... 3.anti-freezing at -40 4. high strengh and high heat insulation Thi-target vacuum solar tube Thi ... 6mm, Φ=70±0.7mm &=2.0mm Internal pipe diameter and thickness Φ=37±0.7mm &=1.6mm, Φ=47±0.7mm &=1 ... , cold water flows down into the vacuum tube, and there it is heated with solar energy absorbed by ... of heat because of the vacuum between the inner and outer glass tubes, and when cold water becomes hot, hot ...