An oilless screw compressor incorporating water-cooled cooling units for cooling compressed air discharged from compressor bodies having a pair of male and female screw rotors which can be rotated in a contactless and oilless manner, the cooling units comprising a plate type heat-exchanger, and the amount of cooling water for the plate type heat-exchanger being adjustable. Embodiments of the double tread molding and retreading methods also remove the need for cementing the cured tread to prevent future oxidation buildup. It includes a first cooling-element side and a second cooling-element side which is on the opposite side from the first cooling-element side. A plurality of central optical switches connected between the plurality of input switches and plurality of output switches is provided.