I have two main questions. The first is how do you use a yeast brink? I realize that that is a bit of an open question, but I am really unclear on how they are used in the real world. Do you use load cells or do you rely on some sort of electronic feedback like a turbidity meter? My second question is, can you describe a back-of-the-envelope calculation for determining the size needed?
Yeast brinks are pretty commonly used to hold yeast in suspension at a temp of about 2C, they are usually fitted with a sanitary, cleanable stirring agitator or sometimes they are used with a recirculation pump system.
The brinks I am familiar with use a stirring agitator fitted from a top plate on the top of the tank. The agitator top plate also has CIP inlets, and safety valving for over and under pressure. Alfa Laval sells these among others. We used load cells on the brinks and added a calculated weight of yeast slurry to each brew based on cell count in the slurry and target cell count in the wort. We would enter the weight onto a controller and a CO2 valve would open to push the yeast out into the wort stream through an automatic valve. This worked very well and could be done manually as well.
In regards to sizing, you usually yield about 3-4 times that amount of yeast per fermentation that you put in. I would consider sizing at 1.5 times your normal pitch rate. Make sure it has a jacketed cone and sides for maximum cooling capacity and a temp well inside the cone (pointed up). Hope this helps, let me know if you have any other questions. I would also encourage you to attend your next district MBAA meeting if possible and use the opportunity to discuss this issue with other brewers who could add their insights.