Cold Storage Ammonia and Fire: NFPA 13 Dry-Pipe, IIAR-2 and Machinery Room

Cold stores (-25 °C freezers, +2 °C chillers) mostly use ammonia (NH₃) as refrigerant. Ammonia is both flammable (LFL 15%) and toxic (IDLH 300 ppm). Wet-pipe sprinklers cannot be used in spaces below freezing, so NFPA 13 dry-pipe or preaction systems are installed (NFPA 13-2025, 8.8). Refrigerated spaces maintained above 0 °C are outside that section (8.8.1) and can use wet-pipe systems. IIAR-2 defines machinery room and ventilation rules.

Dry-Pipe Sprinkler Design

• Air pressure: 25–50 psi (1.7–3.4 bar) in pipe
• Water delivery time: max 60 s with a single-orifice test connection (8.2.4.2); by calculation, Table 8.2.4.3.2 applies: high-piled storage, 4 most remote sprinklers open, max 40 s
• System volume: up to 500 gal (1,900 L) without a quick-opening device, or up to 750 gal (2,850 L) with one, no specific water delivery requirement (8.2.4.2.1, 8.2.4.2.2); larger systems must demonstrate delivery time by test or calculation
• Sprinkler: upright or dry pendent
• Low-point drum drips: drain condensate regularly

IIAR-2 Machinery Room

• Separation: 2-hour fire-rated wall
• Ventilation: normal 0.5 ACH, emergency 30 ACH
• NH₃ detector: 25 ppm alarm, 150 ppm high alarm
• Egress: 2 independent doors
• ESD: emergency shutdown button accessible from outside the room

Ammonia Fire/Explosion Risk

• LFL: 15% (150,000 ppm)
• UFL: 28%
• MIE: 680 mJ (very high)
• Explosion pressure: Pmax about 6 bar
• Ignition difficulty: most leaks do not ignite; gas must accumulate in a confined space

Ventilation Calculation

500 m² machinery room, 30 ACH emergency ventilation:

• Volume: 500 × 4 = 2,000 m³
• Flow: 2,000 × 30 = 60,000 m³/h
• Fans: 2 × 30,000 m³/h (redundancy)
• Exhaust: at roof level — NH₃ is lighter than air and rises
• Inlet: outside wall, low level, motorized damper

Cold storage with MEP Calc

MEP Calc sizes machinery-room ventilation, NH3 detector setpoints and dry-pipe volume per IIAR-2/NFPA 13.

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SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report. One of the few tools that unifies all three major fire protection standards.

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