July/August 2020

prone to overheating, as compression of the gas film raises its temperature. As a result, machines of 150 kW or larger typically require a separate liquid cooling system. Magnetic bearings can tolerate higher forces, allowing their use on large machines without the need for additional cooling. They also have good damping characteristics, which help them accommodate the transient forces generated by power surges or problems elsewhere in a machine. Innovations in control The biggest perceived weakness of magnetic bearing systems comes from their complexity. Active magnetic bearings require sophisticated control electronics and a continual source of power. The former means that magnetic bearings tend to have a higher upfront cost than air bearings, and the latter has implications for machine design. Turbocompressors that use magnetic bearing systems usually incorporate a backup power source, either capacitors or batteries, which can provide sufficient energy to stop the machine safely in the event of a loss of external power. To further protect the machine in the event of a major problem, magnetic bearing designs incorporate back-up mechanical bearings that sit a few tenths of mm away from the shaft. In the highly unlikely event that the magnetic bearing system fails, these back-up units “catch” the shaft, allowing the machine to stop safely. As a further benefit in the Industry 4.0 world, magnetic bearing systems provide “built-in” condition monitoring capabilities for the compressor. The control system continually collects data on the precise position of the shaft, allowing vibration, misalignment and other early warning signs of potential problems to be detected and analysed. That helps to reduce their cost, a major strength of the design, especially when used in smaller machines. When an air bearing starts up, however, its parts run in contact with each other until the shaft is rotating fast enough to establish the pressurized film. Typically, an air bearing does not “take off” until a shaft speed of 5’000 rpm is achieved. Over time, multiple starts and stops result in wear to the surface of the foils, which will eventually need to be replaced. To minimise the number of stop-start cycles, air bearing compressors are sometimes left running at idle, a practice which increases energy consumption significantly over the lifetime of a machine. For applications that call for a discontinuous supply of air, this characteristic of air bearings can be a significant limitation. Size limitations Other limitations of air bearings arise on larger machines. The air foil design has a relatively low overall load carrying capacity, especially in the axial direction, so big air bearing turbocompressors often adopt a double impeller design, reducing the overall efficiency of the machine. Large air bearings can also be 32 | Plant & Works Engineering www.pwemag.co.uk July/August 2020 Process, Controls & Plant Focus on: Seals, Bearings & Lubrication Tel: 01142 889464 Email :sales@dpdoorsandshutters.co.uk Website:www.dpdoorsandshutters.co.uk Aluminium Support Legs and Head meaning easier and quicker installation. Built in integral Windbar system giving excellent wind resistance, no need for additional add on wind bars. Designed for external door locations where wind resistance is Key. Variety of activation options available, from basic push button, to remote handsets, movement detectors, pull chords and induction loops. Safety features as standard, Multi Light beam and electronic safe edge. High Speed Doors