Evacuated Tube Collector Parts

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Much like flat plate collectors, evacuated tube collectors have a solid, durable, insulated frame with one glazed side which contains absorbers. In this case, the absorbers are individual transparent tubes rather than a single, flat plate under a pane of glass.

The glazing of the evacuated tubes is normally either a single or double layer of Pyrex. As always, there's a tradeoff involved:

Single-layer tubes

Using a single layer of glazing means that the sun's energy only has to travel through one solid layer before it starts heating the absorber plate. This is obviously more efficient.

Unfortunately, it also means that the tube itself contains a vacuum and therefore needs a very good, efficient seal at the ends, where the metal and glass meet. The seal must be able to withstand very high temperatures but must also be flexible enough that it can expand and contract without allowing air to enter the tube.

Double-layer tubes

Also called Sydney tubes, Dewar tubes or twin tubes, a double-layer setup includes a second, smaller tube within the larger, outer one. The inner tube either contains or is coated with absorber material and the air is evacuated from between the two tubes to create a vacuum.

The upside of this is that there's no need for such a complicated, expensive seal at the ends, which can extend the life of the tubes... but the price paid is a second layer of glazing between the absorber and the sun's radiation.

Getters

A "getter" is a neat little chemical system that indicates when a tube in your solar thermal setup has failed. It's not used in every type of tube since it adds to the cost, but it can make maintenance a lot simpler.

The standard getter is pure barium. A small amount is inserted into the tube before it is sealed and absorbs any residual gas, turning into a thin layer of silver that coats the bottom of the tube. If the seals on the tube fail and air gets in, the barium absorbs the oxygen and turns white or foggy –giving you a clear signal that the tube's integrity is compromised and it needs replacing.

Racks

As you might expect, evacuated tubes are not an integral part of the box which contains them. They are normally placed on a rack and slotted into connectors, joining them to the manifold. This construction makes it simple to replace failed tubes and also means the rack can be mounted on the roof before the tubes are connected (thus reducing the risk of breakages).

It is important to note that, if you're using evacuated tubes with heat pipes, the rack must be mounted at an angle of at least 25 degrees. This is to ensure that the fluid flows properly when it evaporates and condenses.