The performance of a double-cone device (1) is increased, not only by moving the gap or inlet openings (22) a short distance into the exit cone (47), but also by making the conicity null3 (55) of the so-formed small diffuser less than the conicity null2 (109) of the remaining part (53) of the exit cone. Double cone units (7, 60), particularly the ones with this improved diffuser, may be used in pump installations (1, 60), like well-pumps (1), where liquids must be pumped from great depths.