Liquid-vapour separators are used to separate vapour stream from a mixed stream of liquid and vapour. They are also called knockout drums. The vessel diameter must be large enough to slow down the gas to a sufficiently low velocity so that the dropplets settle out. Demisting pads can be used to improve the efficieny of the knockout drums. Equation to determine the height of heavy liquid outlet:

where D in the above equation is the vessel diameter, V_v is the volumetric flowrate of gas or liquid, u_s is the settling velocity of the droplets.