid
International Hazard
Posts: 3253
Registered: 13-7-2003
Location: Wisconsin
Member Is Offline
Mood: Rock n' Roll
|
|
Your attached photo says 2Pa for the ultimate vacuum. Either way, you're nowhere near that. Do not over or under fill the oil, put it right to the
mark and leave it there. Look at the manual and see if there are any adjustments on the pump itself, sometimes there are. If there are no
adjustments, check any connections you made to the pump. If you put a hose barb or connector on the pump make sure it is sealed/leak free. Use a
short piece of vacuum tubing so you get a good seal and connect a monometer you trust. There is a conditioning period where the seals are breaking in
and the oil is working its way through the system but that's over. The void space in the manometer and tubing should be small, it should pull down
quickly. Once your pump has 'warmed up' for 5 minutes you should see pretty much your ultimate vacuum. If you have never ventured to these vacuum
ranges just be aware that tiny leaks (leaks you cannot feel/see/hear) have the potential to prevent you from getting to your target although I think
it would have to be a decent leak for you to pull 70 torr vs expected. With your gauge attached if you do have any fittings connected to the pump
push down on them and see if it helps. Also make sure your exhaust vent (if attached) is properly sized, an obstructed exhaust will hurt your ability
to pull vacuum.
|
|
|
Octavian
Harmless
Posts: 19
Registered: 25-5-2015
Member Is Offline
Mood: No Mood
|
|
Thanks for the quick replies!
The actual plate on the pump claims ultimate vacuum of 5Pa rather than the 2Pa stated in the ad and flow rate is marked as 3CFM on the pump plate.
Those are the only specs to speak of, the booklet contains the same specs.
The only thing on the pump you can do anything with is the on/off switch, the intake port, the exhaust which doubles as a fill port and the drain
plug.
No noticeable leaks, and it pulled the same vacuum on both a greased GG distillation setup and with just the gauge connected directly to the pump via
about 10" of vac tubing with a hose clamp on both ends just to be sure. The gauge was my first suspicion, so I run a small distillation, the boiling
point was dead on the quoted boiling point for 60 torr (Merck Index) and once the distillation come back to room temp I closed the stopcock to the
pump and turned it off, the vacuum in the system didn't change noticeably over the 2 min I watched it indicating to me that my setup was practically
leak free (Backed up by testing it again connected directly to the gauge). Given the b.p (head temp) was dead on to the degree both the first time at
60torr and the second at 70 torr I'm pretty confident in the gauges accuracy. The gauge was purchased new BTW and also seemed to give spot on figures
for my shitty aspirator that pulls ~200 torr (boiling points of several liquids using a trusted thermometer confirmed that)
IrC,
You don't see the problem in something that is supposed to pull 5 Pascals only pulling to around 8,000 Pascals? Seems most people with good water
aspirators are able to obtain better vacuum than 70 torr?
Thanks again guys, hope the additional info helps!
|
|
|
IrC
International Hazard
Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline
Mood: Discovering
|
|
Ignore my last post if you read it I was thinking torr when I meant microns. A typical Air conditioning use single stage pump should get down to at
least 70 microns. What I get for reading posts when I wake up. Yes you seem to be getting defective pumps if the oil is of the proper type and filled
to the line in the sight glass. I think getting a replacement missing screws is a dead giveaway that the company you are dealing with has serious
issues. I have a similar AC pump that IIRC cost around $125 new that specifies 70 microns. With good quality high vacuum oil it actually gets down to
around 60 microns, I use it for a rough pump for my Alcatel 2004a. This helps keep the Alcatel in top shape.
[Edited on 2-18-2016 by IrC]
"Science is the belief in the ignorance of the experts" Richard Feynman
|
|
|
Dr.Bob
International Hazard
Posts: 2748
Registered: 26-1-2011
Location: USA - NC
Member Is Offline
Mood: No Mood
|
|
I have found little success with cheap, Chinese vacuum pumps, and that is one of the smallest ones that I have ever seen. It should be able to get
lower than 70 torr under no load, but even if it did do that, once there is a load, it will die and do poorly still. It may even have internal leaks.
Also, remember that the numbers that they give may referenced to some specific conditions, like having the vacuum working on the moon, or a CFM
rating based on no pressure or vacuum at either end. So under real use, those numbers fail quickly.
You should be able to buy an AC vacuum for $100 that will do fine, but it will be bigger than that model by at least twice. My cheap diaphragm pump
can do almost 10 mm of Hg vacuum, any mechanical one should do much better. If you look around and can find a used Welch belt drive rotary pump, it
will do amazingly if not too abused. I have been using the same pump at work for 5+ years with no issues, it pulls down to 0.1 torr, according to
our lousy gauge. And it was old when I started here.
|
|
|
JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
|
|
Single stage rotary vane vacuum pumps don't pull a high vacuum. Period. You generally have to use a screw pump or multiple stages for high vacuum.
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
I have a Harbor Freight, $100, 2.5 CFM, made in China, single stage rotary vane pump that pulls down to 1mmHg per my Bennert manometer. If you have
no leaks I can't offer any suggestions.
[Edited on 18-2-2016 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
zed
International Hazard
Posts: 2284
Registered: 6-9-2008
Location: Great State of Jefferson, City of Portland
Member Is Offline
Mood: Semi-repentant Sith Lord
|
|
Seal all of the seams and cracks and connectors with shellac or whatnot. Just crank it up, and brush on your selected sealant, while the pump is
running. The solvent with be evaporated off and the residue will plug all the leaks.
I don't know about your specific pump, but I had a similar problem with a brand new Nelson, I once purchased. Once it was sealed, it was a honey.
Before it was sealed, it couldn't reach full vacuum.
|
|
|
IrC
International Hazard
Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline
Mood: Discovering
|
|
The problem I have is I don't see how a single stage rotary vane can get down to either 5 or 2 Pa as mentioned in the text and pic in the OP. I have
owned many of different brands and either 37.5 or 15 microns seems doubtful. The best one I have in that type is a 2.5 CFM VP2.5 and the spec states
70 microns ultimate vacuum (although as I stated above I have seen it get down to 60 µmHg on fresh new oil). Looking at the one magpie mentioned, or
at least a nearly identical model the rating is 75 µmHg. I do not believe either 37.5 or 15 µmHg is possible even if it is printed in that pic
above. Am I missing something here? I can get down to 0.1 micron with the $850 Alcatel 2004a, don't even turn it on until the single stage has reached
vacuum below 100 microns just to protect the Alcatel from the atmosphere. The best vacuum I have seen from a single stage is as I stated about 60
microns. In any case 60 torr after an hour assuming as mentioned by others there are no leaks in the system indicates they sent you two bad pumps in a
row. Any chance you can get a refund and grab a Harbor Freight off eBay? Every one I have bought from that company has been perfect, getting down to
at least their stated specs if not better.
"Science is the belief in the ignorance of the experts" Richard Feynman
|
|
|
Octavian
Harmless
Posts: 19
Registered: 25-5-2015
Member Is Offline
Mood: No Mood
|
|
The replies were extremely appreciated, so thank you all!
I never expected it to actually pull the 2 or 5Pa claimed but I DID expect to be able to pull at least 5mmHg (torr) tho. I asked the seller if he'd be
willing to upgrade it to the cheapest two stage they carry (1.8CFM) for free since I've been screwed around for 3 weeks now and it's cost me $40 to
send both back but they wouldn't come to the party, so I'll be getting a refund and looking elsewhere - unfortunately I've never seen the Harbor
Freight brand in Australia.
Could anyone with local knowledge point me towards a half decent HVAC pump that doesn't cost an arm and leg? Like I said I would be quite happy with 5
or even 10 torr. Anything corrosive, lower boiling or nasty gets stripped off with my aspirator and given I'm only a hobby chemist I'm not keen to
drop over about $150 on a pump.
For reference so none of my countrymen have to deal with this same issue, the eBay seller in question is "aus-auc.2010". They
DO seem to respond quick enough, but it also seems that they're selling products that they know are defective and/or don't come near living up to the
specs advertised - and to top it off they've recently jacked all their prices up by about $30-40 - maybe to make up for all they are loosing via
postage costs of sending replacements? 
[Edited on 19-2-2016 by Octavian]
|
|
|
ziqquratu
Hazard to Others
Posts: 385
Registered: 15-11-2002
Member Is Offline
Mood: No Mood
|
|
That pump looks very similar to the ones we use around the lab - we buy cheap refrigeration pumps for general lab use, since they're better than 1/5
the price of the "real" lab pumps, and work just about as well (certainly more than well enough for our purposes).
In any case, the pumps we use reliably pull down below 1mmHg - last I checked, with fresh oil, my 3 year old pump could easily do 0.3 mmHg - if you
hook the gauge (I use a mercury manometer) directly to the pump; once you connect it to a Schlenk line, of course, there are some leaks, but I rarely
have trouble keeping it around the 1mm mark even with rather old oil (often below with a little fresh grease on the glass joints).
As to alternative suppliers - look for a refrigeration supply business around you. Actrol and Totaline each have a number of outlets around the country, and have happily helped me out (cash sales) a number of times. Last quote I got from
Actrol was around $230 for a pump (ask for a "Robinaire 15151 or equivalent", they should be able to advise you on what they have or can get - that's
the type we buy and it's rather excellent, we've subjected ours to severe abuse - and I do mean severe, such as one guy who decided that removing HCl
without a trap, several times before we noticed him, would be a good way to use his pump - over many years, and the failure rate in the face of even
that sort of treatment has been quite low. I'd estimate average life, at 12h/day, 7 days a week, to be >4 years). Unfortunately that's the cheapest
option I've found in Australia, but I've found them well worth the money in university labs - they should be entirely awesome for hobby use!
Oh, I do recommend having some spare oil on hand for a quick oil change in case you badly contaminate the pump (as well as for routine maintenance, of
course); a 4L should cost around $25-30, and should last quite some time (the pumps hold around 200mL, as I recall).
|
|
|
Octavian
Harmless
Posts: 19
Registered: 25-5-2015
Member Is Offline
Mood: No Mood
|
|
ziqquratu, Thank you!!! I will contact them as soon as my money gets refunded from this one. Sounds like they can take FAR more use and abuse than I
could ever throw at one - and the fact that some have still performed well after what you described, I wouldn't mind spending a bit more for something
like that!
That's also REALLY cheap for vacuum oil from what I've seen so far, I've seen a lot from google searches selling for around $20+/500ml!
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
