## 2 Heat Recovery System  
In this study, the developed integrated system controls the temperature and recovers the heat generated in the cutting tool-workpiece contact area. This system not only regulates the temperature of the coolant but also performs the heat energy recovery function. This original cooling and heat recovery system was specially designed and manufactured and then integrated into an existing CNC machining center. The developed system mainly uses a heat pump, and the heat is recovered from the cutting fluid (coolant). From a thermodynamic point of view, heat pumps collect the heat contained in low-temperature energy sources by allowing the refrigerant, which is mostly in the liquid phase in the evaporator (manufactured by MIT, Istanbul, Turkey), to pass into the vapor phase. In terms of the heat source and the sources to which the heat is transferred, heat pumps are called by different names, such as air-to-air, water-to-water, soil-to-air, and water-to-air [13; 14]. In this study, the heat source is cooling water, and the source to which the heat is transferred is water. In this case, the heat pump application is of the water (cooling water)-to-water type. In this respect, the study is also unique in terms of heat pumps because no heat pumps application of the coolant to water type was found within the scope of the literature research. In the developed system, the cooling-water mixture is pumped directly to the evaporator in the system and then sent back to the work area from the system outlet after being cooled to the appropriate level in the system. The schematic diagram showing the main elements of the developed system is presented in Figure 1 as detail. The developed system can operate in the desired mode, namely constant temperature and recovery mode and offline natural cooling mode. In the constant temperature mode, the temperature of the cooling liquid is kept constant at a predetermined level. In this mode, if the temperature is adjusted to the optimum level, the processing quality is improved, and it can be used to heat the workshop environment and obtain hot water thanks to heat recovery. This mode directly supports energy efficiency and sustainability goals. In the offline cooling mode, the developed system is disabled and the system performs its function as the existing classical cooling circuit. In this context, the selection of circuit elements in the preparation of the heat recovery system was made through the manufacturer's catalogs, taking into account the capacities and features required for the system. In the preparation of the heat recovery system, the circuit elements were selected directly from the manufacturer catalogs, taking into account the capacities and features required for the system. However, in order to provide the theoretical background in the circuit design, the equations related to the energy balance of the main elements in the circuit diagram are given below. The heat transfer mechanism in the heat exchanger occurs with the following equation:  
\[Q=U\cdot A\cdot\Delta T_{lm}\]  
where \(U\) is the overall heat transfer coefficient, \(A\) is the heat transfer area, and \(\Delta T_{lm}\) is the logarithmic mean temperature difference. The Equation related to the fluid dynamics of the refrigerant is as follows:  
Figure 1: Schematic diagram of the circuit elements of the developed heat recovery system. \[\Delta P=-f\cdot\frac{L}{D}\cdot\frac{\varphi v^{2}}{2}\]  
where \(\Delta\)P is the pressure drop, f is the Darcy friction factor, L is the length of the pipe, D is the pipe diameter, \(\varphi\) is the fluid density, and v is the velocity. The thermodynamic cycle of a heat pump consists of several stages, including compression and expansion processes. Detailed Equations for each stage are given below:  
* **Compression Process** In this stage, the refrigerant is compressed in the compressor (manufactured by LG, Seoul, Republic of Korea). Thus, its pressure and temperature increase.