Start/Stop service and varying discharge pressure capabilities are important factors when selecting blowers for sequential batch reactors (SBR’s) and aerobic digesters. The Inovair geared blower utilizing non-contact oil film bearings has proven very effective for these applications.
A common issue with many high speed turbo blowers (HST) is wear of the air bearing. In instances where a blower may need to stop/start several times per hour the blower could experience nearly 10,000-12,000 start/stop cycles per year. This would mean based off of published data, in as little as 2.5 years the air bearings would need to be replaced. This occurs because during the start/stop cycle as the blower initially accelerates during start-up or decelerates during shutdown, contact is made between the bearing surface and the shaft. The published data suggests 25,000 starts/stops is the limit.
Additionally, air-bearings generate stiffness with speed. Under normal operation in the 80-100% speed range, there is adequate stiffness to provide stability to the rotor. However, operation on a widely varying liquid levels requires the blower to operate will below this 80% threshold and the bearing begins to lose stiffness and rotor instability can occur. The instability of the rotor can lead to contact with the bearing foils, significantly reducing the life of the bearings.
When designing the Inovair geared blower, the ability to start/stop without limitation and maintaining high shaft stability were driving factors. To avoid unnecessary wear, Inovair engineers designed the gear box with proven reliable non-contact floating ring oil-film journal bearings and tapered-land thrust bearings. These bearings have been utilized for decades in the turbo-machinery market and operate on the same basic principles as the crankshaft bearings in most cars. The simplicity and reliability of the bearings made it the obvious choice for the Inovair IM and IO geared blowers.
The forced lubrication system provides pressurized oil to the bearings. A mechanical pump ensures adequate oil is delivered during start-up and shutdown cycles. This ensures there is oil between the bearing and shaft at all times, preventing contact. As the shaft increases speed, a high-pressure oil wedge (generally 100-500 psi) is formed between the shaft and bearing. This wedge creates hydrodynamic pressure in the lubricant film keeping the surfaces separated and 100% non-contact. Since this oil-film wedge is continually produced, the bearings are not subject to wear during start-up and shutdown cycles. Likewise, this self-generated high-pressure oil film, allows the shaft to operate at extremely low speeds, while still maintaining high stiffness and rotor stability. Additionally, since there are no parts in contact or subject to fatigue (as with ball bearings), this type of bearing is considered an unlimited life bearing.
Besides being unlimited life, these bearings offer superior dampening (the force to resist radial motion) and can handle high impact loads without issue. This is especially beneficial as the rotor accelerates through its first critical or if the blower were to be subject to a surge condition. The dampening ability of this type of bearing ensures the shaft is stable throughout its entire operating range. Additionally, since oil-film bearings generate their high-pressure oil film independently of the system air pressure, they are not subject to instability issues when exposed to sudden increases or losses of system air pressure.
The use of oil-film bearings has allowed Inovair geared blowers to successfully operate in varying liquid level applications that vary from 3 to 18 psi and in start/stop application with cycles as short as 15 minutes.