What are the different types of heat exchangers?

Three common heat exchangers  are tube-in-tube, shell-in-tube, and plate type.

Tube-in-tube heat exchanger is a common type of heat exchanger used in heat pump water heaters.  Tube-in-tube suffers from moderate pressure drops.  Shell-in-tube improves on the tube-in-tube with better flow rate and thus, low pressure drop. 

Plate heat exchanger has very good heat transfer rate.  It is also space saving, but is not normally suitable for direct applications as the plates are relatively easily contaminated.