Triple Vacuum Evacuated Glass Tube

Related product: Triple Vacuum Evacuated Glass Tube

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Just like an automobile engine, the computer CPU, vacuum tube is the core component of the solar water heater. Its performance lies on the actual film coating technology.

Vacuum tubes are the absorber of the solar water heater. They absorb solar energy converting it into heat for use in water heating. Vacuum tubes have already been used for years in Germany, Canada, China and the UK. There are several types of evacuated tubes in use in the solar industry. Our collectors use the Triple vacuum tubes. This type of tube is chosen for its reliability, performance and low manufacturing cost.

Each vacuum tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. 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 temperatures. This "evacuation" of the gasses forms a vacuum, which is an important factor in the performance of the vacuum tubes.

Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the vacuum tube absorbs the radiation from the sun and converts it to heat, we don’t 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 150oC / 304oF , the outer tube is cold to touch. This means that vacuum tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-T conditions).

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

Vacuum tubes are aligned in parallel, the angle of mounting depends upon the latitude of your location. In a North South orientation the tubes can passively track heat from the sun all day. In an East West orientation they can track the sun all year round.

The efficiency of a vacuum water heater is dependent upon a number of factors, one important one being the level of vacuum radiation (insolation) in your region. To learn more about insolation and the average values for your area click here.