Choosing gas turbines: Single- vs two-shaft

Below is an excerpt from a tutorial on gas turbine performance by Rainer Kurz of Solar Turbines and Klaus Brun of Southwest Research Institute at the 3 rd Middle East Turbomachinery Symposium in February, 2015.

Siemens SGT-200 as single shaft turbine[/caption]

The choice of whether to use a single-shaft or two-shaft power plant is largely determined by the characteristics of the driven load. If the load speed is constant, as in the case of an electric generator, a single-shaft unit is often specified; an engine specifically designed for electric power generation would make use of a single-shaft configuration. An alternative, however, is the use of a two-shaft engine. If the load needs to be driven with varying speeds (compressors, pumps), two-shaft engines are advantageous.

The two types have different characteristics regarding the supply of exhaust heat to a cogeneration or combined cycle plant, primarily due to the differences in exhaust flow as load is reduced; the essentially constant air flow and compressor power in a single-shaft unit results in a larger decrease of exhaust temperature for a given reduction in power, which might necessitate the burning of supplementary fuel in the waste heat boiler under operating conditions where it would be unnecessary with a two-shaft. In both cases, the exhaust temperature may be increased by the use of variable inlet guide vanes.

Siemens SGT- 200 as twin-shaft turbine[/caption]

Cogeneration systems have been successfully built using both single-shaft and two-shaft units. The torque characteristics are very different and the variation of torque with output speed at a given power may well determine the engine's suitability for certain applications. The compressor of a single-shaft engine is constrained to turn at some multiple of the load speed, fixed by the transmission gear ratio, so that a reduction in load speed implies a reduction in compressor speed. This results in a reduction in mass flow, hence of power and torque. This type of turbine is only of limited use for mechanical drive purposes.

However, the two shaft engine, having a free power turbine, has a very favorable torque characteristic. For a constant fuel flow, and constant gas generator speed, the free power turbine can provide relatively constant power for a wide speed range. This is due to the fact that the compressor can supply an essentially constant flow at a given compressor speed regardless of the free turbine speed. Also, at fixed gas generator operating conditions, reduction in output speed results in an increase in torque. It is quite possible to obtain a stall torque of twice the torque delivered at full speed. The actual range of speed over which the torque conversion is efficient depends on the efficiency characteristic of the power turbine. The efficiency penalty will not be greater than about five or six percent over a speed range from half to full speed.