Control Valve Sizing

exosquad-

Without knowing the GPM of your process fluid it is just a "guess" on sizing the correct valve. Without spending any additional money, can you measure the coil size or find a coil and/or air handler make/model/serial number and contact the manufacturer for specifications? Also, depending on the control accuracy, a larger or smaller valve than required will produce variable results for your process. A valve that is too small will not provide sufficient capacity. A valve that is too large will produce temperature swings at normal loads and produce noisy operation at low load conditions.

If you are in the facilities or control industry, you may want to purchase an ultrasonic flow meter. You then can measure the flow rate at the location of where you will install the valve. You must measure this process at full load with no restrictions such as an old valve, etc., therefore giving you the GPM of the process.

Typical ultrasonic flow meter:

http://www.omega.com/Green/pdf/FD613_614.pdf

Once the GPM is known, accurate valve sizing is simple:

Find the Cv of the required valve:

Cv=(GPM) / SqRt ((delta P) / (specific gravity))

Most controls contractors use 5 psig for the delta P

Specific Gravity of water is 1

So your calculation is highly dependent on knowing the GPM

Once the Cv is known, you can go to any valve manufacturer's catalog and select the appropriate valve. NEVER undersize the valve. A slightly oversized valve is acceptable, unless you are controlling steam. Then you would use a 1/3 - 2/3 set up. But that discussion is for another day! Good Luck!