What is the role of a captive air tank, also known as an expansion or bladder tank?

A captive air tank, or expansion tank, is primarily designed to maintain minimum system pressure within a water system. In typical applications, these tanks accommodate the thermal expansion of water as it heats up, which occurs in closed water systems. When water heats, it tries to expand, and without a mechanism to accommodate this expansion, pressure within the system can rise excessively, potentially leading to system failure or leaks.

The tank contains a flexible bladder that moves as water enters and exits, allowing it to absorb the pressure changes caused by temperature fluctuations. This mechanism stabilizes system pressure, helping to ensure consistent water flow and preventing issues such as water hammer, which can be both damaging and noisy.

While the other options may represent functions related to water systems, they do not accurately describe the specific role of a captive air tank. Purifying wastewater or increasing water temperature are not functions associated with this type of tank. Also, while storage of excess water can be a characteristic of different types of tanks, captive air tanks specifically focus on pressure management rather than general water storage.