Right now i rack with CO2. I purge my kegs, then connect a SS racking cane to the liquid side, dip it about halfway down the fermenter. Then apply low co2 pressure to the ferm. I get a very clear transfer in about 8 mins.
What i was thinking to do instead is to draw a vacuum on the keg, then fill with co2, then draw another vacuum on the keg. Then submerge the racking cane, and when connected to the liquid post, a siphon would start right away. Idea here being that the keg is purged of oxygen to a very high degree and the vacuum would accelerate the speed of the siphon, at least part of the time.
Few issues i see:
1. Not sure how much vacuum pressure a keg can handle
I don't know how much a keg can handle but a couple times I have forgot to pull the thermwell out of my conical causing my pump to make negative pressure in the keg. I haven't collapsed a keg yet.
2. will the co2 dissolved in the beer come out of solution from the vacuum leading to foaming?
I never had issues with foaming transferring this way.
3. o2 exposure as o2 is drawn into the fermenter (although contact time is minimal).
Yes you are drawing in o2 as you transfer out but the transfer happens quickly so the exposure time isn't worth worrying about. I have been doing this for 2 years and have never had a batch oxidize. I also purge the keg with co2 to seat the lid then vacuum pump that pressure out before connecting the liquid post.