Multi-layer pipe with single corrugation in inner wall

Claim 1

A mutil-layer, spirally wound pipe with a substantially smooth interior wall having a single, helical corrugation in the inner wall and a corrugated exterior wall, comprising a. an inner pipe layer consisting of an elongated sheet of material curled into adjacent, helical convolutions, said inner pipe layer convolutions being joined by a helical seam, b. a single, continuous helical corrugation located in said inner pipe layer having the same helix angle as said helical convolutions, said helical seam being located within said single corrugation on a neutral axis of said single corrugation, c. an outer pipe layer consisting of an elongated sheet of corrugated material curled into adjacent, helical convolutions about said inner pipe layer, said outer pipe layer convolutions being joined by a helical seam, said seam being located on a neutral axis of a corrugation of said outer pipe layer, d. the helix angle of the helical convolutions of said outer pipe layer being substantially the same as the helix angle of said single corrugation, e. said neutral axis of said single corrugation having said helical seam being coextensive with at least a portion of a neutral axis of one of the corrugations of said outer pipe layer and said single corrugation lying within one of the corrugations of the outer pipe layer thereby locating the helical seam of inner pipe layer within said one corrugation of the outer pipe layer, and f. means joining said inner and outer pipe layers.