Abstract: A trough-type scale trap for use with a refinery or chemical plant reactor having a catalyst bed subject to plugging by fine particles. Compared to prior art traps, the trough-type scale trap of the invention is characterized by a larger volumetric capacity and screen area and is substantially easier to construct and to install or remove from closed reactor vessels than conventional traps. Its typical embodiment is as a series of parallel troughs having rectangular openings and disposed chordally across the reactor normal to the direction of flow. In one embodiment, the trough is a rectangular parallelepiped; in another embodiment, the substantially rectangular opening is reduced by tapering to a bottom significantly smaller than the inlet opening.