The crude oil temperature, which highly influenced by solar heat and ambient temperature, has strong effects on the sludge formation and evaporative loss. Here, a numerical method has been presented for predicting the transient average crude oil temperature in storage tanks and the evaporative losses. Results show that the annual evaporative losses rise up to 125% if the tank exterior surface absorptivity is increased as 90% of the solar irradiation is absorbed. It was found that, the sludge height is reduced about 50 mm per months, when the absorptivity of the tank exterior surface is increased from á = 0.1 to á = 0.9 during winter. Considering the opposing effect of the in-tank crude oil temperature on the evaporative losses and sludge formation, the optimum absorptivity of the tank paint could be determined according to the minimal point in the summation curve of both losses.

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