Based on the effect of fouling on heat transfer process，the critical fouling layer thickness was firstly put forward． The expressions of the critical fouling layer thickness were given and the effect of relative parameters on the critical fouling
layer thickness was discussed．In the meantime， the critical fouling layer thickness was compared with the critical thermal insulation layer thickness．The results show that the critical fouling layer thickness may be theoretically used to explain the phenomena of enhancement heat transfer at the initial forming period of fouling under fixed conditions． Furthermore， the critical fouling layer thickness is determined by mass flow rate of fluid， the physical properties of fluid and the thermal conductivity of fouling layer itself， etc． Meanwhile， the presence of the critical fouling layer thickness is subjected to the specific conditions of convective heat transfer，and the critical fouling layer thickness is different from the critical thermal insulation layer thickness although the fouling layer and thermal insulation layer both have poor thermal conductivity．

A numerical method for simulating the developments and distributions of heat transfer and flow fields was proposed. The effects of Reynolds number, curvature ratio, and coil pitch on the average friction factor, average Nusselt number at different axial cross-sections and the total entropy generation rate in a helical coiled tube with uniform heat flux was presented when Reynolds number is 200~1 000, dimensionless pitch 0.1~0.2 and dimensionless curvature ratio 0.1~0.3. The results show that the effect of the secondary flow is enhanced with the increase of turning angle, the maximum velocity perpendicular to axial cross section shifts toward the outer side of helical coiled tube and two Dean roll cells appear with the increase of axial turning angle. Furthermore, the average friction factor, average Nusselt number at different axial cross-sections and the total entropy generation rate present different characteristics when the Reynolds number, curvature ratio and pitch change. Compared with the curvature ratio, the pitch has relatively little influence on convective heat transfer performance and the total entropy generation. In the meantime, the entropy generation caused by viscous flow is much less than that caused by heat transfer and can be neglected.