Steam Turbine Exhaust Annulus Loss & Choke Calculator
Determine LP turbine exhaust hood annulus velocity, leaving kinetic energy loss, Mach number, and condenser backpressure aerodynamic choking limit.
LP Turbine Exhaust Conditions
Exhaust Aerodynamics & Choke Margin
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LP Steam Turbine Exhaust Aerodynamics
In condensing steam turbines, steam expands to deep vacuum pressures ($30 - 80\text{ mbar abs}$). Because specific volume increases exponentially as pressure drops (reaching $20 - 40\text{ m}^3/\text{kg}$), the volumetric flow rate exiting the last-stage blade (LSB) becomes immense.
Aerodynamic Choking & Exhaust Hood Losses
The kinetic energy of steam exiting the blades is largely dissipated as heat in the exhaust hood unless recovered by internal guide vanes and diffusing contours. Furthermore, if condenser vacuum improves (colder cooling water) beyond the choking threshold, velocity through the last blade annulus reaches sonic speed ($ ext{Mach} \approx 1.0$). At this point, the turbine cannot expand further, and lowering condenser backpressure produces zero additional megawatt output.