If you are tossing the coil into hot liquid it might be better to circulate the hot liquid through the fins. The liquid will cool quicker with flow created by some source, than statically. Maybe, place the coil slightly above the liquid level and design some way to continually circulate hot liquid through the fins without aerating. That way you can slow down or increase the volume of liquid through the fins, to find the sweet spot of heat transfer. Acting like the fan speed selector in a window shaker or variable drive fan. The device will act kind of like an evaporative condensor found in water towers. Without the high evaporation rate of a water tower. An auto A/C compressor is driven by an engine. The compressors can produce as much as 7.5 tons of cooling at engine RPM of 2300, 90K BTU/hr. A condensor condenses the heat laden vapor and changes it to a liquid at the refrigerant's condensing temp. Condensing temp of most refrigerant is 105F at 95F ambient across the coil. Condensing takes place in the top 1/4 of the condensor. The liquid is sub cooled as it flows through the coil. To increase sub cooling affect, coil builders use a few tubes at the bottom of the coil as a sub cooler. Sometimes, a suction to liquid heat exchanger is used to further lower liquid temp, to reduce flash gas in the expansion device. If you know the GPM of coolant flowing through the inside of the tubes, fins per inch and a couple of other numbers that you will know, since, you work with refrigeration. You can figure out the cooling capacity of the coil. Some evaps are coated with a special lacquer that keeps them from corroding. If the coil is coated, vinegar nor salt will reach the copper. Scrape a U-bend with a knife edge to see if anything peels off. It's hard to tell by looking at the coil if it is coated. Coated copper feels smoother than non coated. It will have a similar smoothness that regular copper gets after wort does it's thing to it. High temps will remove the coil coating. If the tubes are coated, so are the fins. If it is coated, find out if the coating is OK to use in something that will be poured down the throat. Brew on!