A substrate processing apparatus (100) comprising a process tunnel (102) including a lower tunnel wall (122), an upper tunnel wall (142), and two lateral tunnel walls (128), said tunnel walls being configured to bound a process tunnel space (104) that extends in a longitudinal transport direction (7) and that is suitable for accommodating at least one substantially planar substrate (180) oriented parallel to the upper and lower tunnel walls (122, 142), the process tunnel being divided in a lower tunnel body (120) comprising the lower tunnel wall and an upper tunnel body (140) comprising the upper tunnel wall, which tunnel bodies (120, 140) are separably joinable to each other along at least one longitudinally extending join (160), such that they are mutually movable between a closed configuration in which the tunnel walls (122, 128, 142) bound the process tunnel space (104) and an open configuration that enables lateral maintenance access to an interior of the process tunnel.