A plate-type flow reactor device comprising: at least a first plate having first and second opposing surfaces (22, 24) and a plurality of through-holes extending from the first to the second surface; at least a second plate having a second plate surface sealed against the first surface of the first plate by at least two first O-rings positioned between the first surface of the first plate and the second plate surface; at least a third plate having a third plate surface sealed against the second surface of the first plate by at least one second O-ring positioned between the second surface of the first plate and the third plate surface; two or more first elongated channels defined between the first surface of the first plate and the second plate surface by contours on or in the first surface, and one or more second elongated channels defined between the second surface of the first plate and the third plate surface by contours on or in the second surface, each channel (70a,70b,80a,80b) of said first or second channels (70,80) extending from a first end of the channel (70a,70b,80a,80b) to a second end of the channel (70a,70b,80a,80b); wherein: for each one first channel of the two or more first channels: said one first channel communicates, at the first or second end (94,96) of the one first channel or both, with the at least one second channel via one or more of the through-holes through the first plate, and said one first channel communicates with another first channel of the two or more first channels only via a path including said at least one second channel and the through holes, and each first channel (70 (70a,70b)) is individually surrounded by at least one of the first O-rings and the at least one second channel (80 (80a,80b)) is individually surrounded by the at least one second O-ring. at least a first plate having first and second opposing surfaces (22, 24) and a plurality of through-holes extending from the first to the second surface; at least a second plate having a second plate surface sealed against the first surface of the first plate by at least two first O-rings positioned between the first surface of the first plate and the second plate surface; at least a third plate having a third plate surface sealed against the second surface of the first plate by at least one second O-ring positioned between the second surface of the first plate and the third plate surface; two or more first elongated channels defined between the first surface of the first plate and the second plate surface by contours on or in the first surface, and one or more second elongated channels defined between the second surface of the first plate and the third plate surface by contours on or in the second surface, each channel (70a,70b,80a,80b) of said first or second channels (70,80) extending from a first end of the channel (70a,70b,80a,80b) to a second end of the channel (70a,70b,80a,80b); for each one first channel of the two or more first channels: said one first channel communicates, at the first or second end (94,96) of the one first channel or both, with the at least one second channel via one or more of the through-holes through the first plate, and said one first channel communicates with another first channel of the two or more first channels only via a path including said at least one second channel and the through holes, and each first channel (70 (70a,70b)) is individually surrounded by at least one of the first O-rings and the at least one second channel (80 (80a,80b)) is individually surrounded by the at least one second O-ring. said one first channel communicates, at the first or second end (94,96) of the one first channel or both, with the at least one second channel via one or more of the through-holes through the first plate, and said one first channel communicates with another first channel of the two or more first channels only via a path including said at least one second channel and the through holes, and each first channel (70 (70a,70b)) is individually surrounded by at least one of the first O-rings and the at least one second channel (80 (80a,80b)) is individually surrounded by the at least one second O-ring.