Gas turbines are made up of three major components: compressor, combustor, and power turbine. Multiple compressors, turbines and combustors can be used in various arrangements (Figure 1). In the simple cycle gas turbine, air enters the compressor and is compressed. Then enters the combustor and is ignited with fuel, and then the combustion products turn on the turbine. A second turbine can be added to extract more energy from the exhaust gases. In theory, another combustion chamber can be added to reheat the gases after the first turbine expansion. Additional compressors can be added to further compress the air, increasing the overall pressure ratio and efficiency.
In actual practice, it is common for gas turbines to have two compressors and two turbines with one combustor. The first compressor that the air enters is called the low pressure compressor. If there is a second compressor, it is called the high pressure compressor, since it raises the pressure of the air higher than the low pressure compressor. The low pressure compressor has a larger diameter than the high pressure compressor, and the compressor blades on the low pressure compressor are larger than those of the high pressure compressor. This type of design is called a “split compressor” or “twin spool” engine.

Figure 1. Various Open Cycle Gas Turbine Arrangements

If there are two turbines after the combustor, the first turbine after the combustor is called the high pressure turbine. The second turbine that the exhaust gases flow through is called the low pressure turbine. When the air enters the first turbine it is near its highest pressure, and by the time it exits the high pressure turbine and enters the low pressure turbine the pressure of the gases has decreased substantially. The diameter and blade size of the high and low pressure turbines is analogous to the size in the high and low pressure compressor. The low pressure turbine has a larger diameter and larger blades.
The low pressure compressor is driven by the low pressure turbine and the high pressure compressor is driven by the high pressure turbine. A coaxial shaft is often used in gas turbines with high and low pressure compressors and turbines. The shaft between the low pressure compressor and low pressure turbine rotates within the shaft for the high pressure compressor and the high pressure turbine.
On stationary gas turbines, the high pressure compressor and turbine can be run at the most optimum speed while the low pressure compressor and turbine are varied to meet load requirements. Stationary gas turbines may be called “heavy duty” or “industrial” engines or manufacturers may have specific names for their stationary engines; some are very large.