Dry-Pipe / Preaction Air Compressor CFM
Compressor free-air CFM to restore a dry-pipe system's air pressure within the NFPA 13 limit.
Example
You enter
- Dry system volume (gal, from pipe schedule) 400
- Normal air pressure (psig) 40
- Restore time (min, NFPA 13 <= 30) 30
You get
- System volume 53.5 ft3
- Compressor free-air CFM (spec next size up) 4.85
Checked against published worked examples from: The Viking Corporation.
Details, formula, and sources
ScopeThe 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 or tablesystem_ft3 = dry_volume_gal / 7.48; free_air_cfm = system_ft3 x (normal_pressure_psig / 14.7) / restore_minutes.
Source and editionNFPA 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.
Free accessThe 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.
What governsEstimate. AHJ and licensed professional govern.
API field namesdry_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