Oil Free Magnetic Bearing Chiller
– Energy Advantage
By TK NG
Oil free magnetic bearing chiller (oil free chiller in short) is a highly energy efficient piece of air conditioning equipment, which is characterized by a partial load COP (Coefficient of Performance) higher than that at full load.
An oil free chiller consists of compressor and permanent magnet synchronous motor (PM motor) with magnetic bearings, and variable frequency/speed drive (VFD), besides other heat exchange equipment and accessories.
Because of the use of magnetic bearings, no lubrication oil is required. The advantages are that the much lower bearing friction helps increase the motor and compressor efficiencies, while the absence of lubrication oil in the refrigerant circuit minimizes fouling of the evaporator and condenser, hence helps maintain their heat transfer efficiencies over time.
PM motor has only a few percent efficiency gain over induction motor at full speed/load. In the case of chiller application which is usually under partial load condition for most of the time, the substantial efficiency drop of induction motor at lower speed/load does give PM motor an edge due to the latter’s relatively flat efficiency curve (see Fig 1).
On the other hand, the efficiency of a VFD invariably drops with its output power, and is more prominent at low partial load condition for pulse-width-modulated drive (see Fig 2).
Obviously, the aforesaid factors contribute to the higher efficiency of oil free chiller as compared to conventional ones employing induction motors. Furthermore, higher COP at partial load is also claimed for oil free chiller.
As the efficiency of VFD drops with its output power, while variation in the efficiency of PM motor is not significant, the input power to the compressor must drop at reduced speed/load to more than offset the VFD efficiency loss so as to achieve a higher chiller COP (or lower input kW/ton). This is what happens with a chiller employing centrifugal compressor (see Fig 3)), which greatly benefits air conditioning systems operating under relatively long duration of partial load.
There are also other benefits that worth mentioning. The provision of lubrication oil sump and heater, regular oil analysis and replacement, issues associated with oil migration and fouling, and oil return problem at low cooling load, etc. are all eliminated in the installation, operation and maintenance of oil free chiller.