The objective of this study is to investigate the effects of particle humidity on the inlet particle size distribution, overall efficiency, grade efficiency and cut size diameter for an axial cyclone separator with inner diameter of 150 mm. The collection and grade efficiencies of the cyclone separator were measured by on-line method for inlet velocities, particle concentration and particle humidity in the ranges of 1218 m/s, 30-500 mg/m(3) and 8-30 parts per thousand, respectively. By employing a set of fixed parameters for inlet velocity and particle concentration, the effect of particle humidity on separation efficiency was investigated. The experimental results show that the volume ratio of larger particle increases with the increasing of particle humidity due to particle agglomeration. When the inlet velocity and particle humidity remain constant, the collection and grade efficiencies improve greatly as the increasing of the particle concentration because of the particle aggregation. However, it was noticed that the grade efficiencies did not always increased with the increasing of particle humidity under the same conditions of inlet velocity and particle concentration. The trends of grade efficiency curves for different particle humidity change at the particle diameter of approximately 10 mu m. The grade efficiency improves with the increasing of particle humidity when the particle diameter is larger than 10 mu m, while a contrary tendency is observed when the particle diameter is smaller than 10 mu m. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.