The Critical Flow Function for Superheated Steam

Abstract

Computations are made of the theoretical critical flow rate of superheated steam through nozzles or other passages. Flow maximization was used in determining the critical function φ = GT11/2/p1. Tabulations of the critical flow function are given for the “superheated vapor” region defined by Keenan and Keyes “Steam Tables” with a pressure limitation of 5000 psia. Two approaches are presented to relate the critical flow function to an ideal state. In the first approach the temperature of the throttled vapor, from inlet conditions, to a very low pressure (p = 0.08854 psia) was used. The second approach utilized the equivalent ideal-gas stagnation temperature that steam would have if its compressibility factor was unity. A dimensionless correlation on reduced coordinates is presented for the second approach, which permits the correlation of steam data to hydrocarbons or other nonideal gases the compressibility factors of which are available as functions of reduced coordinates. A practical method for calculating theoretical critical flow is presented utilizing the properties of steam as taken directly from the Keenan and Keyes “Steam Tables.”