Page 260 - Chemical process engineering design and economics

240 Chapter 5

= 2627 kJ/kg(l 129 Btu/lb)

Therefore, h 2 > hav, which means that the steam leaves the turbine superheated.
Although in the isentropic process condensation occurs, friction in the turbine in-
creases the steam temperature and therefore the enthalpy of the steam, preventing
condensation.
The steam flow rate can now be calculated from Equation 5.11.1. The ad-
justed shaft power is 110 hp (82.0 kW). The steam flow rate,

0.87 HO.Ohp 745.7 J/s 1 kg
m = —— ———— ————— ———————————— = 250.9 kg/s (553 Ib/s)
0.36 1 1 hp (2954.0-2164.0) J

Gas Turbines

If the working fluid is a combustion gas, formed by burning a gaseous or liquid
fuel, the expander is called a gas turbine. The gas turbine is a relatively recent
developed driver for process plants. Figure 5.22 shows a flow diagram for a sim-
ple-cycle gas turbine. Fuel is burned with excess compressed air in a combustor at
constant pressure. The gas entering the turbine is limited to 760 to 1,000 °C
(1,400 to 1,830 °F) because of temperature limits on the materials of construction
[37]. The gases are maintained in this temperature range by using excess air. After
combustion, the pressurized gas expands through a turbine to about 0.025 bar (10
in H 2O) above atmospheric pressure to allow for the exit-duct losses [38]. The gas
turbine drives the air compressor and provides excess power for other process ma-
chinery. Inlet-duct pressure losses are about 0.0075 bar (3 in HO, 0.109 psia)
2
[36]. The combustion gas typically contains 14 to 19 % oxygen [37]. An effi-
ciently-operated system requires recovering the enthalpy of the hot exhaust gas.
The ratio of the output power to the total power generated varies from 0.33 to 0.50
[34].
The gas turbine requires an electric starting motor or steam turbine for start-
ing until the gas-turbine speed reaches 55 % of its final speed and becomes self-
supporting. For most applications gears are required to match the speed of the
driven equipment [34]. Molich [34] recommends a safety factor of 10 % when
justifying sizing gas turbines. The gas turbine for process applications ranges from
1,000 (746 kW) to greater than 100,000 hp (74,600 kW) [34].
Turboexpanders

When the source of high-pressure gas is a process stream, the expander is referred
to as a turboexpander. Some process applications of turboexpanders are: the sepa-

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