ENGR217 2017 Q B2 (ENGR5003)

ENGR217 2017 Q B2Past paperOld spec ENGR2172:225 marks30 min

Answer ALL parts (a) - (d)

Formulas you may need

SFEE: $\dot Q - \dot W = \dot m\left[\left(h + \tfrac{C^2}{2} + gZ\right)_{out} - \left(h + \tfrac{C^2}{2} + gZ\right)_{in}\right]$ (on the formula sheet)
Wet steam: $x = m_g/m_{total}$ , $h = (1-x)h_f + x h_g = h_f + x h_{fg}$ (on the formula sheet)
Thermal efficiency: $\eta = \dfrac{W_{net}}{Q_{in}}$ (on the formula sheet)
Steam property values $h_f$ , $h_g$ , $h_{fg}$ at the given pressures and temperatures (learn this: read from steam tables, provided in the exam)

(a)
A boiler fired by natural gas, and containing water and steam, operates continuously to supply 0.25 kg/s of dry saturated steam at 40 bar to a turbine. Feed water at $30^\circ\mathrm{C}$ is pumped into the boiler at the same mass flow rate, so as to maintain steady conditions. Estimate the rate of heat supply needed to operate the boiler, assuming no losses. (Extracts from steam tables are provided in the Appendix.)
Q_boiler =kW
[9]
(b)
In practice, the steam leaves the boiler slightly wet, and when it enters the turbine it is at pressure 40 bar and dryness fraction 0.96. The steam exhausts from the turbine at pressure 0.5 bar and dryness fraction 0.82. Estimate the power delivered by the turbine, assuming negligible heat losses. You may also neglect the kinetic and potential energies of the steam.
h_in =kJ/kg
h_out =kJ/kg
W_turbine =kW
[9]
(c)
More energy could be extracted from the steam by taking the exhaust steam to a lower value of dryness fraction. Explain what places a practical limit on the wetness of the steam leaving the turbine.
[4]
(d)
Find the efficiency of the boiler and turbine as a system for producing mechanical power, and suggest how the efficiency might be improved.
eta =-
[3]