Abstract
Based upon the discussion of steam flow through non-divergent nozzles with oblique exit faces in “Die Dampfturbinen”, by Gustav Flügel, this paper presents a procedure wherewith steam-turbine vacuum corrections may be calculated with a fair degree of accuracy when the design characteristics of the last row of blades and one set of water rates for specified steam conditions and loads are known. The procedure outlined is designed primarily for consulting engineers and power engineers, not for the turbine engineer. On that account certain simplifying assumptions have been introduced. These simplifying assumptions, although perhaps not acceptable where precise results are required, will be found satisfactory for the purposes for which the formulas presented herein have been developed. A comprehensive sample calculation is used to exemplify the procedure outlined.
In addition to a procedure for evaluating vacuum corrections, the paper presents simple formulas for evaluating small increments of isentropic heat drop when the expanding steam is saturated. These formulas give evaluations of small increments of heat drop more accurately than they can be read from ordinary steam charts and may be solved with little if any more labor than is used in reading charts.
Certain formulas are also presented which evaluate the mass rate of steam flow through a blade or nozzle when the critical velocity is reached therein, the mass rate of steam flow through a nozzle or blade when the terminal nozzle or blade pressure is the minimum attainable (that is, when the exhaust-chamber pressure is at or below that corresponding to the limiting vacuum), the rate of change of internal shaft work with a change in the exhaust-chamber vacuum temperature for expansions beyond the critical pressure, and the approximate rate of change of internal shaft work with vacuum temperature when the terminal blade pressure is above the critical pressure. Approximate formulas are given which express the specific volume and latent heat of dry saturated steam in terms of the temperature of the steam. These formulas have been developed for use where the exhaust steam is saturated and where, in addition, the temperature of the exhaust steam does not exceed approximately 140 F.
