Air Vent Sizing

There is no hard and fast process that I am aware of, but- to be most effective, you should circulate warm water (prior to turning on the chiller) through the system for at least 12 hours after the system is filled and before you start the chiller.

The reason for this action is that the warm water will reject most oft he dissolved air where the chilled water will tend to keep it dissolved and circulating.

I would also suggest that you install the air vent on top of a 12" or longer pipe that is one of the branches of a tee. The tee should be at least 2 pipe sizes larger than the main feeding it. For example- a 4 inch CHW main at the top of the system would discharge into the "branch" of a 6 inch tee with a 12" long x 6"pipe rising from the tee to a vent installed in its cap. The other branch of the tee will be a 6" by 4" reducer an then the 4" branch main should continue to serve the system. This way, the slow water flow will assist in releasing water rather than keeping it moving through the pipes.

During the 1^st stage of piping design, the piping isometrics must be furnished with vents and drains at upper and lower points for the purpose of hydrostatic test. And the most common sizes for vents and drains are 1/2" or 3/4". And those vents and drains can be used during the life operation of piping system, and I think that size 1/2" or 3/4" is enough to perform its function.

Use an air vent device that automatically vent entrapped air using controlled pilot valve operated diaphragm venting valve. These are manufactured accdg to your specs and there's no need for you to calculate the discharge capacity...