Wednesday, December 3, 2008

principle of working of steam turbine

“A steam turbine is a prime mover that derives its energy of rotation due to conversion of the heat energy of steam into kinetic energy as it expands through a series of nozzles mounted on the casing or the fixed blades.”
Water is converted to steam by application of heat in the boiler, which makes the steam at specified pressure and temperature. To convert the steam’s energy into work, it must go through a thermodynamic cycle that combines expansion compression, heat input, and heat rejection. The most efficient thermodynamic cycle for an ideal fluid is Carnot cycle. It consists of an isothermal heat input, isentropic expansion, isothermal heat rejection, and an isentropic compression. Regardless of the combination, the efficiency of the cycle, assuming constant mass flow is based on the difference in the enthalpy and between the beginning and end of the cycle.

• 1 to 2: Isentropic expansion
• 2 to 3: Isothermal heat rejection
• 3 to 4: Isentropic compression
• 4 to 1: Isothermal heat supply

Steam can be used as the working fluid in the Carnot Cycle. But its properties adversely impact its usefulness. In this case the steam expansion process takes place completely in the moisture region. This requires compression of a vapour/moisture mixture to return to the cycle’s starting point. Moisture is an expansion process imposes large mechanical efficiency losses. Also, vapour compression is inefficient and consumes relatively large amounts of power.
To avoid a two-phase vapour compression process, turbines are based on the Rankine cycle. It is similar to the Carnot Cycle, except that the initial pressure of the steam is raised and the condensation process that accompanies heat rejection continues until the liquid saturation point is reached. At the end of the cycle, then, condensate is simply pumped back to the boiler to begin the cycle. The role of the steam turbine is to expand the steam from high pressure and temperature to lower pressure and temperature.

Rankine cycle is a heat engine with vapor power cycle. The common working fluid is water. The cycle consists of four processes:

1 to 2: Isentropic expansion
(Steam turbine)
2 to 3: Isobaric heat rejection
3 to 4: Isentropic
compression (Pump)
4 to 1: Isobaric heat supply

Several things can be done to steam to improve the Rankine Cycle efficiency. Raise initial steam condition and reduce the amount of moisture near the end of expansion stage. The first is accomplished by superheating the steam before it does any work. The second involves re- heating steam to near initial-conditions after it is partially expanded by directing it back to the heat source, then completing the expansion. In converting the thermal energy of steam into mechanical energy turbines takes advantage of this facts- as it expands or drops in pressure, through a small nozzle or opening, it accelerates and forms a high-speed jet. Directing this momentum in a rotating blade provides mechanical energy.


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