Experimental assessments on a pressure swirl oil atomizer for positive displacement vane compressors

Abstract

The current work presents an experimental characterization of a pressure swirl nozzle that is used to spray the lubricant in positive displacement vane compressors such that the overall convective heat transfer between oil droplets and air is able to lower the specific energy consumption of the machine, bringing the closed volume compression phase towards an isothermal transformation. An experimental test bench was designed and built to reproduce the compressor operating conditions. Tests at different injection pressures and temperatures allowed to estimate some macroscopic features of the spray, namely break-up length and cone spray angle. In particular, measurements showed that while break-up length decreased at high pressure and temperature up to 3.1 mm at 65◦C and 10 barg, the cone aperture tended to diverge from the nominal value of 80◦ up to 106◦. Furthermore, measurements with a laser diffractometric particle size analyzer allowed to retrieve the droplet size distribution and to estimate a key parameter for the heat transfer capabilities of the spray, namely the Sauter Mean Diameter (SMD). At 55◦C and 9 barg the droplet size distribution fitted a Rosin Rammler function with shape parameter of 1.87 and scale parameter of 228 μm while the spray SMD was 122 μm. In these operating conditions, a series of nozzles equally distributed along the closed volume compression phase of a mid-size industrial vane compressor would lead to specific power savings up to 0.3 kW/(m3/min), that corresponds to 20-25% of the saving potential achievable considering an isothermal compression. This methodology will allow to calibrate a simulation platform of the sprayed injection technique such that further refinements of the energy saving strategy will be addressed.