Turbocompressors have become the backbone of modern water treatment technology, with their mechanically simple design, reliability and good energy efficiency. Ideal for producing large volumes of compressed air at relatively low pressures, they rely on bearings that can spin at high speeds with minimal resistance and for extended periods of time. Today, the market is dominated by two competing technologies: air foil bearings and active magnetic bearings. This article compares their advantages and disadvantages.
The rotor of the turbocompressor core is powered by a high-speed electric motor, usually without intermediate gears. In operation, the motor shaft and rotor assembly rotates at speeds of up to 50,000 rpm and may run continuously for extended periods of time between major repairs. These conditions require special types of bearings, and two methods are commonly used in modern turbocompressor designs.
What is an air foil bearing?
An air foil bearing is a gas bearing designed to use pressure in a gas film to keep moving and stationary surfaces apart, allowing for low friction, high speed rotation. The pressure required for gas bearing operation can be provided externally or generated internally by the rotation of the bearing itself. Air foil bearings use the latter method, and as such, they are a type of dynamic gas bearing.
Air foil bearings were developed by NASA in the 1960s and 1970s specifically for turbomachinery applications. In the air foil design, the space between the rotating shaft and the stationary housing contains two thin foils made of metal coated with a low friction material such as PTFE (polytetrafluoroethylene). The outer foil (called the bump foil) is corrugated and can be bent to accommodate movement or misalignment in the bearing. The inner foil, called the top foil, is cylindrical and shaped to wrap tightly around the shaft or journal. As the shaft rotates, air is drawn in between the shaft and the top foil, creating a narrow gap that allows rotation with minimal friction.
What is an electromagnetic bearing?
Magnetic bearings support the rotating shaft by magnetic levitation. Most magnetic bearing designs use magnets arranged around the bearing housing and shafts containing ferromagnetic material. The magnet attracts the shaft, and when the forces acting on the shaft are balanced, the magnet is held away from the housing, allowing low friction rotation. This design uses an electromagnet, which is powered by an electronic control system that adjusts the force it produces. Sensors in the housing constantly monitor the position of the shaft, and the control system varies the power delivered to the magnets to keep the shaft in the best position.
Advantages of both bearings
Air foil bearings and magnetic bearings are viable solutions for high speed applications. They all have low friction and operate without lubrication, making them essentially maintenance-free. The design benefits from years of refinement and experience in demanding applications. But both designs have advantages and disadvantages, and these differences can have a major impact on choosing the most appropriate bearing technology for a given application. Air foil bearings operate without an external power source or control system.
This helps reduce its cost, which is a major advantage of the design, especially when used in smaller machines. However, when the air bearing is activated, its parts contact each other until the shaft rotates fast enough to form a pressurized film. Typically, the air bearing does not "take off" until a shaft speed of 5,000rpm is reached. Over time, multiple starts and stops can cause wear on the surface of the foil, which will eventually need to be replaced.
To minimize the number of start-stop cycles, air bearing compressors sometimes run idling, a practice that can significantly increase energy consumption over the life of the machine. For applications requiring a discontinuous supply of air, this feature of air bearings can be a significant limitation. Air bearings are also susceptible to damage if particulate matter enters the bearing and becomes trapped between the shaft and the foil. To reduce this risk, most air designs require strict filtration of incoming air. Active electromagnetic bearings overcome many of these limitations.
Disadvantages of the two bearings
Other limitations of air bearings arise on larger machines. The overall load-carrying capacity of the air foil design is relatively low, especially in the axial direction, so large air bearing turbo compressors are often designed with a double impeller, reducing the overall efficiency of the machine. Large air bearings are also prone to overheating because the compression of the air film raises its temperature.
As a result, machines of 150kW or larger often require a separate liquid cooling system. Magnetic bearings can handle higher forces, allowing them to be used on larger machines without additional cooling. They also have good damping properties, which helps them accommodate transient forces caused by electrical surges or problems elsewhere in the machine.
The biggest disadvantage of the electromagnetic bearing system is its complexity. Active electromagnetic bearings require complex control electronics and a constant source of power. The former means magnetic bearings have a higher upfront cost than air bearings, while the latter has implications for machine design. Turbocompressors that use magnetic bearing systems often have integrated backup power sources, either capacitors or batteries, that provide enough energy to safely shut down the machine in the event of an external power interruption.
To further protect the machine in the event of a major problem, electromagnetic bearing designs, such as those used in Sulzer's HST turbocompressor range, incorporate a backup mechanical bearing that is only tenths of the way from the shaft a few millimeters.
In the highly unlikely event that the electromagnetic bearing system fails, these backup units "jam" the shaft, bringing the machine to a safe stop. Although the control systems required for electromagnetic bearings are complex, recent advances in electronic technology have greatly improved their reliability and reduced costs. Inductive sensors for shaft position detection are simple, robust and highly reliable components.
As another benefit in an Industry 4.0 world, the electromagnetic bearing system provides compressors with "built-in" condition monitoring. The control system continuously collects data about the precise position of the shaft, allowing vibration, misalignment and other early warning signs of potential problems to be detected and analyzed. Sulzer has been using magnetic bearings in the HST turbocompressor range for 25 years.
The first HST machine was installed in 1996 and is still in operation. The bearings and control systems used in these units were developed in-house, and the company will continue to develop its own bearings and control systems. This experience has given people confidence in the technology and has helped its performance and reliability continue to improve.