Problem 8.3 - A Reheat Steam Power Plant with a Closed Feedwater Heater

Consider the reheat steam power plant shown below which has been augmented with a Closed Feedwater Heater in an attempt to increase the thermal efficiency.

• a) Neatly sketch the complete power plant cycle on the pressure-enthalpy ( P-h ) diagram provided below, clearly indicating all stations (1) through (10) on the diagram. Using the Steam Tables determine the enthalpy values at each of the stations. Indicate the values on the ( P-h ) diagram at each of the stations and check them for feasibility.

• b) Assuming that the water is incompressible determine the increase in enthalpy (Δh) across the feedwater pump, and use this value to determine the enthalpy at station (7).
  [Δh = 19.2 kJ/kg, h ₇ = 270.4 kJ/kg]

• c) Assuming that the shut-off valve is fully open, determine the mass fraction (y) of the steam bled from the output of the HP turbine at station (2) in order to obtain the conditions indicated at stations (8) and (9) on the diagram. [y = 0.23]

• d) Determine the overall thermal efficiency (η _th ) of this power plant. (Recall that thermal efficiency is defined as the net work done (turbines + feedwater pump) divided by the total heat supplied to the boiler (including the reheat system). [η _th = 37%]

• e) Consider the above reheat steam power plant operating under the condition that the bleed inlet to the Closed Feedwater Heater has been shut off at station (2), thus station (8) is at the same temperature as station (7) (60°C), and the mass fraction (y) of steam is reduced to zero. Under this condition determine the overall thermal efficiency (η _th ) of this power plant. [η _th = 35%]

Justify all values used and derive all equations used starting from the basic steady flow energy equation, the basic equation of enthalpy change (Δh) for an incompressible liquid. and the basic definition of thermal efficiency (η _th )

Consider the two adiabatic turbines only in the reheat steam power plant shown above.

• f) Neatly sketch the two turbine processes (1)-(2) and (3)-(4) on the enthalpy-entropy ( h-s ) diagram provided below. Plot also the isentropic turbine processes on the diagram and indicate the actual and isentropic work for both turbines on the diagram. Using the Steam Tables determine the enthalpy and entropy values at each of the four stations. Indicate the values on the ( h-s ) diagram at each of the stations and check them for feasibility.

• g) Determine the turbine adiabatic efficiencies (η _T ) for both turbines.
  [η _HP = 82%, η _LP = 88%]

Justify all values used and derive all equations used starting from the basic steady flow energy equation and the basic definition of turbine adiabatic efficiency η _T .