Dry systems and pre-action systems are both types of fire sprinkler systems, but they operate differently.
A dry system is a fire sprinkler system that uses pressurized air or nitrogen instead of water in the pipes until a fire is detected. When a fire is detected, the air or nitrogen is released, allowing water to flow through the pipes and extinguish the fire. Dry systems are typically used in buildings with temperatures that may fall below freezing, as water in the pipes could freeze and damage the system. Dry systems are also used in areas with high humidity, such as parking garages and loading docks, to prevent the accumulation of moisture in the pipes.
On the other hand, a pre-action system is a fire sprinkler system that combines elements of a dry and a wet system. In a pre-action system, the pipes are filled with pressurized air or nitrogen, similar to a dry system. However, in a pre-action system, the water is held back by an electrically operated valve. The valve is only activated when a fire is detected, allowing water to flow through the pipes and extinguish the fire. Pre-action systems are typically used in areas where water damage must be minimized, such as computer rooms, libraries, and museums.
In summary, while both dry systems and pre-action systems use pressurized air or nitrogen to prevent water from flowing through the pipes until a fire is detected, the main difference is that a pre-action system uses an electrically operated valve to hold back the water, while a dry system relies on the release of the pressurized air or nitrogen to allow water to flow through the pipes