Abstract: A two-piece outer casing encloses a volume with a central divider plate which extends the length of the casing. The two portions of the casing are bolted or otherwise clamped about the divider plate. Each half of the outer casing has angled inner ribs or diverters. The diverters in one casing extend at an opposite angle to the longitudinal axis of the central plate as do the diverters of the other outer casing. The inner edges of the diverter seal against the surface of the central plate. The transverse dimension of the central plate is less than the transverse dimension of the diverters and the outer casing such that an exchange aperture is defined between the central plate and the inner surface of the outer casing between adjacent pairs of diverters. The casing has an exhaust input port and an exhaust outlet port at opposite ends and opposite sides.