vacuum tube for solar water heaters

Max strength:0.8mpa

Heat loss<0.8 w

Stagnation temperature>200 degree

Vacuum pressure<0.005pa

Emittance<8%(80c)

Absorptive coating graded AL-N/AL

Material:borosilicate glass 3.3

Thermal expansion:3.3*10-6 C

Glass thickness:1.6mm

Inner tube diameter:47mm

Outer tube diameter:58mm

Length(normal):1500mm,1800mm

Evacuated tube basic specifications:

In order to maintain the vacuum between the two glass layer, a barium getter is used(the same as in television tubes). During manufacture of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO,CO2,O2,H2O,H2,N2,out-gassed from the evacuated tube during storage and operation, thus helping to maintain the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine wheather or not a tube is in good condition.

Why a vacuum? As you know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat. We dont want to lose it! The vacuum helps to achieve this. The insulation properties are so good that while the inside of the tube may be 150 degree, the outer tube is cold to touch. This means that evacuated tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss(during high delta-T condition).

The inner tube is coated with a special selective coating(AL-N/AL) which features excellent solar radiation absorption and minimal reflection properties. The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperature. This evacuation of the gas forms a vacuum which is an important factor in the performance of the evacuated tubes.

Each evacuated tube consists of two glass tubes made of extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection.

Vacuum tube

high heat absoption, anti-freezing,anti-corrosion