A substrate processing apparatus comprising • a housing which bounds a process tunnel extending in a longitudinal direction and that is configured to accommodate a plurality of substantially flat substrates that, in use are simultaneously transported through the process tunnel in the longitudinal direction, wherein the housing includes: • an upper wall of which a downwardly directed planar tunnel side bounds an upper side of the process tunnel, wherein the downwardly directed planar tunnel side includes a plurality of gas injection openings through which, in use, gas is injected in the process tunnel for forming an upper gas bearing between each substrate (S) of the plurality of substrates and the upper wall ; • a lower wall of which an upwardly directed planar side extends parallel to the downwardly directed planar tunnel side and bounds a lower side of the process tunnel , wherein the upwardly directed planar tunnel side includes a plurality of gas injection openings through which, in use, gas is injected in the process tunnel for forming a lower gas bearing between each substrate (S) of the plurality of substrates and the lower wall ; • two lateral side walls that extend in the longitudinal direction and that bound two lateral sides of the process tunnel ; • gas exhaust openings which are accommodated in one of the above-mentioned walls and that open into the process tunnel for exhausting gas from the process tunnel ; characterized by • two endless transport belts , wherein each side wall is implemented as a belt part of an associated one of the two endless transport belts ; and • at least one drive for driving the endless transport belts . • a housing which bounds a process tunnel extending in a longitudinal direction and that is configured to accommodate a plurality of substantially flat substrates that, in use are simultaneously transported through the process tunnel in the longitudinal direction, • an upper wall of which a downwardly directed planar tunnel side bounds an upper side of the process tunnel, wherein the downwardly directed planar tunnel side includes a plurality of gas injection openings through which, in use, gas is injected in the process tunnel for forming an upper gas bearing between each substrate (S) of the plurality of substrates and the upper wall ; • a lower wall of which an upwardly directed planar side extends parallel to the downwardly directed planar tunnel side and bounds a lower side of the process tunnel , wherein the upwardly directed planar tunnel side includes a plurality of gas injection openings through which, in use, gas is injected in the process tunnel for forming a lower gas bearing between each substrate (S) of the plurality of substrates and the lower wall ; • two lateral side walls that extend in the longitudinal direction and that bound two lateral sides of the process tunnel ; • gas exhaust openings which are accommodated in one of the above-mentioned walls and that open into the process tunnel for exhausting gas from the process tunnel ; • two endless transport belts , wherein each side wall is implemented as a belt part of an associated one of the two endless transport belts ; and • at least one drive for driving the endless transport belts . <EOS>