Heat exchangers operating in the power and process industries are fouled to a greater or lesser extent depending on surface temperature, surface condition, material of construction, fluid velocity, flow geometry and fluid composition. This fouling phenomenon is time-dependent and will result in a decrease in the thermal effectiveness of a heat exchanger. Once the thermal effectiveness decreases to a minimum acceptable level, cleaning of the equipment becomes necessary to restore the performance. In this paper, we present a simple probabilistic approach to characterize various fouling models that are commonly encountered in many industrial processes. These random fouling growth models are then used to investigate the impact on risk-based thermal effectiveness, overall heat-transfer coefficient and the hot- and cold-fluid outlet temperatures of a shell-and-tube heat exchanger. All the results are presented in a generalized form in order to demonstrate the generality of the risk-based procedure discussed in this paper. (C) 2000 Elsevier Science Ltd. All rights reserved.