Characterising resource usage for a supercritical carbon dioxide cooling and lubricating system used in machining processes

Cryogenic or sub-zero media are increasingly used for cooling and lubrication (CL) of machining processes. Supercritical carbon dioxide (scCO2) shows potential in both CL functions, being able to carry dissolved lubricant (MQL) into the cutting zone.

Evaluating environmental performance for different CL options requires detailed supporting information, via characterising system resource use. This represents a knowledge gap.

In this research a scCO2 plus MQL delivery system was characterized for its consumption of CO2 and MQL, system electrical energy and pneumatic air resources, in response to CO2 exit nozzle orifice diameters. 72 flowrate tests were run with multiple repeats to assess variability. At 13.1 MPa (131 bar) supply pressure, as the nozzle diameter increased from 0.15 to 0.30 to 0.45 mm, the CO2 flowrate increased from 5.2 to 17.0 then to 34.0 kg per hour. Pneumatic air flow through the system also increased, approximately in proportion to nozzle diameter, whilst system electric power consumption barely changed.

Calculated emissions were dominated (over 98 percent) by two resource flows: the direct release of CO2 through the nozzle, plus the electricity driving pneumatic air input. MQL resources accounted for less than 1 percent of emissions. Annual projected running costs were dominated by the use of CO2.

The efficient use of CO2 is important to best deploy this promising CL technology into industry. This can be realised via means such as post-system CO2 delivery design, and lowering system pressure. Future work will characterise scCO2 plus MQL in full machining systems to optimize the process performance.