Wet Systems: In wet systems, the overhead sprinkler piping connected is filled with water under pressure. When a fire causes one or more of the connected sprinklers to activate, water immediately discharges from the sprinkler head.
Wet pipe systems are the most common type of fire sprinkler system and the simplest. Because of their simplicity, they are very reliable and require less installation and maintenance. These are usually the first choice in fire sprinkler systems because they’re easy to work with, modify, and are often more reliable.
However, these systems cannot be installed in environments were the temperatures reach below 40 degrees F. And of course there can be severe property damage if there’s a leak or other malfunctions in the piping, especially if the area the sprinklers are protecting has sensitive equipment or documents.
Dry Systems: These do not have water in the direct non-heated sprinkler piping, but only pressurized air or nitrogen. The pressurized air holds back the water supply at a main dry-pipe valve. When a fire occurs and the sprinkler activates, the air pressure drops and the main dry-pipe valve opens to release the water into the piping system to discharge from the sprinklers.
These fire sprinkler systems are thus designed for environments with freezing temperatures like walk-in refrigerators, loading docks, and warehouses. Because of its special air pressure needs and other components, dry pipe sprinklers are more complex than wet fire sprinklers and are thus more costly to install and maintain.
Because water must travel from the valve to the piping and sprinklers, there can be a delay of up to 60 seconds in fire suppression. Dry fire sprinkler systems also have certain design restrictions as to their individual system size and thus can make additions to the system very difficult or not possible. These systems can, however, be an addition to a larger wet pipe sprinkler system and just used in the environments that see freezing temperatures.