Thank you! (always) Very well thought out response. It is nice to know one can beat the crap out of a pump and not have to worry about a catastrophic failure.
(this is where I check my understanding) I know this piston units boasts it “has a built-in liquid ingestion prevention system; the ball in the cup floats up if any liquid overflow streams in, blocking the inlet.” which to me translates to “It’s gonna stop before it breaks itself, hopefully!” where a rotary vein pump will keep on chugging if one was to make a mistake or two.
Cascade puts the Welch pumps on their filtration kits and I happen to peddle those, so the knowledge is hugely appreciated!
@sidco, I don’t have nearly as much experience with vacuum pumps as I do with air compressor pumps, but if I had to guess, the oil free pumps have shorter MBTF times and are considerably louder, at least thats how things are on the compression side of things.
@Beaker I definitely appreciate the effort to write that explanation, and in a deep vacuum scenario it makes a lot of sense.
The reason I was revisiting this concept was because my AI Cleanvac 11 CFM scroll vacuum pump decided to crap a bearing and eat a tip seal and I’ll be ordering parts to rebuild it shortly. In the meantime I am forced to substitute an oil lubricated Welch or Edwards to provide vacuum to the rotovaps we run, and while I can definitely see some value to aerosolizing a small amount of oil in the course of achieving higher vacuum for ones process, I don’t need ultra-high vacuum as much as I desire less maintenance & mess and consistent vacuum for the equipment we are running. As well, since I’m not here all the time and don’t trust others to care about expensive equipment they didn’t pay for, I worry about making sure that oil levels are checked at regular intervals and want to do everything I can to avoid relying on others to maintain oil levels and killing equipment.
I do already locate my vacuum pump outside to avoid having to deal with fume containment and evacuation inside of the building, I definitely see your perspective that mist collectors are worthless, but I see a considerable amount of oil loss due to it just bubbling/splashing out of the outlet with the oil level between the lines, the mist collector seems to help somewhat just by adding some height to the outlet flange.
A less expensive oil free vacuum solution is AI’ s WaterVac07. I have a freezer full of water frozen in Solo cups and Nestle’ s Chocolate drink mix containers, the 6"x5"x2". I can maintain 33’F on my condensor water and by wrapping 30 feet of thin walled Tygon tubing around the outside of the condenser I’ve greatly increased the chilled surface area in the condenser increasing the solvent recovery rate.
I have a couple of larger 5-head water aspirators that came with my rotos, unfortunately they don’t hit very good vacuum unless (as you’ve noted) you keep the water really cold, I’ve also found they don’t flow enough CFM to keep good vacuum on more than 2 rotos. I had really good luck with my scroll vacuum pump up until it decided to crap out, and have parts on order to get it going (and a backup scroll pump on order too in case this happens again)
Alright so this blew my mind a little bit because I a have been using the gas ballast while distilling through my volatiles and usually close it off when head temps reach about 120C. Just to clarify, in the middle of a run do you completely break vacuum? Have you ever had issues with bumping when you re-apply the vac?
Hey thank you for this explanation. I’m confused by how you let it gulp air though. I definitely experience problems with pressure fluctuations during the heads removal and maybe this would help that.
some quick-fix conditions that might lead to an increased amount of oil mist from a rotary pump:
i) too much oil in reservoir (oil vapor space in reservoir too small and some blows out)
ii) too little oil in reservoir (not enough oil reservoir can get overheated and evolve more vapor)
iii) oil contaminated with solvent or terpenes (low boiling components increase vapor pressure of multicomponent systems, evolve more vapor)
iv) leaks in system or ballast open (increased flow rate of gas over hot oil reservoir could entrain via mass transfer, or “whisk” for lay people, excessive oil mist droplets)
If I saw this my first fixes would be: change oil entirely, test a CLEAN system for leaks by pulling vacuum and isolating system section by section. I havent had zero luck with oil mist filters, but they dont entrain 100% of oil mist. They do some amount of work and dont hurt anything unless theres (improbably) a clog, so I would still continue using one. Cheap oil mist filters sometimes come with shitty entrainment mesh, so maybe swapping that out for some quality mesh could be night and day.
didnt read the previous comments hope this was helpful and not redundant.
I have a vent valve just above the vacuum sensor. It has a filter to filter particles but this restricts the flow enough that the vacuum seal is not broken. So I just vent to atmosphere during the run from that point. All incoming air will flow toward the pump and not the other way towards the hot compound which is still under vacuum but reduced during the venting. This fresh air seems to blow contaminate down through the pump and out without risking oxidation by breaking vacuum and flooding the compound with oxygen.
This is mostly what I do. I also always run full open with vacuum broken when I am cleaning up. I give it a minute or two pumping just fresh air through the pump with no restriction. You can really smell the terps and nasty stuff when you do this but my oil still pulls blank off vacuum now after months whereas before I changed oil monthly.
I can both attest to the amount of junk a rotary vane pump can ingest and just keep chugging along, and provide testament to how little debris it takes to damage a scroll pump.
ya know, i’m really tired of changing out this pump oil. It seems like keeping the volatiles cold enough is not practical but how much less of a problem is it to do that than it is to clean the pump every day?
I’m wondering if there’s a better way using adsorbents. can anybody put ten bags of charcoal in the line.
This vacuum pump situation is ridiculous to me. We have a -90C electrical cold trap and a dry ice / ethanol bath in line and they accomplish next to nothing. The room my equipment is in smells god awful.
I know you said you have a -90C mechanical cold trap, but how cold is it actually maintaining? What temp is the inlet and outlet to the cold trap? Could be you need to put something between your short path (or other equipment) and your cold trap to make sure your line temps are sufficiently cold to actually trap solvent and volatiles before they make it into your pump oil. And IMO, putting vacuum pumps (or at least their vapor outlet) indoors is a guaranteed way to make your lab stink even if you are doing everything else right.
Well the display on the cold trap gets to -90C and it keeps a bath of methanol cold. but the dry ice and ethanol cold finger is in line. I think the vapor rate is just that high that it gets through. It seems strange that they have all these respirators and things that filter out everything but everybody is still dealing with this. It seems like a huge drain on everybody’s time, but other than going to extremely low temp I don’t know if anything can be done. There’s probably a reason it’s still a problem unfortunately.
There is an alternative method for removing volatiles than trying to distill them out. I have not scaled it up but have received email back from those that did and this method does take a lot of water and methanol which can all be recovered. Typically I run LLE on raw crude and not decarbed. I use heat during solvent purge of the LLE to decarb the stuff.
The method is similar to a brine scrub insofar as it is a liquid to liquid extraction but my method leaves out salt and uses instead methanol and water in an LLE that removes the bulk of the volatiles when Hexane, water, and methanol are used. The LLE has problems to cope with too and likely an R&D effort to refine the technique. However it is one possible link in a chain that can cope with terpenes but without the hit on expensive pump oil. I use this LLE every month now to take out terpenes prior to short path distillation. LLE like this makes it possible to run decent tasting disty in one pass if temps can be adjusted downward. You cannot really know how to adjust until you run this idea on your own compound.
