Posts Tagged ‘fire behavior’

My last post, Hazards Above, provided a brief overview of three incidents involving extreme fire behavior in the attic or truss loft void spaces of wood frame dwellings. This post will examine the similarities and differences between these lessons and identify several important considerations when dealing with fires occurring in or extending to void spaces. At the conclusion of Hazards Above, I posed five questions:

1. What is similar about these incidents and what is different?
2. Based on the limited information currently available, what phenomena do you think occurred in each of the cases? What leads you to this conclusion?
3. What indicators might have pointed to the potential for extreme fire behavior in each of these incidents?
4. How might building construction have influenced fire dynamics and potential for extreme fire behavior in these incidents?
5. What hazards are presented by fires in attics/truss lofts and what tactics may be safe and effective to mitigate those hazards?

Similarities and Differences

The most obvious similarities between these incidents was that the buildings were of wood frame construction, the fire involved or extended to an attic or truss loft void space, and that some type of extreme fire behavior occurred. In two of the incidents firefighters were seriously injured, while in the other firefighters escaped unharmed.

Given the limited information available from news reports and photos taken after the occurrence of the extreme fire behavior events, it is not possible to definitively identify what types of phenomena were involved in these three incidents. However, it is interesting to speculate and consider what conditions and phenomena could have been involved. It might be useful to examine each of these incidents individually and then to return to examine fire behavior indicators, construction, and hazards presented by these types of incidents.

Minneapolis, MN

In the Minneapolis incident the fire occurred in an older home with legacy construction and relatively small void spaces behind the knee walls and above the ceiling on Floor 3. The triggering event for the occurrence of extreme fire behavior is reported to be opening one of the knee walls on Floor 3. As illustrated in Figure 1, the fire appeared to transition quickly to a growth stage fire (evidenced by the dark smoke and bi-directional air track from the windows on Floor 3 Side A. However blast effects on the structure are not visible in the photo and were not reported.

Figure 1. Minneapolis MN Incident: Conditions on Side A

Note: Photo by Steve Skar

Potential Influencing Factors: While detail on this specific incident is limited, it is likely that the fire burning behind the knee wall was ventilation controlled and increased ventilation resulting from opening the void space resulted in an increase in heat release rate (HRR). Potential exists for any compartment fire that progresses beyond the incipient stage to become ventilation controlled. This is particularly true when the fire is burning in a void space.

Extreme Fire Behavior: While statements by the fire department indicate that opening the knee wall resulted in occurrence of flashover, this is only one possibility. As discussed in The Hazard of Ventilation Controlled Fires and Fuel and Ventilation, increasing ventilation to a ventilation controlled fire will result in increased HRR. Increased HRR can result in a backdraft (if sufficient concentration of gas phase fuel is present), a vent induced flashover, or simply fire gas ignition (such as rollover or a flash fire) without transition to a fully developed fire.

Harrisonburg, VA

The Harrisonburg incident involved extreme fire behavior in Exposure D (not the original fire unit). The extreme fire behavior occurred after members had opened the ceiling to check for extension. However, this may or may not have been the precipitating event. As illustrated in Figure 2, as members prepare to exit from the windows on Floor 3 , Side C, flames are visible on the exterior at the gable, but it appears that combustion is limited to the vinyl siding and soffit covering. There are no indicators of a significant fire in Exposure D at the time that the photo was taken. However, it is important to remember that this is a snapshot of conditions at one point in time from a single perspective.

Figure 2. Harrisonburg, VA Incident: Conditions on Side C

Note: Photo by Allen Litten

Potential Influencing Factors: The truss loft was likely divided between units by a 1 hour fire separation (generally constructed of gypsum board over the wood trusses). While providing a limited barrier to fire and smoke spread, it does not generally provide a complete barrier and smoke infiltration is likely. Sufficient smoke accumulation remote from the original fire location can present risk of a smoke explosion (see NIOSH Report 98-03 regarding a smoke explosion in Durango, Colorado restaurant). Alternately, fire extension into the truss loft above an exposure unit can result in ventilation controlled fire conditions, resulting in increased HRR if the void is opened (from above or below).

Extreme Fire Behavior: Smoke, air track, and flame indicators on Side C indicate that the fire in the truss loft may not have continued to develop past the initial ignition of accumulated smoke (fuel). It is possible that smoke accumulated in the truss loft above Exposure B and was ignited by subsequent extension from the fire unit. Depending on the fuel (smoke)/air mixture when flames extended into the space above Exposure B ignition could have resulted in a smoke explosion or a less violent fire gas ignition such as a flash fire.

Sandwich, MA

In the Sandwich incident, the extreme fire behavior occurred shortly after the hose team applied water to the soffit. However, this may or may not have been the precipitating event. As illustrated in Figure 3, the fire transitioned to a fully developed fire (likely due to the delay in suppression as the injured members were cared for). Blast effects on the structure are obvious.

Figure 3: Sandwich, MA: Conditions on Sides C and D

Note: Photos by Britt Crosby (http://www.capecodfd.com)

Potential Influencing Factors: The roof support system in this home appears to have been constructed of larger dimensional lumber (rather than lightweight truss construction). In addition, it is likely that the attic void spaces involved in this incident were large and complex (given the size of the dwelling and complex roof line). It appears that at least part of the home had a cathedral ceiling. Fire burning in the wood framing around the metal chimney would have allowed smoke (fuel) and hot gases to collect in the attic void in advance of fire extension.

Extreme Fire Behavior: The violence of the explosion (see blast damage to the roof on Side D in Figure 3) points to the potential for ignition of pre-mixed fuel (smoke) and air, resulting in a smoke explosion. However, it is also possible that failure of an interior ceiling (due to water or steam production from water applied through the soffit) could have increased ventilation to a ventilation controlled fire burning in the attic, resulting in a backdraft).

Fire Behavior Indicators

The information provided in news reports points to limited indication of potential for extreme fire behavior. One important question for each of us is how we can recognize this potential, even when indicators are subtle or even absent.

Important! A growth stage fire can present significant smoke and air track indicators, with increasing thickness (optical density), darkening color, and increasing velocity of smoke discharge. However, as discussed in The Hazard of Ventilation Controlled Fires, when the fire becomes ventilation controlled, indicators can diminish to the point where the fire appears to be in the incipient stage. This change in smoke and air track indicators was consistently observed during the full-scale fire tests of the influence of ventilation on fires in single-family homes conducted by UL earlier this year.

Even with an opening into another compartment or to the exterior of the building, a compartment fire can become ventilation controlled. Consider building factors including potential for fire and smoke extension into void spaces in assessing fire conditions and potential for extreme fire behavior. A ventilation controlled fire or flammable mixture of smoke and air may be present in a void space with limited indication from the exterior or even when working inside the structure.

Building Construction

Each of these incidents occurred in a wood frame structure. However, the construction in each case was somewhat different.

In Minneapolis, the house was likely balloon frame construction with full dimension lumber. As with many other structures with a “half-story”, the space under the pitched roof is framed out with knee walls to provide finished space. This design is not unique to legacy construction and may also be found with room-in-attic trusses. The void space behind the knee wall provides a significant avenue for fire spread. When involved in fire, opening this void space can quickly change fire conditions on the top floor as air reaches the (likely ventilation controlled) fire.

The incident in Harrisonburg involved a fire in a townhouse with the extreme fire behavior phenomena occurring in an exposure. While not reported, it is extremely likely that the roof support system was comprised of lightweight wood trusses. In addition, there was a reverse gable (possibly on Sides A and C) that provided an additional void. As previously indicated, the truss loft between dwelling units is typically separated by a one-hour rated draft stop. Unlike a fire wall, draft stops do not penetrate the roof and may be compromised by penetrations (after final, pre-occupancy inspection). Installed to code, draft stops slow fire spread, but may not fully stop the spread of smoke (fuel) into the truss lofts above exposures.

Firefighters in Sandwich were faced with a fire in an extremely large, wood frame dwelling. While the roof appeared to be supported by large dimensional lumber, it is likely that there were large void spaces as a result of the complex roofline. In addition, the framed out space around the metal chimney provided an avenue for fire and smoke spread from the lower level of the home to the attic void space.

Hazards and Tactics

Forewarned is forearmed! Awareness of the potential for rapid fire development when opening void spaces is critical. Given this threat, do not open the void unless you have a hoseline in hand (not just nearby).

Indirect attack can be an effective tactic for fires in void spaces. This can be accomplished by making a limited opening and applying water from a combination nozzle or using a piercing nozzle (which further limits introduction of air into the void).

If there are hot gases overhead, cool them before pulling the ceiling or opening walls when fire may be in void spaces. Pulses of water fog not only cool the hot gases, but also act as thermal ballast; reducing the potential for ignition should flames extend from the void when it is opened.

Lastly, react immediately and appropriately when faced with worsening fire conditions. Review my previous posts on Battle Drill (Part 1, Part 2, and Part 3). An immediate tactical withdrawal under the protection of a hoseline is generally safer than emergency window egress (particularly when ladders have not yet been placed to the window).

Ed Hartin, MS, EFO, MIFireE, CFO

It has been a number of months since the last Reading the Fire post. It is essential to continue the process of deliberate practice in order to continue to improve and refine skill in Reading the Fire.

As we start the New Year it is a good time to reaffirm our commitment to mastering our craft. Developing and maintaining proficiency in reading the Fire using the B-SAHF (Building, Smoke, Air Track, Heat, and Flame) organizing scheme for fire behavior indicators, requires practice. This post provides an opportunity to exercise your skills using a video segment shot during a residential fire.

Residential Fire

In mid-January 2010, the Gary, Indiana Fire Department was dispatched to a residential fire on Massachusetts Street at East 24^th Avenue, on arrival Battalion 4 advised of a working fire in a 2 story dwelling. While the first arriving engine was laying a supply line from a nearby hydrant, the first in truck forced entry.

Download and the B-SAHF Worksheet.

Watch the first 35 seconds (0:35) of the video. This segment was shot from Side A. �First, describe what you observe in terms of the Building, Smoke, Air Track, Heat, and Flame Indicators; then answer the following five standard questions?

1. What additional information would you like to have? How could you obtain it?
2. What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
3. What burning regime is the fire in (fuel controlled or ventilation controlled)?
4. What conditions would you expect to find inside this building? If presented with persons reported (as the first arriving companies were) how would you assess potential for victim survival?
5. How would you expect the fire to develop over the next two to three minutes

Now watch the remainder of the video clip and answer the following questions:

1. Did fire conditions progress as you anticipated?
2. A voice heard in the video states that this was a backdraft. Do you agree? Why or why not?

It is likely that the first in truck company in this incident made entry to search for occupants and to locate the fire. Regardless of your perspective on search with or without a hoseline, this video clip provides lessons.

• It is essential to read the fire, recognize the stage(s) of fire development and burning regime(s) in the involved compartments.
• In addition to reading current conditions, anticipate likely fire development and potential for extreme fire behavior.
• Making entry (and leaving the door fully open) creates a ventilation opening (inlet, exhaust, or both). Recognize the potential influence of changes to the ventilation profile on fire behavior.
• To borrow a phrase from a number of National Institute for Occupational Safety and Health Death in the Line of Duty reports; �Ventilation and fire attack must be closely coordinated�. One key element in this coordination is that charged lines must be in place before completion of ventilation openings. This is critical when dealing with a ventilation controlled fire.

Ed Hartin, MS, EFO, MIFIreE, CFO

Firefighters are often provided with an oversimplified explanation of fundamental scientific concepts related to fire behavior. This is done with the intent to make training manuals and texts understandable and to focus on the information that firefighters �must know�. However, a tremendous opportunity to develop the ability to make sense of fire dynamics and the impact of tactical operations is lost in the process. This series of posts continues to explore ways of building a scaffold to allow firefighters to develop a deeper understanding of firefighting as science.

Electromagnetic Radiation

The term radiation is used to describe many different things ranging from visible light, infrared light, and ionizing radiation such as x or gamma rays. Each of these is an example of radiation as an electromagnetic wave produced by the motion of electrically charged particles. Electromagnetic radiation can travel through empty space and air. Radiation can also penetrate through other materials depending on the characteristics of the material and the radiation�s energy. Some ionizing radiation is in the form of particles (rather than waves), but that is outside the scope of our examination of radiation as a mechanism of heat transfer.

As illustrated in Figure 1, electromagnetic waves can be described in terms of their wavelength, amplitude, frequency, and energy.

Most of the electromagnetic spectrum cannot be detected by the human eye. While the electromagnetic spectrum includes radiation in a broad range of wavelengths, those of most interest in the study of fire behavior are categorized as infrared

Figure 1. Electromagnetic Wave

From longest to shortest wavelengths, the spectrum is usually divided into the following sections: radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma-ray radiation. Humans can only see a narrow band of visible light, which is a small fraction of the electromagnetic spectrum. We perceive this radiation as the colors of the rainbow ranging from red to violet, with reds having longer wavelengths �and violet having shorter wavelengths

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object’s temperature. Any material that is above absolute zero gives off some radiant energy. Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation.

Figure 2. Electromagnetic Spectrum

Figure 3. Planck’s Curve

Thermal radiation occurs at a wide range of frequencies. However, as illustrated in Figure 2, the power emitted at each wavelength is dependent on temperature, with the main frequency and power of emitted radiation increasing as temperature increases. This can be observed when color changes from red, to yellow, and then white as an object is heated. While color change is visible, most of the radiant energy is still in the infrared spectrum.

Electromagnetic waves of any frequency will heat surfaces that absorb them. However, temperatures of hot surfaces, gases, and flames in the fire environment result in emission of electromagnetic waves predominantly in the infrared and visible portion of the spectrum.

Stefan-Boltzmann Law: The amount of energy per square meter per second that is emitted by a black body is related to the fourth power of its Kelvin temperature. As temperature increases, emission of radiant energy increases exponentially.

A black body is a theoretical object that completely absorbs all incoming radiant energy and is also a perfect emitter of radiant energy. Materials encountered in the fire environment do not completely have the characteristics of a black body and may be classified as gray bodies. A gray body absorbs or emits a portion of the radiative flux depending on the emissivity (?).

Emissivity is the relative ability of the surface of a material to emit radiant energy. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. Emmisivity of a black body would be 1.0 with the emissivity of actual materials ranging from approximately 0.1 for highly reflective materials (e.g., polished silver) to 0.97 for fairly efficient absorbers and emitters of radiant energy (e.g., carbon particulate).

Emission of radiant energy is measured as heat flux (energy transfer per unit of time over a given surface area). The SI unit of measure is joules/second/square meter or (since a watt is a J/s) watts/square meter (w/m²).

Figure 4. Stefan Boltzmann Law

Electromagnetic radiation spreads out as it moves away from its source. As a result, the intensity of the radiation decreases as distance from the source becomes greater (as illustrated in Figure 5). The simplest example involves a point source of radiation (distance from the source is much greater than the size (e.g. surface area) of the emitter). With a point source, reduction in radiation intensity follows the inverse square law.

Figure 5. Radiation Intensity Decreases With Distance

Inverse Square Law: For point sources, intensity of the radiation varies inversely with the square of the distance from the source. Doubling the distance reduces intensity of the radiation by a factor of four (1/4 of its original value).

When radiation is emitted from other than a point source (as it is under fire conditions), variation of the radiation intensity with distance is more complex. If the area of the source is large compared with the distances involved, intensity decreases with distance but does not follow a simple law. As a rough guide, if the distance from the source is greater than about 5 times the dimensions of the source, the inverse square law can be applied.

More to Follow

Subsequent posts in this series will examine physical and chemical changes and the process of combustion.

Ed Hartin, MS, EFO, MIFireE, CFO

Things to Think About

Methods of heat transfer are often presented to firefighters in a simplistic way with the expectation that they will understand the basic concepts and are assessed on their ability to recall the definitions of conduction, convection, and radiation. Unfortunately this does not provide a solid basis for understanding phenomena encountered on the fireground.

Convection

In general terms, convection refers to movement of molecules within fluids (i.e., liquids and gases). Convection results in both heat and mass transfer (these are interrelated as extensive properties such as thermal energy are dependent on mass). Convection involves diffusion due to random movement of individual molecules (Brownian motion) and large scale motion of currents in the fluid (advection).

Natural Convection

Natural or free convection results from temperature differences within a fluid. As a fluid is heated, it expands while mass remains the same. Decreased density (mass/unit volume) makes the heated fluid more buoyant, causing it to rise. As the heated fluid rises, cooler fluid flows in to replace it. Natural convection is one of the major mechanisms of heat transfer in a compartment fire, heated air and smoke rise and cooler air moves in to replace it. This process transfers thermal energy, heating other materials in the compartment and also transfers mass as smoke moves out of the compartment and cool air (containing oxygen necessary for continued combustion) moves into the compartment.

In the late 1700s, French scientist Jacques Charles studied the effect of temperature on a sample of gas at a constant pressure. Charles, found that as the gas was heated, the volume increased. As the gas was cooled, the volume decreased. This finding gave rise to Charles Law.

Charles’ Law: At a constant pressure, the volume occupied by a fixed mass of gas is directly proportional to its thermodynamic temperature (V?T).

Figure 1. Charles Law

Density is mass per unit volume (?=m/V). As the volume of a given mass of gas increases, it becomes less dense (and more buoyant). If unconfined, gases that are less dense than the surrounding air will rise (resulting in natural convection currents). In a compartment fire, conditions are not as simple as stated in Charles Law. Initially, hot gases resulting from a fire in a compartment are relatively unconfined as the volume of smoke and hot gases is small in relation to the size of the compartment and there may be some leakage of smoke from the enclosure. However, as the fire continues to develop, the volume of smoke increases and conditions change.

Gay-Lussac�s Law: At a constant volume and mass, the pressure exerted by a gas is directly proportional to its thermodynamic temperature (P?T).

The pascal (Pa) is the standard international unit of measure for pressure (force per unit area) and is defined as one newton per square meter (N/m²). To provide a point of reference for firefighters more familiar with pounds per square inch (psi) as a unit of measure for pressure, 1 Pa = 0.000145 psi. Low pressures (such as the pressure generated by temperature increases in a compartment fire) are measured in Pa while higher pressures (such as fire pump discharge pressure) are more commonly measured in kilopascals (kPa=1000 Pa).

As illustrated in Figure 2, if the volume is constant (e.g., the gas is confined) doubling the temperature in Kelvins, doubles the pressure in pascals (Pa).

Figure 2. Gay-Lussac’s Law

When a fire is unconfined (e.g., outdoors), convection is influenced primarily by differences in density between hot fire gases and cooler air. Convection as a result of a fire in an enclosure (e.g., compartment fire) is significantly influenced by differences in density and differences in pressure.

Figure 3. Natural Convection

Forced Convection

In forced convection, energy is carried passively by fluid motion which occurs independent of the heating process.

While at first glance, it may appear that this type of convection would not be encountered in the fire environment, but it is extremely important. Forced convection can be caused by natural effects such as wind blowing into an opening (e.g., window broken due to fire effects). This type of forced convection can quickly create untenable conditions both inside the compartment and in adjacent spaces (e.g., rooms, hallways). Forced convection can also have a positive influence on the fire environment. One example would be the use of positive pressure ventilation, in which a blower (fan) is used to create an air flow from an inlet to an exhaust opening, removing hot smoke and gases from the compartment.

Figure 4. Forced Convection/Wind Driven Fire

Note: Adapted from Fire Fighting Tactics Under Wind Driven Fire Conditions: 7-Story Building Experiments.

Factors Influencing Convective Heat Transfer

Heat transfer by convection is more complex than conduction as there is no single property such as thermal conductivity that can be used to describe the mechanism of heat transfer. Factors that influence heat transfer by convection in the fire environment include temperature difference between the fluid (gas) and surfaces, fluid velocity, and turbulence (related to surface roughness and compartment configuration).

Figure 5 illustrates convective heat transfer with laminar (smooth) fluid flow. Energy is transferred from higher temperature fluid molecules to the cooler surface. Bulk fluid temperature (T_b) is the temperature of the fluid some distance away from the surface. As heat is transferred, the temperature of the fluid molecules is lowered (with a corresponding rise in surface temperature). These cooler molecules insulate the surface from the higher temperature molecules further away from the surface, slowing convective heat transfer.

Figure 5. Convection-Laminar Flow

When velocity and/or turbulence increases, cooler molecules that have transferred energy to the surface are quickly replaced by higher temperature molecules, resulting in increased convective heat transfer as illustrated in Figure 6. This is the same phenomena as wind chill, except in this case, energy is transferred from a hot fluid (gas) to a solid surface rather than from a hot surface (i.e., your skin) to a cooler fluid (air).

Figure 6. Convection-Turbulent Flow

More to Follow

Subsequent posts in this series will examine radiant heat transfer and then move on into discussion of the process of combustion.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Kerber, S. & Madrzykowski, D. (2009) Fire Fighting Tactics Under Wind Driven Fire Conditions: 7-Story Building Experiments, NIST Technical Note 1629. Gaithersburg, MD: National Institute of Standards and Technology.

I am using this series of posts to work through the process of developing a chapter on the foundational scientific concepts related to practical fire dynamics and fire control theory. My hope is to take the middle ground between the oversimplified and unsupported explanations provided in most texts intended for firefighter training and the higher level materials intended for fire protection engineers. This is proving to be no small task! Your feedback on my success (or lack thereof) in providing scientifically sound, but reasonably simple explanations would be greatly appreciated.

Back to Everyday Concepts Part 1

When faced with the challenge of developing firefighters understanding of energy, temperature, heat, and power in a limited timeframe, I generally avoid detailed discussion of the actual definition of the SI unit for energy, the Joule, and the mechanical equivalent of thermal energy. I have found that illustrating the concept of the Joule as it relates to thermal energy in terms of heating water to serve the purpose. However, as I looked back at the first post in this series, I think it would be useful to go back to the source, and examine James Joule�s experiments that made the connection to the equivalence of mechanical and thermal energy.

While not commonly used in scientific work, the American fire service has typically used the British thermal unit (Btu) as a measure of thermal energy. The Btu is defined in terms of the heating effect of energy transferred to water. One Btu is the energy required to raise the temperature of one pound of water by one degree Fahrenheit.

As discussed in the first post in this series, the SI unit of measure for energy is the Joule (J) which is defined in mechanical terms, but is applicable to all forms of energy.

In the mid 1800�s English physicist James Joule demonstrated the equivalence of mechanical and thermal energy by using a mechanical apparatus to stir water in an insulated container with paddles driven by a falling weight (see Figure 1).

Joule (1845) reported that based on analysis of data from a number of experiments, that expenditure of mechanical energy of 817 ft/lbs (the energy required to raise 817 pounds to a height of one foot) was the equivalent of an increase in temperature of one pound of water by one degree Fahrenheit. Conversion to SI units of measure is a bit complex, but 817 ft/lbs is equal to 1107 Newton/meters (the energy required to raise a mass of 1107 N to a height of 1 meter). While a non-standard measure of energy, the Newton/meter (N/m) provides a direct comparison to ft/lbs. In mechanical terms, a N/m equals the SI unit for energy, the Joule. Expressed in SI units, 1107 Joule of energy were required to raise the temperature of 0.454 kg (1.0 lbs) of water 0.56^o C (1^o F). This is quite close to the currently accepted conversion value in which 1055 J = 1 Btu.

Figure 1. Demonstration of the Mechanical Equivalent of Heat

Note: Joule used a lesser weight falling over a greater distance, repeated a number of times. This drawing is simplified to provide a conceptual illustration.

Heat Transfer

In everyday language the word heat is used in a variety of ways (many of which are incorrect from a thermodynamic perspective). In thermodynamics, heat is a method of energy transfer. Heat is not a form of energy (a commonly stated misconception), but simply the name of the process of energy transfer based on temperature difference. Objects do not �have� heat, they have thermal energy, and heat is thermal energy in the process of transfer to objects having a lower temperature.

Even though it involves energy transfer, heat is not the same as work. Remember that work involves force causing movement in a direction influenced by that force (and if no movement in that direction occurred, no work is done). Energy transferred by heat results in an increase in molecular movement, but not in a specific direction, therefore no work is done. However, this does not mean that energy transferred by heat cannot be transformed into mechanical energy and accomplish work.

Transfer of energy from one object to another must be classified as heat or work. When energy content changes, it must be the result of heat, work, or a combination of both. Heat and work are processes by which energy is exchanged rather than energy itself.

The word flow is often used in discussing heat transfer (e.g., energy flows from objects with higher temperature to those with lower temperature). This helps visualize patterns of movement, but it is important to remember that neither energy nor heat is a fluid. Heat is the process of energy transfer due to temperature differences. This energy transfer takes place in a variety of different ways.

Second Law of Thermodynamics: There are several ways to state this law. The simplest is that heat cannot spontaneously flow from a material at lower temperature to a material at higher temperature. However, thermal energy moves from materials at high temperature to those having lower temperatures until they have the same temperature (equilibrium).

There are three methods of heat transfer, conduction, convection, and radiation. Each of these has significant impact on the processes of combustion, fire development, and fire control.

Conduction

Conduction of heat occurs when adjacent atoms vibrate against one another or as electrons move from atom to atom. Heat transfers through solid materials and between solid materials in direct contact with one another by conduction. The atoms in liquids and gases are further apart, reducing the probability of collision and transfer of thermal energy.

Figure 2. Conduction

Factors Influencing Conductive Heat Transfer

The factors influencing conduction are temperature difference, length (or thickness), cross sectional area, and the thermal conductivity of the conductor.

Thermal conductivity is the measure of the quantity of thermal energy which flows through a conductor. In addition to form, there are a number of factors influencing thermal conductivity of materials including molecular bonding, structure, and density. Units of measure for conductivity must account for the amount of energy transferred in a given amount of time, thickness (or distance), and temperature difference. The SI units of measure for thermal conductivity are Watts per Kelvin per Meter (W?K?m). While appearing to be complex, this measure is fairly straightforward; indicating the number of Watts (Joules/second) transferred a distance of one meter for each Kelvin of temperature difference (Figure 3)

Figure 3. Thermal Conductivity

When the temperature of one surface of a solid material is higher than another, heat will move through the material. Depending on the characteristics of the material, this conductive heat transfer may be slow or it may occur quickly. The rate of heat transfer is defined by the coefficient of thermal conductivity.

As illustrated in Figure 3, the total amount of heat transfer is dependent on the coefficient of thermal conductivity, difference in temperature, and cross sectional area of the conductor. It is difficult to measure thermal conductivity as it describes a semi-static situation with a constant temperature gradient. However, heat transfer results in temperature changes towards equilibrium (equal temperature at all points in the conductor).

A high coefficient means heat moves very quickly; a low coefficient means heat moves very slowly. As illustrated in Table 1, the thermal conductivity constant (k) for different materials varies considerably.

Table 1Thermal Conductivity Table

Metals are usually the best conductors of thermal energy due to their molecular bonding and structure. Metallic chemical bonds have free-moving electrons and form a crystalline structure which aids in transfer of thermal energy as illustrated in Figure 4.

Figure 4. Conduction in Metals

Because the outer electrons in metals are shared by all the atoms, they are not considered to be associated with any one atom. Since these electrons are attracted to many atoms, they have considerable mobility that allows for the good thermal conductivity seen in metals.

In general, density decreases so does conduction (some unusual materials such as carbon foam, have low density and high conductivity). Therefore, most fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas: fewer collisions between atoms means less conduction. Conduction is dependent on the area being heated, temperature differential, and thermal conductivity of the material.

What�s Next

The next post in this series will examine convection and radiation as mechanisms of heat transfer. In addition, I will be starting a series of posts to discuss a comprehensive approach for nozzle testing from an operational perspective.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Joule, J. (1845). On the existence of an equivalent relation between heat and the ordinary forms of mechanical power. Philosophical Magazine, 3(xxvii), p. 205.

Quick Review

The previous post in this series presented a video clip of an incident on the afternoon of February 18, 2010 that injured four Chicago firefighters during operations at a residential fire at 4855 S. Paulina Street.

First arriving companies discovered a fire in the basement of a 1-1/2 story, wood frame, single family dwelling and initiated fire attack and horizontal ventilation of the floors above the fire. Based on news accounts, the company assigned to fire attack was in the stairwell and another firefighter was performing horizontal ventilation of the floors above the fire on Side C when a backdraft or smoke explosion occurred. Two firefighters on the interior, on at the doorway and the firefighter on the ladder on Side C were injured and were transported to local hospitals for burns and possible airway injuries.

In analyzing the video clip shot from inside a nearby building, we have several advantages over the firefighters involved in this incident.

Time: We are not under pressure to make a decision or take action.

Reduced Cognitive Workload: Unlike the firefighters who needed to not only read the fire, but also to attend to their assigned tactics and tasks, our only focus is analysis of the fire behavior indicators to determine what (if any) clues to the potential for extreme fire behavior may have been present.

Repetition: Real life does not have time outs or instant replay. However, our analysis of the video can take advantage of our ability to pause, and replay key segments, or the entire clip as necessary.

Perspective: Since the field of view in the video clip is limited by the window and the fidelity of the recording is less than that seen in real life, it presents a considerably different field of view than that of the firefighters observed in operation and does not allow observation of fire behavior indicators and tactical operations on Sides A, B, and D.

Initial Size-Up

What B-SAHF indicators could be observed on Side C up to the point where firefighters began to force entry and ventilate the basement (approximately 02:05)?

Figure 1. Conditions at 01:57 Minutes Elapsed Time in the Video Clip

Building: The structure is a 1-1/2 story, wood frame, dwelling with a daylight basement. The apparent age of the structure makes balloon frame construction likely, and the half story on the second floor is likely to have knee walls, resulting in significant void spaces on either side and a smaller void space above the ceiling on Floor 2. One window to the left of the door on Side C appears to be covered with plywood (or similar material). Given the location of the door (and door on Side A illustrated in the previous post in this series), it is likely that the stairway to the basement is just inside the door in Side C and a stairway to Floor 2 is just inside the door on Side A.

Smoke: A moderate volume of dark gray smoke is visible from the Basement windows and windows and door on Floor 1 as well as a larger volume from above the roofline on Side B. While dark, smoke on Side C does not appear to be thick (optically dense), possibly due to limited volume and concentration while smoke above the roofline on Side B appears to be thicker. However smoke on Side C thickens as time progresses, particularly in the area of the door on Floor 1. The buoyancy of smoke is somewhat variable with low buoyancy on Side C and greater buoyancy on Side B. However, smoke from the area of the door on Floor 1 Side C intermittently has increased buoyancy.

Air Track: Smoke on Side C appears to have a faintly pulsing air track with low velocity which is masked to some extent by the effects of the wind (swirling smoke due to changes in low level wind conditions). Smoke rising above the roofline on Side B appears to be moving with slightly greater velocity (likely due to buoyancy).

Heat: The only significant heat indicators are limited velocity of smoke discharge and variations in buoyancy of smoke visible from Sides B and C. Low velocity smoke discharge and low buoyancy of the smoke on Side C points to relatively low temperatures inside the building. The greater buoyancy and velocity of smoke observed above the roofline on Side B indicates a higher temperature in the area from where this smoke is discharging (likely a basement window on Side B).

Flame: No flames are visible.

Initial Fire Behavior Prediction

Based on assessment of conditions to this point, what stage(s) of development and burning regime(s) is the fire likely to be in?

Dark smoke with a pulsing air track points to a ventilation controlled, decay stage fire.

What conditions would you expect to find inside the building?

Floors 1 and 2 are likely to be fully smoke logged (ceiling to floor) with fairly low temperature. The basement is likely to have a higher temperature, but is also likely to be fully smoke logged with limited flaming combustion.

How would you expect the fire to develop over the next few minutes?

As ventilation is increased (tactical ventilation and entry for fire control), the fire in the basement will likely remain ventilation controlled, but will return to the growth stage as the heat release rate increases. Smoke thickness and level (to floor level) along with a pulsing air track points to potential for some type of ventilation induced extreme fire behavior such as ventilation induced flashover (most likely) or backdraft (less likely). Another possibility, would be a smoke explosion; ignition of premixed gas phase fuel (smoke) and air that is within its flammable range (less likely than some type of ventilation induced extreme fire behavior)

Ongoing Assessment

What indicators could be observed while the firefighter was forcing entry and ventilating the daylight basement on Side C (02:05-02:49)?

There are few changes to the fire behavior indicators during this segment of the video. Building, Heat, and Flame indicators are essentially unchanged. Smoke above the roofline appears to lighten (at least briefly) and smoke on Side C continues to show limited buoyancy with a slightly pulsing air track at the first floor doorway.

What B-SAHF indicators can be observed at the door on Side C prior to forced entry (02:49-03:13)?

Figure 2. Conditions at 03:06 Minutes Elapsed Time in the Video Clip

Figure 3. Conditions at 03:08 Minutes Elapsed Time in the Video Clip

Building, Smoke, Heat and Flame indicators remain the same, but several more pulsations (03:05-03:13) providing a continuing, and more significant indication of ventilation controlled, decay stage fire conditions.

What indicators can be observed at the door while the firefighter attempts to remove the covering over the window adjacent to the door on Floor 1 (03:13-13:44)?

No significant change in Building, Heat, or Flame Indicators. However, smoke from the doorway has darkened considerably and there is a pronounced pulsation as the firefighter on the ladder climbs to Floor 2 (03:26). It is important to note that some of the smoke movement observed in the video clip is fire induced, but that exterior movement is also significantly influenced by wind.

What B-SHAF indicators do you observe at the window on Floor 2 prior to breaking the glass (03:44)?

Figure 4. Conditions at 03:43 Minutes Elapsed Time in the Video Clip

The window on Floor 2 is intact and appears to be tight as there is no smoke visible on the exterior. It is difficult to tell due to the angle from which the video was shot (and reflection from daylight), but it would be likely that the firefighter on the ladder could observe condensed pyrolizate on the window and smoke logging on Floor 2. It is interesting to note limited smoke discharge from the top of the door and window on Floor 1 in the brief period immediately prior to breaking the window on Floor 2.

What indicators are observed at the window on Floor 2 immediately after breaking the glass (03:44-03:55)?

Figure 5. Conditions at 03:52 Minutes Elapsed Time in the Video Clip

No significant changes in Building, Heat, or Flame indicators. Dark gray smoke with no buoyancy issues from the window on Floor 2 with low to moderate velocity immediately after the window is broken.

What B-SAHF indicators were present after the ventilation of the window on Floor 2 Side C was completed and 04:08 in the video clip (03:44-04:08)?

Buoyancy and velocity both increase and a slight pulsing air track develops within approximately 10 seconds. In addition, the air track at the door on Floor 1 shifts from predominantly outward with slight pulsations to predominantly inward, but with continued pulsation (possibly due to the limited size of the window opening on Floor 2, Side C.

Anticipating Potential Fire Behavior

Unlike the firefighters in Chicago who were operating at this incident, we can hit the pause button and consider the indicators observed to this point. Think about what fire behavior indicators are present (and also consider those that are not!).

Initial observations indicated a ventilation controlled decay stage fire and predicted fire behavior is an increase in heat release rate with potential for some type of extreme fire behavior. Possibilities include ventilation induced flashover (most likely) or backdraft (less likely), or smoke explosion (less likely than some type of ventilation induced extreme fire behavior).

Take a minute to review the indicators of ventilation controlled, decay stage fires as illustrated in Table 1.

Table 1. Key Fire Behavior Indicators-Ventilation Controlled, Decay Stage Fires

Which of these indicators were present on Side C of 4855 S. Paulina Street?

Building: The building appeared to be unremarkable, a typical single family dwelling. However, most residential structures have more than enough of a fuel load to develop the conditions necessary for a variety of extreme fire behavior phenomena.

Smoke: The dark smoke with increasing thickness (optical density) is a reasonably good indicator of ventilation controlled conditions (particularly when combined with air track indicators). Lack of buoyancy indicated fairly low temperature smoke, which could be an indicator of incipient or decay stage conditions or simply distance from the origin of the fire. However, combined with smoke color, thickness, and air track indicators, this lack of buoyancy at all levels on Side C is likely an indicator of dropping temperature under decay stage conditions. This conclusion is reinforced by the increase in buoyancy after ventilation of the window on Floor 2 (increased ventilation precipitated increased heat release rate and increasing temperature).

Air Track: Pulsing air track, while at times quite subtle and masked by swirling smoke as a result of wind, is one of the strongest indications of ventilation controlled decay stage conditions. While often associated with backdraft, this indicator may also be present prior to development of a sufficient concentration of gas phase fuel (smoke) to result in a backdraft.

Heat: Velocity of smoke discharge (air track) and buoyancy (smoke) are the only two heat indicators visible in this video clip. As discussed in conjunction with smoke indicators, low velocity and initial lack of buoyancy which increases after ventilation is indicative of ventilation controlled, decay stage conditions.

Flame: Lack of visible flame is often associated with ventilation controlled decay and backdraft conditions. However, there are a number of incidents in which flames were visible prior to occurrence of a backdraft (in another compartment within the structure). Lack of flames must be considered in conjunction with the rest of the fire behavior indicators. In this incident, lack of visible flames may be related to the stage of fire development, but more likely is a result of the location of the fire, as there is no indication that flames were present on Side C prior to the start of the video clip.

What Happened?

Firefighters had entered the building for fire attack while as illustrated in the video clip, others were ventilating windows on Side C. It is difficult to determine from the video if a window or door at the basement level on Side C was opened, but efforts were made to do so. A window on Floor 2 had been opened and firefighters were in the process of removing the covering (plywood) from a window immediately adjacent to the door on Floor 1. At 04:12, an explosion occurred, injuring two firefighters on the interior as well as the two firefighters engaged in ventilation operations on Side C.

Starting at approximately 03:59, velocity of smoke discharge from the window on Floor 2 Side C increases dramatically. At 04:08 discharge of smoke begins to form a spherical pattern as discharged from the window. This pattern becomes more pronounced as the sphere of smoke is pushed away from the window by increasing velocity of smoke discharge at 04:12, immediately prior to the explosion. Velocity of smoke discharge at the door increases between 03:59 and -4:12 as well, but as the opening is larger, this change is less noticeable. As pressure increases rapidly during the explosion a whooshing sound can be heard. After the explosion, there was no noticeable increase in fire growth.

Figure 6. Conditions at 04:08 Minutes Elapsed Time in the Video Clip

Figure 7. Conditions at 04:09 Minutes Elapsed Time in the Video Clip

Figure 8. Conditions at 04:10 Minutes Elapsed Time in the Video Clip

Figure 9. Conditions at 04:11 Minutes Elapsed Time in the Video Clip

Figure 10. Conditions at 04:12 Minutes Elapsed Time in the Video Clip

Figure 11. Conditions at 04:13 Minutes Elapsed Time in the Video Clip

Based on observation of fire behavior indicators visible in the video clip, we know that a transient extreme fire behavior event occurred while a crew was advancing a hoseline on the interior and ventilation operations were being conducted on Side C. What we don�t know is what firefighting operations were occurring on the other sides of the building or in the interior. In addition, we do not have substantive information from the fire investigation that occurred after the fire was extinguished.

The Ontology of Extreme Fire Behavior presented in an earlier post classifies these types of phenomena on the basis of outcome and conditions. As a transient and explosive event, this was likely a backdraft or smoke explosion. In that this occurred following entry and during ongoing ventilation operations, I am inclined to suspect that it was a backdraft.

Indicators visible on Side C provided a subtle warning of potential for some type of ventilation induced extreme fire behavior, but were likely not substantially different from conditions observed at many fires where extreme fire behavior did not occur.

As the title of the wildland firefighting course S133 states; Look Up, Look Down, Look Around! Anticipation of fire development and extreme fire behavior requires not only recognition of key indicators, but that these indicators be viewed from a holistic perspective. Firefighters and/or officers performing a single task or tactical assignment may only see part of the picture. It is essential that key indicators be communicated to allow a more complete picture of what is occurring and what may occur as incident operations progress.

Ed Hartin, MS, EFO, MIFireE, CFO

Everyday Concepts

Firefighters, like most everyone else, have a commonsense, everyday understanding of energy, heat and temperature. However, this everyday understanding is likely to be considerably different than the way these concepts are defined and used in science. Think about how heat and temperature are used on a day-to-day basis. On a sunny summer day, people are likely to say that it is hot because they feel hot or because the thermometer indicates a temperature is high. This may lead people to believe that temperature is a measure of hotness or heat. On the other hand, scientists view these concepts considerably differently!

So what! What difference does it make if we use our commonsense, everyday understanding of energy, heat, and temperature in our effort to make sense of fire dynamics? Why is this important?

If you are going to take a trip, it is useful to understand the concept of distance and have some type of units (e.g., kilometers or miles) to describe how far away your destination is. When describing a building, firefighters indicate the number of stories and dimensions (e.g., meters or feet). Having a good grasp of the concepts of energy, heat and temperature provides a way to describe the fire potential of different types of fuel, the size of a fire in terms of energy and power, and the thermal environment encountered by firefighters.

Take a minute and think about how you would define energy, heat, and temperature. Write your ideas down on a piece of paper so you can come back to them later. Don�t worry about the textbook definition, just write down what you think these words mean. After reading the rest of this post, come back to your notes and see how your understanding of energy, heat, and temperature has changed.

Energy, Heat, and Temperature in Firefighting

For many years, firefighters in the United States learned about British thermal units (Btu) as a measure of energy. A British thermal unit is the amount of energy required to raise the temperature of 1 pound of water (at 60^o Fahrenheit (F)) 1^o F. Firefighters often can state a reasonable approximation of this definition and the Btu seems to be a fairly simple unit of measure with direct applicability in the firefighting context.

�Before an observer can formulate and assent to an observation statement, he or she must be in possession of the appropriate conceptual framework and a knowledge of how to appropriately apply it� (Chalmers, 1999, p. 11). It is one thing to recognize a definition, but it is another thing entirely to be able to use this information in a broader context and make sense of things! For example a young child may be able to identify a red apple, but may not have a good understanding of what makes this fruit an apple (as opposed to a pear) or how a red apple and a green apple can both be apples. Developing an understanding of the fundamental scientific concepts that underlie fire dynamics and firefighting is much the same. Knowledge and understanding must extend beyond simple recognition of, or the ability to restate definitions and concepts presented in a text of lecture.

Thermodynamics

Thermodynamics is a branch of physics that describes processes that involve changes in temperature, transformation of energy, and the relationships between heat and work. Fire and firefighting also involves changes in temperature, transformation of energy, heat and work. �Thermodynamics, like much of the rest of science, takes terms with an everyday meaning and sharpens them � some would say, hijacks them � so that they take on an exact unambiguous meaning� (Atkins, 2007, p. 3).

Thermodynamics deals with systems. A thermodynamic system is one that interacts and exchanges energy with the area around it. A system could be as simple as a block of metal or as complex as a compartment fire. Outside the system are its surroundings. The system and its surroundings comprise the universe.

While in general terms the universe includes everything, we will generally look at things on a smaller scale. For example we might consider a burning fuel package as the system and the compartment as the surroundings. On a larger scale we might consider the building containing the fire as the system and the exterior environment as the surroundings.

Figure 1. Thermodynamic Systems

Thermodynamic systems can be classified on the basis of their interaction with the surroundings.

• Isolated systems do not exchange energy or matter with their environment.
• Closed systems exchange energy but not matter with their environment.
• Open systems exchange energy and matter with their environment. A boundary allowing matter exchange is called permeable.

Laws of Thermodynamics: These laws summarize the properties of energy and its transformation from one form to another. Numbered from zero to three, these laws are both simple and extremely complex. This series of posts examines the laws of thermodynamics in the context of fires and firefighting to move from theoretical to practical application.

Energy

Energy is a fundamental concept in physical science, but is difficult to define in a way that is meaningful on an everyday basis. Energy is the ability to do mechanical work or transfer thermal energy from one object to another. Energy can only be measured on the basis of its effects. There are basically two kinds of energy, kinetic and potential. Kinetic energy is associated with motion of an object and potential energy is that which is stored and may be released at a later time.

There are a number of different forms of energy; mechanical, chemical, electrical, radiant, and thermal. However, each has the ability to be transformed into work, which is force applied to an object, causing it to be displaced. In thinking about energy and work it is important to keep two things in mind:

• Energy is the capacity to do work.
• Work involves force causing movement in the direction of that force.

If the force does not influence movement in the direction of the force, no work was done.

Newtons (named after Isaac Newton) are the standard international (SI) unit for force. A Newton is the amount of force required to give a mass of one kilogram an acceleration of one meter per second squared. However, it may be easier to visualize force in terms of weight. In our everyday environment, weight is the force exerted as a result of our mass and the effects of gravity. For example, a kilogram (which is a unit of mass) exerts a downward force of 10 Newtons (or 2.2 pounds). To make things more complicated, kilograms are used in everyday language to express weight (rather than Newtons). This is because on earth, weight and mass are directly proportional.

The SI unit for energy (capacity to do work) is the Joule. A Joule is a force of one Newton causing displacement of an object a distance of one meter. For example, approximately one Joule of energy is required to lift a small apple (which weighs one Newton (or 0.22 pounds) a distance of 1 meter. In that energy is the capacity to do work, the Joule is also used to measure energy (regardless of its form).

While mechanical energy may be of interest to firefighters, what does this have to do with thermal energy and fire behavior? One really big puzzle is how Joules which are defined in terms of mechanical energy can be used to measure thermal energy? This is a really good question, but several more scientific concepts are needed in order to make sense of the answer.

Substances have potential chemical energy based on the bonds within and between their atoms and molecules. Formation, breaking, or rearrangement of these chemical bonds results in transfer of energy into or out of the substance. For example, in combustion the reaction of an oxidizer and fuel results in transformation of chemical potential energy into thermal and radiant kinetic energy. Thermal energy is molecular kinetic energy resulting from molecules moving around in random directions as well as molecular rotation and vibration. Radiant energy is comprised of electromagnetic waves in the infrared region of the electromagnetic spectrum although some is in the visible region. The term thermal radiation distinguishes this form of electromagnetic radiation from other forms such as radio waves and ionizing radiation

First Law of Thermodynamics: Energy cannot be created nor destroyed only transformed from one form to another. For example, in combustion the chemical reaction between oxygen and fuel results in transformation of chemical energy to thermal and radiant energy. However, the total amount of energy remains the same.

Temperature

Temperature is a measure of the average kinetic energy. Temperature of any substance, whether solid, liquid, or gas, is directly related to all motion (kinetic energy) of its molecules. This is especially important for liquids and solids because the kinetic energy of these substances is almost entirely vibrational and rotational. All molecules above a temperature of absolute zero (the temperature at which molecular motion stops) are in a continual state of motion and possess kinetic energy.

The Kelvin is the standard international unit for temperature. In this scale, temperature is measured in Kelvins (K), not degrees (as with the Celsius and Fahrenheit scales). While the least common in everyday use, the Kelvin thermodynamic temperature scale is important in understanding thermal energy, temperature, and heat. With the Kelvin scale, 0 K is absolute zero, the theoretical absence of all thermal energy.

While the Kelvin is the standard international unit for temperature, the Celsius scale is commonly used as both an everyday (outside the United States) and scientific measure of temperature. The degree Celsius (^o C) is the same increment of measure as the Kelvin, the difference between these two scales is the zero point on the scale. With the Celsius scale 0^o C is the freezing point of water (273.15 K) while as previously noted 0 K (-273.16^o C) is absolute zero.

In the United States, the Fahrenheit scale is commonly used to measure temperature on an everyday basis. Unlike the Celsius scale where the difference between the freezing and boiling points of water is 100^o, the Fahrenheit scale places the freezing point at 32^o and boiling point at 212^o, a difference of 180^o.

Figure 2. Common Temperature Scales

Note: Equivalent temperatures have been rounded to the closest whole unit (i.e., degree, kelvin).

The Kelvin temperature scale is used in scientific work involving thermodynamics, because this scale starts at absolute zero (the point at which a substance has no thermal energy). This means that temperature in Kelvins is a measure of the absolute temperature. Use of an absolute temperature scale allows expression of physical laws and mathematical formulas more simply.

For example, 100^o C is not twice as high a temperature as 50^o C (even though at first glance it appears that it is). This becomes clear when using the Kelvin scale. A temperature of 50^o C is 323.15 K while 100^o C is 373.15 K, an increase of just over 13% in absolute temperature.

Third Law of Thermodynamics: In the complete absence of molecular kinetic energy, the temperature of a substance would be absolute zero. Absolute is 0 K or -273.15� C.

Measuring Energy

As previously discussed, the SI unit of measure for energy is the Joule (J). While defined in terms of mechanical work, the joule is used for all forms of energy. In the standard international system of units, prefixes such as kilo (thousand) and mega (million) are used to provide incrementally larger units of measure. In the case of energy, a kilojoule (kJ) would be a thousand joules and a megajoule (MJ) would be one million joules.

While not commonly used in scientific work, the American fire service has typically used the British thermal unit (Btu) as a measure of thermal energy. The Btu is defined in terms of the heating effect of energy transferred to water. In order to provide a simple explanation of the Joule as a unit of measure for thermal energy and allow a direct comparison to the Btu, Figure 3 describes the J in terms of energy transfer to water and provides a comparison to the Btu.

Figure 3. Joule and British Thermal Unit

* In this case, Ounces is a measure of volume not mass (or weight). Another confusing aspect of the traditional system of measure used in the United States!

As illustrated in Figure 3, addition of 4186 joule of energy to a kilogram of water raises its temperature (average internal kinetic energy) by one degree Celsius. Similarly, adding one British thermal unit of energy to a pound of water raises its temperature by one degree Fahrenheit. Directly comparing these two examples is a bit complex as the units of measure for both energy (J & Btu) and temperature (^o C & ^o F) are different.

Some properties of materials are independent of their mass, color would be one example. Other properties are dependent on mass. Weight, would be the most obvious, but other properties are also dependent on the mass of material present.

Figure 4. Energy and Temperature Simulation

Note: This illustration was adapted from a simulation in Energy: Thermal Energy, Heat and Temperature, a National Science Teachers Association knowledge object.

The example provided in Figure 4 examines the difference between temperature and thermal energy as related to mass. The container labeled A initially contains a specific mass of liquid with a temperature of 30^o C and a total thermal energy of 8 J. Liquid is moved from the container labeled A to the one labeled B. How does the temperature of the liquid and thermal energy in each container change as this transfer takes place? The temperature of the liquid remains the same regardless of the quantity in each of the containers. However, as the mass of liquid in each container changes, the thermal energy of the liquid in the container changes as well. Related to average thermal energy, temperature is independent of mass, while the total thermal energy relates directly to mass. Properties of materials fall into two categories. Extensive properties (like energy) are dependent on the quantity (mass) of material while intensive properties (like temperature) are not.

What�s Next?

The next post in this series will examine the concept of heat and the relationship between heat, energy, and temperature.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Atkins, P. (2000). Four laws that drive the universe. Oxford, UK: Oxford University Press

Chalmers, A. (1999) What is this thing called science? (3^rd ed.). Indianapolis, IN: Hackett.

National Science Teachers Association (NSTA). (2006). Energy: Thermal energy, heat and temperature. Retrieved February 27, 2010 from http://www.nsta.org/store/product_detail.aspx?id=10.2505/7/SCB-EN.3.1

A Quick Review

As discussed in the previous posts in this series, military battle drills are an immediate response to enemy contact that requires fire and maneuver in order to succeed. Battle drills are initiated with minimal commands from the unit leader. Soldiers or marines execute preplanned, sequential actions in response to enemy contact (see Figure 1).

Figure 1. Battle Drill

Battle Drill Part 2 addressed the appropriate reaction of a team of firefighters on a primary hoseline when confronted with rapidly worsening fire conditions that are not readily controllable once they occur (e.g., flashover, wind driven fire conditions). As when a military unit is ambushed, the fire and maneuver of battle drill involves more than one weapon. This post will address the role and reaction of backup lines in the extreme fire behavior battle drill.

Backup Lines

Once a hoseline has been deployed for fire attack it is good practice to stretch a backup line. Klaene and Sanders (2008) observe that backup lines are needed to protect the crew on the initial attack line and to provide additional flow if needed (p. 216). Unfortunately, many firefighters see the backup line as simply another attack line and miss the first and primary function of this hoseline to protect crews on primary hoselines.

The first priority in fire attack operations is to get a hoseline in position to apply water effectively to the fire. To this end, hoselines are deployed in series (attack line first, then backup line) not in parallel, where both lines are attempting to advance and maneuver in the same space. The crew of the backup line can often assist in pulling up additional hose for the attack line (particularly when crews are lightly staffed). As illustrated in Figure 2, the backup line is positioned to protect the means of egress and if necessary support fire attack.

Figure 2. Attack and Backup Line Placement

Extreme Fire Behavior Battle Drill

As discussed in Battle Drill Part 2, the thermal insult experienced in an extreme fire behavior event is dependent on temperature (of gases and compartment linings) and flow of hot gases. The higher the temperature and faster the speed of gas flow, the higher the heat flux. Survival requires that crews on hoselines extinguish or block the flames, cool hot gases, and maneuver out of the flow path to a point of egress or area of safer refuge.

Crews engaged in fire attack or search are often first to encounter rapidly deteriorating fire conditions. Hose Handling and Nozzle Technique Drill 8 outlined the immediate actions that should be taken to support a tactical withdrawal under severe fire conditions. In these circumstances, the crew staffing the backup line has a critical role in supporting withdrawing crews.

Fire conditions that are beyond the capability of a single hoseline may be controlled by the higher flow rate from multiple lines. As noted by Klaene and Sanders (2008) one of the functions of backup lines is to provide additional flow if needed (p. 216). The attack line and backup line operating in a coordinated manner may be able to control fire conditions and allow continuation of fire attack. If this is the case, these lines should be reinforced by deployment of one or more additional backup lines.

If fire conditions cannot be controlled, and the attack line must be withdrawn while maintaining water application to protect the crew, the crew on the backup line can aid in withdrawal of attack and/or search hoselines. If the hoseline is not withdrawn as the firefighter on the nozzle retreats, the hose may kink or become exposed to flames (either of which may result in loss of water supply to the nozzle).

While the attack or search crew is likely to be first to encounter worsening fire conditions, this is not always the case. Depending on fire location and building configuration, fire spread may cut off the attack or search line from behind. In this situation, the backup line becomes the primary means of defense for operating crews.

Regardless of how deteriorating conditions develop, safe and effective tactical withdrawal requires a coordinated effort between interior crews and as soon as possible, report of conditions to Command and if necessary transmit a Mayday message.

Drill 9-Extreme Fire Behavior Battle Drill-The Backup Line: Key hose handling and nozzle techniques when faced with extreme fire behavior are the ability to apply long pulses of water fog or maintaining a continuous flow rate while maneuvering backwards. However, the backup line may initially need to advance to support fire attack, and then if necessary cover and support other crews as they withdraw.

Hose Handling & Nozzle Technique Drill 9 Instructional Plan

Skill in operation and maneuver of a single hoseline is a foundational firefighting skill. However, in the extreme fire behavior battle drill, coordinated operation of the attack and backup line is essential, making Hose Handling & Nozzle Technique Drill 9 an important step in skill development.

References

Klaene, B. & Sanders, R. (2008) Structural Firefighting Strategy and Tactics (2^nd ed.). Sudbury, MA: Jones & Bartlett.

At a formal dinner on 23 January 2010, Chief Ed Hartin was recognized as an honorary member of Company 1 �Germania� of the Valdivia, Chile Fire Department. In addition, he was awarded a commendation for supporting the ongoing professional development of the members of Company 1 �Germania� of the Valdivia, Chile Fire Department and encouraging them in their efforts to share their knowledge with Chile�s fire service.

Commendation for Support of Company 1 �Germania�

Left to Right: Teniente Juan Esteban Kunstmann, Chief Ed Hartin, Capit�n Francisco Silva V.

On 24-27 January 2007, the Company 1 �Germania� of the Valdivia, Chile Fire Department hosted the first international fire service congress to be held in South America. Participants included over 150 firefighters and officers from Chile, Peru, Argentina, and the United States. The congress provided an opportunity to participate in both classroom and hands-on workshops on a wide range of fire service topics including fire behavior, ventilation, search, rapid intervention, technical rescue, and extrication. While topical areas were diverse, the congress had a substantive emphasis on compartment fire behavior with lectures presented by CFBT-US Chief Instructor Ed Hartin and Geraldo Crespo of Contraincendio in Buenos Aires, Argentina and practical training sessions conducted by Ed Hartin and Juan Esteban Kunstmann of the Valdivia Company 1 �Germania�.

Lecture Presentation

Lecture presentations by CFBT-US Chief Instructor Ed Hartin included (click on the links for a copies of the presentations):

• Fire Development and Flashover: Foundational Knowledge
• Extreme Fire Behavior: Understanding the Hazard
• 3D Firefighting: Controlling the Fire Environment
• Reading the Fire: B-SAHF
• Modern Buildings: Fires in the Built Environment
• Large Area Fires: Tactical Implications
• Fire and Smoke in Void Spaces: Hidden Hazards
• Live Fire Training as Simulation: The Role of Fidelity

CFBT practical skills sessions were held at the Valdivia Fire Department�s training center and focused on developing basic skill in nozzle technique and understanding fire development in a compartment.

This is My Nozzle! There are many like it, but this one is mine�

Center: Ed Hartin

Practicing Nozzle Techniques

Right: Teniente Juan Esteban Kunstmann

International Collaboration

Left to Right: Battalion Chief Danny Sheridan, FDNY and Capit�n Giancarlo Passalacqua Cognoro, Lima, Pe?u Cuerpo General de Bomberos Voluntarios

Congratulations to the members of Company 1 �Germania� for their success with the first Congreso Internacional Fuego y Rescate! I look forward to working with these outstanding fire service professionals in their ongoing efforts to learn and share knowledge with the fire service throughout Chile, Latin America, and the World.

Ed Hartin, MS, EFO, MIFireE, CFO

Two recent events in Baltimore, Maryland and Gary, Indiana point to the criticality of recognizing key fire behavior indicators and understanding practical fire dynamics.

Five Firefighters Injured in Baltimore

Early on the morning of Friday, January 15, 2010, the Baltimore City Fire Department was dispatched to a residential fire Southeast Baltimore. First arriving companies observed a row house of ordinary construction with a large volume of smoke and flames issuing from the basement and extending to the first floor.

According to a department spokesperson, the first engine took a line through the front door to the rear kitchen area where crew had some trouble finding the basement stairs. Another engine company went to the rear with a line to the outside stairwell leading to the basement and was just starting down the stairs. The first truck vented some skylights on the roof as well as the front basement windows. As crews were attempting to access the fire, some type of transient extreme fire behavior resulted in flames blowing through the unit and out the front door, rear stairwell, second floor windows, and skylights. The firefighter from the first arriving truck assigned to the roof described the sound of a freight train coming through.

Five firefighters injured as a result of this explosive fire behavior phenomenon were transported to area hospitals. The officer of the first in engine company was admitted to the Bayview Burn Center, where he is listed in stable condition

Find more videos like this on firevideo.net

What Happened?

As always when a video of an incident involving extreme fire behavior is posted to the web, there is ongoing debate about what happened. Was it a backdraft? Was it a flashover? An interesting debate, but the value is not so much in being �right�, but in understanding how these phenomena occur, what might have happened in this incident, key indicators that may (or may not) be visible in the video, and most importantly how to prevent this from happening to us and the firefighters that we work with!

Flashover: sudden transition to fully developed fire. This phenomenon involves a rapid transition to a state of total surface involvement of all combustible material within the compartment.

Given adequate fuel and ventilation, a compartment fire may reach flashover as it develops from the growth to fully developed stage. However, when fire development is limited by the ventilation profile of the compartment, changes in ventilation will directly influence fire behavior.

For many years firefighters have been taught that ventilation reduces the potential for flashover. However, when a fire is ventilation controlled, heat release rate is limited by the available oxygen. Under these conditions; increasing air supply by creating opening results in increased heat release rate. This increased heat release rate may result in flashover.

If a fire is sufficiently ventilation controlled and a high concentration of excess pyrolizate and unburned flammable products of combustion accumulate in a compartment, the outcome of increased ventilation may be different.

Backdraft: Deflagration of unburned pyrolyzate and combustion products following introduction of air to a ventilation controlled compartment fire and ignition of the fuel/air mixture. This deflagration results in a rapid increase in pressure within the compartment and extension of flaming combustion through compartment openings. Occurrence of this phenomenon requires an atmosphere in which the fuel concentration is too high to deflagrate without introduction of additional oxygen.

As introduced in Extreme Fire Behavior: An Organizational Scheme, extreme fire behavior phenomena can be classified on the basis of outcome and conditions (see Figure 1)

Figure 1. Extreme Fire Behavior Classification.

Use of this approach may aid in making sense of what may have occurred in the Baltimore incident. But, it is often difficult to classify extreme fire behavior phenomena into discrete, black and white categories. What is the dividing line between a ventilation induced flashover and a backdraft. One key difference may be the speed with which heat release rate increases, but where is the dividing line (see Figure 2)?

Figure 2. The Gray Area.

Keep in mind that while being right is great, it is more important to work through the process of figuring things out to improve your understanding.

Near Miss in Gary

Monday morning January 18, 2010 firefighters in Gary, Indiana were operating at a residential fire at 24^th and Massachusetts when they experienced a near miss involving rapid fire progression. Have a look at video of this incident and give some thought to what influenced fire behavior. Also look at the similarities and differences between the extreme fire behavior that occurred in the Baltimore and Gary incidents.

Back on Task!

I have been extremely busy working on a project for the National Institute for Occupational Safety and Health and preparing for the International Fire & Rescue Congress in Valdivia, Chile. Next week�s post will provide a quick update on training conducted at the Congress.

After returning from Chile, I will be back on task with examination of the concept of battle drills to develop effective reaction to worsening fire conditions while operating in an offensive mode.

Ed Hartin, MS, EFO, MIFireE, CFO