"Dry-pipe sprinkler systems are intended for protecting low-temperature environments such as unheated loading docks, cold storage facilities, or any property where the ambient temperature is less than 40 °F (4.4 °C). These systems contain pressurized air or dry nitrogen that is released when the sprinkler system operates."
Coffee Break Training - Fire Protection Series No. FP-2011-17
United States Fire Administration: http://www.usfa.fema.gov/