Not too long ago, a mechanical engineer could size the smoke management system for an atrium by simply calculating the volume of the atrium. The building codes were prescriptive when determining the capacity of smoke management systems, and there was never a need to consult with another consultant or specialist.

Today, a more complex set of equations and methodology is mandated by both building and fire codes to determine the design fire size, its resultant smoke generation capacity, and the corresponding size and operation of a building's smoke management equipment.

The need for a collaborative effort in the design of life safety systems was outlined in the first part of this series. "Rx for Life Safety Design: Collaboration, Part One," which appeared in the November 2005 issue of Engineered Systems (page 34), explained why a growing number of projects require additional design team members to facilitate a full understanding of the complex issues related to fire and smoke control design. For example, a specialist in code consulting is frequently needed to present code equivalencies to the AHJ and get "buy-in" to the proposed fire/smoke control approach to the project. The architect and other team members often have so many of their own design issues and tasks that the code issues become too casual and are overlooked. A code specialist is mandatory on certain projects such as arenas, theaters, large shopping malls, or hospitals.

Complexity of building systems justifies and mandates the need for a team approach rather than each design professional operating on his own. The architect, mechanical engineer, electrical engineer, code consultant, AHJ, and other specialists must function as a team in order to design the most efficient life safety systems for a given facility.

Overall geometry of a small atrium showing a 2-MW fire. (Used by permission from RWDI, Inc.)

The Coming of Age of Fire Sciences

The need for collaboration in the design of complex smoke management systems is increasingly apparent in regard to smoke management in atria and other large spaces. (Throughout this article, reference to atria or atrium implies not just atria, but other large spaces such as shopping malls, arenas, convention halls, indoor stadiums, etc.)

Pick up any of the updates from ASHRAE and NFPA regarding smoke management systems, and it becomes obvious that fire sciences has come of age. During the last decade, extensive research and development has been conducted to predict smoke characteristics and migration.

Previous building codes required 40,000 cfm or a set number of ach based on the volume of the atrium for the smoke exhaust capacity in an atrium. Now, NFPA 92B, "Guide for Smoke Management Systems in Malls, Atria, and Large Areas," as well as model building codes offer engineers extensive algebraic equations and other methodology to determine smoke generation capacity in large areas. This has created a need for architects and engineers to be knowledgeable about smoke and its characteristics during a fire event. While this makes these areas safer for building occupants, it requires additional design focus and understanding of the smoke management system.

For example, in NFPA 92B there are numerous equations for determining the mass flow rate for smoke production. These equations are dependent upon whether the fire is being modeled in a clear open space or whether there are conditions in the space such as vertical walls, corners, balconies, or windows. Knowing which equation to apply for a particular building may be challenging for an HVAC engineer whose primary role is determining what size chiller to put in the building.

With the promulgation of NFPA 92B, the concept of a design fire was introduced to the HVAC engineer and the entire design team. The design fire is expressed in terms of rate of heat release in Btu/sec. Fuel loading in the atrium will determine the design fire size. Again, this is a new and challenging concept to an HVAC engineer, so collaboration with a code consultant, CFD specialist, and the AHJ is warranted.

Design fires are categorized as either steady or unsteady, and calculations and NFPA 92B address both design fire types. Determining the fuel load is the challenge for the design team in order to determine the design fire.

Many publications, including NFPA 92B, suggest the minimum design fire size be 1,000 Btu/sec, even if there is minimal fuel in the space. A 2- to 5-MW fire size is typical in most atria. Fire sizes as large as 10 to 20 MW may be warranted in convention centers, arenas, and indoor stadiums.

The NFPA 92B calculations will result in significantly larger exhaust volumes being required to remove the smoke from the atrium. For example, the NFPA 92 equations on a six- to eight-story atrium would generate a smoke production capacity of roughly 600,000 cfm.

The rate of smoke production for an axis-symmetric plume where the smoke interface layer is above the flame limiting height is given by the equation:

m = 0.022Qc1/3 z5/3 + 0.0042 Qc

Where m is the mass flow rate (lb/sec) in the smoke plume at height z for a fire with convective heat output of Qc.

For a fire with a total heat release of 5,000 Btu/sec (5,270 kW), located on the floor of an arena and the smoke layer interface at 100 ft above the floor, the mass and volumetric flow rates are:

m = 0.022 (0.7 x 5,000)1/3 (100)5/3 + 0.00042 (0.7 x 5,000) = 700 lb/sec

Qc = 60 x 700/0.075 = 560,000 scfm

Obviously, this is larger than the previous code requirement for capacity of only 40,000 cfm. A smoke exhaust and the required makeup air (usually 70% of the exhaust capacity) systems will add significant cost to the project. Sometimes the cost can be so prohibitive that it may result in omitting the atrium from the project. But it is realized and accepted in the industry that the old 40,000 cfm or ach-based criteria is inadequate for life safety of the building occupants.

"A design based on a number of air changes per hour is not appropriate for a smoke management system in an atrium or other large space," according to ASHRAE's Design of Smoke Management Systems by John H. Klote and James A. Milke. "Providing a fixed air change rate does not have a consistent impact on the hazard development due to the following factors:

  • Variety of possible smoke management goals
  • Atrium shape and size
  • Function or use of atrium
  • Location of communicating space
  • Means of separation between communicating spaces and atrium.

These factors influence the feasibility and selection of appropriate smoke management approaches as well as the specific design parameters."

Using a logical, systematic design process is likely to help reduce the confusion that can develop as a result of considering the impact of many factors. Four objectives in designing a smoke management system for an atrium include:

  • Establish goals and objectives
  • Identify an effective means of actuation
  • Determine the design requirements
  • Consider the details of design

The Need for CFD Analysis

Unfortunately, NFPA 92B has caused some engineers to unnecessarily oversize atrium smoke exhaust systems. It's not just about the equations, said Ray Sinclair, Ph.D., principal with Ontario, Canada- based Rowan Williams Davies & Irwin, Inc. (RWDI). It's about under-standing the physics of smoke migration in a building.

Sinclair, a recognized expert and leader in building physics and CFD analysis, said you may plug in the numbers, but not have a true understanding of what they mean. It's more than just a code issue. There are practical issues behind the physics.

"The justification for using a CFD specialist would be to analyze the atrium using CFD modeling. In our experience, we have often found reductions in smoke exhaust system capacity of two to six times the NFPA 92B code equations without compromising the life safety of the building occupants," Sinclair said.

Atrium smoke design starts with architectural considerations, not the capacity of the smoke exhaust system. If the architectural elements of the building are conducive to exiting the building occupants in a timely manner, the exhaust system capacity can be significantly reduced.

While the NFPA 92B equations generate extremely high smoke exhaust volumes, an integrated solution which focuses both on architectural elements and occupant exiting times, as well as design of the exhaust and makeup air systems, can produce significant construction cost savings for the project. This can only be done by using a CFD specialist (not just a focus on prescriptive code analysis) to generate the CFD model.

CFD modeling is not just a black box concept. There are excellent CFD tools available to both engineers and code consultants, such as "Fire Dynamics Simulation" (FDS) by N.I.S.T., but without a practical knowledge of the CFD techniques, the results can be costly to the project.

There should always be calibration and validation of the model, according to Sinclair. Generally, it is not the core business of engineers and code consultants to have the broad expertise in CFD modeling necessary to properly and consistently calibrate and validate a CFD model.

Collaboration with architects, HVAC engineers, code consultants, and CFD specialists will produce more effective design solutions. While this is the better approach, it is frequently not used because it results in higher design fees. The owner of the facility should encourage the collaborative approach to produce more efficient designs even though the upfront cost will be higher.

To further illustrate the importance of having a CFD specialist as a member of the design team, Sinclair said there are a number of is-sues overlooked by some CFD users. These issues include:

  • Appropriateness of the soot generation rate for the design fire
  • Suitable prediction of visibility in egress routes
  • Incorporation of activation of detection systems and consideration of the effects of sprinklers and deployable smoke barriers
  • Incorporation of potentially adverse effects that the normal ventilation system can have prior to detection and shutdown of the HVAC sys-tems
  • Lighting systems and signage
  • Sprinklers
  • Effects of exhaust re-entrainment of smoke back into makeup air intakes
  • Effects of temperature of makeup air
  • Wind effects on building facades.

Testing - Agree Early

Collaboration is mandatory to determine the testing procedures of a smoke management system in an atrium. Chapter 8 and Appendix A of NFPA 92B outline procedures for testing smoke management systems.

"It is recommended that the building owner, designer, and AHJ meet during the planning stage of the project to share their thoughts and objectives concerning the smoke management system contemplated and agree on the design criteria and the pass/fail performance tests for the systems," according to NFPA 92B A.8.11. "Such an agreement helps to overcome the numerous problems that occur during final acceptance testing and facilitates obtaining the certificate of occupancy."

Other systems that will be affected by the operation of the smoke management system include the fire alarm system, energy management system, HVAC system, standby power, elevators, automatic operators on doors, and dampers. The need for collaboration and coordination with these systems is required to ensure completely functional smoke management systems.

More Than A Catchy Phrase

"Rx for Life Safety Design: Collaboration." This is more than just a catchy phrase. It's a reminder that complex systems require the input, advice, and expertise of a variety of professionals. As stated throughout this article and part one of this series, the architect and design engineers cannot effectively proceed in the design of life safety systems without the advice of other professionals.

Owners will be required to pay additional design fees in order to assemble a team of professionals with a wide array of experience who can successfully address the many issues of life safety design. There are a variety of tools available to design professionals, but misuse of these tools can result in oversized or improperly designed life safety systems.

Collaboration by the most qualified design professionals will almost always result in a more efficient design, provide maximum benefit to the building occupants, and reduce initial costs. With a solid collaborative effort, which involves an AHJ, system expectations will be defined early in the design process. This will eliminate costly system modifications throughout the construction of the project. ES

For More Information:

Klote, John. H. and James A. Milke, Design of Smoke Management Systems, ASHRAE, Atlanta.

Phillips, Duncan A., Ph.D., and Ray Sinclair, Ph.D., Reducing Exhaust Quantities for Atrium Smoke Control., Building and Fire Research Laboratory.