Fire Dynamics and its Role in Sprinkler Activation
By John Swanson
Firefighters during their initial training learn about the concepts of conduction, convection, and radiation and how fire and heat spread in buildings. But it is also important for the sprinkler designer, layout technician, installer, and sprinkler plan reviewer to have a basic understanding of fire dynamics. The reason is, there is a direct correlation between fire dynamics and sprinkler placement and performance.
Before we can discuss why fire dynamics is critical to sprinkler activation, we need to first define what it is. Fire dynamics is the study of how chemistry, fire science and heat transfer influence fire behavior. To put it in simpler terms, fire dynamics studies how fires start, spread, and develop. Why is this important? Since sprinklers are heat actuated, it is important for professionals who design, install, and review sprinkler systems to understand how heat transfer occurs in buildings.
You may have heard the term, heat release rate. Heat release rate (HRR) is the rate at which fire releases energy. NFPA 13 indirectly addresses heat release rate by assigning commodity classifications for storage occupancies. The commodity classification then determines the protection and density requirements. Class I commodities have a lower HRR than Class II commodities. Class III commodities have a lower HRR than Group A plastics, for example. A straightforward way of explaining heat release rate is to use the candle scenario.
One candle will produce approximately 80 Watts of energy and burn at a temperature anywhere between 930°F to 2500°F. Ten candles, on the other hand, burn at roughly the same temperature, but the heat release rate is 10 times greater. The heat release rate for the ten candles jumps to approximately 800 Watts. To better understand this example, hold your hand a few inches over one burning candle and then hold your hand a few inches above ten candles. You will quickly realize the difference between temperature and heat release rate.
Why is this important for professionals who design, install or review sprinkler systems? Heat release rate directly correlates to how quickly fire will spread to adjoining commodities, combustibles, or impact the structure as a whole. This is why NFPA 13 is concerned about certain types of commodities and why it is so important to apply the commodity classification criteria correctly.
Next, let’s review the three different forms of heat transfer.
Conduction: Heat transfer within solids or between contacting solids.
Convection: Heat transfer by the movement of liquids or gases.
Radiation: Heat transfer by electromagnetic waves.
Of the three types of heat transfer, which do you think has the biggest impact on sprinkler operation?
Studies have shown convection, or heat transfer through the movement of liquids and gases, has the greatest impact on sprinkler activation. NFPA 13 has specific installation criteria for sprinklers in relation to the ceiling based on obstructed or unobstructed construction. NFPA 13 defines obstructed construction as, “… construction where beams, trusses, or other members impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire”. Composite wood joist construction, for example, is considered obstructed construction (NFPA 13 Section A.184.108.40.206 #3). The channels created by the composite wood joists “impede heat flow” which impacts the sprinklers ability to activate and control (or suppress) a fire.
When a fire occurs, the fire plume will carry the heated air and products of combustion vertically toward the ceiling. When the fire plume reaches the ceiling, the hot gases transition to a horizontal flow over the ceiling. The horizontal travel of the hot gases is referred to as a ceiling jet. The ceiling jet is critical for sprinkler activation. The more obstructions in place that prevent or slow the hot gases (convection) from reaching the actuating element of the sprinkler, the slower the response time. This is why NFPA 13 is concerned about certain types of ceiling configurations that can impact the ceiling jet, which impacts heat flow, which affects overall sprinkler performance.
In summary, firefighters need to understand the three types of heat transfer because it directly impacts firefighting tactics and strategy. But we also know the presence of sprinklers and the placement of those sprinklers directly correlates to whether it can control the fire. Which is why it is important for the sprinkler designer, layout technician, installer, and sprinkler plan reviewer to understand the basic concepts of fire dynamics and the impact it has on sprinkler effectiveness.
More about the author
John Swanson currently serves as NFSA’s Codes and Standards Specialist. In this role he provides training and education and represents NFSA on codes and standards technical committees. He currently serves as a Principal member of the NFPA 72 – National Fire Alarm and Signaling Code Technical Committee and is a past member of the International Building Code (IBC) Fire Safety Committee and International Fire Code (IFC) Interpretation Committee. From 2013-2017, John served as a fire service representative appointed by Minnesota Governor, Mark Dayton to the Minnesota Board of Architecture and Engineering.