An experimental unit which is adequately instrumented to measure flowrates and temperatures of both fluids accurately is required.  Experiments should focus on change in heat transfer rates over time to determine not only the fouling resistance, but the type of fouling which is experienced.  Typical fouling curves include asymptotic, linear and falling rate fouling.  A good treatment of heat exchanger fouling, mitigation and cleaning technologies was published by Institution of Chemical Engineers in 2000 and edited by Hans Muller-Steinhagen:  "Heat Exchanger Fouling  --  Mitigation and Cleaning Technologies."