Page 95 - Handbook of Energy Engineering Calculations

Btu/lb (2927.0 kJ/kg). This is the actual enthalpy of the steam at the inlet to
2
heater 2. Plot this enthalpy on the 200-lb/in (abs) (1379.0-kPa) pressure line
of the Mollier chart, and find S = 1.613 Btu/(lb · °F) [6.8 kJ/(kg · °C], Fig.
4′
19.
Using the same procedure with constant-S expansion from H , we find H 5
4′
2
= 1059.5 Btu/lb (2464.4 kJ/kg) at 16 lb/in (abs) (110.3 kPa), the inlet
pressure to heater 3. Next, find H = H − e (H – H ) = 1258.4 −
4′
5′
5
4′
e
0.8(1258.4 − 1059.5) = 1099.2 Btu/lb (2556.7 kJ/kg). From the Mollier chart
find S = 1.671 Btu/(lb · °F) [7.0 kJ/(kg · °C)], Fig. 19.
5′
Using the same procedure with constant-S expansion from H to H , find
5′
6
H = 898.2 Btu/lb (2089.2 kJ/kg) at 1 inHg absolute (3.4 kPa), the condenser
6
inlet pressure. Then H = H − e (H – H ) = 1099.2 − 0.8(1099.2 − 898.2)
6
e
5′
6′
5′
− 938.4 Btu/lb (2182.7 kJ/kg), the actual enthalpy of the steam at the
condenser inlet. Find, on the Mollier chart, the moisture in the turbine
exhaust = 15.1 percent.
2. Determine the overall engine efficiency
Overall engine efficiency e is higher than the engine-section efficiency
e
because there is partial available-energy recovery between sections.
Constant-S expansion from the throttle to the 1-inHg absolute (3.4-kPa)

exhaust gives H , Fig. 19, as 838.3 Btu/lb (1949.4 kJ/kg), assuming that all
3s
the steam flows to the condenser. Then, overall e = H – H /(H − H ) =
e
2
6′
2
3S
1474.5 − 938.4/1474.5 − 838.3 = 0.8425, or 84.25 percent, compared with
0.8 or 80 percent, for individual engine sections.
3. Compute the bleed-steam flow to each feedwater heater
For each heater, energy in = energy out. Also, the heated condensate leaving

each heater is a saturated liquid at the heater bleed-steam pressure. To
simplify this calculation, assume negligible steam pressure drop between the
turbine bleed point and the heater inlet. This assumption is permissible when
the distance between the heater and bleed point is small. Determine the pump

work by using the chart accompanying the compressed-liquid table in Keenan
and Keyes—Thermodynamic Properties of Steam, or the ASME—Steam
Tables.