A vorticity form of mean mechanical energy equation is derived in equipments with inlets and outlets, which establishes a relation between the mean mechanical energy losses and flow parameters such as vorticity, Reynolds stress, etc. The causes of mean mechanical energy losses and distribution of relatively larger losses in the fluid field have been found. The approach in this paper presents a universal tool for energy analysis and mechanism research of drag reduction in complex equipments. (c) 2005 Elsevier Ltd. All rights reserved.