## 5 Discussion  
One of the main contributions of the article is the derivation of relations for the practical estimation of the thickness of the oil layer, which is applied by means of the dispersed oil fraction under the given conditions.  
The flow of the dispersed oil fraction inside the lean tube is influenced by a number of variables and factors entering this process. In addition, the measurement errors, the nozzle angle adjustment and the depth of its insertion into the pipe, as well as the design and condition of the inner surface of the pipe itself, have a considerable impact on the results of measuring the distance of the impact  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Method** & \(\mathbf{p_{1}}\) & \(\mathbf{p_{2}}\) & \(\mathbf{p_{3}}\) \\ \hline
1 & 9.238 & 1.559 & \(-0.182\) \\
2 & 9.010 & 1.822 & \(-0.228\) \\
3 & 9.071 & 1.561 & \(-0.172\) \\
4 & 9.040 & 1.652 & \(-0.188\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Optimum dependency coefficients (6) for criteria 1–4 for average values of 25 mm pipes.  
Figure 10: The course of the thickness of the oil layer in model dependencies for different pipe diameter values—15, 25 and 35 mm (critical value is \(h_{c}=1.68675\)\(\upmu\)m).  
of oil microparticles. For these reasons, the basic conditions for approximation were defined so as to guarantee a pessimistic (i.e., in graphical representation the lower) estimate of the oil film thickness.  
The results also show the large influence of tube diameter on the distance of the microparticles of the application liquid. Three tube diameters were investigated, with two methods applied, and, in one case, given the parameters, the method did not provide sufficient results suitable for application. However, after adjusting, the designed parameters of the applied layer were attained, such as: flow rates, application time, fraction density, temperature, etc.--this is a prerequisite for fulfilling the desired parameters of the coated layer.  
As can be seen from the previous graphical representation, under the same application conditions, the dispersed oil fraction in different diameter pipes behaves differently. In the test pipe of middle diameter, the oil microparticles fell the furthest and in greatest abundance, to form a coherent oil film of sufficient thickness. For other pipe diameters, it is recommended to apply a dispersion oil fraction from both sides of the lean pipe, to obtain a sufficiently thick oil film as prescribed by the oil manufacturer. In the case of the largest-diameter pipe, it is advisable to extend the time span so that a sufficient quantity of oil microparticles travels a sufficient distance up to the mid-length of the pipe. In each of these cases, when applying a dispersed oil fraction, it is recommended to rotate the coated pipe around its longitudinal axis to distribute the oil film uniformly in the radial direction. This rotation, as well as application of the dispersion oil fraction from both sides of the pipe, is not a problem in practical technical applications.  
From the viewpoint of monitoring the coating process, it is a good choice for this challenging process to use an experimental system designed and developed for multi-parametric simulated monitoring of technical systems [25], but it is still necessary to adhere to commonly used [26] standards. It is appropriate to optimize the proposed process. In general, computer-aided optimization technologies are recommended [27].