I have the Bonanza with the evap AC. The square canister at the back with the little latches on it has a set of frangible wicks that absorb water very well. There is a recessed(NACA style) scoop on the top. Pour about 1/2 gal of water in the scoop and it flows down over and saturates the wicks. Then you take off, and the air blows over the wicks and the water evaporates out at a measured rate.
In very humid weather it doesn't work well of course. In hot dry weather it's fantastic. It not only cools the interior, but it also helps with the natural reduction in humidity inside the plane at altitude. When I go to NM or CO in the summer, I fill it before take off and even on a 100F day on the ground, once I get over about 3000' it's gets rather chilly inside. Above 6000' it's quite cool. The water lasts about 2-4 hours depending on the dew point spread. Just carry a jug of distilled water, and fill it at each fuel stop.
In humid weather an actual recirculating closed AC system would work much better, but the Beech evap system is light, simple, low drag, and effective when in drier conditions.