In a typical Air Shaft Duct Assembly, the smaller duct is inserted through the larger diameter duct. Cold air is blown through the smallest duct, then returned through the larger duct. Beer lines are run through the small duct. The assembly is covered with tube insulation. Aluminum Ducts are semi-rigid, corrugated, and easily bent. UL listed with 1 end crimped for male/female assembly. Corrugated Vinyl Ducts connect easily to ES602A Blower Assembly and ESDAK Underbar Draft Arm Kit. Galvanized Steel Ducts make a strong, flexible housing for simplified installation.