A substrate processing apparatus comprising: - a process tunnel including a lower tunnel wall, an upper tunnel wall, and two lateral tunnel walls, said tunnel walls being configured to bound a process tunnel space that extends in a longitudinal transport direction and that is suitable for accommodating at least one substantially planar substrate oriented parallel to the upper and lower tunnel walls (122, 142); - a plurality of gas injection channels (124, 144) provided in both the lower and the upper tunnel wall, wherein the gas injection channels in the lower tunnel wall are configured to provide for a lower gas bearing, while the gas injection channels in the upper tunnel wall are configured to provide for an upper gas bearing, said gas bearings being configured to floatingly support and accommodate said substrate there between; and - a plurality of gas exhaust channels, provided in both said lateral tunnel walls or, alternatively in at least one of the lower and the upper tunnel wall (122, 142) directly adjacent both of said lateral tunnel walls, wherein the gas exhaust channels in or directly adjacent each lateral tunnel wall are spaced apart in the transport direction; characterized in that: - the process tunnel is divided in a lower tunnel body comprising the lower tunnel wall and an upper tunnel body comprising the upper tunnel wall, which tunnel bodies (120, 140) are separably joinable to each other along at least one longitudinally extending join, such that they are mutually movable between a closed configuration in which the tunnel walls (122, 128, 142) bound the process tunnel space and an open configuration that enables lateral maintenance access to an interior of the process tunnel; and by - a press or at least one clamp configured to force the upper and the lower tunnel bodies (120, 140) together along the at least one longitudinally extending join in order to effect the closed configuration. - a process tunnel including a lower tunnel wall, an upper tunnel wall, and two lateral tunnel walls, said tunnel walls being configured to bound a process tunnel space that extends in a longitudinal transport direction and that is suitable for accommodating at least one substantially planar substrate oriented parallel to the upper and lower tunnel walls (122, 142); - a plurality of gas injection channels (124, 144) provided in both the lower and the upper tunnel wall, wherein the gas injection channels in the lower tunnel wall are configured to provide for a lower gas bearing, while the gas injection channels in the upper tunnel wall are configured to provide for an upper gas bearing, said gas bearings being configured to floatingly support and accommodate said substrate there between; and - a plurality of gas exhaust channels, provided in both said lateral tunnel walls or, alternatively in at least one of the lower and the upper tunnel wall (122, 142) directly adjacent both of said lateral tunnel walls, wherein the gas exhaust channels in or directly adjacent each lateral tunnel wall are spaced apart in the transport direction; - the process tunnel is divided in a lower tunnel body comprising the lower tunnel wall and an upper tunnel body comprising the upper tunnel wall, which tunnel bodies (120, 140) are separably joinable to each other along at least one longitudinally extending join, such that they are mutually movable between a closed configuration in which the tunnel walls (122, 128, 142) bound the process tunnel space and an open configuration that enables lateral maintenance access to an interior of the process tunnel; - a press or at least one clamp configured to force the upper and the lower tunnel bodies (120, 140) together along the at least one longitudinally extending join in order to effect the closed configuration.