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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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included on these types <strong>of</strong> graphs by adding a family <strong>of</strong> plots. As an<br />

illustration for the Laval–Jeffcott rotor, consider the shaft stiffness, kt, asa<br />

design parameter and allow the rotor mass to take on values <strong>of</strong> 15, 20, and<br />

25 kg (33.07, 44.09, and 55.12 lbm). Define the operating spin-speed range<br />

(including design margins) for the rotor to be between 2,000 and 4,000 rpm.<br />

Typically the upper and lower speeds in the operating range include a<br />

margin <strong>of</strong> approximately 30% to provide for inaccuracies in simulated<br />

results associated with deficiencies in modeling and the inability to<br />

accurately evaluate many system parameters. A critical speed map for this<br />

rotor system is illustrated in Fig. 11.<br />

This critical speed map indicates that, for a rotor mass <strong>of</strong> 20 kg<br />

(44.09 lbm), a shaft stiffness less than 0.88 MN/m (5,025 lbf/in.) places the<br />

synchronous forward critical speed below the lowest operating speed and<br />

that a value above 3.51 MN/m (20,043 lbf/in.) positions the critical speed<br />

above the highest operating speed. This critical speed map then provides a<br />

designer with information that may be used to assist in selecting the shaft<br />

stiffness. The family <strong>of</strong> curves associated with the rotor mass generalizes the<br />

critical speed map and provides additional information to assist a designer<br />

in sizing the system.<br />

Figure 11 Critical speed map for a Laval–Jeffcott rotor.<br />

Copyright © 2003 Marcel Dekker, Inc.

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