The term Heavy Duty designates those GTs designed to be stationary, differently from those designed to be used on transport, the Land Marithmic (LMs) or also called Aeroderivatives because their design is basically the same of a jet turbine, with some modifications.
The Heavy Duty GTs have two main configurations: One-Shaft, designed for electric power generation, or Two-Shafts, the second one with variable speed, designed to be used as mechanical drive.
As every thermal machine, Gas Turbines work based on a Thermodynamic Cycle, compressing athmospheric air, compressing it, adding thermal energy by means of combustion of a fuel and finally expanding the hot gases to obtain mechanical power, that will drive the compressor (which consumes about half of the power) and the remaining power will be delivered through the shaft.
The Thermodynamic Cycle used by the gas turbines is named Brayton Cycle. Differently from the reciprocating engines which create the thermodynamic cycle (Otto or Diesel cycle) by sequentially compressing, burning and expanding a certain ammount of gas inside a chamber, the gas turbines operate as a continuous process, performing the compression, combustion and expansion as the gases flow through the turbine.
Because of that, GTs can opperate with large flows of gases so they can deliver large ammounts of power with a compact size and with high efficiency.
For power generation applications, the GTs drive a Generator that operates synchrounsly with the power grid, either at 3000 RPM if the frequency of the grid is 50 Hz or at 3600 RPM if the grid operates at 60 Hz. 5100 RPM turbines use a Reduction Gear.
A rule that applies for all gas turbines is that the bigger the turbine, the slower is the speed. Because of that, turbines designed to operate at 3600 RPM are smaller than those designed to operate at 3000 RPM. And for applications that demand smaller turbines, a reduction gear is used to allow a GT operating at 5100 RPM to drive a 3000 or 3600 RPM generator.
GTs can be analyzed as a system composed of 4 main subsystems: Compressor Section, Combustion Section, Turbine Section and Auxiliaries Systems.
The Compressor Section is responsible for compressing the air, making it pass through several stages, each stage will add kynetic energy through the rotating movement of the compressor blades and transform the kynetic energy into pressure by the compressor stator vanes. The air leaving the compressor is called Compressor Discharge (CD) Air.
The Combustion Section will increase the temperature of the CD Air by burning a fuel that may be liquid, but usually natural gas is used. The mixture air-fuel is actually a lean mixture, orelse the temperature of the gases would be so high they would melt the metal parts.
For turbines that operate at combined cycle, the firing temperature will dictate the efficiency of a turbine. The higher the temperature, the better. Consequently the combustion and turbine parts operate at very high temperatures and they need efficient means of cooling to allow them to work at temperatures below the gases temperature.
The turbine section operates in an inverse way of the compressor, making the hot gases pass through the nozzles, that direct the hot gases over the turbine buckets, making them rotate. The turbine has 2 throuth 4 stages, while the compressor has 17 or 18 stages.
