Sciencemadness Discussion Board

at the risk of being flamed.. another Vacuum Pump question

RedFox - 19-1-2014 at 01:49

Hi All,

im sorry it had to come to this but while UTFSEing and looking around i still cannot come to a definitive answer to the question of a good (solvent safe) vacuum pump that can pull atleast 10mmHg, im my search i have come across KNF diaphragm pumps and they look fairly tough, but i cannot get an answer as to what they will pull vacuum down to, allot are rated at around 2CFM.. from my research (on non diaphragm pumps) one would want something around the 5CFM mark to get down to 10mmHg.. so it only leads me to the conclusion that these 2CFM pumps will get close to the mark but may not be stable? has anyone got any words of wisdom on the topic.. i have UTFSE.. all the way to the end, googled, read site after site etc etc. another question though, is a cold trap required with a diaphragm pump or would it just sit as a safeguard? im looking to let go of around $400 on a pump so i would like it to be able to suit all reasonable needs, and it doesnt have to be new, just a solid unit. ive been all over ebay but no real answers on what vacuum will be developed.

thanks in advance guys

unionised - 19-1-2014 at 02:21

"i have come across KNF diaphragm pumps and they look fairly tough, but i cannot get an answer as to what they will pull vacuum down to"
How hard did you look?

http://www.knfusa.com/products/laboratory-pumps/product/cate...

Also, what are you hoping to do with the pump?

RedFox - 19-1-2014 at 02:36

Hi unionised,

thanks for the reply, i did find the KNF website. thankyou for the link to what looks like the "N800 series of pumps" ill have a look through the info, what i have seen so far on ebay is "MODEL UN726.3 TTP" im not sure where these pumps will get to in the way of pressure, im looking for a bit of an all round pump that will serve me for a few years, i dont want to be too limited with not being able to distil over some of the higher boiling point products from reactions, and also i want to still off and recover solvents, i realise this probably wont need <10mm but ill hook up a bleed valve to regulate pressure, i really just want a pump that im not going to kick myself in 6-12 months when i want some more serious vacuum. thanks for the reply!
Quote: Originally posted by unionised  
"i have come across KNF diaphragm pumps and they look fairly tough, but i cannot get an answer as to what they will pull vacuum down to"
How hard did you look?

http://www.knfusa.com/products/laboratory-pumps/product/cate...

Also, what are you hoping to do with the pump?

Organikum - 19-1-2014 at 04:39

A PTFE corrosion resistant diaphragm pump with at least 2 stages (better 3 or 4) is what you want now.

KNF is good but expensive, Pfeiffer, ILMVAC and Vaccubrand are very good and there is something funny as they all have the same pumps just a little different looking.
ILMVAC is best for the money.

Or buy a used one which is rated "kaputt" (no good vacuum anymore), those one gets often very cheap and the repair set including tools is less then 200$ for a two stage.
After this its better then new.
Used ones often come with controller an stuff whats worth it.

Vaccubrand MZ2-C (C for chemical resistant) or the similar from ILMVAC. Thats it now.
Used with controller and exhaust condenser + valves for 250€ in Germany. Ebay classifieds ("Kleinanzeigen") is the secret tip.
Was secret. lol

If you later need high vacuum you use the diaphragm as forepump and get a cheap two-stage rotary vane oil pump for less then 100.-, invest another 100.- for fluoro-something oil and you have high vac without frozen CO2 trap and all the hassle. Without changing oil every time. And the rotary corroding away in no time.

Good luck
Take time and hunt something decent down, its worth to wait and search!

/ORG

Dr.Bob - 19-1-2014 at 19:53

A diaphragm pump can work pretty well for most rotovaps and simple distillations, even for DMF, and other mid high boiling solvents. Most will pull down to 10-20 mm (maybe less), depending on condition, temperature, etc. The CFM rating is only valid for an open system, so it does not directly determine the ultimate vacuum, as that is based on pump design and leakage, as much as flow, since there will be little flow at high vacuum.

I have used several brands, and they mostly all work similarly (in the same price/CFM rating range) but some of the cheaper ones will break more often. The KNF and Vaccubrand are both good in my experience. I have also used VWR ones, and they seem to have slightly more issues, but still work OK.

RedFox - 20-1-2014 at 09:21

Thanks Organikum, exactly the info i was looking for!

thanks Dr.Bob, i thought it was strange why i was getting allot of CFM ratings and no real pressure ratings, obviously i found out i was looking at the wrong type of pump, it seems like with a 2nd hand chemical resistant pump even if you look at a decent brand/model, it really comes down to the age and wear on the unit, i would rather spend a couple hundred extra to get something thats going to last instead of a quick solution, or even a rebuild on a 2nd hand purchased unit. the Vacuubrand MZ2C is looking very attractive at the moment.