Improved oil level detection and equalization system for CO2 high pressure shell hermetic rotary compressor.

The development of hermetic CO2 compressors provided the economical ability to design transcritical systems that are efficient, reliable and cost effective. Moreover, if combined with variable speed technology, these compressors are able to provide a broad rotor speed working range, low noise and low vibrations. In order to ensure a reliable operation of these systems, a key feature is a proper oil management accomplished via oil level monitoring and equalization among the devices. This paper presents an improved method for oil level control in a high-pressure shell hermetic rotary compressor rack for CO2 vapour compression cycles. The solution is based on the temperature variation measured during the fluid expansion from the compressor shell to the suction line through a modulating throttling device. Thanks to a dedicated logic, the modulation of the valve opening degree allows to perform both the detection and the equalization of the oil levels among all the rack compressors, increasing reliability. Unlike other available methods, the oil level control is achieved by taking into account the current system working conditions in terms of pressures, temperatures and compressor rotor speeds, minimizing the required monitoring time and the bypassed flow rate, increasing the unit efficiency thus. The required layout is simple and suitable both for single and multi-compressor units. Tests have been carried out for various working conditions with transcritical and subcritical regimes, different compressor speeds and evaporating pressures in a real unit designed as a booster system.