1,138,144. Pipe joints. GENERAL ELECTRIC CO. 19 April, 1966, No. 17027/66. Heading F2G. [Also in Division F4] A composite heat exchanger tube, able to withstand corrosive atmospheres and temperatures in the order of 2000‹ F., comprises an inner member 11, preferably metallic, and outer sleeve members 12, 12<SP>1</SP>, preferably ceramic, the inner member having a greater coefficient of expansion than the outer member and being spaced therefrom in the cold state. The outer members have an internal axial taper, with opposite ends abutting one another to form a joint 13. On subjecting the exchanger to external heat both members expand longitudinally and radially to the configuration shown in Fig. 2, wherein the inner member fits tightly against the outer members and a gap 14 is formed between the outer members, the gap, if necessary, being covered by any suitable joint structure, (Figs. 4-9, not shown). On cooling, member 11 is permanently deformed and this together with the taper of members 12 prevents the latter from sliding along member 11 and so ensures equal gaps between the same on subsequent heating. In use the composite exchanger tube may be connected to a header by means of an expansion loop, (54, Fig. 11, not shown), the loop being formed from an unclad extension of the metallic tube 11, or alternatively by means of a slip joint 56 effected between the header 52 and pipe 11, a bellows 55 or a spring, (58, Fig. 13, not shown), being interposed between the cladding and header. In an alternative embodiment, the ceramic cladding may be arranged within the metallic tube, in which case the former has a greater coefficient of expansion than the latter.