A reactor pipe 150 which receives reductant, eg urea solution, from an injector into a flow of exhaust from an engine comprises an inlet portion, a plurality of flow directing louvers 350, (410, 420, fig.4), an outlet portion 310, and a body 230 which curves to form an exterior radial loop which directs the exhaust flow radially around an in-line path between eg a DPF 130 and an SCR catalyst 160. The radial loop may be helical or non-helical. The radial loop reduces the axial length of the decomposition chamber and also directs the flow of the exhaust toward the outlet portion and reduces the velocity of the flow of the exhaust such that the reductant has an increased amount of time to react with the exhaust. The reactor pipe 150 may have a port 360 for a diesel exhaust fluid injector. The reactor pipe may also comprise a number of perforated plates 320 eg to distribute the reductant uniformly over the face of the catalyst. The reactor pipe 150 may have pinhole openings (1100, fig.10) allowing some exhaust to exit the reactor pipe and a structure (1200) to promote mixing.
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