The multistage centrifugal blower is primarily a 60-Hz market product. A two-pole AC induction motor directly drives the blower at 3,600 rpm. These blowers have been used widely for the last three decades in the U.S. for wastewater treatment plant (WWTP) applications due to their simple design and rugged construction.
The cost of energy drives the market towards not only an efficient single design point, but also an efficient operating range when both flow and pressure are varied. On the other hand, if a blower is running at 3,000 rpm, as in the 50-Hz market, about 20% of flow and 40% of pressure are lost compared to a 60-Hz application.
To meet these new market challenges, the Variable Frequency Drive (VFD) multistage centrifugal blower was designed, where the VFD serves not only as speed adjusting device to maximize efficiency, but also as a speed increaser to minimize the blower size. For existing WWTP installations, retrofit to a VFD drive requires minimum investment and lead time while achieving drastic energy savings. A comparison with the gear drive high-speed single stage blower concept is also made.