How do I calculate Gas turbine efficiency calculation?

The efficiency depends primarily on size (different manufacturtes have use similar designs). Bigger Gas Turbines tend to more effiecient. Additional factors such as waste heat recovery and heat exchangers can improve the system efficiency (but not the effiecency of the actual Gas Tubine).

Smaller gas turbines such as the Rolls Royce Avons have around 25% Thermal Efficiency while larger units such as the Rolls Royce Trent-50 have efficiencies of around 42%. A full list of Tubine Efficiencies for different manufacturers can be found at http://www.vibronurse.com/turbmath.HTML

Note that the above comment does not make the Avon a bad unit its simply the case that it is designed primarily for a high power to weight ratio. Some people will also quote very high figures for G.E Frame machines (+50%) but these numbers usually include co-gen (a steam turbine used to recover waste heat) and are therefor not just Gas Turbine Efficiency.

The full calculation for calculating Gas Turbine effieciency is avalaible at VibroNurse at the following page (http://www.vibronurse.com/turbmath.HTML) and program which does the calculation is available at http://www.vibronurse.com/gaseff.HTML. We use this program at our oil and gas platforms and it works well particlauarly as a condition monitoring tool.