What type of sprinkler system contains air under pressure?

A Dry Pipe system is characterized by the presence of air under pressure. In this type of system, the sprinkler pipes are filled with pressurized air instead of water. When a sprinkler head activates due to heat, the air pressure in the pipes is released and allows water to flow into the system from a water supply, extinguishing or controlling the fire.

This system is particularly beneficial in environments where the temperature might drop low enough to cause water in the pipes to freeze, making it ideal for unheated buildings or those subject to freezing temperatures. By keeping the pipes filled with air, the system also prevents the water from sitting in the pipes, which maintains normal operating conditions and minimizes the risk of freezing.

In contrast, Wet Pipe systems are filled with water at all times, while Pre Action systems have both dry and wet components that require some form of activation (like an alarm) before water is allowed to flow, and Deluge systems have open sprinkler heads that release large quantities of water simultaneously, often used in high-hazard areas. Understanding these differences clarifies why the Dry Pipe system is the correct answer.