for additional information see this thread 'http://www.egpet.net/vb/threads/27092-Excel-library-for-process-calc-s-including-distillation' Prode Properties allows to size / rate a PSV for critical or two-phase flows with different models including HEM (Homogeneous Equilibrium Model) The HEM model for the nozzle applied in Prode Properties solves hin+1/2*vin^2 = ho+1/2*vo^2 (in = inlet, o = orifice) where vo (for a critical flow) is the speed of sound ho, vo calculated at vena contracta conditions this model works well for critical and two-phase flows, it is also applicable for subcritical flows within certain limits. For subcritical flows Prode calculates the area solving a constant energy operation but since usually for a nozzle we do not specify the internal area of pipe (which is required for calculating vin in above formula the procedure estimates the value, this may generate errors when the rate vo/vin is low. For such cases it is suggested to use as alternative to ISPF() the method EPF double t = EPF(integer stream, double p, double E, double aout, double et) this method allows to specify both inlet and outlet condition thus modeling a adiabatic, irreversible expansions when the contribute of kinetic energy cannot be neglected.
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