An adsorption gas dryer has a first drying tower (13) and a second drying tower (14) each having a first port (24) and a second port through which gas can pass into or out of the tower, a first manifold (15) and a second manifold (16) connected respectively to the first and the second ports of both towers (13,14), the manifolds (15,16) each including integral gas passages, each port of the towers (13,14) being in communication with a gas passage, and the manifolds (3,4) each including at least one integral valve seat upon which a valve actuator is mounted to constitute avalve for controlling flow of gas through the gas passages, wherein the second manifold (16) has two exhaust valves (17,18) for controlling purge gas.