Abstract: A structure of, and process for making, an improved heat exchanger. The exchanger may be used for heat transfer between air going to a space for human comfort, and humid, cooled air from an evaporative cooling system. The heat exchanger is made from a series of layers of thermoplastic sheeting spaced apart in a stack by uniquely shaped flat-topped dimples formed therein. The plates are joined together at their edges by adhesive and sealants and aerodynamically designed U-shaped clips. The clips are installed in a manner to form alternating channels of cooled and non-cooled air streams flowing perpendicularly to each other in the stack of plates. Special alignment detentes provide easier stacking and assembly of the plates.