Page 100 - Handbook of Energy Engineering Calculations

[7.4 kJ/(kg · °C)]; H = 1522.4 Btu/lb (3541.1 kJ/kg) from the superheated-
4
steam table.

5. Compute the exhaust-steam properties
Use the Mollier chart and an assumed constant-entropy expansion to 1 inHg

(3.4 kPa) absolute to determine the ideal exhaust enthalpy, or H = 947.4
5
Btu/lb (2203.7 kJ/kg). The ideal work of the low-pressure section of the

turbine is then H – H = 1522.4 − 947.4 = 575.0 Btu/lb (1338 kJ/kg). The
5′
4
actual work output of the low-pressure section of the turbine is W = H −
4
e2
H = 575.0(0.8) = 460.8 Btu/lb (1071.1 kJ/kg).
5′
Once W is known, H can be computed from H = H − W = 1522.4 −
e2
4
5′
5′
e2
460.0 = 1062.4 Btu/lb (2471.1 kJ/kg).
The enthalpy of the saturated liquid at the condenser pressure is found in
the saturation-pressure steam table at 1 inHg absolute (3.4 kPa) = H = 47.1
6
Btu/lb (109.5 kJ/kg).
The pump work W from the compressed-liquid table diagram in the steam
p
tables is W = 5.5 Btu/lb (12.8 kJ/kg). Then the enthalpy of the water entering
p
the boiler H = H + W = 47.1 + 5.5 = 52.6 Btu/lb (122.3 kJ/kg).
p
1
6
6. Compute the cycle thermal efficiency

For any reheat cycle,

Figure 22 is an energy-flow diagram for the reheat cycle analyzed here.
This diagram shows that the fuel burned in the steam generator to produce
energy flow Q is the largest part of the total energy input. The cold-reheat
a1
line carries the major share of energy leaving the high-pressure turbine.