FURTHER CLASSIFICATION OF TURBINES
As the output capacities and working conditions have affected the construction of each particular make it has been suggested that the following particulars be given for each turbine: 1) number of shafts, 2) number of cylinders, 3) number of exhausts, 4) the speed.
Many types of industrial turbines are in use today, depending upon the conditions under which they must operate. They are classified as high-or-low-pressure turbines, according to the inlet pressure of the steam, and as superposed, condensing, and noncondensing turbines, according to the exhaust steam pressure. A superposed or high backpressure turbine is one that exhausts to pressures well above atmospheric pressure, 100 to 600 psi. A superposed turbine operates in series with a medium-pressure turbine. The exhaust steam of the superposed turbine drives the medium-pressure unit. The noncondensing turbine has lower exhaust pressures, but the steam still leaves at atmospheric pressure or above 15 to 50 psi. The exhaust steam may be used for drying or, heating processes.
The condensing turbine operates at exhaust pressures below atmospheric pressure and requires two auxiliaries: a condenser and a pump. The condenser reduces the exhaust steam to water. As the steam is condensed and the water is removed by a pump, a partial vacuum is formed in the exhaust chamber of the turbine. This type of turbine is used chiefly for the low-cost electric power it produces.
If steam is required for processing, a turbine may be modified by extracting or bleeding the steam.
Extraction takes place at one more point between inlet and exhaust, depending upon the pressures needed for the processes. The extraction maybe automatic or nonautomatic. Generally, factory processes require steam at a specific pressure, in the case, and automatic-extraction turbine is necessary. When steam is needed within the power plant itself for heating boiler feed-water, nonautomatic extraction is generally used.
Turbines may be classified according to their speed and size. Small turbines, varying in size from a few horsepower to several thousand horsepower, are used to drive fans, pumps, and other auxiliary equipment directly. The speed of these units is adjusted to the speed of the driven machinery or is converted by a suitable gear arrangement. These turbines are used wherever steam is readily available at low cost or where exhaust steam is needed.
Turbines for the production of electric power range in size from small units to those of over 500,000 kw, and the trend is toward even larger units.
Sometimes turbogenerator units are constructed to operate at 3.600 or 1,800 rpm. The selection of the speed depends almost entirely on the size of the turbogenerator desired. The speed of 3,600 rpm is preferred whenever the size of the turbine permits. The turbine operating at the higher speed has the following advantages: lighter weight, more compactness, and great suitability for high-pressure, high-temperature operation.
With a few exceptions turbines larger than 100,000 kw will operate at 1,800 rpm. All turbines of smaller capacity will run at 3,600 rpm. However, because of the advantages of the 3,600 rpm unit and because of the greater efficiency of large units turbine manufacturers will continue to raise the upper limit of speed and capacity.
Generally, turbogenerators on a, single shaft and within a given speed range are constructed with either a single or a double-rotor.
The double-rotor arrangement is used for only the largest turbines falling within a given speed range. A double-rotor unit is called a tandem-compound turbine, and the flow is double-exhaust to accommodate the large volumes of steam occurring at the low-pressure end.
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