irwinben

Recipes 

I am designing a HERMs system right now as well. It seems to me that the smaller the diameter of the tubing the more surface area of the wort is subjected to the heat of the MLT water, thus heating it more quickly.
My question is whether the goal really is to heat the wort that quickly. Doesn't a higher swing in temps make controlling the mash temp more difficult? Is it better to pass the wort through the heat exchanger more times and accomplish a slower, more consistent temp change?

mpcluever

Recipes 

I guess that's a good question. I was thinking I'd like to be able to bump up as quick as possible and stop.
As far as smaller tubing being more efficient, I won't doubt it, however, it's all about surface area and I calculate (probably wrong) that 75' of 1/2" tubing has the same surface area as 100' of 3/8". I'm thinking I'll probably get 100' of 1/2" and lop off 25-30' for the HERMS coil.

irwinben

Recipes 

You are right... There is more surface area in the larger tube. I was thinking that a higher proportion of the liquid toches the surface, but that might be wrong too. Either way, I am going with 1/2 inch in order to maintain a higher potential flow rate. I also think I am going to run the sparge water through the tube, hopefully cleaning (or pre-cleaning) it after running wort through it in the mash process.