Sciencemadness Discussion Board

Question about cold traps

Hilski - 8-1-2007 at 23:38

I have a Gast lab vacuum pump, which has carbon fiber vanes, and uses a separate siphon type oiler, instead of actually having oil inside the pump. I plan to use the pump to vac distill things like benzaldehyde and other non-corrosive substances. I want to make a basic vacuum trap to prevent damage to the pump if some non compressable were to get into the suction line. But what I wanted to ask was if I really need to use a cold trap? Like I said earlier, there is no oil in the pump to contaminate, and I will not be using the pump to distill corrosive liquids.

This pump is made specifically for this type of application, and the literature makes no mention of using a trap other than the filters that came with t he pump.

Here's a picture of the pump showing the oiler plumbed into an intake port on the side.


Thanks in advance.

[Edited on 9-1-2007 by Hilski]

matei - 9-1-2007 at 03:29

Hilski, I think that kind of pump doesn't require a solvent trap, because from what I can tell from thet picture it does't make a vacuum better than maybe 10 torr. At that pressure the quantity of solvent which can get into the pump is small. However when you distill something you must use an efficient condenser.
Cold traps are used only for rotary oil pumps when you want to achieve a vacuum of 10e-2 torr.
For a rotary oil pump a good alternative for a cold trap would be to use 2 traps connected in series, one filled with activated carbon and one with KOH pellets, but in this case the vacuum would be 1 torr or so.

Hilski - 9-1-2007 at 08:02

Quote:
Cold traps are used only for rotary oil pumps when you want to achieve a vacuum of 10e-2 torr.

OK. That's good to know.
For some reason I was thinking that you were pretty much always supposed to have one, but like you said, my pump doesn't go to less than 1 torr. This pump is good for about 27 inches of mercury which, if I remember correctly is somewhere around 60 - 70 torr.

Thanks

Maya - 9-1-2007 at 10:19

You might as well use a water aspirator which can go as low as 15-20 torr.


Otherwise , if looking for a cold trap then you may want to invest in a SAVANT, You can sometimes pick them up fairly cheap from a famous place to get 'em. I got two , one for $9 the other for $50 and one works down to -86 deg and the other one doesn't :o

Hilski - 9-1-2007 at 14:19

Quote:
You might as well use a water aspirator which can go as low as 15-20 torr.

If I had a aspirator vacuum station built, I would definitely use it. But since I don't, I will be using the pump.
But for some reason, I find it hard to believe that an aspirator can pull more than the 27 inches of mercury that this (or any good vacuum pump) can pull. I've worked with all sorts of vacuum pumps in refrigeration, and even the best of them can't get much better than about 28 - 29 in Hg. The only system I've used that can get very close to 30 inHg had a huge diesel driven compressor that was used to pull a vacuum on the high side of the system before shutting it down.

matei - 9-1-2007 at 15:09

Quote:
Originally posted by Maya
You might as well use a water aspirator which can go as low as 15-20 torr.


I think 15-20 torr is the theoretical limit which a water aspirator can achieve (that is the vapour pressure of water at 10 deg. C). In practice I don't think they go below 40-50 torr.

MargaretThatcher - 9-1-2007 at 16:24

Quote:
Originally posted by matei
Quote:
Originally posted by Maya
You might as well use a water aspirator which can go as low as 15-20 torr.


I think 15-20 torr is the theoretical limit which a water aspirator can achieve (that is the vapour pressure of water at 10 deg. C). In practice I don't think they go below 40-50 torr.


Not so. Most lab. water-jet pumps will pull down to the vapour pressure of water. Even homemade ones get down that far. Just float ice in a recirculating setup and you will achieve a really good pressure.

Maya - 10-1-2007 at 04:29

If I had a aspirator vacuum station built, I would definitely use it. But since I don't, I will be using the pump.
But for some reason, I find it hard to believe that an aspirator can pull more than the 27 inches of mercury that this (or any good vacuum pump) can pull. I've worked with all sorts of vacuum pumps in refrigeration, and even the best of them can't get much better than about 28 - 29 in Hg. The only system I've used that can get very close to 30 inHg had a huge diesel driven compressor that was used to pull a vacuum on the high side of the system before shutting it down. >>>>>>>>>

I've got down to exactly 15-17 torr measured with a high precision homemade mercury manometer which was precise to 1 mm. I didn't even have to cool it below 10 deg

I think most pumps will do better than 28-29 in Hg, not putting your pump down. Maybe you should check the vanes or oil for cleanliness or contamination which is not letting it achieve a better vacumm?

Chemist514 - 10-1-2007 at 08:51

Quote:
Originally posted by Hilski
I have a Gast lab vacuum pump, which has carbon fiber vanes, and uses a separate siphon type oiler, instead of actually having oil inside the pump. I plan to use the pump to vac distill things like benzaldehyde and other non-corrosive substances. I want to make a basic vacuum trap to prevent damage to the pump if some non compressable were to get into the suction line. But what I wanted to ask was if I really need to use a cold trap? Like I said earlier, there is no oil in the pump to contaminate, and I will not be using the pump to distill corrosive liquids.

This pump is made specifically for this type of application, and the literature makes no mention of using a trap other than the filters that came with t he pump.

Here's a picture of the pump showing the oiler plumbed into an intake port on the side.


Thanks in advance.

[Edited on 9-1-2007 by Hilski]


This site has amazing info on vacuum...
www.belljar.net
and there is this guy who is amateur-aware so to speak...
http://www.ee.ualberta.ca/~schmaus/vacf/index.html

The very best of luck! I'm having a blast with my donated ED 35 High Vacuum pump, but really must locate a real dessicator now. You happen to know anything about an Alcatel CFA 212 pirani/penning monitor? I have one of those but no probes... always the case. :< (I'd love to get it working, reads down to 10-7.
again, the very best of luck to ya.

chemrox - 19-1-2007 at 09:42

Quote:
Originally posted by Maya
You might as well use a water aspirator which can go as low as 15-20 torr.


Otherwise , if looking for a cold trap then you may want to invest in a SAVANT, You can sometimes pick them up fairly cheap from a famous place to get 'em. I got two ,

Could you be a little more specific here? Or send me a U2U? Thanks

cold trap

chemrox - 19-1-2007 at 09:48

I just flashed off a liter of xylene using and inefficient trap at around 30mm Hg and messed up my pump oil big time. I keep a gallon of oil on hand. I need a better trap. Improvising with a flask and ice bath didn't do it and I need to do some plumbing before I can use my aspirator. That Buchi coil condenser just isn't that efficient.

Maya - 19-1-2007 at 10:05

Try using a regulator with that rotovap, sometimes you get too much vacuum at first and solvent flashes over, I don't think rotovaps are suppossed to be used less than 10 torr, at least not with large quantities of solvent

Hilski - 19-1-2007 at 10:18

Quote:
I think most pumps will do better than 28-29 in Hg, not putting your pump down. Maybe you should check the vanes or oil for cleanliness or contamination which is not letting it achieve a better vacumm?

I didn't know it at the time I bought the pump, but despite the fact that this pump is designed specifically for laboratory use (distillations, filtering etc) it is only good for 26 inches of mercury (about 100 torr) right out of the factory. The pump is distributed by Fisher scientific, and is stupid expensive if purchased new (I only paid 50 bucks) so it doesn't make a whole lot of sense that it is so limited. I did disassemble the pump and clean it, but everything looked great. The thing hadn't been used much at all.
Granted, one can do a lot with 100 torr, but I'd much rather be a lot lower. So, I decided to make an aspirator like the one on Frogfot's website. I was able to get to under 30 torr with the homemade jet pump, and with some tweaking I think I can get more out of it. This thing is almost too good to be true, since it cost me all of 0$ to make. Now I just need to get a spa pump so I can build a recirculating system, instead of wasting so much water.
Quote:
The very best of luck!

Thanks. I for one need all the luck I can get.

Hilski





[Edited on 19-1-2007 by Hilski]