Smoke management system design for large spaces

In large-volume spaces such as atria and covered malls, smoke management systems are often an important aspect of fire protection, with their primary goal being to ensure that  the impact of smoke and heat on occupants is not life threatening This involves keeping the height of the smoke layer above the highest level of occupancy for a defined period, longer than the expected time to evacuate the building.

Smoke Control Vs Smoke Management

Smoke control

systems use fans to pressurize appropriate areas to limit smoke movement in a fire situation.

Smoke management

 systems include pressurization and all other methods that can be used singly or in combination to modify smoke movement.

Smoke Management Approaches

The following approaches can be used to manage smoke in atriums:

Smoke filling:  This approach allows smoke to fill the atrium space while occupants evacuate the atrium. It applies only to spaces

where the smoke filling time is sufficient for both decision making and evacuation. Nelson and Mowrer (2002), Chapter 4 of  Klote and Milke (2002), and Proulx (2002) have information on people movement during the evacuation. The filling time can be estimated either by zone fire models or by filling equation

Unsteady clear height with upper layer exhaust: This approach exhausts smoke from the top of the atrium at a rate such that occupants have sufficient time for decision making and evacuation. It requires analysis of people movement and fire model analysis of smoke filling. Steady clear height with upper layer exhaust:  This approach exhausts smoke from the top of the atrium to achieve a steady clear height for a steady fire. our calculation method is presented here based on this approach.  

Steady clear height with upper layer exhaust

Definitions

Atrium: A large-volume space created by a floor opening or series of floor openings connecting  two or more stories that is covered at the top of the series of openings and is used for purposes other than an enclosed stairway ; an elevator hoist way; an escalator    opening; or as a utility shaft used for plumbing, electrical, air-conditioning,  or communications facilities.  

 

 

Clear height: Is the distance from the top of the fuel to the interface between the “clear” space and the smoke layer. Also called design height.

Clear height

 

Design Fires: The design fire has a major effect on the atrium smoke management system. Fire size is expressed in terms of rate of heat release.

Steady fires have a constant heat release rate.

Steady design fire sizes for atriums

‐ Minimum fire for fuel‐restricted atrium              = 2000  kW

‐ Minimum fire for atrium with combustibles       = 5000   kW

‐ Large fires                                                                   = 25000 kW

ASHRAE Handbook 2007

52 ‐ Table (2)

Plume:   A column of smoke that rises above a fire

Axisymmetric Plume: A plume that rises above a fire, does not come into contact with the wall or other obstacles, and is not disrupted or deflected by airflow.

Smoke Layer: The accumulated thickness of smoke below a physical or thermal barrier.

 

Plugholing The condition where air from below the smoke layer is pulled through  the smoke layer into the smoke exhaust  due to a high exhaust  rate. 

Makeup Air:  Makeup air has to be provided to ensure that the exhaust fans are able to move the design air quantities and to ensure that door opening force requirements are not exceeded.

The large openings to the outside can consist of open doors, open windows, and open vents.   The large openings to the outside do not include cracks in the construction, gaps around closed doors, gaps around closed windows, and other small paths. It is recommended that make up air is designed at 85 percent to 95 percent of the exhaust not including the leakage through these small paths.

The maximum value of 200 ft/min (1.02 m/sec) for makeup air is to prevent significant plume deflection and disruption of the smoke interface.         

 Calculations  Procedures: this calculation is based on steady fires and axisymmetric plume assumptions to calculate smoke exhaust flow rate from the top of the atrium to achieve a steady clear high.

1-Qc = ξ Q

where :

Qc: convective proration of the heat release rate (KW).

ξ: convective fraction of heat  0.7 is often used.

Q:   heat release rate (KW) based on the fire size.

ASHRAE Handbook 2007

  • 52 ‐ Equation (24)  

Steady clear height with upper layer exhaust

2-       Z f = 0.166 Q c^(2/5)    NFPA92B ‐ [6.2.1.1a(1)]

Zf is a mean flame height (m)

3a-   M’ = (0.071Qc ^1/3 * Z^5/3)+0.0018Qc  NFPA92B ‐ [6.2.1.1b(1)]                                                               

use this equation when Z > Zf

M’: Mass flow rate in plume at height z (kg/s)

3b-  M’ = 0.032Qc^3/5* Z  NFPA92B ‐ [6.2.1.1C(1)]                                                                

use this equation when Z > Zf

M’: Mass flow rate in plume at height z (kg/s)                                          

4- Tp= Ta + (Qc/ M’ Cp)  NFPA92B ‐ (6.2.5)

Where

Tp : Average temperature

 at elevation z (°C)

Ta: Ambient temperature (°C)

Qc: Convective portion of heat

release rate (kW)

M: Mass flow rate of the

plume at the elevation z (kg/s)

Cp: Specific heat of plume

 gases (1.0 kJ/kg.°C)

7-  Vmax =4.16 γd^(5/2) *((Ts-To)/To)^1/          NFPA92B ‐ (6.3.3a)

Where

Vmax = maximum volumetric flow rate without plug holing at Ts, m3/s

γ = exhaust location factor, dimensionless value is 1 or 0.5

Di = Diameter of the exhaust inlet  Di = (2 a* b)/a+b  

d = depth of smoke layer below lowest point of exhaust inlet, m

Ts = absolute temperature of smoke layer, K    

To = absolute ambient temperature, K    

8- S min = 0.9 Ve ^1/2   NFPA92B ‐ (6.3.9a)

Where

Smin = minimum edge-to-edge separation between inlets, m

Ve = volumetric flow rate of one exhaust inlet, m3/s

9-  M’ make up = 0.85 M’ exhaust

 

References

  • NFPA 92B Guide for Smoke Management System In Malls,Atria, and Large Areas.
  • ASHRAE APPLICATIONS 2007, CH 52, Fire and Smoke Management.
  • Principles of Smoke Management by John Klote and James Milke, CH14, ATRIUM SYSTEM.

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