An arrangement in which the flows of two separate process streams are fed into a process in the same direction for the purpose of carrying out heat and mass transfer. It is typically used for heat-sensitive materials and in packed gas absorption columns where the chemical reaction in the liquid phase tends to be rapid. It is generally inefficient as a flow arrangement, since once equilibrium between the process streams has been reached, no further transfer takes place. It is also known as parallel flow. Compare countercurrent flow.