F1 - (Flow Recovery Coefficient)
F1 = The valve pressure recovery factor, a dimension less quantity. (Measured when valve is not choked.)
 
The Vena Contracta is the place along the axis of flow, just beyond the orifice, where the jet steam contracts to its minimum cross-sectional area. Note: It is at this point that the velocity is at its highest, and the fluid pressure is at its lowest.