Terminal load distribution and pipe network structure are the key factors that affect the energy-saving potential of central air-conditioning chilled water systems, nonlinear thermodynamic performance of an air-conditioning system with large inertia will mainly exert influence on the stability and reliability of energy-saving operation control. Unreasonable variable flow control strategy can neither achieve an ideal energy-saving effect nor meet the air-conditioning comfortableness requirements. With a direct-return chilled water system as study object, this paper built a hydraulic calculation model of pipe network topology, bypass loop hydraulic calculation model. AHU thermodynamic model, and water pump variable frequency operation model. Operating frequency of a water pump for different flow ratio, pump power, temperature difference of pipe network supply and return water, pressure difference of pipe network supply and return water, bypass control valve characteristics, system adjustability coefficient, and pipe network resistance characteristics of a chilled water system are studied under the condition of given supply water temperature, and pipe network's AHU node thermal and humid load. And energy consumption characteristics of constant temperature difference control and variable temperature difference control are also analyzed with comparison. The results can provide theoretical guidance for the stable and reliable energy-saving operation of a chilled water system. (C) 2012 Elsevier Ltd. All rights reserved.