In short
- Protect the whole volume: room plus raised floor and ceiling voids if they are in the same enclosure.
- Chemical agents: W = V ÷ S · C ÷ (100 − C), in kg.
- Inert gases: X = 2.303 · (Vs ÷ S) · log₁₀(100 ÷ (100 − C)), in m³ of agent per m³ of room.
- S, the specific vapour volume, depends on the minimum room temperature; altitude reduces the quantity above sea level.
- A 912 m³ hall needs about 466 m³ of IG-541 (21 cylinders of 23 m³) or about 710 kg of FK-5-1-12.
Protected volume
Multiply length by width by the full height that the gas will fill: room height plus the raised floor void plus the ceiling void, if they are not separated by a sealed barrier. Volumes of solid, impermeable structures can be deducted; IT racks normally are not.
Design concentration
The design concentration is the agent's extinguishing concentration for the fuel class plus a safety factor, taken from the listed system's design manual. Typical Class A surface-fire values:
| Agent | Design C | NOAEL | GWP | Discharge |
|---|---|---|---|---|
| FK-5-1-12 | 5.3 % | 10 % | < 1 | ≤ 10 s |
| HFC-227ea | 7.9 % | 9 % | 3,220 | ≤ 10 s |
| IG-541 | 39.9 % | 43 % | 0 | ≤ 60 s |
| IG-55 / IG-100 | 40.3 % | 43 % | 0 | ≤ 60 s |
| IG-01 | 41.9 % | 43 % | 0 | ≤ 60 s |
The gap between design concentration and NOAEL (no observed adverse effect level) is the safety margin for anyone still in the room. All values are editable in the agent calculator.
Chemical agents (halocarbons)
For FK-5-1-12, k₁ = 0.0664 and k₂ = 0.000274, so at 20 °C S = 0.0719 m³/kg. A lower minimum temperature gives a smaller S and therefore more agent; use the coldest temperature the room will see.
Inert gases
Inert gas is stored as compressed gas, so the quantity is counted in cylinders. With 23 m³ of agent per cylinder (a typical 80 l, 300 bar bottle), the cylinder count is the agent volume divided by 23, rounded up.
Altitude correction
Air is thinner at altitude, so less agent reaches the same concentration. The standards give a correction factor: 1.00 at sea level, about 0.93 at 610 m, 0.86 at 1,220 m and 0.78 at 1,830 m. Multiply the quantity by the factor.
The 912 m³ hall at 20 °C, sea level
| Agent | Quantity | Per m³ | Oxygen after |
|---|---|---|---|
| IG-541 at 39.9 % | 466 m³ · 21 cylinders | 0.51 m³ | 12.6 % |
| IG-55 at 40.3 % | 472 m³ · 21 cylinders | 0.52 m³ | 12.5 % |
| FK-5-1-12 at 5.3 % | ≈ 710 kg | 0.78 kg | ≈ 19.8 % |
| HFC-227ea at 7.9 % (existing systems) | ≈ 570 kg | 0.63 kg | ≈ 19.2 % |
The inert option needs about 21 large cylinders, the chemical option a handful of containers. That is the space trade-off in one line.
What the quantity does not cover
- Room integrity: a door fan test must show the concentration holds for about 10 minutes above the protected height.
- Pressure relief: inert gas raises room pressure during discharge; chemical agents can first pull it negative. Vents are sized to the wall strength.
- Pipe network and nozzles: flow calculations from the listed manufacturer's software decide pipe sizes and nozzle positions.
- Multiple rooms: one bank with selector valves can serve several rooms; it is sized for the largest one.
Questions about agent quantity
Why use the minimum temperature?
Gas is denser when cold, so a given mass fills less volume. Designing at the coldest expected temperature makes sure the concentration is reached on the worst day.
Do I include the raised floor?
Yes, if the floor void is open to the room or has its own nozzles. Each void that is part of the enclosure adds to the protected volume.
How much FK-5-1-12 per cubic metre?
About 0.7–0.8 kg/m³ at a 5.3 % design concentration around 20 °C. The exact figure depends on temperature and the listed design concentration.
How many inert gas cylinders do I need?
Roughly one 23 m³ cylinder per 45 m³ of room for IG-541 at 39.9 %, before altitude correction and the manufacturer's flow limits.
Sources
- NFPA 2001; ISO 14520; EN 15004: total-flooding equations, k₁ / k₂ values and altitude factors.
- Design concentrations are typical Class A values; the listed system's design manual and the AHJ decide.
- Worked figures are screening estimates, not a design.