What does the calculator do?
It uses the NFPA 2001-2025 total flooding equations to find the clean agent quantity that brings an enclosure to the design concentration. Halocarbons (HFC-227ea, FK-5-1-12) are sized by mass with Equation 7.3.1; inert gases (IG-541, IG-55, IG-100) by gas volume at 21 °C (70 °F) with Equation 7.3.2. The same quantity is then checked at the maximum design temperature with 7.3.1.1 / 7.3.2.3 and compared with the personnel safety limits.
| Agent type | Equation | Clause |
|---|---|---|
| Halocarbon | W = (V/s) · C/(100 − C) | 7.3.1 |
| Inert gas | X = 2.303 · (s₀/s) · log₁₀(100/(100 − C)) | 7.3.2 |
| Inert gas mass | W = (V/s) · ln(100/(100 − C)) | 7.3.2.2 |
V is the net volume: gross volume minus fixed structures impervious to the agent. s is the specific volume of the agent vapour at 1 atm, taken at the minimum anticipated temperature, so a colder room needs more agent for the same concentration. s₀ is the specific volume of the inert gas at 21 °C and 1.013 bar. The s values are the s = k1 + k2·t approximations printed under the Annex A flooding tables (Tables A.7.3.1(b), (j); A.7.3.2(d), (f), (h)).
Where does the design concentration come from?
The Class A minimum extinguishing concentration (MEC) is found by test in a listing programme (7.2.2.1.1; UL 2166 for halocarbons, UL 2127 for inert gases, 7.2.2.3). The minimum design concentration then uses these safety factors:
- Class A: MEC × 1.2 (automatic detection and actuation) or × 1.3 (manual-only actuation), and not less than the n-heptane extinguishing concentration (7.2.2.1.2).
- Class B: the greater of the listed or cup-burner value for the fuel × 1.3 (7.2.2.2.1, 7.2.2.2.3).
- Class C: Class A MEC × 1.35 (7.2.2.4.1).
The defaults in the calculator are the informative Annex A values of Tables A.7.2.2.3(b) and A.7.2.2.2.1. For a real design, enter the listed value of your system.
| Agent | Class A MEC | Class A design | Class C design | n-heptane MEC |
|---|---|---|---|---|
| HFC-227ea | 5.2 % | 6.7 % | 7.0 % | 6.62 % |
| FK-5-1-12 | 3.3 % | 4.5 % | 4.5 % | 4.5 % |
| IG-541 | 28.5 % | 34.2 % | 38.5 % | 34.4 % |
| IG-55 | 31.6 % | 37.9 % | 42.7 % | 36.2 % |
| IG-100 | 31.0 % | 37.2 % | 41.9 % | 32.6 % |
Personnel safety check
For halocarbons the concentration is compared with the NOAEL and LOAEL of Table 4.3.2.4(a): HFC-227ea NOAEL 9.0 %, LOAEL 10.5 %; FK-5-1-12 NOAEL 10.0 %, LOAEL >10.0 %. Normally occupied spaces may be designed up to the NOAEL; above it only where the PBPK table allows 5 minutes of exposure (4.3.2.4(1)–(3)). For HFC-227ea that is Table 4.3.2.4(c): 5 min up to 10.5 %, 1.13 min at 11.0 %.
For inert gases the limit is oxygen depletion (4.3.3.5): below 43 % with exposure limited to 5 min, 43–52 % to 3 min; 52–62 % only in normally unoccupied spaces with exposure under 30 s; above 62 % only where personnel are not exposed.
Times and altitude
Discharge time is at most 10 s for halocarbons (7.5.1.1) and, for inert gases, 60 s for Class B and 120 s for Class A and C hazards (7.5.1.2). After discharge, at least 85 % of the minimum design concentration must be held at the highest height of protected content for 10 minutes or long enough for trained personnel to respond (7.4.1). Where pressure differs from sea level by more than 11 % (about 3,000 ft / 915 m), the quantity is corrected with the Table 7.3.3.3 factor (7.3.3.3), for example 0.82 at 5,000 ft.
Not included: the piping tee design factor (Table 7.3.3.1), additional design factors such as unclosable openings and re-ignition (7.3.3.2), and the pipe flow calculation (Section 6.2). The designer assigns and documents these separately.
Frequently Asked Questions
At what temperature is the clean agent quantity calculated?
At the minimum anticipated temperature: NFPA 2001 7.3.1 and 7.3.2 take the agent specific volume (s) at the minimum design temperature of the enclosure. The concentration the same quantity produces at the maximum temperature is also calculated and recorded (7.3.1.1 / 7.3.2.3).
Can the default concentrations be used for design?
No, they are informative. The defaults come from NFPA 2001 Annex A Tables A.7.2.2.3(b) and A.7.2.2.2.1; a design uses the value from the system's UL 2166 or UL 2127 listing (7.2.2.1.1, 7.2.2.3).
Up to what concentration can HFC-227ea be used in a normally occupied space?
Up to the NOAEL of 9.0 % directly (4.3.2.4(1)). Up to 10.5 % Table 4.3.2.4(c) allows 5 minutes, so it is permitted if exposure is limited by that table (4.3.2.4(2)); above that it is not permitted in normally occupied spaces (4.3.2.4(3)).
Why is the inert gas discharge time 60 or 120 seconds?
NFPA 2001 7.5.1.2 limits inert gas discharge to 60 s for Class B fuel hazards and 120 s for Class A surface-fire and Class C hazards. For halocarbons the limit is 10 s (7.5.1.1).
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