# Study on Heat Transfer Performance and Anti-Fouling Mechanism of Ternary Ni-W-P Coating
## 3 Results
### Heat Transfer Behavior  
Figure 9a shows the test data obtained from the flow fouling experiment. Water heating tank is used for the heating of flowing water. CaCO\({}_{3}\) solution is heated after until temperature range of fouling deposition is \(328\pm 1\) K through the heat transfer of the wall of the inner tube in the measurable anti-fouling heat exchanger temperature. Logarithmic mean temperature difference (LMTD) of measurable anti-fouling heat exchanger is calculated according to the Equation [2]. \[\Delta t_{m}=(\Delta t_{max}-\Delta t_{min})/\ln(\Delta t_{max}/\Delta t_{min}) \tag{13}\]  
where \(\Delta t_{max}\) and \(\Delta t_{min}\) represent the maximum and minimum temperature difference of the inlet and outlet in the process of re-fluent heat transfer, K. Equation (14) is then used for determining the heat load of CaCO\({}_{3}\) solution and heated water. \[\Phi_{4}=q_{m1}c_{1}\big{(}t_{1}^{\prime}-t_{1}^{\prime\prime}\big{)}=q_{m2}c _{2}\big{(}t_{2}^{\prime}-t_{2}^{\prime\prime}\big{)} \tag{14}\]  
where \(q_{m1}\) and \(q_{m2}\) are mass flow rate of water and CaCO\({}_{3}\) solution, respectively, kg/h. \(c_{1}\) and \(c_{2}\) are specific heat capacity of water and CaCO\({}_{3}\) solution, respectively, J/(kg-K). \(t_{1}^{\prime}\) and \(t_{1}^{\prime\prime}\) are the hot side and cool side temperature of water, respectively, K. \(t_{2}^{\prime}\) and \(t_{2}^{\prime\prime}\) are the hot side and cool side temperature of water, respectively, K.  
After that, the heat transfers coefficient of the measurable anti-fouling heat exchanger can be calculated by  
\[k=\Phi_{4}/A^{\prime}\Delta t_{m} \tag{15}\]  
where \(A^{\prime}\) is the heat transfer area of inner tube of measurable anti-fouling heat exchanger, m\({}^{2}\). Figure 9b displays the heat transfers coefficient change of the measurable anti-fouling heat exchanger. It can be known that the heat transfer coefficient of the designed heat exchanger is stable at about 2500 W/m\({}^{2}\cdot\)K after 1 h.  
The worst problem caused by fouling and oxide deposition is the terrible effect on the heat transfer performance of the material surface. In Figure 7, the uncoated specimen has obvious fouling deposition. Besides, thermal resistance of the Ni-W-P coating itself has contributed to the decrease of the overall heat transfer coefficient of the coated material. Therefore, we specially compare and analyze the heat transfer coefficients of different specimens after fouling at the same time. Meanwhile, the thermal conductivity of ternary Ni-W-P coating was measured to determine the process of thermal conductivity changing with W content, which is utilized to ascertain the optimum coating parameters. Figure 10a shows the heat transfer coefficient curves of ternary Ni-W-P coatings with different W contents. Notwithstanding the W content has changed, there is un-conspicuous law of corresponding  
Figure 9: (**a**) Test data of experimental rig; (**b**) Heat transfer of measurable anti-fouling heat exchanger. heat transfer coefficient. Whereas, the decreasing rate of heat transfer efficiency of the specimen coated with ternary Ni-W-P coating has decreased significantly after 72 h, which proves that the heat transfer performance is better than that of the uncoated mild steel. Although there is still some fouling deposited on the surface of ternary Ni-W-P coating, the thickness of ternary Ni-W-P coating is merely 20 \(\upmu\)m. Compared with the influence of fouling layer, the effect of the coating thickness on the heat transfer of coating can be ignored. As shown in Figure 10a, we can easily know that the heat transfer efficiency of specimen 5 increases with the up of the temperature of the heating source. The up of temperature can improve the heat transfer efficiency of coating while it can also accelerate the deposition rate of CaCO\({}_{3}\) fouling, resulting in the further increase of fouling layer thickness. However, the temperature of the heat source should not be reduced unilaterally just in terms of deposition of CaCO\({}_{3}\) fouling.