Abstract: In a process for controlling a particle flow system which is to be operated at slightly above the velocity at which saltation occurs, a cross correlation technique is used to determine the spread of particle velocities in the system and this is compared with a signal representing the desired spread to produce an error signal indicating the desired mean velocity of particle flow. A second cross correlation technique, having a shorter integration period, is used to determine the actual mean particle velocity and this is compared with the first error signal to produce a second error signal to control the flow rate of the conveying fluid.