Sheet Metal Connectors, Inc. has a complete line of Double Wall Spiral Pipe and compatible fittings. Double Wall Spiral Pipe is fabricatedfrom a coil of metal into a rigid steel tube with a 4-ply spiral lockseam, which has a smooth interior for low friction loss with the grooved seam entirely on the outside. Inner and outer pipes are manufactured separately, then the inner pipe is wrapped and stuffed into the outer. Inner pipe is available with perforated or solid inner material. All sizes shall be listed using inside dimensions.