A membrane dryer is a compressed air dryer that consists of a semi-permeable membrane through which water vapor, and possibly some of the air, can permeate. The membrane material is selected to promote water diffusion whilst limiting the progress of other gas molecules that make up the compressed air. The water vapor that has diffused through the membrane layer is then conducted away to atmosphere through an exhaust passage in a protective casing surrounding the membrane layer. Typically a small amount of the dried compressed air, often called sweep gas, is used in ejecting the collected water vapor to atmosphere. All units are fully packaged ready to be hooked-up and are customizable in terms of heavy duty supplies severe, dangerous or explosive environmental conditions, ATEX, CE, ASME, ISO7183 compliance.