The paper presents an approach for robust PI controller design for a system affected by parametric uncertainty. The method is based on plotting the stability boundary locus in the plane of controller parameters that is called (kp, ki)-plane. Designed robust PI controller is implemented for control of two counter-current tubular heat exchangers in series with uncertain parameters, in which kerosene as a product of distillation in a refinery has to be cooled by water. The controlled variable is the temperature of the outlet stream of the kerosene from the second heat exchanger and the control input is the volumetric flow rate of the inlet stream of the cold water in the second heat exchanger. Simulation results of robust PI control of heat exchangers are also presented.