Compressors consume more than 17% of the energy produced in developed countries. This pollutes the environment with more than 3000 Mt of carbon dioxide per year.
Oil injected screw compressors have large masses of oil injected with the working fluid. This injection of oil provides a separating film between contacting surfaces, acts as a cooling medium for the compressed fluid, and seals the gaps between the rotors to reduce leakage. The sealing and cooling brought about by fluid injection is extremely advantageous, but the lubricating film is essential to allow reliable operation as it removes friction and wear between the rotor surfaces.
Elastohydrodynamic lubrication is the best operational regime for screw compressors. A high viscosity fluid required for full hydrodynamic lubrication would induce high frictional forces between the rotor tip and the housing and would require extra power input to overcome these losses. A low viscosity fluid would cause the extremely undesirable effects of boundary lubrication.
Using specialized computer simulation packages, Rotor Design Solutions Ltd (RDS) has analyzed the distribution of the oil within a screw compressor. This shows contact between the rotors can be limited to regions of relatively low temperature oil. Rotors with partial helical contact have been shown to have a film thickness in the order of 3 times bigger than that for the same set of rotors with full length contact.
This method has the potential to achieve thicker films to give more reliable contact conditions within screw compressors and expanders. It also provides the option of using lubricants with lower viscosity whilst still maintaining a thick enough film for elastohydrodynamic lubrication. This perhaps offers the opportunity to lubricate with fluids that reduce the viscous drag forces within the machine or have improved cooling capabilities for more economical operation. In the best eventuality, the adaptation may allow working process fluid lubrication within screw machines.
The RDS concept is patent pending. It can be added to OEM current rotor designs, with no requirement for a machine re-design and there is minimal added cost to manufacture the rotor set. The concept has clear environmental benefits and is complementary to other emerging technologies, including rotor surface coating, rotor profile design and lubricant chemistry.
The research has been co-funded by the UK’s innovation agency, Innovate UK. We are now actively seeking a compressor manufacturer who may be interested in conducting some experimental studies to explore the potential further. For more information about the concept, please contact me directly.
14th March 2019. Jo Davey, Director, Rotor Design Solutions Ltd