IC diameter

I have the copper tubing to make a chiller but I'm not sure what size to make the coils. Is there an optimum radius for the coils to cool the wort?

The diameter of the coil depends on your kettle, both how deep and wide it is. You want as much surface area as possible basically, with no part of the coil outside the wort. Make note of how high your wort is in the kettle, and how wide your kettle is. That should give you the optimal dimensions for your kettle. Ideally, you want an inch or so between the chiller tubing and the side of the kettle. Aside from that, just build/buy a chiller that is going to be immersed in the hot wort as much as possible.

Ideally, you want an inch or so between the chiller tubing and the side of the kettle.
I guess that's what I was wanting to know.

I'd say an inch or so from the sides is about what my pre-made chiller runs.....but if you're making your own, there are other considerations, to put it mildly. Heat transfer depends on the heat differential between the two liquids in question, and the conductivity of the metal being used (copper and stainless are the most common, the former being better). Then you have to think about the movement of the liquids (both the wort outside the chiller and the water inside) such that the temperature gradient between the two is kept as large as possible. The greater the gradient, the greater the transfer, which principle is most simply illustrated by how hot the water is discharging from the filter immediately after beginning to chill.
There are things going on that may not be immediately obvious, but if you visualize the chiller sitting in the wort and cooling it, as the wort right next to the chiller becomes cooler, this narrows the gradient, and cooling is less efficient. The bottom line is to stir or agitate the wort in such a way that this is kept to a minimum. As a practical rule, shown by my own times to pitch temperature, more stirring / agitation is better.
Then there is the same principle, carried to the inside of the chiller tubing. the water is flowing through the tubing, but not all the water is transferring heat equally through the metal. The water next to the metal obviously transfers the most, while that in the center of the tubing conducts least. The greater the flow through the tubing, the more transfer......although as the flow increases, efficiency decreases (You're using X more water, but transferring X-Y less heat, so the increment by which you can speed up chilling grows less and less as flow increases.

An experience with my own wort chiller made me start thinking about this. I was getting a leak on one of the connections, so I decided to discard the hose clamps for regular compression fittings. Upon removal of the hoses and clamps, I discovered that there was a yellow plastic tape running through the entire 25 ft. length of 3/8 in. copper tubing. I e-mailed the vendor and asked about the tape, wondering whether this would not retard the flow of water. They replied immediately that that was the point. The tape slowed the water and agitated it, mixing it as it flowed through, thus increasing efficiency. They told me that it would take 15% longer to chill the wort WITHOUT the tape in the tubing. I haven't the knowledge or the math skills to prove this with numbers, but I can sort of see how this might work.

I am making a 50ft (3/8)this weekend, I am going to use a Corny as a form. I expect that the coils will be about 10inches in diameter, and stand about 14 inches tall.

I am going to be using mine in a converted Sanke with a 12inch hole in the top. It will be too big for the mostly 5gal batches i am doing now, but will be perfect for 10g batches.

I based the diameter of mine on the fact that I would someday be using it in a converted Sanke kettle. Most people say they are able to cut a 12inch opening in their sanke so I made mine about 10 inches. I didn't care about thermodynamics.

mnm129 said:

I based the diameter of mine on the fact that I would someday be using it in a converted Sanke kettle. Most people say they are able to cut a 12inch opening in their sanke so I made mine about 10 inches. I didn't care about thermodynamics.

Click to expand...

I should have done the same, I just had to unsolder and recoil a 50' chiller to fit it into a sankey.
The architecture of the chiller matters little as long as you are getting agitation. Your main concern should be how does it fit in your vessel (or vessels) and how sturdy/convenient is the layout.

2 easiest ways to agitate:
Stir
Whirlpool return arm

I'll be shortly adding the whirlpool part.. if you need info on that, read up on mrmalty.com.