Gas Turbine vs Steam Turbine
Turbines are a class of turbo machinery used to convert the energy in a flowing fluid into mechanical energy by the use of rotor mechanisms. Turbines, in general, convert either thermal or kinetic energy of the fluid into work. Gas turbines and steam turbines are thermal turbo machinery, where the work is generated from the enthalpy change of the working fluid; i.e. The potential energy of the fluid in the form of pressure is converted into mechanical energy.
Based on the direction of the fluid flow turbines are categorized into axial flow turbines and radial flow turbines. Technically a turbine is an expander, which delivers mechanical work output by the decrease in pressure, which is the opposite operation of the compressor. This article focuses on the axial flow turbine type, which is more common in many engineering applications.
The basic structure of an axial flow turbine is designed to allow a continuous flow of fluid while extracting the energy. In thermal turbines, the working fluid, at a high temperature and a pressure is directed through a series of rotors consisting of angled blades mounted on a rotating disk attached to the shaft. In between each rotor disks stationary blades are mounted, which act as nozzles and guides to the fluid flow.
More about Steam Turbine
Even though the concept of using steam to do mechanical work was used for a long time, the modern steam turbine was designed by English engineer Sir Charles Parsons in 1884.
The steam turbine uses pressurized steam from a boiler as the working fluid. The superheated steam entering the turbine loses its pressure (enthalpy) moving through the blades of the rotors, and the rotors move the shaft to which they are connected. Steam turbines deliver power at a smooth, constant rate, and the thermal efficiency of a steam turbine is higher than that of a reciprocating engine. The operation of steam turbine is optimal at higher RPM states.
Strictly, the turbine is only a single component of the cyclic operation used for power generation, which is ideally modelled by the Rankine cycle. The boilers, heat exchangers, pumps, and condensers are also components of the operation but are not parts of the turbine.
In modern days, primary use of the steam turbines is for the electrical power generation, but at the early 20th century steam turbines was used as the power plant for ships and locomotive engines. As an exception, in some marine propulsion systems where the diesel engines are impractical, such as aircraft carriers and submarines, the steam engines are still used.
More about Gas Turbine
Gas turbine engine or simply a gas turbine is an internal combustion engine, using gases such as air as the working fluid. Thermodynamic aspect of the operation of the gas turbine is ideally modelled by the Brayton cycle.
Gas turbine engine, unlike the steam turbine, consists of several key components; those are the compressor, combustion chamber, and turbine, which are assembled along a rotating shaft, to perform different tasks of an internal combustion engine. Gas intake from the inlet is first compressed using an axial compressor; which performs the exact opposite of a simple turbine. The pressurized gas is then directed through a diffuser (a diverging nozzle) stage, in which the gas loses its velocity, but increases the temperature and the pressure further.
In the next stage, gas enters the combustion chamber where a fuel is mixed with the gas and ignited. As a result of the combustion, the temperature and pressure of the gas rise to an incredibly high level. This gas then passes through the turbine section, and when passing through produces rotational motion to the shaft. An average size gas turbine produces shaft rotation rates as high as 10,000 RPM, while smaller turbines may produce 5 times as much.
Gas turbines can be used to produce torque (by the rotating shaft), thrust (by high speed gas exhaust), or both in combination. In the first case, as in the steam turbine, the mechanical work delivered by the shaft is merely a transformation of enthalpy (pressure) of the high temperature and pressure gas. Part of the shaft work is used to drive the compressor through an internal mechanism. This form of the gas turbine is used mainly for electric power generation and as power plants for vehicles such as tanks and even cars. The US M1 Abrams tank uses a gas turbine engine as the power plant.
In the second case, the high pressure gas is directed through a converging nozzle to increase velocity, and the thrust is generated by the exhaust gas. This type of gas turbine is often called Jet engine or turbojet engine, which powers the military fighter aircraft. The turbofan is an advanced variant of above, and the combination of both thrust and work generation is used in turboprop engines, where shaft work is used to drive a propeller.
There exist many variants of the gas turbines designed for specific tasks. They are preferred over other engines (mainly reciprocating engines) due to their high power to weight ratio, less vibration, high operation speeds, and reliability. The waste heat is dissipated almost entirely as the exhaust. In electrical power generation, this waste thermal energy is used to boil water to run a steam turbine. The process is known as combined cycle power generation.
What is the difference between Steam Turbine and Gas Turbine?
• Steam turbine uses high pressure steam as the working fluid, while the gas turbine uses air or some other gas as the working fluid.
• Steam turbine is basically an expander delivering torque as the work output, while a gas turbine is a combined device of compressor, combustion chamber, and turbine executing a cyclic operation to deliver work as either torque or thrust.
• Steam turbine is only a component executing one step of the Rankine cycle, while gas turbine engine executes the whole Brayton cycle.
• Gas turbines can deliver either torque or thrust as the work output, while steam turbines almost all the time delivers torque as the work output.
• The efficiency of the gas turbines is much higher than the steam turbine due to higher operating temperatures of the gas turbines. (Gas turbines ~1500 0C and steam turbines ~550 0C)
• The space required for the gas turbines is much less than steam turbine operation, because steam turbine requires boilers and heat exchangers, which should be connected externally for heat addition.
• Gas turbines are more versatile, because many fuels can be used and working fluid, which has to be fed continuously, is readily available everywhere (air). Steam turbines, on the other hand, require large amounts of water for the operation and tend to cause problems in lower temperatures due to icing.