After finding out the hard way with a 20 gallon conical that ambient cooling will not remove enough heat, we went with internal 1/2" SS coil and chilled water for cooling. As in most ferment processes when the volume and heat generation goes up the ability of the fermenter to shed heat through the walls is usually inadequate at 60-70 degrees, it needs to be near freezing. Whether you go with internal or external cooling methods you will still have to use something to get enough of a temperature difference to remove the heat. Trying ambient air cooling of fermenters will be a challenge as the ambient air temperature would have be quite low for an active fermentation, and end up much to cool for initial and end stages. The other draw back of ambient cooling is moisture control as moisture flows from warm area to cooler area and builds up on cooling coil as ice. As always I advise against relying on window airconditioners for cold rooms even though they seem to work for a while, they are not equiped to deal with low cooling coil temperatures and ice buildup unless you do some controls work to incorporate a second temperature controller to de-ice, and seal all the openings that let in outside air.
