What type of tank has its pressure provided by air mechanically compressed against the water surface?

The type of tank that has its pressure provided by air mechanically compressed against the water surface is a hydropneumatic tank. These tanks utilize a combination of air and water, where air is compressed in the upper portion of the tank. This compression creates pressure that pushes the water out when a demand for water occurs.

Hydropneumatic tanks are particularly effective for maintaining a constant pressure in water distribution systems. They help manage fluctuations in water demand and ensure an adequate supply while minimizing the need for additional pumping. The mechanical compression of air allows for a more stable pressure system without the need for continuous pump operation, which can save energy and reduce wear on system components.

In contrast, standpipes primarily rely on gravity or elevated structures to maintain pressure, while storage tanks serve to simply store water with no pressure control incorporated. Elevated tanks utilize height to create pressure through gravity but do not involve mechanical compression of air. Therefore, the hydropneumatic tank is specifically designed to use compressed air to assist in managing pressure above the water surface.