The annular fouling test section has been used extensively to study the fouling characteristics of fluids. Fouling occurs inside circular tubes and in many other complicated geometries including the shell-side of tubular heat exchangers, plate exchangers and other enhanced heat transfer surfaces. The main criteria for simulating industrial fouling conditions are: identical water to that used in the plant, same bulk temperature, same surface temperature and same surface shear stress as exists in the operating heat exchanger. When fouling occurs inside a heat exchanger tube, these criteria are quite easily met with an annular fouling test section particularly if the annular test section can be located in a side stream of the water that goes directly to the problem heat exchanger. This paper describes experiments in which a tubular and annular test section were compared. In several tests, the velocity and surface temperature of the test sections were identical and in several other tests the surface shear stress and surface temperature were equal. In general, better agreement between the two geometries was obtained when shear stresses were equal.