Steam turbine isentropic efficiency

For large steam turbines, isentropic efficiency can be between 50 and 70% while efficiency for small turbines is much lower with a wide variation depending on speed, horse power (hp), and pressure conditions. Efficiency varies with turbine loading, which can be described by an efficiency curve. This curve can be used to determine actual steam rates based on specific turbine loading. 

Steam enters a turbine of a Rankine power plant at 5 MPa and 400°C, and exhausts to the condenser at lOkPa. The turbine produces a power output of 20 MW. Given a turbine isentropic efficiency of 90% and a pump isentropic efficiency of 100% ... 

Two parameters, namely, theoretical steam rate and turbine isentropic efficiency, can be used to determine actual steam rate required by the steam turbine to make a certain amount of power. The theoretical steam rate is described by ideal expansion. The assumption behind ideal expansion is that there is no thermal and hydraulic losses in the expansion process resulting in zero change in entropy (5) (see Figure 15.5). Thus, the theoretical steam rate is the minimum required for making a certain amount of power. In reality, losses occur in steam expansion and isentropic efficiency describes the irreversible losses in the real expansion causing deviation from the ideal expansion ... 

Exampie A.3.1 The pressures for three steam mains have been set to the conditions given in Table A.l. Medium- and low-pressure steam are generated by expanding high-pressure steam through a steam turbine with an isentropic efficiency of 80 percent. The cost of fuel is 4.00 GJ and the cost of electricity is 0.07 h. Boiler feedwater is available at 100°C with a heat capacity... 

The steam turbine is basically a steam expander, expanding the steam over a given pressure range along an isentropic path. Space does not permit a complete development of all the aspects of the turbine. However, a few fundamentals will be reviewed to help give an understanding of how turbines fall into different efficiency ranges. 

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 ... 

Figure 23.10a illustrates the relationship between turbine efficiency and mass flow through the turbine. Because hMECH his, the major contribution to the nonlinear trend of the overall efficiency with part-load is from the isentropic efficiency his A turbine model needs to capture this behavior. It should also be noted that there will be an efficiency associated with an electricity generating set coupled to the steam turbine (typically 95 to 98%). 

The isentropic power from a steam turbine can be related to the maximum shaft power through the overall turbine efficiency ... 

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 ... 

A proposal is made to use a geothermal supply of hot water at 1 MPa and 170°C to operate a steam turbine as shown in Fig. 2.24a. The high-pressure water is throttled into a flash evaporator chamber, which forms liquid and vapor at a lower pressure of 400 kPa. The liquid is discarded while the saturated vapor feeds the turbine and exits at lOkPa. Cooling water is available at 15°C. The turbine has an isentropic efficiency of 88%. Find the turbine power per unit geothermal hot-water mass flow rate. Find the optimized flash pressure that will give the most turbine power per unit geothermal hot-water mass flow rate. 

The third step is to model the components of the conceptual plant. For example, a steam turbine may be modeled as an adiabatic process with 85% isentropic efficiency. 

Steam is fed to a turbine at 614.7 psia and 825°F and is discharged at 64.7 psia. (a) Find the theoretical steam rate, lb/kWh, by using the steam tables, (b) If the isentropic efficiency is 70%, find the outlet temperature, (c) Find the theoretical steam rate if the behavior is ideal, with Cp Cv = 1.33. 

Separately driven centrifugal compressors are adaptable to low-pressure cryogenic systems because they can be coupled directly to steam turbine drives, are less critical from the standpoint of foundation design criteria, and lend themselves to gas turbine or combined cycle applications. Isentropic efficiencies of 80 to 85% are usually obtained. 

Figure 5.19 Isentropic efficiencies for single-stage noncondensing turbines - dry saturated steam. From Ref. 31 with permission. 

Figure 5.21 Isentropic efficiencies for multistage steam turbines. From Ref. 34 with permission.

A steam turbine operates adiabatically with a steam rate of 30 kgs. The steam is supplied all 1,050 kPa and 37S°C and discharges at 20 kPa and 7S°C. Determine tthe power output of the turbine and the efficiency of its operation in comparison with a turbine that operates isentropically from the same initial conditions to the same final pressure. 

Having embarked on the quest of simplification, we show that the steam tables may be represented in the regions of interest to steam turbines by relatively simple approximating functions, examples of which are given. Further, it is shown that small changes in efficiency have little effect on the calculated mass flow, and it will often be sufficient for control engineering purposes to use the isentropic mass flow equation. 

Willans Line - The Willians line is used to show steam rates at different loads on the turbine. Often, a willans line for a specific turbine is not available, and one only has has incomplete information about an installed steam turbine namely the rated power and (perhaps a guesstimate) the isentropic efficiency or specific steam consumption at full load. But what you need and don t have is the steam consumption at the turbine operating load. 

Consider an HP-MP turbine (Figure 15.6) with isentropic efficiency of 50% and steam conditions given as HP inlet at 750 °F and 600psig MP exhaust at 150psig. The turbine is used to drive a pump with power demand of 300kW. What is flie actual... 

For Power Generation Subsystem Power is generated from a steam turbine (k), theoretic steam rate ( turbine k) and isentropic efficiency (turbine k)- Th isentropic efficiency depends on the inlet and outlet steam conditions and steam rate (Afturbine k)- For a given total power demand, power import can be determined from on-site power generation ... 

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