Cold stores (-25 C freezer, +2 C chiller) use ammonia (NH3) refrigerant. Ammonia is flammable (LFL 15 percent) and toxic (IDLH 300 ppm). Wet-pipe sprinklers freeze; NFPA 13 dry-pipe or pre-action systems are used. IIAR-2 defines machinery room and ventilation rules.
Dry-Pipe Sprinkler Design
- Air pressure: 25-50 psi in pipe
- Water delivery time: max 60 s (system requirement)
- System volume limit: 750 gal (2,840 L)
- Sprinkler: upright or dry pendant
- Low-point drum drips: drain condensate
IIAR-2 Machinery Room
- Separation: 2-hour firewall
- Ventilation: normal 0.5 ACH, emergency 30 ACH
- NH3 detector: 25 ppm alarm, 150 ppm high alarm
- Egress: 2 independent doors
- ESD: external emergency shutdown button
Ammonia Fire/Explosion Risk
- LFL: 15 percent (15,000 ppm)
- UFL: 28 percent
- MIE: 680 mJ (very high)
- Explosion pressure: Pmax 6 bar
- Ignition difficulty: rare - confined space needed
Ventilation Calculation
500 m2 machinery room, 30 ACH emergency:
- Volume: 500 x 4 = 2,000 m3
- Flow: 2,000 x 30 = 60,000 m3/h
- Fan: 2 x 30,000 m3/h (redundancy)
- Exhaust: roof level - hot gas up
- Inlet: outside wall low level, auto damper
Cold storage with MEP Calc
MEP Calc sizes machinery-room ventilation, NH3 detector setpoints and dry-pipe volume per IIAR-2/NFPA 13.
Download MEP Calc on the App Store
Sources & Further Reading
NFPA 13, IIAR-2, IIAR-9, ASHRAE 15, OSHA 29 CFR 1910.111. NFPA 13 Sprinkler standard.