Calculating the size of the Smoke Exhaust Fan (for exhausting smoke)

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Calculating the size of the Smoke Exhaust Fan (for exhausting smoke)

In a hall with a height of 20 meters, it is necessary to consider safety standards.

In designing smoke ventilation systems such as NFPA 92, BS 7346-4, or MYP.

of Thailand, which generally uses the following principles:

🔥 1. Know the types of Smoke Exhaust System.
 • Smoke Control: Have a smoke layer high enough for people to escape the fire.
 • Smoke Extraction: Quickly sucks out all smoke.
 • Halls 20 meters high often use the Smoke Layering system – emphasizing that the smoke floats at the top. It doesn't reach the people below.


2. Calculate the smoke exhaust rate (Smoke Exhaust Airflow Rate).

General case (simple calculation)

Using general principles from BS or NFPA standards:

Q = A × v

 • Q = amount of air to be sucked (m³/s)
 • A = horizontal area of the hall (m²) (i.e. area 20 x 40 = 800 sq m)
 • v = velocity of smoke extraction at smoke layer level (recommended ≥ 0.5–1.0 m/s)

Example:
 • Hall area: 20 x 40 = 800 m²
 • Requires fume extraction speed 1.0 m/s.
 • So
➜ Q = 800 × 1.0 = 800 m³/s

Convert to CMH (cubic meter per hour):

800 m³/s × 3600 = 2,880,000 CMH


3. Consider Volume Method (if you want to drain everything)

Q = Volume / Time

 • Volume = Hall volume = Width x Length x Height = 20 x 40 x 20 = 16,000 m³
 • To drain in 10 minutes (600 seconds):

Q = 16,000 / 600 = 26.7 m³/s = ~96,000 CMH

(Suitable for all smoke ventilation systems in a rush)

4. Additional factors in choosing a fan
 • Fire resistance temperature: The standard usually uses a fan that can withstand heat 250°C or 300°C for 2 hours.
 • Back pressure in the system (Static Pressure): may be between 500–1500 Pa.
 • Number of fans: Must be selected to suit the smoke flow direction. and zoning
 • Redundancy (backup fans) for critical systems.

✅ Preliminary summary

If the hall is 20 meters high:
 • Use Smoke Layering ➜ Calculate at 0.5–1.0 m/s at the smoke layer (~2.5–3 meters above)
 • If hall 20x40 m² ➜ requires 800–1600 m³/s or 2,880,000–5,760,000 CMH
 • Axial or Centrifugal fans that are heat resistant should be used.