Stodola's cone law

The Law of the Ellipse, or Stodola's cone law,[1][2] is a method for calculating highly nonlinear dependence of extraction pressures with a flow for multistage turbine with high backpressure, when the turbine nozzles are not choked.

[3] It is important in turbine off-design calculations.

Stodola's cone law, consider a multistage turbine, like in the picture.

The design calculation is done for the design flow rate (

, the flow expected for the most uptime).

The other parameters for design are the temperature and pressure at the stage group intake,

, respectively the extraction pressure at the stage group outlet

is used for the pressure after a stage nozzle; pressure does not interfere in relations here).

For off-design calculations, the Stodola's cone law off-design flow rate is

, respectively, the temperature and pressure at the stage group intake are

Stodola established experimentally that the relationship between these three parameters as represented in the Cartesian coordinate system has the shape of a degenerate quadric surface, the cone directrix being an ellipse.

[4][5] For a constant initial pressure

the flow rate depends on the outlet pressure

as an arc of an ellipse in a plane parallel to

For very low outlet pressure

, like in condensing turbines, flow rates do not change with the outlet pressure, but drops very quickly with the increase in the backpressure.

, flow rates change depending on the inlet pressure

as an arc of hyperbola in a plane parallel to

Usually, Stodola's cone does not represent absolute flow rates and pressures, but rather maximum flow rates and pressures, with the maximum values of the diagram having in this case the value of 1.

The maximum flow rate has the symbol

and the maximum pressures at the inlet and outlet have the symbols

The pressure ratios for the design flow rate at the intake and outlet are

If the speed of sound is reached in a stage, the group of stages can be analyzed until that stage, which is the last in the group, with the remaining stages forming another group of analysis.

This division is imposed by the stage working in limited (choked) mode.

axis direction, appearing as a triangular surface, depending on the critical pressure ratio

is the outlet critical pressure of the stage group.

[6][7] The analytical expression of the flow ratio is:[8] For condensing turbine the ratio

is very low, previous relation reduces to: simplified relationship obtained theoretically by Gustav Flügel (1885–1967).

[8][9] In the event that the variation of inlet temperature is low, the relationship is simplified: For condensing turbines

, so in this case: During operation, the above relations allow the assessment of the flow rate depending on the operating pressure of a stage.