Page 61 - Handbook of Energy Engineering Calculations

turbine backpressure, Btu/lb (kJ/kg). Using the steam tables and Mollier chart
and substituting in this equation, CTE = (3413/10)(1/[1340.6 − 47.06]) =
0.2638, or 26.38 percent.

2. Find the combined engine efficiency
(b) Combined engine efficiency, CEE = (w )/(w ) = (weight of steam used by
i
e
ideal engine, lb/kWh)/(weight of steam used by actual engine, lb/kWh). The
weights of steam used may also be expressed as Btu/lb (kJ/kg). Thus, for the

ideal engine, the value is 3413 Btu/lb (7952.3 kJ/kg). For the actual turbine,
h − h is used, where h is the enthalpy of the wet steam at exhaust
2x
2x
1
conditions; h , is as before.
1
Since the steam expands isentropically into the wet region below the dome
of the T-S diagram, Fig. 1, we must first determine the quality of the steam at

point 2 either from a T-S diagram or Mollier chart or by calculation. By
calculation using the method of mixtures and the entropy at each point: S =
1
S = 0.0914 + (x )(1.9451). Then x = (1.6508 − 0.0914)/1.9451 = 0.80, or 80
2
2
2
percent quality. Substituting and summing, using steam-table values, h =
2x
47.06 + 0.8(1047.8) = 885.3 Btu/lb (2062.7 kJ/kg).

FIGURE 1 T-S diagrams for steam turbine.

(c) To find the CEE we first must obtain the ideal steam rate, w = 34l3/(h 1
i
− h ) = 3413/(1340.6 − 885.3) = 7.496 lb/kWh (3.4 kg/kWh).
2x
Now, CEE = (7.496/10)(100) = 74.96 percent. This value is excellent for
such a plant and is in a range being achieved today.

Related Calculations. Use this approach to analyze the efficiency of any