Energy conservation of HENs (heat exchanger networks) has been attaching more and more attentions with all kinds of methods for operation optimization. Several methods optimize the temperature and/or pressure differential set points for HENs, but cannot control the components directly, which has to seek the help of some control strategies. This paper introduces a direct optimal control strategy of VSPs (variable speed pumps) based on the newly proposed thermal resistance-based optimization method together with the physical models of each component in HENs, which can directly calculate the optimal rotation frequencies of each VSP for optimal operation. To illustrate this method, a series of experiments are performed with a VWV (variable water volume) HEN, including a group of experiments to determine the characteristic parameters in the physical models of each component, and the others to test the HEN performances with the optimal operating parameters and other alternative ones. The results show that the newly proposed direct control strategy can directly get the optimal rotation frequencies of each VSP with the least total power consumption under specific system requirements and constraints. On this basis, for different system requirements, different operating frequencies of VSPs are optimized to demonstrate the universality of the direct optimal control strategy. (C) 2015 Elsevier Ltd. All rights reserved.