SteamTurbine - ME 416 CAD of Thermal Systems Computer...

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ME 416 CAD of Thermal Systems 1 ME 416 Computer Assisted Design of Thermal Systems Steam Turbine Calculations In an ideal thermodynamic sense steam turbines are treated as isentropic devices. Though actual steam turbines deviate from this ideal performance, the treatment of a steam turbine as an isentropic device still forms the foundation for actual turbine calculations. Calculations for steam turbines are demonstrated from three perspectives in this document: using an isentropic assumption, using a specified adiabatic or isentropic efficiency, using actual manufacturers operating data. Also a second law analysis for steam turbines is provided. Ideal or Isentropic Steam Turbine Calculations We consider the schematic shown in Figure 1 where state 1 represents the inlet conditions for the turbine and state 2s represent the ideal/isentropic outlet state for the turbine. Figure 1. Schematic of Steam Turbine Normally the inlet state for the turbine is fixed by specifying the temperature and pressure of the incoming steam. For the outlet state only the pressure is specified, which mean that state 2s is not fixed. Often the mass flow rate of steam passing through the turbine is given, however, since the calculations for the ideal turbine will be linear they may be done on a per mass basis and scaled up according to the mass flow rate. We begin by going to the steam tables and obtaining the entropy and enthalpy at state 1 with the given temperature and pressure. Next the outlet state may be fixed by using the isentropic condition or State 1 State 2s Turbine
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ME 416 CAD of Thermal Systems 2 s 2s = s 1 At this point we know both our pressure and entropy at state 2s, which fixes the state. This allows us to go to the steam table and identify the fluid phase, followed by an evaluation of the enthalpy and temperature of the outlet state. We can now calculate the specific work (work per mass) and power produced by w = h 1 - h 2 ( ) W = m h h 1 2 - Adiabatic Turbine Calculation For our purposes an adiabatic turbine will have an adiabatic or isentropic efficiency that has been specified. The specific work is calculated with w = w s ideal η ⋅ where η s is the isentropic efficiency of the turbine and w ideal is the work that would be produced if the turbine behaved isentropically. Hence, an adiabatic turbine calculation first requires an ideal turbine calculation. Once the ideal work is calculated the actual work is calculated with the equation provide above. The final step of the calculation is to determine the actual outlet conditions, since up to this point in time only the isentropic outlet conditions have been found. Using the fist law we can write w = h - h 1 2a where the subscript 2a denotes the actual outlet state, state 2a. To demonstrate this
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SteamTurbine - ME 416 CAD of Thermal Systems Computer...

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