August 19, 2020
With the ever-increasing pressure on our planet earth for clean water for various domestic and industrial consumption, it is inevitable that we treat most of the domestic and Industrial Waste Water and reuse the same wherever possible. Having said that it is important that we find the best and most energy-efficient way of treating large volumes of wastewater.
It is important to note that approximately 60% of the energy consumption of a wastewater treatment plant is consumed by the aeration blowers. Hence many companies are in the pursuit of developing energy-efficient blowers for aeration.
Over the last 15 years, the High-speed Turbo Technology for Low-pressure blowers has evolved significantly. With the introduction of high-frequency converters and new motor technologies, the speed limit was pushed up quite a lot.
All high-speed blowers are designed with permanent magnet motors. The permanent magnet motor and the Blower’s Impeller are connected to the same common shaft and the speeds/RPM can range anywhere between 25,000 – 40,000 RPM.
In a magnetic bearing motor, ball bearings are replaced by magnetic bearings which will ensure the rotor levitation through the generated magnetic field.
Since the shaft rotates in the thin air gap, such high speeds are possible.
In a normal induction motor, the RPM of the motor is defined by the formula N = 120 X f / P
N= RPM, f = frequency of the power supply, and P = number of poles in an induction motor.
Hence for a 2 pole motor which runs on a power supply of 50 Hz the RPM is = (120 X 50) / 2 = 3000.
Now if a motor has to achieve an RPM of 25000 then we can easily calculate that the frequency of the power supply to the motor has to be 416 Hz.. which explains why these machines have to be designed always with a Variable Frequency Drive (VFD).
Also, managing the heat emitted by the VFD at such high switching frequencies and motor at such high RPMs poses a great challenge to the stable operation of the machine in tropical countries like India, where the mercury soars well above 40 degrees Celsius during the summer season.
Keeping these challenges in mind, Atlas Copco has improved the reliability of these machines to a great extent by using water-cooled VFDs and motors.
Since there is no mechanical friction, these technologies are one of the most efficient in terms of energy efficiency. The only difference will come from the manufacturer's experience and knowledge from designing the best impellers and allied components for these blowers.
Now when it comes to reliability the industry is divided between two prevailing technologies:
In magnetic bearing technology, the shaft’s position is measured by more than 10 position sensors continuously and feedback is given to the Magnetic bearing controller on a real-time basis.
This helps in keeping the high-speed shaft in a safe position throughout the operation of the blower. When there is a power failure the power stored in the capacitor banks of the VFD powers the Magnetic bearing controller and brings down the rotor RPM to safe levels, from where the shaft lands on the secondary bearings (physical bearings) till it comes to a halt.
Whereas in airfoil bearings there is no such comprehensive shaft management system. The shaft rotates in the thin pressurized air film which is drawn from the discharge side of the blower.
When there is a sudden change in the pressure on the process side the air wedge which acts as the bearing might fail and the high-speed shaft lands on the bump foil with a crash. When such high-speed shafts land on a bump foil, there is only a limited no.of times the system can survive such crash.
Therefore, the chances of failure of the blowers with airfoil bearing are much more frequent.
Further, the heat generated in the VFD and the motor will require the blower to be installed in an air-conditioned room to avoid frequent high-temperature trips. However, these machines are economical from the initial investment point when compared to magnetic bearing machines.
Atlas Copco after close to 18 years of experience in designing, manufacturing, and servicing these types of machines strongly believes in magnetic bearing technology as the more reliable out of the above two options. Hence, our air experts recommend buying aeration blowers with magnetic bearing technology.
To conclude, It is good to see that the industry accepts such a radical technology change in the blower industry. However, it is the fact that high speeds/RPMs technology is yet to be adopted by the majority of the industries but with the current trends, it can be inferred that the high-speed turbo technology is gaining acceptance slowly and steadily.
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