What is the benefit of the air that is trapped in the compression tank when the system is being filled?

The trapped air in the compression tank plays a crucial role in accommodating the natural expansion of water as it heats up. Water expands when heated, and without a proper system in place to handle this expansion, pressure could build up excessively within the piping system. The air cushion provided by the compression tank allows for this expansion to occur safely, absorbing the pressure changes and preventing potential damage to the system.

In contrast, filtering impurities, maintaining constant temperature, or preventing corrosion involves processes that are not directly related to the function of the air in the compression tank. The primary feature in this context is the air's ability to act as a buffer against pressure changes due to thermal expansion, which is why the correct answer is centered around the cushion effect and its role in allowing water expansion.