Increasing demand for oil storage tanks required we develop new models for large tanks that are adapted to use under extreme conditions. In order to avoid solidification of the oil in the tank at low temperatures, we need to accurately represent the behavior of heat flux in the tank. In this paper, we propose a step-by-step algorithm for determining the heat transfer coefficient based on the law of conservation of energy, when is then applied in a discrete unsteady heat transfer equation derived using the Taylor series expansion method and solved numerically. We studied the effect of the liquid level and the average temperature in the tank on the heat flux density, and using this as the basis we were able to set up a theoretical model for optimizing the design and use of floating-roof tanks.