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.
Formula and source
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.
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