Cost of Coolant

For several years, DAMRC has carried out productivity optimizations and industrial research with the purpose to strengthen the Danish production industry, in particular the machining industry.

Through this project, DAMRC has gained knowledge and understanding of what happens to the machining process and energy consumption, when you reduce the pressure and flow of the coolant supply. The project is based on literature, which has shown that the process of delivering coolant to the machining process accounts for a significant proportion of the total energy consumption of CNC machines. The project aims to translate this knowledge into a practical demonstration of the extent to which it is possible to reduce energy consumption of CNC milling, by reducing the pressure and flow of the coolant supply. An important aspect is to understand the derived effects it has on machining quality and working environment as well as economic and environmental aspects, which are examined in a combination of geometric measurements, visual analyzes and measurements on aspects related to the work environment and health issues.

The project is built up as a comparative pre-project that can form the basis for a larger project. As a foundation for the project DAMRC has made a mapping of the potential for adjustments in pressure of coolants today in danish/Nordic machining industry. This was done by mapping the danish market size for CNC machines, including estimations on the number of CNC machines with the present ability to adjust the flow and pressure of the coolant supply. The mapping showed that approximately 5% out of the overall 15.000 danish CNC-machines can adjust the pressure of the coolant supply, whereas the flow is a derived effect. Given that these approximately 750 machines can reduce their energy consumption by 25%, this will amount to an annual energy saving of approx. 1,67 GWh and a reduction of 485,2 tons of CO2.

To make the remaining 95% of the CNC-machines in Denmark capable of adjusting coolant supply pressure, it would require an investment in retrofitting the CNC-machine with a pressure adjustable coolant supply unit. This will likely be an investment in the range of DKK 75,000-250,000 per coolant supply unit, which means that at least 30’000 kWh must be saved before break-even. However, if pressure-adjustable systems were installed in all CNC-machines in Denmark, there would be a potential national energy saving of 33.4 GWh, if the pressure adjustments cause a reduction in energy consumption by 25%.

After mapping out the market potential DAMRC has conducted a series of practical machining tests, evaluating the actual saving potential, using a CNC Milling Machine, to which a retrofitted pressure adjustable coolant-supply-system has been added.

The tests consist of three test series – each series with its own material. The materials tested is aluminum (AW6082), steel (S275), and stainless steel (AISI304), as these are widely used in the danish industry.

For each series of the tests, the pressure has been adjusted cf. pre-defined increments, to obtain a broad understanding of the connection between the pressure and flow settings, and the energy consumption. This has resulted in 12 different pressure and flow settings that have been tested for the three materials, while the energy consumption has been monitored.

Through the practical machining experiments, the project has shown that there is a significant energy saving to be gained by reducing the coolant supply pressure. At a pressure reduction from 40bar to 18bar, the experiments have shown that the total energy consumption for the CNC-machine can be reduced by 15-25%. This corresponds to reductions in the kW consumption of 1.2kW, 1.3kW, and 0.5kW, for respectively aluminum, steel, and stainless steel (stating consumption was averagely 4.8kW, 5.5kW and 3.8kW at 40bar pressure).

The project has also shown that there is a large correlation between the flow rate and the “basic energy consumption”. By halving the flow rate the study has shown that, energy consumption was reduced by 20- 30% while maintaining the supply pressure at maximum setting (in this project, 40bar). This was done by reducing the cross-section of the flow channels in the used bushing. In kW, this corresponds to a reduction in power consumption in the size 1-1.7kW, which also highlights this approach to energy savings as important and relevant for the industry. This result further indicates that in the cases of “dry machining” there is a potential greater energy gain – given that the machining conditions allows it.

Through the project, it has not been possible to define a specific rule for the energy savings by pressure and flow reduction. This is due, that the saving potential is highly correlated with the share of total energy consumption that coolant supply system accounts for. However, the project has shown that there are significant energy savings to be achieved if the machining companies actively work to identify the processes to which coolant is non-critical.

The workpiece quality analyzes have NOT shown any tendentious negative impact from the reduced coolant pressure and flow.

In terms of tool condition and tool wear, there has been a tendency of increased wear on the tools used for the tests in steel. This is assumed that this wear is related to the hardened conditions of the shell of the steel bar, which limits the relevance for pressure reduction of coolant supplies in materials with similar conditions. The analysis has also shown challenges with chip evacuation from pockets when machining heavier, high-density materials (steel and stainless steel in these experiments). This was observed at pressure settings below 25 bar, indicating that there is a critical limit below which the refrigerant lubricant does not have enough power to flush the chips away.

For the experiments in stainless steel, measurements of the particle concentration of wet particles (10my and 1my particle size, respectively) have been taken. These have shown a tendency for a decreased concentration at lower pressures for both particle sizes, with tendential changes of approx. 50% and 33% for resp. 10my and 1my particles. This indicates that oil fumes and mists are formed to a lesser extent, which has a positive impact on the health of the machine operators and thereby the working environment.

The result of this project - together with results from other projects from DAMRC and Aarhus University regarding energy consumption on CNC-machines - has established a foundation for a future project for which funding will be sought. This project will study the energy consumption of support processes and stand-by modes in CNC-machines, with the aim of identifying energy saving potentials related to these processes. This builds on the projects, which have shown that up to 90% of a CNCmachine’s energy consumption goes to these support processes and " stand-by modes”.

In addition, DAMRC will continue to communicate and highlight the findings and results achieved in the project as a sustainable method to reduce companies’ energy consumption on CNC-machines - and potentially their bottom line.