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Air Flow Rate Calculations

Updated: Jan 28


1- Calculate the Crack Length (which cause the leakage of air flow)

2- Calculate The Leakage Area

Table (1.1) Standard Leakage Area

3- Air Leakage

∆P: difference pressure between elevator and outside space which equal 50 Pa (Maximum 60 Pa). Leakage Area: Total area for all doors.

4- Multiply Air Leakage by Safety Factor (25%)

Elevator Pressurization Calculation Example

One Door (Lift Landing) with 0.9m width (Height as standard is equal 2.1 m)

1- Crack Length



2- The Leakage Area =

3- Air Leakage

4- Take 25% Factor of Safety

Staircase pressurization

-Pressurization System: Control the smoke by pressure difference, to make sure that the smoke will not leak to stairwell.


  • Wind Effect:

  • Smoke Feedback:

  • Design Pressure Difference:

(F MAX= 133N), to calculate the force:

Table (1.2): Pressure design Difference (NFPA 92,2012)

  • Type of Stairwell Pressurization:

“CFM = (300-550) Per floor”

-Multiple Injection (One Fan or more)

- Fan Location: The stairwell pressurization fan can be located anywhere, but since the entry/exit doors at ground level will remain open during emergency for fire personnel entry as well refuge escape operations, the top location is preferred to avoid the short-circuiting of the air.


Flow rate leak by crack of doors and by open doors (if there is any door open), so we will calculate the air flow rate in two steps :

Step one: Calculate Leakage air flow rate We need:
  • Door dimensions (W*L)

  • Door Leakage length (By Excel Sheet)

  • Number of doors

  • By Excel sheet, calculate the total leakage area for doors.

  • Then using 𝑄 = 0.827 𝑥 𝐴 𝑥 𝑉 𝑥 P^(1/2) (m^3 /s) (By Excel Sheet)

Step two: Calculate Leakage air flow from open doors
  • Number of open doors

  • Using 𝑄 = 𝐴 𝑥 𝑉 (m^3 /s) (By Excel Sheet)

Final Step: Total Air flow rate
  • 𝑄 = 𝑄1 + 𝑄2

  • Add 25% factor of safety

Stair Pressurization Calculations :

Stair cases will be provided with pressurization fans to pressurize the stair wells in case of emergency as follows :

Escape Door Dimensions = 2.1 x 1.05 m

Escape Door Leakage Length = ( 2.1 + 1.05 ) x 2 =

= 6.3 m

Total Number of Doors = 10 Doors

So, The total doors actual leakage area =

= 10 x 0.01 x 6.3 ÷ 5.6 = 0.11 m²

So, Q1 = 0.827 x 0.113 x 50 ½ = 0.66 m^3 /s

= 1394 cfm

Assume 3 doors are fully opened during fire mode;

So, Q = 2.205 x 0.75 x 3 = m^3 /s = cfm

Total air flow rate = Q1 + Q2 = 1394 + 10512 = 11906 cfm

Assume 25% Safety Factor :

So, Q = 11906.3 x 1.25 = 14883 cfm


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