Page 70 - Handbook of Energy Engineering Calculations

2. Find the Rankine engine efficiency for the actual turbine
The Rankine engine efficiency for this turbine is: (0.7986/0.98) = 0.814 = (H 1
− H )/(H − H ). Solving, (H − H ) = 0.814(555) = 452.3 Btu/lb (1053.8
c
1
c
c′
1
kJ/kg).
At the end of the actual expansion of the steam in the turbine, H = 1453 −
c′
452.3 = 1000.7 Btu/lb (2331.6 kJ/kg) enthalpy.

3. Determine the moisture of the steam
Referring to the Mollier chart where H crosses the pressure line of 1.5 in
c′
(3.81 cm) Hg, the moisture percent is found to be 9.6 percent.

Related Calculations. The Mollier chart can be a powerful and quick
reference for solving steam expansion problems in plants of all types—utility,

industrial, commercial, and marine.

STEAM FLOW FOR STEAM-TURBINE NO-LOAD AND
PARTIAL-LOAD OPERATIONS

A 40,000-kW straight-flow condensing industrial steam turbogenerator unit is
2
supplied steam at 800 lb/in (abs) (5512 kPa) and 800°F (426.7°C) and is to
exhaust at 3 in (76 cm) Hg absolute. The half-load and full-load throttle
steam flows are estimated to be 194,000 lb/h (88,076 kg/h) and 356,000 lb/h
(161,624 kg/h), respectively. The mechanical efficiency of the turbine is 99
percent and the generator efficiency is 98 percent. Find (a) the no-load

throttle steam flow; (b) the heat rate of the unit expressed as a function of the
kW output; (c) the internal steam rate of the turbine at 30 percent of full load.

Calculation Procedure:

1. Find the difference between full-load and half-load steam rates and the
no-load rate

(a) Assume a straight-line rating characteristic and plot Fig. 10a. This
assumption is a safe one for steam turbines in this capacity range. Then, the
difference between full-load and half-load steam rates is 356,000 − 194,000 =