Valve Flow Coefficient (Cv) is the flow capability of a control valve at fully open conditions relative to the pressure drop across the valve. It is defined as the volume of water (GPM in the US) at 60°F that will flow through a fully open valve with a pressure differential of 1 psi across the valve. It is useful to know how to calculate Cv because it is the standardized valve sizing and selection method for control valves used throughout the HVAC industry.
The equation used to find Cv is
Q = Flow in Gallons per Minute
G = Specific gravity of fluid (estimated as 1 for water systems)
ΔP = 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 ΔP of 4, first find the square root of the ΔP √ 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 ΔP 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 ΔP 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 ΔP.
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