SMOKE STRATIFICATION

The result of rooftop solar radiation, a hot layer of air often forms under an atrium's ceiling. The temperature of this layer often exceeds 120°F. As previously mentioned, plume temperature decreases as plume height increases. For a 1,800-Btu-per-second fire in an atrium with a floor-to-ceiling height of 60 ft, the ceiling-smoke temperature would be only about 95°F.

When a plume's average temperature is less than a hot-air layer's, smoke will stratify under the hot-air layer, preventing smoke from reaching ceiling-mounted smoke detectors. Projected-beam smoke detectors should be used for applications in which smoke stratification is possible. Three commonly accepted arrangements of projected-beam smoke detectors can be used for atrium smoke control: upward beams that detect the smoke layer, horizontal beams that detect the smoke layer at various levels, and horizontal beams that detect the smoke plume. For these arrangements, the spacing of detectors is critical. Spacing recommendations can be found in the previously mentioned book published by the ICC.°

DESIGN FIRES

Design fires, which can be deemed steady or unsteady, have a major impact on an atrium smoke-control system. Design-fire size is expressed in terms of heat-release rate. Typically ranging from 1,800 to 8,000 Btu per second, design fires should be evaluated as part of a smoke-control system's engineering analysis. For a discussion of the concepts behind design fires, see the HPAC Engineering article “Design Fires: What You Need to Know”⁷ and the conference paper “Determining Design Fires for Design-Level and Extreme Events.”⁸ A detailed treatment of design fires can be found in the previously mentioned book published by the ICC.²

Designers should not make the blunder of thinking that an atrium with almost no materials should have a very small design fire. This kind of thinking does not account for changes in space use or transient fuels. Transient fuels are materials that reside in a space temporarily, such as holiday decorations, paint and solvents used for redecorating, cardboard boxes awaiting removal, and upholstered furniture. Transient fuels must not be overlooked when analyzing design-fire size.

PLUGHOLING

FIGURE 4: Plugholing can result in system failure.

Plugholing occurs when air below is pulled through a smoke layer and into smoke exhaust (Figure 4). Plugholing lowers the smoke-layer interface and can expose people to smoke. Lowering the interface can result in system failure; however, plugholing can be prevented by keeping flow relatively low at each smoke-exhaust inlet. To avoid plugholing, the maximum flow rate at smoke-exhaust inlets must be calculated correctly and the number of inlets chosen carefully.

CFD MODELING

CFD modeling divides a space into a large number of cells and solves the governing equations for each. (Governing equations are nonlinear, partial differential equations for conservation of mass, momentum, and energy.) Atrium applications can have 100,000 to 1 million cells. Obstructions, such as walls, balconies, and stairs, should be considered. Boundary conditions, including smoke exhaust and makeup air, should be defined.

Atrium smoke-control designs based on the conventional algebraic-equation approach tend to be conservative and have exhaust flow rates that are somewhat high. Conversely, CFD modeling can be the basis for exceptions for the requirements to smoke-layer depth, the 200-fpm limitation on makeup air, and plugholing.

CFD modeling provides a high degree of confidence that a tenable environment will be preserved. CFD modeling's strength is that it can simulate fire-induced smoke flows, which algebraic equations cannot.

There are some good general-purpose CFD models, but the NIST's Fire Dynamics Simulator (FDS)⁹ is for fire applications. While the annual fee to use a commercial CFD model can be tens of thousands of dollars, the FDS and its associated documents can be downloaded for free at www.fire.nist.gov/fds/downloads.html. For a non-mathematical discussion of CFD modeling, see the previously mentioned HPAC Engineering article. The previously mentioned book published by the ICC includes a detailed introduction to CFD modeling.³

REFERENCES

1. Klote, J.H. (2006, June). CFD: A new way to design atrium smoke control. HPAC Engineering, pp. 19-27.

2. Klote, J.H., & Evans, D.H. (2007). A guide to smoke control in the 2006 IBC. Country Club Hills, IL: International Code Council.

3. International Code Council. (2006). 2006 International Building Code. Country Club Hills, IL: International Code Council.

4. NFPA. (2005). NFPA 92B: Standard for smoke management systems in malls, atria, and large areas. Quincy, MA: National Fire Protection Association.

5. Klote, J.H., & Milke, J.A. (2002). Principles of smoke management. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

6. Peacock, R.D., Jones, W.W., Reneke, P.A., & Forney, G.P. (2005). NIST special publication 1041: CFAST — Consolidated model of fire growth and smoke transport (version 6): User's guide. Gaithersburg, MD: National Institute of Standards and Technology.

7. Klote, J.H. (2002, September). Design fires: What you need to know. HPAC Engineering, pp. 43-51.

8. Bukowski, R.W. (2006, June). Determining design fires for design-level and extreme events. Paper presented at the 6th International Conference on Performance-Based Codes and Fire Safety Design Methods, Tokyo, Japan.

9. McGrattan, K. (2004). NIST special publication 1018: Fire dynamics simulator (version 4): Technical reference guide. Gaithersburg, MD: National Institute of Standards and Technology.

For past HPAC Engineering feature articles, visit www.hpac.com.

Known worldwide as a smoke-control expert, John H. Klote, DSc, PE, is a consulting engineer based in Leesburg, Va. Formerly, he conducted fire research for the National Institute of Standards and Technology.