Insulated double-walled exhaust pipe

Claim 1

An improved double-walled exhaust pipe including an inner pipe, an outer pipe surrounding the inner pipe to produce a uniform annular gap therebetween, and an annular flange attached to each end of the inner and outer pipes, wherein the improvement comprises: filling the annular gap between the inner pipe, the outer pipe and the annular flanges with low-density, high-temperature resistant, inorganic, fired, hollow, ceramic spheroids comprising A. a continuous phase made of a material selected from the group consisting of aluminum phosphate, sodium silicate (Na 2 Ox 1 SiO 2 ) in which x 1 is the molar ratio of SiO 2 to Na 2 O and is between about 2.75 and 3.65 and potassium silicate (K 2 Ox 2 SiO 2 ) in which x 2 is the molar ratio of SiO 2 to K 2 O and is between about 2.5 and 4; and B. an insolubilizing agent, selected from the group consisting of kaolin clay, iron oxide, titanium dioxide, alumina trihydrate an zinc oxide, which combines with the continuous phase during firing to make the continuous phase insoluble in water; said hollow, ceramic spheroids having a cellular shell, in which the shell material contains a multiplicity of irregular, non-spherical, hollow cells.