The effect of scale on heat transfer in a jacketed vessel is to add another resistance to heat transfer. The overall heat transfer coefficient is always calculated by summation of resistances (inside, outside, wall, glass liner, etc). The additional resistance to heat transfer of a scale can be estimated if one knows the thermal conductivity of the scale material. Frequently, the scale material is not known. However, if the scale is porous, then it will be filled with the fluid that is present inside the vessel. In that case, an approximation can be made by using the thermal conductivity of the fluid. If the scale is a composite of known materials, then the composite thermal conductivity can be estimated using a volume fraction average. The additional heat transfer resistance would then be calculated as k/x where k is the thermal conductivity and x is the scale thickness.