Systematic investigations of pressure drop type oscillations and their thresholds were conducted for steam-water two-phase How in a uniformly heated, helically coiled tube. Experiments were conducted to obtain the critical conditions for the occurrence of pressure drop type dynamic oscillation in a closed-circulation helically coiled tubing steam generator. The study showed that different locations of the tank which provided a compressible gas volume (called compressible volume for short) corresponded to different oscillation initial boundaries as well as different oscillating amplitudes and time periods. Moving compressible volume from the inlet of the steam generator upstream could dramatically suppress the occurrence of pressure drop oscillation. Also, non-uniform heat flux distribution along the evaporating channel could change oscillating boundaries significantly, especially when higher heat flux was applied to the higher mass quality region. A study of pressure drop oscillation in various steam generator inclinations showed that gravity has little influence upon the oscillating boundaries. Based on the experimental observations, a set of new methods was first proposed to eliminate the occurrence of pressure drop oscillation. (C) 2001 Elsevier Science Ltd. All rights reserved.