A gas turbine is a device that uses high-pressure combustion gases to turn a series of turbine wheels to provide rotational energy to turn an axle or a shaft. Gas turbines are used to operate electric generators, ships, and racing cars and are a primary component of jet aircraft engines. There are three primary parts of a gas turbine system: an axial compressor, a combustion chamber, and a gas turbine. The gas turbine system mixes compressed air with fuel in a combustion chamber. A spark plug ignites the mixture, which is directed into the suction side of the gas turbine. The hot combustion gases rush into the gas turbine, causing the turbine wheels to turn. Hot exhaust gases are discharged from the body of the gas turbine. The air compressor and the gas turbine are mounted to the same axle, which is connected to the workload.
When the air compressor and the combustion chamber are used in combination, the device is frequently referred to as a gas generator. During operation, a fraction of the power generated by the turbine is used to run the compressor. When the air compressor pulls air into the system, the pressure increases. When the compressed air mixes with the fuel and is ignited, the higher pressure allows the mixture to burn better. The fuel used to operate a gas turbine is natural gas or oil. The hot combustion gases produced by the gas or oil are used in the same way a steam turbine uses steam to turn the rotor. The air for combustion is generally filtered through a bag-house arrangement to remove airborne contaminants, which would deposit on turbine components.
The compressor consists of the compressor rotor assembly, stator blades, rotor blades, compressor case assembly, air inlet filter assembly, bearings and seals, and compressor diffuser assembly. The combustion chamber consists of a fuel injector, combustor housing assembly, gas fuel manifold, bleed air valve, and igniter. The gas turbine consists of a gas producer rotor assembly, power rotor assembly, moving turbine wheels and stationary blades, nozzle case and assembly, turbine exhaust diffuser, and exhaust collector. The workload consists of the driven shaft and the driven device.
Each part of the gas turbine system is an integral part of the whole unit. Axial flow compressors have replaced most other compressor designs because of the large volume they can handle. The combustion chamber combines two feed components to produce a continuous, high-pressure flow into the turbine. The gas turbine has a number of stages that increase in size to accommodate the expanding hot gases that jet through the moving turbine wheels and stationary blades.
The basic gas turbine cycle is termed the Brayton cycle. The ideal Brayton cycle is a closed cycle consisting of an isentropic compression process; a constant pressure external heating process; an isentropic expansion process; and finally a constant
pressure external cooling process which returns the working substance to the inlet state of the compression process.
Simple Open Cycle: The simple open cycle gas turbine takes atmospheric air into the compressor as the working substance. Following compression, the air enters the combustion chamber where the temperature is raised by the combustion of fuel. The gaseous combustion products are then expanded back to the atmosphere through a turbine. The turbine in this system derives enough power from the high temperature gas to drive both the compressor and load.
Regenerative Ideal Brayton Cycle: The use of a regenerator in an ideal Brayton cycle acts to reduce the amount of available energy lost by external heat exchange. This available energy loss is due to irreversible heat input. A heat exchanger or regenerator is placed in the system to transfer heat internally from the hot exhaust gas to the cooler air leaving the compressor. This preheating of the combustion air thus reduces the amount of external heat input needed to produce the same work output.
Combined Cycle: Instead of using the hot exhaust gas for regeneration, this approach uses exhaust gas to generate steam. This steam can be used either as a supplement to the plant steam system or to generate additional horsepower in a Rankine cycle. In the basic Rankine cycle, the hot exhaust gas passes successively through the superheater, evaporator, and economizer of the steam generator before being exhausted to the atmosphere. The steam leaving the boiler is expanded through a steam turbine to generate additional power. The cycle is closed by the addition of a condenser and feed water pump completing a basic Rankine cycle. Since the steam cycle does not require any additional fuel to generate power, the overall thermal efficiency is increased.
The gas turbine was first widely used as an aircraft power plant. However, as they became more efficient and durable, they were adapted to the industrial marketplace. Over the years the gas turbine has evolved into two basic types for high-power stationary applications: the industrial or heavy duty design and the aircraft derivative design.
1. Heavy Duty
The industrial type gas turbine is designed exclusively for stationary use. Where high power output is required, 35,000 hp and above, the heavy duty industrial gas turbine is normally specified. The industrial gas turbine has certain advantages which should be considered when determining application requirements. Some of these are:
• Less frequent maintenance.
• Can burn a wider variety of fuels.
• Available in larger horsepower sizes.
In a single shaft design, all rotating components of the gas turbine are mounted on one shaft. The single shaft design is simple, requiring fewer bearings, and is generally used where the speed range of the driven equipment is narrow or fixed (as in generator sets). It requires a powerful starting system since all the rotating components (including the driven equipment) must be accelerated to idle speed during the start cycle.
In a split shaft design, the air compressor rotating components are mounted on one shaft, and the power turbine rotating components are mounted on another shaft. The driven equipment is connected to the power turbine shaft. A split shaft design is advantageous where the driven equipment has a wide speed range or a high starting torque. The air compressor is able to run at its most efficient speed while the power turbine speed varies with the driven equipment. The split shaft design allows a much smaller starting system since only the air compressor shaft is accelerated during the start cycle.
2. Aircraft Derivative
An aircraft derivative gas turbine is based on an aircraft engine design which has been adapted for industrial use. The engine was originally designed to produce shaft power and later as a pure jet. The adaptation to stationary use was relatively simple. Some of the advantages of the aircraft derivative gas turbines are:
• Higher efficiency than industrial units.
• Quick overhaul capability.
• Lighter and more compact, an asset where weight limitations are important such as offshore installations.
Components of a Steam Turbine
The basic components of a gas turbine system fall under four primary areas: the compressor, combustion chamber, gas turbine, and workload . Each of these areas has a number of critical components, and they all are linked by a common axle.
The compressor consists of the compressor rotor assembly, stator blades, rotor blades, compressor case assembly, air inlet filter assembly, bearings and seals, and compressor diffuser assembly.
The combustion chamber consists of a fuel injector, combustor housing assembly, gas fuel manifold, bleed air valve, and igniter.
The gas turbine consists of a gas producer rotor assembly, power rotor assembly, moving turbine wheels and stationary blades, nozzle case and assembly, turbine exhaust diffuser, and exhaust collector.
The workload consists of the driven shaft and the driven device.
1-ENGINEERING DATA BOOK by Gas Processors Suppliers Association
2-Process Technology - Equipment and Systems by Charles E. Thomas