Multistage turbines

FIG. 10-46 Vertical multistage turbine, or mixed-flow, pump. 

Price range from the minimum single-stage turbine to the most efficient multistage turbine... 

FIG. 29-17 Approximate efficiency for multistage turbines. To convert horsepower to kilowatts, multiply by 0.7457. 

The upper limit in exhaust pressure for back pressure multistage turbines varies between 350 psia and 500 psia. For small, single-stage, back-pressure turbines, the standard is somewhere between atmospheric and 65 p ia. 

Increasing the average temperature during the heat-addition process can be accomplished with a superheater. The moisture content of steam at the turbine exhaust can be decreased by reheating the steam between the stages of a multistage turbine. 

An increase in the average temperature during the heat-addition process can also be accomplished by regenerating the steam. A portion of the partially expanded steam between the stages of a multistage turbine is drawn off to preheat the condensed liquid before it is returned to the boiler. In this way, the amount of heat added at low temperature is reduced. So the average temperature during the heat-addition process is increased. 

Increasing the average temperature during the heat-addition process with a reheater without increasing the compressor pressure ratio increases the net work of the Brayton cycle. A multistage turbine is used. Gas is reheated between stages. 

These larger turbines often let down the steam in two or more stages. They are called multistage turbines. Their principle of operation is not significantly different from a simple single-stage machine. 

Figure 4.1. Efficiencies of (a) single-stage and (b) multistage turbines (Gartmann, De Laval Engineering Handbook McGraw-Hill, New York, 1970, pp. 5.8-59, Figs. 5.2and 5.3).

Because of increased emphasis on maximising cogenerated power, newer plants are trying to utilize back-pressure turbines only in applications where efficiencies above 70% can be attained. This typically means limiting the applications to the large (>1000 kW) drives, and using small machines only where they are necessary for the safe shutdown of the unit. Multistage turbines are used even on the smaller loads. 

Obtain the turbine efficiency, t b> for a single-stage turbine from Equation 5.11.5 or Equation 5.11.8 for a multistage turbine. 

Steam turbines are most often used in processes to provide power to compressors or electric generators as shown in Figure 3.28. Multistage turbines may also admit or extract steam between stages. Turbine speed is a fast loop, controlled by manipulating the supply steam valve or valves, as often there will be a rack of parallel steam valves supplied as part of the turbine system. 

To improve efficiency it was apparent that catalysts would, first, permit complete oxidation of the fuel under all operating conditions of the turbine, second, permit reaching the maximum temperature tolerated by the turbine blade in a single step, and third, reduce cost of the multistage turbine for the reason that only fuel would have to be added between stages in order to consume the oxygen contained in the compressed air. 

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