Abstract: An evaporator including a frame which defines a fluid inlet plenum and an interchangeable baffle plate positioned in the plenum along with an interchangeable fluid spray bar. A transition element carries fluid from a standard round tubular fluid inlet and distributes the fluid equally across the width of the spray bar. By forming a plurality of interchangeable baffling plates each having at least one of the placement, configuration and size of holes therethrough different than the other interchangeable baffling plates and forming a plurality of interchangeable spray bars each having a different fluid outlet configuration, the baffle plates and spray bars can be interchanged to design an evaporator for air conditioners and the like with the least amount of internal fluid pressure drop and the best fluid inlet flow balance with the best heat performance.