Flow coefficient of a device represents the ease for a fluid flow to pass through the device. High flow coefficient of a device such as restriction orifice or a control valve indicates low resistance to the passage of flow. Similarly low coefficient indicates higher resistance to the passage of flow and high pressure drop for a given flow.
Mathematically flow-coefficient can be defined as,
Cv = Flow coefficient
V = Volumetric flow
ρ = Fluid density
ΔP = Pressure drop
Depending on the different units used for each of the term on right side of the equation, range of values for flow-coefficient (Cv) changes. Sometimes specific gravity of a fluid can be used instead of the density.
A flow-coefficient (Cv) is a commonly used parameter for the design of flow devices like control valves, flow meters, sprinklers etc.