Heat transfer refers to the transfer of thermal energy between physical systems. A heat exchanger is a device designed to perform heat transfers from one medium to another.
Heat exchangers are used to transfer heat from a fluid on a side of a barrier to a fluid on the other side without allowing the fluids to mix together. Heat exchangers maximize the surface area of a wall that is used between the two fluids while minimizing any resistance to the flow of a fluid through the exchanger.
Parallel-flow, a major type of heat exchangers, allow two fluids to enter the exchanger at the same end. The two fluids then travel in parallel to the other side of the exchanger. The hot fluid transfers heat to the wall via convection. Parallel-flow heat exchangers are often used when two fluids must be brought to close to the same temperature.
In counter-flow heat exchangers, the fluids enter the exchanger from opposite ends. As the two flows move toward each other from opposite directions, the system is able to maintain almost a constant gradient between the two liquid flows as they travel the length of the exchanger. This enables nearly all the heat properties from one flow to be transferred to the other flow.