Selection of the oil-free bearing system for a 30 kW ORC microturbine

AUTORZY: Łukasz Breńkacz, Grzegorz Żywica, Małgorzata Bogulicz

The article discusses results of the analyses of various beari ng systems for the rotor of an ORC (Organic Rankine Cycle) microturbine with an electric po wer of 30 kW. It is impossible to choose the appropriate bearing system taking into account on ly basic parameters of the bearings. When designing a new power turbine, it is important to conduct the dynamic and strength analysis of the entire rotor-bearings-supporting structure system. The n ominal rotational speed of the newly designed single-stage axial-flow turbine is 40,000 rpm. The tur bine will be powered using the vapour of a low-boiling working medium. The chosen working medi um cannot be used in combination with all materials that are commonly used for turbi ne constructions. An additional requirement was that the turbogenerator must be oil-free. The t emperature of the working medium directed to the rotor blades cou ld exceed a value of 200 °C. Th ree bearing systems were considered: bearings lubricated with a low-boiling fluid (in th e liquid form), gas bearings lubricated with the vapour of a low-boiling medium and rolling bearings. Since the rotors used in those three systems have differe nt geometries, their dynamic pr operties vary as well. The rotor dynamics analyses were carried out using computer programs belo nging to the MESWIR environment, which had been developed at the Institute of Fluid -Flow Machinery of the Polish Academy of Sciences (IMP PAN) in Gdańsk. The computational mode l, based on the finite element method, was used to prepare graphs on which are present ed vibration amplitudes as functions of the rotational speed. The computational model, bas ed on the finite element method, served to perform calculations on the basis of which graphs pre senting vibration amplitudes as functions of the rotational speed were prepared. Moreover, vibr ation trajectories of individual nodes of the computational model were shown. Besides analyses o f the bearings themselves, calculations were also carried o ut to assess the dynamic proper ties of the rotors supported by those bearings in a wide range of rotational speeds. As a result of t he conducted analyses, the concept of an innovative turbogenerator was created. Its rotor can oper ate at a very high rotational speed, and the bearings do not re quire oil lubrication. Keywords: oil-free bearing, microturbine

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