... vacuum tube length:47/1500mm,58/1800.2,vacuum tubes:15pcs,18pcs,20pcs,24pcs,30pcs3,Outer tank material: ... has 15years long life span ,and the vacuum tubes can work individually,so if the only one of ... principle:1,The high quality vacuum tubes ansorb the solar ultraviolet ray;then heat the cold water in the vacuum ... full,it will close the valve automatically copper heat pipe for solar collector