Heater treaters are designed to break welistream emulsions, allowing the separation of crude oil from water and other foreign materials. Gas is liberated prior to the filtering and settling sections, allowing liquid and sediment separation without the agitation of gas breaking out of the liquid.

Vertical treaters use a firebox to heat water to a specified temperature; oil then passes through the hot water in streamlets. This is commonly called the "hot water wash" method. The height of the vertical heater treater is set to provide adequate time for separation and to provide gravity flow from the treater to the storage tank.

A horizontal treater can handle large amounts of emulsion by utilizing a long "U" firetube. This firetube has greater heating capacity, thereby allowing a higher treating rate. The large horizontal interface area in the settling section allows for more efficient separation of emulsion and gas release. Horizontal treaters must operate under pressure to boost oil to storage tanks.