Heat exchanger

A heat exchanger is a system used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contactHeat exchangers, metal shells and tubes, work by transferring heat from one place to another. ... The hot flue gas heats the metal as the gas makes its way to the exhaust outlet of the furnace. As this is happening, the hot metal heats the air circulating over the exterior of the heat exchanger.Simplified heat exchanger concepts

Heat exchangers work because heat naturally flows from higher temperature to lower temperatures. Therefore if a hot fluid and a cold fluid are separated by a heat conducting surface heat can be transferred from the hot fluid to the cold fluid