A hybrid mechanical-electrostatic separator for oil-refrigerant separation

In the United States, water heating, space heating, and space cooling account for 49 % of energy consumption in buildings (residential and commercial). Most space cooling needs and a small portion of space heating needs are met using electrically driven vapor-compression systems. Adequate lubrication of the compressor within these vapor-compression systems is a critical factor in maintaining compressor performance and lifetime. However, oil transport from the compressor to other components in the system can lower the heat transfer coefficient and increase pressure drop. To mitigate this, most compressors rely on coalescing oil separators to separate oil from the refrigerant stream. These oil separators suffer from reduced separation efficiency at high refrigerant mass flow rates. To counter the separation efficiency reduction requires denser and heavier meshes to trap more oil droplets, further decreasing system energy and power densities and increasing pressure drop. Electrostatic separation has been explored as an alternative technology to leverage the tribocharging of oil droplets as they flow through the system. However, these electrostatic parallel-plate systems can be complex. This study demonstrates a hybrid approach to separation of oil from refrigerant by introducing an electric field to traditional coalescing oil separators to capture oil at a higher efficiency and reduce the number of mesh nodes required, ultimately reducing pressure drop. Experimentally, we demonstrate an increase in separation efficiency by a factor up to 7× in the hybrid system when compared to a system with no applied electric field past a critical value for electric field intensity, with a possible 3× reduction in pressure drop—leading to a 1 % enhancement in coefficient of performance. Insights for implementation of this technology into a real HVAC system are also found, such as the limiting behavior in separation efficiency at higher field intensities at a given constant flow rate. The electrostatic separation technology shows significant promise to improve both system performance and lifetime of new and existing vapor-compression systems, increasing the energy density, power density, and heat transfer coefficient of the system while simultaneously lowering their labor cost and carbon footprint of maintenance and replacement.