CV sizing for water HVAC control valves
Water Valve CV Sizing:
Formula to use for Standard Conditions: Assuming 3-5 PSI drop through a coil.
Cv= ((Q x √G)/√∆P)
Variables:
Cv = Valve Coefficient of flow which is what is the flow with a 1 psi drop across the valve.
Q = Flow in Gallons per Minute.
G = Specific gravity of fluid (Water = 1). If using water, you can remove x √G.
∆P = Pressure drop across fully open valve in psi
Example: Standard Practice is to assume you have a 3-5 PSI Drop across a Control Valve. The assumption is the coil is roughly that drop across it. Let’s say the coil needs 44 Gallons Per Minute per print.
Step 1: CV Rating = Cv= ((44 x √1)/(√4 psi))
Step 2: CV Rating = Cv= ((44 x 1)/2)
Step 3: CV Rating = Cv= 22
Example: Let’s say the coil needs 44 Gallons Per Minute per print and a pressure drop of the coil in FT WG 8.5.
Step 1: Convert 8.5 FT WG to PSI by taking it times .433PSI Per Foot. Which Equals 3.6805.
Step 2: CV Rating = Cv= ((44 x √1)/(√3.6805 psi))
Step 3: CV Rating = Cv= ((44 x 1)/(1.918462926407493 ))
Step 4: CV Rating = Cv= 22.93502751309052
Formula used to check if a Modulating Valve CV should work: Assuming 3-5 PSI drop through a coil. For Example: You need a CV of 22 for a coil with a 4psi pressure drop and there isn’t a valve that matches perfectly. The 1.5” has a 28 CV and the 1.25” has a 20 CV. Which Valve should you use?
∆P= □([(Q x √G)/Cv ]^2 )
Variables:
Cv = Valve Coefficient of flow which is what is the flow with a 1 psi drop across the valve.
Q = Flow in Gallons per Minute.
G = Specific gravity of fluid (Water = 1). If using water, you can remove x √G.
∆P = Pressure drop across fully open valve in psi
Step 1: Valve Pressure Drop = ∆P= ([(44 x √1)/28]^2 )
Step 2: Valve Pressure Drop = ∆P= ([(44 x 1)/28]^2 )
Step 3: Valve Pressure Drop = ∆P= ([(44 )/28]^2 )
Step 4: Valve Pressure Drop = ∆P= ([1.57]^2 )
Step 5: Valve Pressure Drop = ∆P= (2.47 for 1.5" valve)
Step 6: Valve Pressure Drop = ∆P= ([(44 x √1)/20]^2 )
Step 7: Valve Pressure Drop = ∆P= ([2.2]^2 )
Step 8: Valve Pressure Drop = ∆P= (4.84 for 1.25" valve)
Technically the 1.25” valve falls within the rule of thumb of 3-5 psi. What if it didn’t? Then you need to look at the system. Is the pump big enough to deal with a higher-pressure drop valve if your CV is low? If your CV is high is there going to be a problem with hunting of the valve?