## 1 Introduction
[PERSON] and [PERSON] [17] extended the available data for a normal gas flow to the small tube diameter banks of various staggered arrangements to lower the Reynolds numbers from 10\({}^{3}\) to 10\({}^{4}\). In the paper by [PERSON] and [PERSON] [18], heat transfer coefficients are obtained for the individual rows of the ten-row inline tube banks, under various pitch ratio arrangements. To study heat transfer rates in the crossflow over the smooth tube banks, the correlating equations obtained by [PERSON] and his co-authors are widely used. One of these investigations, by [PERSON] et al. [19], was of a closely spaced staggered tube bank of large smooth tubes in water streams within 21 turbulent regions. They found out that the efficiency of the banks from an energetic point of view increased as the tube spacing decreased. [PERSON] [20] reported the pressure and hydrodynamic resistance of single tubes and tube banks of various arrangements in flows of gases and v (viscous liquids) at higher Reynolds numbers and various Prandtl numbers from 0.7 to 500. The effect of the fluid properties was considered by [PERSON] and [PERSON] [21; 22] for the inline and staggered tube banks in a water crossflow at Reynolds numbers ranging from 5 \(\times\) 10\({}^{4}\) to 2 \(\times\) 10\({}^{6}\) and Prandtl numbers from 3 to 7. They determined the optimal arrangements and geometries of the tube banks in an oil crossflow at Reynolds numbers ranging from 1 to 2 \(\times\) 10\({}^{4}\). The former Grimison correlations were modified by [PERSON] [23], where the empirical formulas were created instead of a graphical representation. [PERSON] et al. [24] used the Ansys Fluent software and the RNG k-\(\epsilon\) turbulence model with a modified dissipation term in the \(\epsilon\) equation, where the Reynolds numbers (based on the maximum mean velocity inside a tube bank and the tube hydraulic diameter) of 10\({}^{3}\), 5 \(\times\) 10\({}^{3}\), 10\({}^{4}\), and 10\({}^{5}\) were used. According to the results, the optimal tube spacing for a staggered tube array in crossflow was found for \(S_{T}/D\) = 1.5, 1.3 and \(S_{L}/D\) = 1.15, 1.05, respectively (Figures 1 and 2). [PERSON] et al. [25; 26] presented the models for the inline and staggered arrangement of tube banks applicable over a wide range of Reynolds and Prandtl numbers, as well as the longitudinal and transverse pitch ratios. It was determined that the average heat transfer coefficient for tube banks in crossflow depends on the number of longitudinal rows, longitudinal and transverse pitch ratios, and the Reynolds and Prandtl numbers. Moreover, it was proved that the staggered arrangement gave higher heat transfer rates than the inline one. The same findings were observed by [PERSON] et al. [27]. [PERSON] [28] indicated that the heat transfer coefficient might be reduced by 37.1% from the prediction of a well-known correlation by [PERSON], as the longitudinal pitch decreased. The heat transfer degradation can be estimated by using an empirical correction factor, and the Zukauskas correlation can predict existing experimental data when the correlation is combined with the empirical factor developed in the study. [PERSON] et al. [29; 30] extended and developed the formulas of Gnielinski, Gaddis, and Zukauskas for the calculation of the Nusselt number in tube banks. The main contribution is a unique formula for a wide range of the Reynolds numbers, while the Zukauskas correlations are divided into the Reynolds number subintervals. [PERSON] et al. [31] created the CFD model by comparing different boundary conditions, domain dimensions, and turbulence models for unsteady simulations of the inline tube banks. The correlation of Gnielinski is recommended for the tube banks with large transverse spacing as it agrees, within \(\pm\)13%, with the numerically obtained values. The former way to compute the Nusselt number based on the heat transfer coefficient is derived from the Fourier and Newton law. As a part of the formula, the reference temperature should be expressed.