Machining with machine tools generates heat in various parts of the equipment due to the high power involved, the friction related to the high mechanical forces applied together with the high rotation speed of the tools. Heat increases the temperature of all parts including electric motors, drives, spindles, working tools and workpieces, causing the metal to expand, a condition that would jeopardize the machining accuracy if not properly managed by a dedicated thermal control system.
For this purpose, oil and coolant are circulated inside the machine to cool the components/tools and the mechanical part being machined. The chiller cooling system prevents the machine or workpiece during manufacture from heat distortion by recovering the heat that the oil or the coolant transports out of the machine and keeping the liquid temperature ideally perfectly constant.
Operating principle
Unlike water coolers and fan coolers, chillers perform arbitrary temperature control not only by exchanging heat but also by controlling the refrigerant temperature. With reference to Figure 1, the operating principle of chillers is based on the following four steps:
- The refrigerant gas, that is contained in an isolated closed circuit, is compressed by a compressor to produce high temperature, high-pressure compressed gas.
- The compressed gas is cooled and condensed by a condenser to produce a high-temperature, high-pressure liquid.
- In the decompression mechanism, this high-temperature, high-pressure liquid is squeezed and decompressed through an electronic expansion valve to produce a low-temperature, low-pressure liquid that can be easily evaporated by a heat-exchanger (evaporator). The amount of refrigerant circulating is controlled by changing the valve opening in response to the capability command.
- The energy absorbed by the refrigerant in the evaporator is provided by the oil, that therefore cools down. During this process the heat transferred to the refrigerant from the oil (latent heat of vaporisation) produces the evaporation of the liquid, turning it into a low-temperature, low-pressure gas that is conveyed back to the compressor, where the cycle starts again.
The above cycle is managed by dedicated electronics that control each step of the thermal conditioning precisely.
Hybrid technology
In June 2022, Daikin acquired Duplomatic Motion Solutions, a global player active in the design and manufacturing of components and systems for motion applications, with a strong heritage in fluid power. Within its hydraulic division, that includes the Duplomatic Group, Daikin develops and manufactures high-performance energy-efficient chillers for machine tools under the name “Oil-Con”. Among the many players active in the development of air conditioning equipment, Daikin is the only company innovating all critical components used for the refrigeration cycle in-house.
The company’s chemical division develops refrigerants of different kinds with a strong focus on green technologies. Daikin introduced inverter technology for the energy-efficient control of electric motors in 1986 and, thanks to the mass-production of inverters, cost-effective highly-efficient units with sophisticated control algorithms aimed at controlling specially- designed IPM motors form part of all Daikin equipment. In Oil-Con chillers, the combination of these technologies results in high accurate temperature controls and unique energy savings.
In fact, the inverter-controlled compressor allows the user to achieve the required cooling capacity by adapting the compression power to the actual heat load, thus avoiding unnecessary power consumption. The highly efficient IPM motor is driven by its dedicated drive adjusting the compressor speed according to the required cooling capacity, resulting in a highly accurate thermal control with typical accuracies of ± 0.1°C.
The thermal control performance is almost independent from the thermal load (Figure 2). Compared with non-inverter models, power consumption can typically be reduced by 45% (Figure 3).
Chiller configurations
Different chiller configurations have been developed for cooling coolants, lubricants, hydraulic and cutting oils in machine tools.
Four types of chillers are available depending on the type of liquid to be cooled and the installation method, using either oil or water as fluids and two different installation configurations (recirculating or immersion type). The temperature control method can be changed as needed by the application. Three control methods are available: (1) control to match any fixed temperature, (2) control to synchronize to room temperature, and (3) control to synchronize to machine tool temperature.
Temperature Control
High precision is required for controlling liquid temperature for machine tools. A long-term development trend for chillers is the improvement of liquid temperature control accuracy. Current models, such as the recirculating AKZ9 and AKZ10 series, and the tank-immersion AKJ9 series, achieve a standard temperature accuracy of ± 0.1°C, while special specifications support temperature accuracies down to ± 0.01°C, being the accuracy almost independent from the heat load.
Recently, the installation of Internet of Things (IoT) related functions in machine tools has become the standard. User needs for IoT related functions in chillers is increasing year by year, and new developments are underway. Monitoring of chillers has already been commercialized, but the first stage has been limited to just connecting the equipment. Currently, data is collected to establish the correlation between chiller fluid temperature and machine tool processing accuracy. In the future, the use of artificial intelligence and machine learning applied to data will allow the quantitative determination of the relationship between fluid temperature and machine processing accuracy.
This will enable the next step of control accuracy, moving from a feedback system that follows the change in the liquid temperature and cools it, to a predictive control where changes in the liquid temperature are suppressed by predicting the required temperature control from the machine processing content.
