Chilled water circuits

[Goober]

What is the relationship between static pressure and flow rate? I understand static pressure when system standing still, but why is there a need for a particular static pressure to ensure certain flow rates?

What is the difference between a "Flow control" valve and a "constant pressure" valve in a return line when referencing it to the balancing of a system?

But wait.....theres more......I can't quite get my head around the need for the above valves when any given number of FCU's have bypass valves...

[Tesla]

Hi Goober
How's the weather there at the mo? Static pressure is measured at the peripheral or side of the tube where total pressure is measured at the center of the tube which has a direct relationship with the flow rate which is calculated with various measurements. So if you have in the same tube or pipe or duct a difference of static pressures from one end to the other fluid will flow at a certain rate according to the amount of friction from the walls and restrictions in the tube. Really static pressure can exist when there is flow or when there is no flow.
On your second question you need to be a little specific as to what fluid you are referring to - water, refrigerant or air. I must assume from your third question the fluid is water. Constant pressure will control a constant pressure drop across a coil to provide a more stable flow of fluid (hence heat transfer (dependent on load and limits) across a coil independent of velocity within limits. Where flow control will compensate for other valves opening on a circuit increasing total velocity to meet demand or static pressure set point. We have two variables - the pump speed (or bypass) and the valve/s opening and closing.
When bypass valves or three way valves are used at the load end say FCU's they can use the flowing fluid for some of the load which requires it and on top of this the source (chilled water system) may be set up or installed a constant flow control.
I know it is very confusing with all the different types of valves, control and systems especially with relatively newer technology utilising variable speed drives and compensating variable controls. Just wait till you come across a variable primary variable secondary system.
If you are still a little confused let us know or if someone could add to or correct this answer please feel free.
I apologise for not reading the title - Chilled Water which I noticed just after writing the above.