BYO had a good article on open fermentation that touched upon fermenter geometry (see link). I am lucky that my first choice for fermentation is my conical, but when that's full and I just have
to brew, I will ferment beers in my 15 gallon Polarware kettle which is about 19" diameter x 15" tall. That's pretty close to a 1:1 ratio mentioned in the article, or close enough for me at least. Here's what I've noticed about the kettlementor and some tips:
* 30%-40% faster fermentation in the kettle. In four 10-gallon batches of IPA (two fermented in conical, two in kettle) brewed over the course of three months but with virtually identical recipe/brewing/ferm-temp each time, my 1.064 IPA finished out to 1.014 in 11 days (both times) in the conical versus 5-6 days for both batches fermented in the kettle. I won't bore you with more details, but I've experienced relatively the same thing with Saisons and Hefe's.
* Slight increase in phenol & ester production. It's hard to quantify this, so you'll just have to try. Just choose a beer that benefits from some ester flavor/aroma. You can minimize the esters somewhat by aging the beer in a refridgerator. I have received high scores on beers fermented in my kettle and don't recall ever being "dinged" for too much ester character. Would I ferment a Bohemian Pilsner in a kettle? Probably not. You can do it with lots of ale styles though.
* If your kettlementor has drilled/threaded valves, you might want to clean them out from the inside with a toothbrush prior to a final saniztizing rinse. Those threads collect a lot of gunk.
* I place a 5 lb weight on top of the lid to keep any foreign particles out and hold a decent seal. That does allow for some CO2 seepage. I have also used masking tape around the lid as an added precaution, but I'm not sure it's really necessary. Please note, in a 15 gallon squatty kettlementor, a 10-11 gallon batch is not likely to have krausen up to the lid (at least I never have).
* Because of the wide surface area, the trub is evenly and thinly distributed so I don't get much (if any) running out when I transfer the beer to kegs or secondary. I transfer beer out the spigot, through a tube into the next container. Simple. A few minutes before transferring, I usually put a little half-inch thick piece of wood under the kettle on the spigot side to lift up that side. That encourages the trub to float away from the valve.Considering all the wierd and wonderful vessels that have been used for fermentation over the years, I don't think it's worth stressing too much over a nicely sanitized covered steel container, whatever it's shape may be. Use good brewing practices, keep everything sanitary, and you'll most likely make great beer!http://www.byo.com/stories/techniques/article/indices/31-fermentation/1216-open-fermentation-tips-from-the-pros