Study of a dry fixed-vane revolving compressor
Toh, Shaun Jia Hui
Date of Issue2017-05-29
School of Mechanical and Aerospace Engineering
The report serves as a study on a dry fixed-vane revolving compressor and to provide results that support the development of such a variant. The need for a lubricant-free alternative arises from the appealing standpoint of having a compressor that is unrestricted by orientation. This provides possible applications into small-scale cooling or heating uses, this could be very useful in refrigeration or other portable appliances. However, with a dry variant of the RV compressor, there are challenges such as high frictional losses, metal-to-metal bonding as well as material wear. As such there is a need to devise a way to lower these frictional losses and come up with an alternative solution. With lower frictional losses, a possible operational dry fixed-vane revolving compressor can be produced. The report starts with an investigation into prior research and developmental work done for the fixed-vane revolving compressor. The RV compressor was developed through the reduction of the number of moving components from a rolling piston compressor. It was found to be more mechanically efficient and shown improved performance based on prior developed work. However, research into lubricant-free compressors are relatively new, studies were done on oil-free compressors such as scroll and reciprocating compressors. With an oil-free fixed-vane revolving compressor, there is a need to investigate dry friction because this remains an area where there are high frictional losses. The basic characteristics of friction and dry sliding friction were covered to better understand the principals behind the friction losses. Next, the existing mathematical models were used to study the behaviour of the fixed-vane revolving compressor. The compressor’s operational behaviour was expressed in six different models such as geometric, thermodynamic, mass flow, valve dynamics, heat transfer and force analysis. These expressions work together to predict and simulate the behaviour of the fixed-vane revolving compressor. However, due to the absence of lubricating fluid, changes in the force analysis expression must be made. The changes are made in areas where there is high metal-to-metal contact such as the vane-side, rotor end faces and journal bearings. Major changes to equations were made in the rotor end-face expressions, where the rubbing at the upper end-face is not considered as non-contact occurs as a result of gravity. These changes in mathematical expression have been made into the models to produce predicted power friction losses of the compressor. Considering the absence of lubricants, the expected power friction loss of the fixed-vane revolving compressor will be high. As such there is a need to consider coating of other self-lubricating materials onto the compressor. This is to lower the coefficient of friction and thereby lowering overall power friction loss. Two materials were considered, they are Polytetrafluoroethylene (PTFE) and Polyetheretherketone (PEEK). These were considered for their low coefficient of friction values as well as their wear properties. Additionally, thrust bearings were considered as well for their extremely low coefficient of friction, which could bring down friction loss in the compressor. Results show that both PTFE and PEEK decrease overall power friction loss for the three key areas when compared with dry power friction loss results. It was found that dry friction losses at the shaft-journal supports remain significantly higher than the lubricated one. However, when using sealed thrust bearings at the end-face and sealed roller bearings at the shaft-journal-support, results proved similar to lubricated values of power friction losses. Thus, thrust and roller bearings provide a better solution to lower power friction losses at the shaft-journal supports, but at a higher cost. Overall, self-lubricated material such as PEEK proved a viable option in a dry fixed-vane compressor. This is because PEEK has better wear properties than PTFE even though the latter has a lower coefficient of friction. As such, a combination of both PEEK and thrust bearing is recommended as a possible configuration of a dry fixed-vane revolving compressor. With the above-mentioned work that was done, this will lead towards a development of an operational dry fixed-vane revolving compressor.
Final Year Project (FYP)
Nanyang Technological University