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This page explains how the Eco-nomical collector tube works.

A double walled glass tube with a vacuum between the walls contains a heat pipe which transfers heat from the tube to the water in the manifold of the panel.  The purpose of the vacuum is to trap heat which has entered the tube as sunlight.  Heat can move about using three mechanisms, conduction, convection and radiation.  Energy enters as radiation from the sun, which can cross the vacuum without difficulty (after all, it has already crossed 150 million km of space!).  The vacuum prevents the heat leaving via conduction or convection, just like a thermos flask.  The inner tube has a special selective coating which absorbs over 93% of the radiation falling on it, effectively preventing radiation losses.

The heat builds up quickly inside the tube, and warms the heat pipe via thin aluminium fins which also hold the heat pipe in the centre of the tube.  The heat pipe is a copper tube that runs inside the glass tube.  It is held in the glass tube using a silicone rubber bung.  Note that no seal is required here, in fact a small hole allows the pressure inside the tube to be relieved as the pipe warms up.  The heat pipe contains a liquid at low pressure which boils at low temperature.  Because it becomes less dense as a gas, it rises up the heat pipe (which explains why the panel must be installed with a minimum slope of 15).  At the top of the heat pipe is a bulb which fits into a copper socket in the manifold.  Because it is cooled by the water flowing through the manifold, the liquid condenses on the inside of the bulb, thereby transferring heat energy to the water flowing through the manifold.  Good thermal contact between the heat pipe bulb and the manifold socket is ensured by the use of thermal transfer paste supplied with the panel.

                                 

Note that the base of the glass tube has a silver coating; this is barium, used to maintain the vacuum between the glass tubes during manufacture (the same technique is used to manufacture CRT TV tubes).  Should the vacuum be lost, if, for example the tube is cracked, this silver coating turns to a white powdery material, provided a simple check on the integrity of the vacuum.

 

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Last modified: 30-06-10