Expansion tank

As the heat transfer fluid increases in temperature its volume also increases thus an expansion tank is used to capture the addition fluid as it is pushed out of the piping loop. The expansion tank also helps prevent the loss of heat transfer fluid that would escape the piping system through the safety valves during high temperature periods. When the heat transfer cools, fluid in the expansion tank can then flow back into the system.

Most expansion tanks are a steel tank with a rubber membrane inside. On one side of the membrane is a gas such as nitrogen, which is under a pressure equal to the system operating pressure. On the other side of the membrane is the space where the extra heat transfer fluid goes when expansion of the fluid occurs. The membrane material will begin to deteriorate when exposed to temperatures above 160 °F (71 °C). To protect the expansion tank from such high temperatures, they should be connected to the piping that goes to the collector. This will be where the coolest fluid will be found. If temperatures exceeding 160 °F (71 °C) are expected in this pipe then a buffer tank or auxiliary reserve tank should be placed between piping loop and the expansion tank. This is a normal steel tank filled with heat transfer fluid. Since it is not circulating in the collector piping system loop, it will be somewhat cooler. When expansion occurs, the extra hot fluid will enter the auxiliary reserve tank causing the residing cooler fluid then to enter the expansion tank protecting its membrane.

The membrane expansion vessel may need to be designed to compensate not only the additional volume of heated up fluid, but also the volume of steam that is produced by the collector field. That can be more than just the volume of the collectors. Water in pipes above the collector field, for example, can drain into the collectors after they are already empty and then start to produce even more steam.