A flow reactor, comprising a flow reactor module; the flow reactor module comprising: a process fluid module with a process fluid passage extending therethrough, the process fluid module comprising an extended body having a width, a length, and a thickness, the thickness being less than the length and less than the width, the process fluid module having first and second major surfaces (12, 14) on opposite sides of the process fluid module, oriented perpendicularly to a direction of the thickness of the process fluid module; and a first heat exchange fluid enclosure sealed against the first major surface of the process fluid module, the first heat exchange fluid enclosure comprising an interior surface for containing heat exchange fluid against the first major surface to form a heat exchange fluid path for the heat exchange fluid, and an inflow port or location for delivering heat exchange fluid to the heat exchange fluid path and an outflow port or location for receiving heat exchange fluid from the heat exchange fluid path, the outflow port or location spaced from the inflow port or location in a first direction, wherein the interior surface of the first heat exchange fluid enclosure comprises two or more raised ridges extending in a second direction at least partially crosswise to the first direction and having a distance between successive ones of the two or more raised ridges and having a gap between the two or more raised ridges and the first major surface; and wherein the gap is in the range of from 0.2 to 0.5 mm and the distance is in the range of from 10 mm to 30 mm. a flow reactor module; the flow reactor module comprising: a process fluid module with a process fluid passage extending therethrough, the process fluid module comprising an extended body having a width, a length, and a thickness, the thickness being less than the length and less than the width, the process fluid module having first and second major surfaces (12, 14) on opposite sides of the process fluid module, oriented perpendicularly to a direction of the thickness of the process fluid module; and a first heat exchange fluid enclosure sealed against the first major surface of the process fluid module, the first heat exchange fluid enclosure comprising an interior surface for containing heat exchange fluid against the first major surface to form a heat exchange fluid path for the heat exchange fluid, and an inflow port or location for delivering heat exchange fluid to the heat exchange fluid path and an outflow port or location for receiving heat exchange fluid from the heat exchange fluid path, the outflow port or location spaced from the inflow port or location in a first direction, wherein the interior surface of the first heat exchange fluid enclosure comprises two or more raised ridges extending in a second direction at least partially crosswise to the first direction and having a distance between successive ones of the two or more raised ridges and having a gap between the two or more raised ridges and the first major surface; and wherein the gap is in the range of from 0.2 to 0.5 mm and the distance is in the range of from 10 mm to 30 mm. a process fluid module with a process fluid passage extending therethrough, the process fluid module comprising an extended body having a width, a length, and a thickness, the thickness being less than the length and less than the width, the process fluid module having first and second major surfaces (12, 14) on opposite sides of the process fluid module, oriented perpendicularly to a direction of the thickness of the process fluid module; and a first heat exchange fluid enclosure sealed against the first major surface of the process fluid module, the first heat exchange fluid enclosure comprising an interior surface for containing heat exchange fluid against the first major surface to form a heat exchange fluid path for the heat exchange fluid, and an inflow port or location for delivering heat exchange fluid to the heat exchange fluid path and an outflow port or location for receiving heat exchange fluid from the heat exchange fluid path, the outflow port or location spaced from the inflow port or location in a first direction, wherein the interior surface of the first heat exchange fluid enclosure comprises two or more raised ridges extending in a second direction at least partially crosswise to the first direction and having a distance between successive ones of the two or more raised ridges and having a gap between the two or more raised ridges and the first major surface; and wherein the gap is in the range of from 0.2 to 0.5 mm and the distance is in the range of from 10 mm to 30 mm.