In this study, CFD analysis of air-heating vaporizers was conducted. A longitudinally finned vertical pipe was used to represent the air-heating vaporizer in the CFD model. Nitrogen gas was used as the working fluid inside the vertical pipe, and it was made to flow upward. Ambient air, which was the heat source, was assumed to contain no water vapor. To validate the CFD results, the convective heat transfer coefficients inside the pipe, $h_{i-c}$, derived from the CFD results were first compared with the heat transfer coefficients inside the pipe, $h_{i-p}$, which were derived from the Perkins correlation. Second, the convection heat transfer coefficients outside the pipe, $h_{o-c}$, derived from the CFD results were compared with the convection heat transfer coefficients, $h_{o-a}$ which were derived from an analytical solution of the energy equation. Third, the CFD results of both the ambient-air flow pattern and temperature were observed to determine whether they were their reasonability. It was found that all validations showed good results. Subsequently, the heat transfer coefficients for natural convection outside the pipe, $h_{o-c}$, were used to determine the Nusselt number outside the pipe, $Nu_{o.}$. This was then correlated with the Rayleigh number, $R_a$. The results show that $R_a$ and $Nu_o$ a proportional relationship in the range of $2.7414{ imes}10^{12}{leq}Ra{leq}2.8263{ imes}10^{13}$. Based on this result, a relation for the Nusselt number outside the pipe, $Nu_o$, was proposed.