A dynamic compressor is a flow machine where the pressure increase takes place at the same time as the gas flows. The flowing gas accelerates to a high velocity by means of the rotating blades, after which the velocity of the gas is transformed to pressure when it is forced to decelerate under expansion. Depending on the main direction of the flow they are called radial or axial compressors. In comparison with displacement compressors, dynamic compressors have a characteristic where a small change in the working pressure results in a large change in the capacity. Each speed has an upper and lower capacity limit. The upper limit means that the gas's flow velocity reaches sonic velocity. The lower limit means that the counter pressure is greater than the compressor's pressure build-up, which means return flow in the compressor. This in turn results in pulsation, noise and the risk for mechanical damage.