... approved, Pressurized solar thermal collector with copper heat pipe inside of vacuum tubes. Specification:Header tube: seamless copper TU1, Ø22mm×1.2mm ... 40~50mm thicknessVacumm tube: tri-elements coating, copper heat pipe inside, Ø50mm×1.8mCopper heat pipe: 0.6mm thicknessLogo: OEM stickerVacuum tube: 15/20/ ... water in the tank is gradually heated. Evacuated Tube Heat Pipe Solar Heating Collector