An important first step in understanding the gas turbine design process is the knowledge of how individual components act given their particular boundary conditions. However, in order to effectively leverage these individual design processes, a basic knowledge of how these components interact with each other is essential to the overall performance of a gas turbine unit. The power and efficiency outputs of a gas turbine are the result of a complex interaction between different turbomachines and a combustion system. Therefore, performance metrics for a gas turbine are not only based on the respective performances of each turbine, compressor, and combustion system, but also on their interactions. The concept of component matching becomes crucial in understanding how to deal with these systems simultaneously.
In general, gas turbines for industrial applications consist of a compressor, a power turbine, and a gas generator turbine designed into one of two arrangements. The first arrangement invokes the use of the gas generator turbine to drive the air compressor, and a power turbine to load the generator on a separate shaft. This two-shaft arrangement allows the speed of the gas generator turbine to only depend on the load applied to the engine. On a single-shaft arrangement, the system obviously cannot exist at varied speeds and the power turbine coupled with the gas generator turbine would be responsible for driving both the generator and the compressor. A simplified diagram of each arrangement is displayed in Figures 1 and 2.
The efficiency of gas turbine engines can be improved substantially by increasing the firing temperature of the turbine, however, it is important to remember that the surface of the components exposed to the hot gas must remain below a safe working temperature consistent with the mechanical strength and corrosion resistance of the employed materials. Along with this firing temperature limit, obvious upper bounds exist on the speed of the gas generator due to mechanical failures and reduced lifetimes at high RPMs. These two limits help construct a particular range at which the engine can perform. There is a certain “match” temperature that controls whether the engine will be operating at its maximum gas generator speed (speed toping) or its maximum firing temperature (temperature topping). At ambient temperatures above the match temperature, the engine will operate at its max firing temperature and below its max generator speed. In a similar vein, the engine will operate at its max generator speed and below its max firing temperature at ambient conditions below the match temperature. The match temperature is the ambient temperature at which the engine reaches both limits, and it represents the highest efficiency of that engine.
This match temperature is not a trivial or fixed value. Several auxiliary factors cause changes in the gas engine’s match temperature, which must be appropriately accounted for in the gas turbine design. The following factors alter the match point of any gas engine
- – Changes in the fuel properties
- – Reduction in compressor or turbine efficiency due to fouling, increased leakage, tip clearance, and material roughness variations
- – Accessory loads imparted by pumps and other secondary systems
- – Inlet and Exhaust losses
These auxiliary factors along with the routine changes described by varying ambient temperature, ambient pressure, humidity, load, and power turbine speed all contribute to the complexity involved in properly designing a gas turbine. Correctly analyzing off-design conditions becomes an art of variable manipulation and generally requires the use of cohesive design and analysis platforms for proper evaluation. SoftInWay’s integrated software platform allows for streamlined manipulation of your gas turbine design together with immediate off-design analysis based on any prescribed changes. If you would like to learn about how our AxSTREAM platform assists with off-design analysis in gas turbines and other turbomachinery, please visit our software page.