Page 366 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Table 11.5 Heuristics for Drivers and Power Recovery Equipment

1. Efficiency is greater for larger machines. Electric motors are 85%–95%; steam turbines are
42%–78%; gas engines and turbines are 28%–38% efficient (see Figure 8.7).
2. For less than 74.6 kW (100 hp), electric motors are used almost exclusively. They are made for
services up to 14,900 kW (20,000 hp).
3. Steam turbines are competitive higher than 76.6 kW (100 hp). They are speed controllable. They
are frequently used as spares in case of power failure.
4. Combustion engines and turbines are restricted to mobile and remote locations.
5. Gas expanders for power recovery may be justified at capacities of several hundred
horsepower; otherwise any pressure reduction in process is done with throttling valves.
6. The following useful definitions are given:

overall efficiency = ε = ε ε
sh dr
oυ