Steam turbine mechanical efficiency

For a steam turbine to work efficiently, it is neces-saity that the velocity of the tips of the turbine blades be proportional to the velocity of the input steam jet. If the velocity of the input steam jet is high, it is nec-essaity to increase the radius of the blades so that their tip velocity becomes correspondingly high. These long rotor blades subject the entire assembly to unmanageable mechanical stresses at high speed. 

Generally, the efficiency of steam turbines decreases with decreasing load. The overall turbine efficiency can be represented by two components the isentropic efficiency and the mechanical efficiency. The mechanical efficiency reflects the efficiency with which the energy that is extracted from steam is transformed into useful power and accounts for mechanical frictional losses, heat losses, and so on. The mechanical efficiency is high (typically 0.95 to 0.99)6. However, the mechanical efficiency does not reflect the efficiency with which energy is extracted from steam. This is characterized by the isentropic efficiency introduced in Figure 2.1 and Equation 2.3, defined as ... 

In addition to the mechanical efficiency being much higher than the isentropic efficiency, in most cases the mechanical efficiency does not change significantly with load. By contrast, the isentropic efficiency does change significantly with load. The overall steam turbine efficiency can be defined as ... 

For back-pressure turbines, the turbine exhausts to a steam main. Therefore, in order to enable estimation of steam main conditions, it is important to predict the condition of the exhausted steam also. With a value of the mechanical efficiency, the enthalpy of the exhaust steam can be calculated from an energy balance9 ... 

A steam turbine operates with inlet steam conditions of 40 barg and 420°C and can be assumed to operate with an isentropic efficiency of 80% and a mechanical efficiency of 95%. Calculate the power production for a steam flowrate of 10 kg s-1 and the heat available per kg in the exhaust steam (i.e. superheat plus latent heat) for outlet conditions of ... 

Several parameters affect the efficiency of steam turbines, as shown partially on Figure 4.1. Closer examination will need to take into account specific mechanical details which usually are left to the manufacturer. Geared turbines [the dashed line of Fig. 4.1(b)] have higher efficiencies, even with reduction gear losses, because they operate with especially high bucket speeds. For example, for a service of 500 HP with 300psig steam, a geared turbine has an efficiency of 49.5% and one with a direct drive at 1800 rpm has an efficiency of 24%. 

The compressor can be driven by electric motors, gas or steam turbines, or internal combustion (usually diesel) engines. The compressor can also be a steam-driven ejector (Fig. 7b), which improves plant reHabiUty because of its simplicity and absence of moving parts, but also reduces its efficiency because an ejector is less efficient than a mechanical compressor. In aU of the thermally driven devices, turbines, engines, and the ejector mentioned herein, the exhaust heat can be used for process efficiency improvement, or for desalination by an additional distillation plant. Figure 8 shows a flow diagram of the vertical-tube vapor compression process. 

The next step is the conversion of the thermal energy of the steam to the mechanical energy of a turbine. The efficiency of this step is limited by absolute thermodynamic constraints as described classically through an ideal heat cycle such as the Camot cycle. 

The recent efficiency race for natural gas fired power plants with gas-plus steam-turbine-cycle is shortly reviewed. The question can the HTR compete with high efficiencies is answered Yes, it can - in principle. The gas-plus steam-turbine cycle, also called combi-cycle, is proposed to be taken into consideration here. A comparative study on the efficiency potential is made it yields 54.5. % at 1 050 °C gas turbine-inlet temperature. The mechanisms of release versus temperature in the HTR are summarized from the safety report of the HTR MODUL. A short reference is made to the experiences fi-om the HTR-Helium Turbine Project HHT, which was performed in the Federal Republic of Germany in 1968 to 1981. 

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