... -glass double-tube co-axial structure Glass material High borosilicate 3.3 glass External pipe diam eter ... .7mm &=1.6mm, Φ=58±0.7mm &=1.6mm Pipe length 800mm, 1200mm, 1500mm, 1600mm, 1800mm, 1900mm, 2000mm ... absorbed by the coating outside of inner vacuum tube, the solar will be absorbed efficiently by ... water again flows into solar vacuum tube for another heating process. heat pipe vaccum tube evacuated