Originally Posted by tampa911
I don't want to derail your thread, but I saw this after I posted a similar question (or hypothesis). http://www.homebrewtalk.com/f11/immersion-chillers-395062/
In theory (or in my head) I would think a longer length of narrower tubing would cool more effiecently than a short length of fatter tubing. I don't know how much difference there really is in the application, but 100 feet of 1/4" tubing is about the same price as 50 feet of 1/2". I think I might make a go of it.
I work with a lot of chillers at work, and am always using his formula
btuh = 499.8 x gpm x TD
Basically, it is a water heat transfer. The TD is the temperature difference between water entering and leaving the coil.
Essentially, if you drop your flow rates your btu/hr decrease. The easiest way to get a faster cooling rate is to increase your water flow rate. A typical value for desirable TD is about 10F. In other words, your coil temperature enters at 50F and leaves at 60F. There is a point that increasing the flow rates beyond a certain point is counter-productive because of laminar flow that sets up inside the coil.
Think about it this way. With 50' of 1/4 inch tubing, the water in the coil will probably reach the temperature of the wort before the end of the coil. If the coil temp is the same temp as the wort temperature, you will have stagnant space in your coil where there is no cooling happening. You've just waster 10' of copper. Having a rapid flow rate prevents the coil temperature from approaching the temperature of the wort, but this is difficult to achieve with 1/4" tube. Keeping the coil TD low increases your heat transfer rate, because it keeps the temperature difference between the wort and coil as high as possible. The higher the difference between the coil temp and the wort temp, the faster the cooling rate will be.