The flow coefficient α is a proportionality factor for the calculation of the flow rate through flow resistances, for example the following applies for orifice-shaped resistances
with
A = geometrically smallest flow cross-section,
ρ = density of the pressure fluid
p 1 - p 2 = pressure before the pressure resistance minus pressure after the flow resistance.
The flow coefficient takes into account all losses due to fluid friction and turbulence on flowing through the resistance. These losses appear in the energy balance as heating of the fluid.
The flow coefficient is essentially dependent on the Reynolds number Re and the shape of the edge geometry (sharp edges, rounded) of the flow resistance (Figure D 90).
Figure D 90: Flow coefficients αD for the most common 2-way cartridge valves