One application in which pressure exchangers are widely used is reverse osmosis (RO). In an RO system, pressure exchangers are used as energy recovery devices (ERDs). As illustrated, high-pressure concentrate from the membranes [C] is directed [3] to the ERD [D]. The ERD uses this high-pressure concentrate stream to pressurize the low-pressure seawater stream (stream [1] becomes stream [4]), which it then merges (with the aid of a circulation pump [B]) into the highest-pressure seawater stream created by the high-pressure pump [A]. This combined stream feeds the membranes [C]. The concentrate leaves the ERD at low pressure [5], expelled by the incoming feedwater flow [1].
Pressure exchangers save energy in these systems by reducing the load on the high pressure pump. In a seawater RO system operating at a 40% membrane water recovery rate, the ERD supplies 60% of the membrane feed flow. Energy is consumed by the circulation pump, however, because this pump merely circulates and does not pressurize water, its energy consumption is almost negligible: less than 3% of the energy consumed by the high pressure pump. Therefore, nearly 60% of the membrane feed flow is pressurized with almost no energy input.