Chilled Water Air Separators - Full Pipe Flow Inline or Smaller Side-stream???

[BronYrAur]

It has always been my experience that Rol-Air-Trol or similar air separators are usually full pipe size and are located on the main system piping. For a chilled water application, I typically see them upstream of the chillers. For boilers, they are typically downstream. This makes good sense with the solubility of air vs water temp.

I have noticed on a couple of large chilled water plants, that the air separators have been side-stream to the main chilled water line as opposed to inline. For example, I just saw a plant where the chilled water return header was 20" pipe. Rather than installing a 20" air separator, which would of course be very large, heavy, and expensive, a smaller (6" or 8") separator was installed side-stream to the main 20" pipe. This side-stream branch pulled off the return header with a small inline pump, went through the air separator, and then returned right back to the same header. If I had to guess, flow through the air separator was probably only 10% of the total system flow.

The plant has been around for a long time so it apparently "works", but I just haven't seen that arrangement before. That that leads me to the question of how to properly select an air separator. All the selection software and tables I have seen want to know flow and then stay within a velocity and/or pressure drop limit. That makes sense, but if you are not going to send the entire flow through the air separator, how do you determine the appropriate amount? Or is the principle analogous to a side-stream filter where you will eventually get the level down little by little?

Any thoughts?

 

[Compositepro]

A bypass-flow air vent will work just fine but it does need to be installed correctly. The point where the bypass flow is drawn-off from the main line must be at a high point where air would tend to collect if there were no air vent. This is different from a bypass filter. Also there must be enough pressure difference between the supply and return points for there to adequate flow through the bypass and to push out any air pockets that may occur in the bypass pipe. The bypass line is also often used as the place where chemicals can be added to treat the chilled water through a small basket strainer.