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Why are automatic drain valves in solar heating installations always so problematic, and what should their characteristics be? |
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As the automatic drain valves are installed at the highest point of the installation (beside the collectors), they need to withstand very high temperatures. If standard drain valves were installed, they would not normally resist the temperatures they are exposed to. The Technical Building Code stipulates that drain valves must withstand 150ºC in the hottest areas of Spain and 130ºC in cooler areas. As described below, during normal functioning the automatic drain valves are isolated from the circuit and so they are heated up by transmission from the collectors (not by direct contact with the liquid at this temperature).
They must ALWAYS be installed together with a shut-off valve. This prevents the installation from being drained when the temperature vaporises the liquid.
Automatic drain valves must also be resistant to the liquids used in these systems, usually ethylene glycol.
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How is natural circulation produced in the circuit? |
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Natural circulation is the name given to the flow that occurs when there is a temperature difference between two points in the circuit. In a circuit, the hot water tends to rise while the cold water tends to fall. The reason for this is that the density of the hot water is lower than that of the cold water.
In solar heating installations, this effect may occur when the accumulator is hot and the collectors are cold. The liquid in the solar heating circuit is warmed in the heat exchanger and rises to the collectors. Part of the power obtained from solar radiation is therefore lost.
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