Performance enhancement in sliding vane rotary compressors through a sprayed oil injection technology

Abstract

In Sliding Vane Rotary Compressors, as well as in most of positive displacement machines, the oil is injected to accomplish sealing and lubrication purposes. However, the oil injection could produce an additional outcome during the compression phase with a great saving potential from the energetic point of view. Being the air inside the cell at a higher temperature than the oil injected, a cooling effect could be achieved so decreasing the mechanical power required for the compression. At the moment, the oil is introduced inside the compressor vanes simply through a series of calibrated holes that are only able to produce solid jets. In this way any effective heat transfer is prevented, as demonstrated by p-V measurements inside the cells during the compression phase. In the current study, a theoretical model of a sprayed oil injection technology was developed and further experimentally validated. The oil was injected along the axial length of the compressor through a number of pressure swirl atomizers which produced a very fine spray. The conservation equations, solved with a Lagrangian approach, allowed to track the droplets evolution from the injection until the impingement onto the metallic surfaces of the vanes. The theoretical approach assessed the cooling effect due to the high surface to volume ratio of the droplets and a reduction of the indicated power was predicted. The model validation was carried out through a test campaign on an mid-size sliding vane compressor equipped with a series of pressure swirl injectors. The reconstruction of the indicator diagram as well as the direct measurements of torque and revolution speed revealed a reduction of the mechanical power absorbed close to 7% using an injection pressure of 20 bar. The model is in a satisfactory agreement with the tests and it also confirms the experimental trends available in the literature. A parametric analysis on the injection pressure and temperature and on the cone spray angle was eventually carried out in order to identify an optimal set of operating injection parameters.