Steam turbines normally generate electricity as a byproduct of heat generation from steam. In thermodynamics, a steam turbine is ideally considered as an isentropic device. An isentropic process is one in which there is no change of entropy (the measure of the level of disorder). However, steam turbines do not behave ideally. In order to calculate the actual steam turbine power output, you will first have to calculate the ideal power output, for which you will need three variables: mass of steam flow, enthalpy of steam at turbine inlet, and enthalpy of steam at turbine outlet.
Use the following equation to calculate the power output: W_st = m_w (h_6 -- h_7), where W_st = power output of the steam turbine, m_w = mass of steam flow, h_6 = enthalpy of steam at turbine inlet, h_7 = enthalpy of steam at turbine outlet. You also need to find the entropy at both these states. Consider the two states of the turbine: the inlet state and the outlet state. The inlet state for the turbine is fixed since the pressure and the temperature of the steam let in is specified. The outlet state is not fixed because only the pressure is specified.
Consult the steam tables and obtaining the enthalpy and entropy for the first state using the pressure and temperature given. (A steam table is provides in the Resources section of this article.) Using the isentropic condition (s_2s = s_1), the outlet state may be fixed since you know both the pressure and entropy at this state. Consult the steam tables to identify the fluid phase to subsequently obtain the enthalpy at the outlet state.
Evaluate the enthalpy and the temperature of the outlet state. Use your variables to calculate the power output of the isentropic steam turbine, using the equation W_st = m_w (h_6 -- h_7).
Calculate the actual output by obtaining the adiabatic efficiency of the turbine along with the isentropic output, as calculated in the last step. Calculate the actual (adiabatic) output produced by using the equation W = η_s --- w_ideal , 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.
Things you need
- Steam tables