Edition 01/2017

Structure and operating principle of a turbo blower in the current AERZEN series are both simple and effective: An impeller sitting directly on the motor output shaft rotates inside a spiral housing, generating the conveying flow in the process. The flow-optimised impellors rotate in a speed range of between 20,000 and 70,000 revolutions per minute. The high rotating frequency presents three significant challenges for implementation: first, especially high-frequency motors that can even generate these speeds are required. Second, frequency converters rated for these speeds are also needed. And third, the motor shaft must be effectively and reliably mounted with the attached impellor. Standard industrial roller bearings are not suitable in these machines because the speeds are much higher than those roller and ball bearings are able to absorb.  

Simple bearing with simple physics ...

As an effective bearing without any mechanical friction at all, AERZEN uses compressed air in the two radial drive shaft bearings and the axial bearing for absorbing the axial forces. This concept never pumps air into the bearing shell at high pressure from the outside, it simply applies the compressor principle. The fast-rotating shaft generates, as the turbo blower is started, an eccentric circular motion in the bearing air gap through natural imbalance. Because, on the minimal distance to the bearing wall, the shaft increases the pressure in this area, counterforce is generated in the form of a pressure rise. This forces the shaft back in the opposite direction, comparable with a compressor or rotary engine. Due to the rapidly increasing speed, the shaft centres itself in the bearing, thereby increasing the pressure in the air gap to over 30 bar. The prevailing force couples are so high that they hold the shaft, even under strongly fluctuating and challenging operating conditions, continuously free-floating in the centre of the bearing.

...versus to complex bearing with high operating costs

Turbo blowers from other manufacturers work with magnetic bearings, which also aim to overcome the high speeds without any mechanical bearing. However, this calls for electric current so that the coils arranged in a ring around the drive shaft can develop their forces during operation and allow the shaft to rotate without mechanical friction. Fluctuating operating conditions especially put the highly complex control system of the magnetic bearing to the test, the result often being technology-related safety shutdowns of the complete turbo blower.The control electronics, which have to constantly re-adjust the magnetic field forces, have a limiting effect on the magnetic bearing. For this system to remain active in case of a plant fault, an emergency stop or a power outage, turbo blowers with magnetic bearings must always be equipped with an uninterruptible powersupply (UPS). Therefore, sophisticated load cycles are just as necessary as the regular replacement of battery cells within a rigid maintenance plan. The magnetic bearing itself is equally complex, because it continuously uses electrical energy and the complex control system has to be maintained at regular intervals.

Long service life

The air bearing principle requires no electrical, mechanical or pneumatic controls, even with dynamic load cycles. The air bearing also has reserves for absorbing higher speeds than is usually possible with magnetic bearings. Nonetheless, critics complain that when the turbo blower is started, the driven shaft rests on the bearing and develops wear friction until the air cushion is formed. AERZEN counters this theoretical drawback with an innovative air foil bearing with a two component coating, which includes polytetrafluorethylene. PTFE is a thermoplastic which, due to its very low coefficient of friction, is used as a non-stick coating. “With the PTFE, we produce a kind of lubricating film without oil and grease”, explains Steffen Helmert, Product Manager for turbo blowers at AERZEN. A high surface quality is required for this structure to be able to absorb the friction forces that occur in a fraction of a second when the turbo is started. “The quality depends directly on the production process. Over the many years‘ of experience with turbo blowers, we have constantly refined the process.” In short, AERZEN has effectively found for the new turbo generation asolution that is much simpler and more efficient than the highly-complex magnetic bearing. But PTFE as a tough bearing material does not create a durable mounting by itself. Because the compressor effect in the bearing compacts the air so densely that the layer between bearing ring and shaft is practically as hard as steel, AERZEN has designed adamping layer specifically for this application.This is a millimetre thick plate which, when rolled into an undulating shape, supports the lubricating layer of the bearing and absorbs vibrations at the same time.

Conclusion

In wastewater technology, turbo blowers are consistently designed for low life-cyclecosts. Consequently assemblies such as turbo blowers made by AERZEN are used in processes where long distance solutions are needed. 24/7: As the life-cycle costs comprise to a large extent the energy costs of the assemblies, concerning the energy costs of the turbo blowers available in the market, the question is not whether the impeller is mounted on air or a magnetic field. The real question is which device from which manufacturer is best suited to the application with which performance and consumption ratios. Since the activation process accounts for over 50 percent of the energy consumption of a wastewatertreatment plant, it makes sense to include the cost of service and maintenance, aswell as the energy costs, in the calculation for the blower design. The pure purchase price of an assembly will fade into insignificance in the future. The best suitable solutions for the general conditions of a wastewater treatment plant are required. It will be the life cycle costs (LCC) which, along with aspects such as standardisation and service availability, that play a greater role in investment decisions.