Dry-Pipe / Preaction Air Compressor CFM

The compressor free-air CFM to restore a dry-pipe or preaction system's normal air pressure within the NFPA 13 time limit (30 minutes standard): free air = (system gal / 7.48) x (normal psig / 14.7) / restore minutes. A 400-gal dry system to 40 psi in 30 min needs about 4.85 CFM; a 750-gal system 9.1 CFM -- spec the next larger unit. A listed automatic air-maintenance device (not a shop compressor) is required, and a corrosion-mitigating air or nitrogen source is preferred. The system volume comes from the pipe schedule, the pressure is set to hold the clapper closed with margin, and NFPA 13 / the AHJ set the restore time. A sizing estimate; the compressor's rating at the pressure and the AHJ govern.

system_ft3 = dry_volume_gal / 7.48; free_air_cfm = system_ft3 x (normal_pressure_psig / 14.7) / restore_minutes.

NFPA 13 (Standard for the Installation of Sprinkler Systems) dry-pipe / preaction air-restoration time limit (about 30 minutes), by name; the compressor rating and the AHJ govern.

The free-air relation is public first-principles (system volume x gauge-to-atmospheric ratio over the restore time); the system volume comes from the pipe schedule and the pressure and restore time from NFPA 13 and the AHJ.

Estimate. AHJ and licensed professional govern.

Field names used by the API: dry_volume_gal, normal_pressure_psig, restore_minutes, system_ft3, free_air_cfm

• Restore time NFPA 13: restore normal air pressure within ~30 min (60 min allowed for some systems)NFPA 13
• Listed device a listed automatic air-maintenance device is required, not a shop compressor; nitrogen mitigates corrosionNFPA 13 / practice