In the case of a dry system, pipes are filled with pressurized air or nitrogen. Water does not flow into the pipes until a dry pipe valve is activated. If there is a fire, the system will fill with water that will flow through the sprinklers and onto the fire. Often dry systems are seen to be safer, since there isn't the risk of a damaged pipe leaking water.
Dry systems are:
Great for unheated warehouses and commercial freezers since there is no water contained in the systems to freeze.
More expensive to install and maintenance Due to these systems being more complex than wet systems, the costs associated with installation and any alterations or repair will be higher.