# Enhancement of Heat Dissipation from the Hydraulic System Using a Finned Adaptive Heat Exchanger
## 2 Materials and Methods
Hydraulic pipes are not only routed in a straight line, but are mostly bent and curved at different angles, or they change their position continuously (e.g., extending the arm of the hydraulic arm with a piston). In the case of metal tubes, due to their direct gripping to the machine skeleton, they do not change their position. This led to the design of a shorter (30 mm) continuously adjustable heat exchanger composed of the required number of cooling elements, given the location and difficulty of installation (Figure 2). The rotation of the heat exchange segments is ensured by means of hinges with bushings and connecting pins. This design also allows the pipe to change its length during operation due to temperature changes. The specifications of the hydraulic oil type HL32 produced from highly refined base oils, containing additives that inhibit oxidation and corrosion, are showed in Table 1.  
The hydraulic pipe consisted of a hydraulic rubber hose of ethylene propylene diene monomer rubber material with a thermal conductivity of 0.29 W/(m\(\cdot\)K). The adaptive cooler consisted of aluminum with a thermal conductivity of 237 W/(m\(\cdot\)K). The oil pump with power of 60 W achieved a hydraulic oil suction capacity of 3 L/min at an operating \begin{table}
\begin{tabular}{l c} \hline \hline  & **Properties** & **Values** \\ \hline Density at 15 \({}^{\circ}\)C & 875 kg/m\({}^{3}\) \\ \hline Kinematic viscosity at 40 \({}^{\circ}\)C & 32 mm\({}^{2}\)/s \\ \hline Kinematic viscosity at 100 \({}^{\circ}\)C & 5 mm\({}^{2}\)/s \\ \hline Pour point & \(-\)20 \({}^{\circ}\)C \\ \hline Flash point & 215 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hydraulic oil type HL32 specifications [24]. Figure 2: Flexible orientation of the adaptive heat exchanger on the hydraulic pipe. temperature of 40 \({}^{\circ}\)C to 60 \({}^{\circ}\)C. The contact surfaces of the hydraulic pipe and the adaptive cooler were covered with a copper-based thermal conductive paste with a thermal conductivity of 3.1 \(W\)/(m-K) in the case of the third measurement. The oil temperature at the inlet and outlet of the adaptive cooler \(T_{in}\), \(T_{out}\), and the surface temperature of the hydraulic pipe \(T_{s}\) were measured with NTC ZA 9040-FS resistance temperature sensors and a NiCr-Ni FTA 8068 contact sensor. The sensors were connected to an ALMEMO 2590-4S data-logger. The oil flow was measured using the FVA 915 VTTH flow sensor. The heat flow from the hot fluid can be calculated by the equation [25,26]:  
\[Q_{h}=m_{h}\ c_{p,h}(T_{in,h}-T_{out,h}) \tag{1}\]  
where \(m_{h}\)--mass flow rate of the hydraulic oil, kg/s, \(c_{p,h}\)--specific heat capacity of the hydraulic oil at constant temperature, J/(kg\(\cdot\)K), \(T_{in,h}\), \(T_{out,h}\)--inlet and outlet temperature of hot oil, K. The maximum heat flow uncertainty is 6.5% for the lowest temperature difference between the inlet and outlet of the hydraulic oil passing through the adaptive heat exchanger. The overall heat transfer coefficient referred to the outer heat transfer surface area of hydraulic pipe can be calculated by the equation [27]:  
\[U=\frac{Q_{h}}{A_{out}\ \Delta T_{lm}} \tag{2}\]  
where \(A_{out}\)--outer heat transfer surface area of the hydraulic pipe, m\({}^{2}\), \(\Delta T_{lm}\)--logarithmic mean temperature difference for parallel flow, K.  
In general, the machines and equipment are designed to prevent overheating under normal conditions. Overheating is most often caused by a clogged heat exchanger, overheated pump, or restricted flow, but the ambient temperature is also an important factor, reaching close to 40 \({}^{\circ}\)C on tropical days. Even though the system operates without malfunction and the fluid is continuously cooled, overheating of the fluid may occur because of these temperatures. The AHE is mounted on the outer walls of the hydraulic pipes for this purpose, thereby increasing the total heat exchange surface and continuously cooling the fluid during the operation of the machine before it enters the pump.