Question 12.19: A steam power plant operates on a theoretical reheat cycle. ...
A steam power plant operates on a theoretical reheat cycle. Steam at boiler at 150 bar, 550°C expands through the high pressure turbine. It is reheated at a constant pressure of 40 bar to 550°C and expands through the low pressure turbine to a condenser at 0.1 bar. Draw T-s and h-s diagrams. Find :
(i) Quality of steam at turbine exhaust ;
(ii) Cycle efficiency ;
(iii) Steam rate in kg/kWh.
Refer Figs. 12.27 and 12.28.
From Mollier diagram (h-s diagram) :
h_1=3450 kJ/kg ; h_2=3050 kJ/kg ; h_3=3560 kJ/kg ; h_4=2300 kJ/kg
h_{f_4} (from steam tables, at 0.1 bar) = 191.8 kJ/kg
(i) Quality of steam at turbine exhaust, x_4 :
x_4=0.88 (From Mollier diagram)
(ii) Cycle efficiency, \eta_{\text {cycle }} :
\eta_{\text {cycle }}=\frac{\left(h_1-h_2\right)+\left(h_3-h_4\right)}{\left(h_1-h_{f_4}\right)+\left(h_3-h_2\right)}= \frac{(3450-3050)+(3560-2300)}{(3450-191.8)+(3560-3050)}=\frac{1660}{3768.2}= 0 . 4 4 0 5 or 44.05%.
(iii) Steam rate in kg/kWh :
Steam rate =\frac{3600}{\left(h_1-h_2\right)+\left(h_3-h_4\right)}=\frac{3600}{(3450-3050)+(3560-2300)}
= \frac{3600}{1660}=2.17 kg/kWh.
