What is CV, & How to Calculate it?
Valve Flow Coefficient (Cv) is a measure of a control valve's flow capacity at fully open conditions, relative to the pressure drop across the valve. Specifically, it is the volume of water, measured in GPM in the US, that would flow through a fully open valve with a pressure differential of 1 psi across the valve, at a temperature of 60°F. Being able to calculate Cv is important because it is the standardized method for sizing and selecting control valves used in HVAC applications. By properly selecting valves with the appropriate Cv value, engineers and technicians can ensure efficient and safe operation of the HVAC system.
The equation used to find Cv is pictured top right and the variables are listed below.
- Q = Flow in Gallons per Minute
- G = Specific gravity of fluid (estimated as 1 for water systems)
- P = Differential pressure over valve (Delta P) – stated in psi
With this knowledge, we can quickly identify that a valve with a Cv of 10 will flow 10 GPM of water at 1 psi pressure drop across the valve without any math. A practical example, if we wanted to calculate Cv to select a valve for 10 GPM flow with a required Delta P of 4, first find the square root of the Delta P v 4=2, and use that to divide the flow of 10 GPM 10/2 = 5, which results in a Cv requirement of 5. The master specifications of a job plan will typically list the differential pressure to use when calculating Cv. In the absence of this information, a Delta P range for selecting control valves for water coils of 3-5psi has evolved.
When selecting valves, choose the closest to the calculated Cv, typically rounding down unless a specified maximum Delta P of a project is exceeded. An oversized valve may cause control issues such as hunting or poor heat transfer (Delta T) across a coil due to overflow. Conversely, an undersized valve may not provide enough flow and exceed available Delta P.
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