## 4 Result and Discussion  
In this study, the change in the parameters of the refrigerant R134a flowing in the tubes of a counterflow evaporator is investigated as a function of the water temperature flowing on the shell side. The mass flow rate of the cooled water and the mass flow rate of the refrigerant are constant and are 0.28 kg/s and 0.023 kg/s, respectively. The simulation results are shown graphically in Figures 3-10. The decrease in water temperature was continuous along the evaporator (Figure 3). The change in water temperature ranged from 2.92 \({}^{\circ}\)C to 3.47 \({}^{\circ}\)C flowing into the shell space of the evaporator along its entire length. Figure 4 shows the change in temperature of the tube's outer wall along the length of the evaporator. At a water inlet temperature of 13 \({}^{\circ}\)C, the change in wall temperature  
Figure 3: Variation of the temperature of the water. along the length of the evaporator is 2.36 \({}^{\circ}\)C, while at a water temperature of 19 \({}^{\circ}\)C at the inlet of the evaporator, the change in tube wall temperature is 2.29 \({}^{\circ}\)C.  
The change in the quality (content) of the refrigerant vapor is shown in Figure 5.  
Vapor quality can be determined from Equation (65). \[x=\frac{h-h_{l}}{h_{v}-h_{l}} \tag{65}\]  
where:  
\(h\)--specific enthalpy of mixture  
\(h_{l}\)--specific enthalpy of saturated liquid  
\(h_{v}\)--specific enthalpy of saturated vapor  
Enthalpy \(h_{l}\) and \(h_{v}\) can be found in the thermodynamic properties tables. Enthalpy \(h\) can be determined on the basis of temperature and pressure. At the entrance of the evaporator, part of the refrigerant is already in the vapor phase. The evaporation process starts in the expansion valve due to the pressure drop. The change in vapor content showed almost the same trend at all water inlet temperatures. At a length of the evaporator of \(z=1.83\) m, the vapor content takes the value \(x=0.9\), while in the last 25 cm of the evaporator length, overheating of the refrigerant (\(x=1\)) already occurs. The applied correlation of the heat transfer coefficient for water consists of two parts: The first part contains correction factors related to the geometry of the heat exchanger,  
Figure 4: Temperature variation of the tube wall. Figure 5: Variation of refrigerant vapor quality. which affects the turbulence of the flow. These values are constant along the tube, so the turbulence along the tube changes a little, and therefore the part of convective heat transfer changes a little as well. The second part of the correlation considers the physical properties of water as a working fluid. The values of thermal conductivity, heat capacity, viscosity, and density depend on temperature and pressure. As the temperature of the water changes along the pipe, these values also change and so do the dimensionless Reynolds number and Prandtl number. As the water temperature decreases, the density and dynamic viscosity values increase. As a result, the Reynolds number decreases, and the Prandtl number increases, decreasing the heat transfer coefficient. Since the influence of this part of the correlation is greater than the influence of the first part of the correlation, the general trend is a decrease in the value of the heat transfer coefficient along the tube (Figure 6). Cooling capacity is the measure of a system's ability to absorbing heat from water (brine). It is equivalent to the heat supplied to the refrigerant. \[\dot{Q}_{w}=\dot{m}_{w}\cdot Cp_{w}\cdot(T_{w,in}-T_{w,out}) \tag{66}\]  
The cooling capacity increases as the temperature of the water at the inlet of the evaporator increases (Figure 7). The cooling capacity at a water inlet temperature of 13 \({}^{\circ}\)C was 3420 W, while the cooling capacity at a water inlet temperature of 19 \({}^{\circ}\)C was 4103 W. In general, the cooling capacity increases with the increase of the surface (length of evaporator) through which heat is exchanged. Figure 6: Variation of the heat transfer coefficient of the water.