Our computer watercooling loops are all closed loops. Open/closed refers to the water. Open to new (single pass) water or closed (multiple pass) with the same water.
Open and closed can also sometimes refer to whether the water is connected to the atmosphere, if the water is pumped back to the pump (closed to atmosphere) or gravity fed, or pumped back to the pump (open to the atmosphere).
That being said, read Martin's Pump Selection Guide
. Ask about what you don't understand.
You might want to read more on Martin's site and Skinnee's site
Edited by billbartuska - 10/23/15 at 1:45pm