The vapor diffusion theory is supported by several authors in explaining the mechanism of gas-side fouling of not only low temperature but also high temperature heating surfaces of oil-fired boilers. The validity of the theory is well established to the low temperature fouling. However, present shortage of knowledge on the thermal properties of nearly all of the fouling substances detected in the high temperature region makes it difficult to conclude whether the substance is gaseous or misty when they reach the surface of the deposit. The validity of the theory to the high temperature fouling is investigated in this paper.
In this study, NaCl was supplied to a test furnace as the fouling material and the deposit on tubes with predetermined temperatures was measured. The gas temperature arround the test tubes was between 770 and 980°C, and the tube metal temperature was from 250 to 720°C. These temperature ranges were selected so as to include both gaseous and misty NaCl in the boundary layer of the tested tubes. The gas velocity was from 14.4 to 18.1 m/s, and the NaCl concentration was from 130 to 420 ppm. Experiments were also carried out making use of Na, S and V as the fouling materials. NaCl was selected in the first experiment because its thermal properties was the most well known and Na, S and V were selected in the second experiment because they were ordinarily found in the high temperature deposits of oil-fired boilers.
The following results were obtained by these experiments.
1. The vapor diffusion theory can be applied also to the high temperature fouling if the phase change of fouling materials in the boundary layer is properly accounted for.
2. When the temperature is low enough in the neighbourhood of the heat transfer surface, a considerable amount of the fouling material in the flue gas changes into misty form from the vapour phase. The deposition rate due to the vapour diffusion decreases because of the accompanied decrease of the vapour concentration, and the deposition rates due to other mechanisms, particular to the misty form are negligibly small. The total deposition rate therefore decreases in this case. This conclusion explains also the low-level fouling in the intermediate temperature region between the economizer and the superheater.
3. It should be noted that the deposited materials are sometimes in the molten state on the heat transfer surface at much lower temperature than their melting points.

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