Flow coefficient
The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow. It describes the relationship between the pressure drop across an orifice valve or other assembly and the corresponding flow rate. A greater restriction in flow will create a larger pressure drop across a device and thus a smaller flow coefficient, conversely device with little restriction in flow will have a small pressure drop and a larger flow coefficient. For example, the flow coefficient of a 1" ball valve may be 80 while a similarly sized globe valve in the same application may be 10.
Mathematically the flow coefficient can be expressed as
where,
In more practical terms, the flow coefficient is the volume of water at that will flow per minute through a valve with a pressure drop of across the valve.
The use of the flow coefficient offers a standard method of comparing valve capacities and sizing valves for specific applications that is widely accepted by industry. The general definition of the flow coefficient can be expanded into equations modeling the flow of liquids, gases and steam using the discharge coefficient.
For gas flow in a pneumatic system the for the same assembly can be used with a more complex equation. Absolute pressures must be used for gas rather than simply differential pressure.
For air flow at room temperature, when the outlet pressure is less than 1/2 the absolute inlet pressure, the flow becomes quite simple. With = 1.0 and 200 psia inlet pressure, the flow is 100 standard cubic feet per minute. The flow is proportional to the absolute inlet pressure, so the flow in scfm would equal the flow coefficient if the inlet pressure were reduced to 2 psia and the outlet were connected to a vacuum with less than 1 psi absolute pressure.
Flow factor
The metric equivalent flow factor is calculated using metric units:where,
can be calculated from using the equation