Rabu, 15 Juni 2011

condensing steam turbine

With a condensing steam turbine the vacuum to be in the condenser is already established by the turbine design. The cooling water flow is already determined based on the turbine design steam rate and the water supply temperature. When the load on the turbine is reduced the turbine inlet steam supply valve is modulated and less steam is there to condense. The condenser water flow may be reduced. If the water supply temperature changes the flow may be also changed. This is usually done automatically by adjustment of the condenser water supply control valve. The condenser vacuum controller sends the appropriate operating signal to that water valve actuator. Reducing the water flow reduces the horsepower needed by the pump motors.
The Rankine Cycle is inherently inefficient because the energy expended to evaporate the boiler water is all lost through the condenser and ends up either being discharged by evaporation in the cooling towers, by release into the river or bay, or to a cooling pond. However, if there is a need for a steam heating supply elsewhere, the heat of condensation can be utilized there. This can be the case in the winter where building heating is needed or also in a manufacturing process . A back-pressure turbine is employed and the condensing turbine cut back. This is a practice in some industrial manufacturing plants, but not in dedicated utility power plants.

Turbine, rotary engine that converts the energy of a moving stream of water, steam, or gas into mechanical energy. The basic element in a turbine is a wheel or rotor with paddles, propellers, blades, or buckets arranged on its circumference in such a fashion that the moving fluid exerts a tangential force that turns the wheel and imparts energy to it. This mechanical energy is then transferred through a drive shaft to operate a machine, compressor, electric generator, or propeller. Turbines are classified as hydraulic, or water, turbines, steam turbines, or gas turbines. Today turbine-powered generators produce most of the world's electrical energy. Windmills that generate electricity are known as wind turbines (see Windmill).

Hydraulic Turbines

The oldest and simplest form of the hydraulic turbine was the waterwheel, first used in ancient Greece and subsequently adopted in most of ancient and medieval Europe for grinding grain. It consisted of a vertical shaft with a set of radial vanes or paddles positioned in a swiftly flowing stream or millrace. Its power output was about 0.5 horsepower. The horizontal-shaft waterwheel (that is, a horizontal shaft connected to a vertical paddle wheel), first described by the Roman architect and engineer Marcus Vitruvius Pollio during the 1st century BC, had the lower segment of the paddle wheel inserted into the stream, thus acting as a so-called undershot waterwheel. By about the 2nd century AD, the more efficient overshot wheel had come into use in hilly regions. Here the water was poured on the paddles from above, and additional energy was gained from the falling water. The maximum power of the waterwheel, which was constructed of wood, increased from about 3 horsepower to about 50 horsepower in the Middle Ages.

The transition from waterwheel to turbine is largely semantic. The first important attempt to formulate a theoretical basis for waterwheel design was in the 18th century by the British civil engineer John Smeaton, who proved that the overshot wheel was more efficient. The French military engineer Jean Victor Poncelet, however, devised an undershot wheel, the curved blades of which raised efficiency to nearly 70 percent; it quickly came into wide use. Another French military engineer, Claude Burdin, invented the term turbine, introduced as part of a theoretical discussion in which he stressed speed of rotation. Benoit Fourneyron, who studied under Burdin at the School of Mines at St. √Čtienne, designed and built wheels that achieved speeds of 60 or more rpm (revolutions per minute) and provided up to 50 horsepower for French ironworks. Ultimately Fourneyron built turbines that operated at 2300 rpm, developing 60 horsepower at an efficiency of more than 80 percent.

Despite its remarkable efficiency, the Fourneyron turbine had certain drawbacks as a result of the radial outward flow of water that passed through it. This created problems if water flow was reduced or load removed. The British-born American engineer James B. Francis designed a turbine in which the flow was inward, and the so-called reaction, or Francis, turbine, became the most widely used hydraulic turbine for water pressures, or heads, equivalent to a column of water 10 to 100 m (33 to 330 ft). This type of turbine operates by expanding the pressure energy in the water during the flow through the blade passages, resulting in a net force, or reaction, which has a tangential component that turns the wheel.

For installations where water heads of about 90 to 900 m (about 300 to 3000 ft) were available, the Pelton wheel, named after the American engineer Lester Allen Pelton, came into use during the second half of the 19th century. In this turbine, the water is piped from a high-level reservoir through a long duct, or penstock, to a nozzle where its energy is converted into the kinetic energy of a high-speed jet. This jet is then directed onto curved buckets, which turn the flow by nearly 180 degrees and extract the momentum. Because the action of the Pelton wheel depends on the impulse of the jet on the wheel, rather than on the reaction of the expanding water, this type of turbine is also known as an impulse turbine.

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