Design of Auxiliary Passive Magnetic Bearing for Cryogenic Turboexpander

Abstract

In the modern time of industrialisation high performance, high precision, smooth operations are a very vital requirement for a machine system. Such a very essential system is turboexpander which is used in the several applications to provide very low-temperature cooling and refrigeration. Nowadays, turboexpander systems are using gas bearings to operate friction free, noiseless and for smooth operation. The main problem for these turboexpander systems is a heavy contact of rotor parts with the static parts i.e. thrust bearing surface at the time of start-up and shut-down of the machinery. The rotary system of the modern turboexpanders, after using gas bearings to support against radial as well as axial thrust are facing the wearing problem with the static parts at the time of start and stop. To avoid the problem, an auxiliary passive magnetic bearing is modelled by using Finite Element Method Magnetics (FEMM) that can suspend the rotor system without heavy contact with the lower thrust bearing surface during start-up and shut-down. This magnetic bearing has to levitate a designed rotary system of weight approximately 2.64 N, which consists a rotor, a brake compressor and a turbine wheel. This pair of magnet bearings is axially magnetised and can levitate the rotor at an appropriate distance. After the simulation of different bearing dimensions with two types of Neodymium (NdFeB) alloy magnet of Grade N42 and N52, it is found that the grade N52 gives better values of repulsive forces at same gap and dimensions as compared to grade N42. It is also important to know that N52 has lower operating temperature (70oC) as compared to N42 (80oC). So according to the operating conditions any of the above grades can be used as an auxiliary passive magnetic bearing.