Mixing is achieved using an impeller mounted on a centrally located stirrer shaft.the stirrer shaft is driven rapidly by the stirrer motor;the effect of the rotating impeller is to pump the liquid and create a regular flow pattern. Liquid is forced away from the impeller, circulates through the vessel,and periodically returns to the impeller region.in gassed stirred tanks such as bioreactors used for aerobic culture,gas is introduced into the vessel by means of a sparger located beneath the impeller.