Turboexpander Performance (2)

A Note on Turboexpander Aerodynamic Design

Turboexpander polytropic efficiency can be expressed as a function of its specific speed (as defined below):

In which N is turboexpander rotational shaft speed, Q₂ is turboexpander discharge flow and Δh is the ideal enthalpy reduction through the turboexpander.

Turboexpander polytropic efficiency is increased with the increase of its specific speed up to an optimum value. This trend suggests two methods to increase turboexpander efficiency (and hence its power generation):

1. Increasing turboexpander shaft speed (N);
2. Increasing turboexpander number of stages (or decreasing Δh).

Turboexpander shaft speed is limited by the capability of its hydrodynamic bearing. To increase the speed to values higher than this limit, magnetic bearings (with higher capital cost) have to be utilized. On the other hand, increasing the number of stages also means a higher capital cost. These costs need to be justified and balanced by the increase in turboexpander power generation capacity.

Following table shows simulation results for three different proposed turboexpander designs with the same process conditions. Results show that how turboexpander power generation is increased via either increasing number of stages or utilizing magnetic bearings (and therefore designing the turboexpander for higher shaft speeds).
 

Preliminary turboexpander aerodynamic design

 

Following points should be considered when optimizing turboexpander design using such an approach:

• Maximum allowable shaft speed (limited by hydrodynamic bearing) for the specific turboexpander process design conditions;
• Optimum specific speed value for the selected turboexpander design.

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