Parallel flow heat exchanger

The term parallel flow heat exchanger is not correct from a physical-thermodynamic point of view. Heat is not a material quantity, but a form of energy that can be transferred as a result of a temperature difference, but not ‘exchanged’. An ‘exchange’ would imply a mutual, balanced transfer, which contradicts the second law of thermodynamics. The technically correct term is therefore

parallel flow heat exchanger (see this glossary for details)

A heat exchanger is a technical system in which thermal energy is transferred from a warmer medium to a colder medium without the media mixing.