insulation combination of the flange 55 the spirally rolled foil 50 and the casing 54 is moved in the direction indicated by arrow 57 until the insulation abuts a radially disposed annular flange 60 that provides a transverse termination for the thermal insulation that is associated with the conduit 41 with the thermal insulations and casings for the conduits 40 and 41 in the abutting relation that is shown in fig2 a circumferential strip 61 is wrapped around the outer casing 54 and an outer casing 62 for the conduits 40 and 41 respectively to bridge across the plane of contact between the two conduit insulations preferably the circumferential strip 61 is spot welded or otherwise fastened to the casings in order to provide a secure joint that tends to reduce heat flow through the plane of contact between the two abutting insulations turning once more to fig1 the ells 20 and 21 are manufactured in a manner that is similar to that which is described in connection with the stack segment 12 the angular relation between the individual portions of the ells however is provided by cutting a cylindrical stack segment at appropriate angles to the segment 39 s longitudinal axis in order to produce the angular surfaces that are needed for miter joints 63 64 65 and 66 as hereinbefore mentioned thermal expansion and contraction of the exhaust stack structure is accommodated through the illustrative expansion joints 16 and 17 the construction of a typical expansion joint.