Heat Release Rate

 

 

 

By: Eddie Crombie

 

In the last post we extensively covered flashover.  Specifically what has changed in the modern environment we operate in.  We all know we fight explosive fires in disposable buildings because of the widespread use of synthetic materials in our daily lives. This translates to rapid fire development due to an ever increasing heat release rate.

 

In every chapter on fire behavior there is a chart that shows the life cycle of a fire using a bell curve. (Fig 1.)

Fig1.  This modern heat release rate of a compartment fire.

This graph measures the heat release rate of the combustible materials within a container.  So what does this mean to a firefighter?  A lot actually.  In short, the heat release rate is a measurement of heat energy released over a specific unit of time.  In comparison, a British thermal unit is simply a measurement of heat energy.  Think 3D (HRR) versus 2D (BTU).

Simply put, a fuel has a specific heat of combustion or a total amount of energy released when burned.  Different materials give off different levels of potential energy based on their chemical makeup.  This is measured in kilojoules per gram.  See Fig 2.

Fig 2. Notice how similar the amount of potential energy of a synthetic material like polystyrene is to a flammable gas like propene.

How fast this potential energy is released is called the heat release rate.  This is measured in kilojoules per second. See Fig 3.

Fig 3. We have all watched the “Dry Pine Tree Fire” video.  Compare that to a sofa.

 

Until recently, using heat release rate for the study of fires was only accessible in theory.  Prior to 1980, the tools available to quantify the heat release rate of a material were merely bench-scale devices that did not have the capability of gathering data on a more useful large scale.  Combine this with the enormous construction costs of building such a unit (NIST spent $250,000 to build a calorimeter in 1977 1) using heat release rate to quantify the size of a fire was impossible.  Finally in 1982, Dr. Vytenis Babrauskas developed the Cone Calorimeter with enabled scientists to accurately measure heat release rate on a macro scale.  This translated to comprehensive studies of entire structure fires and not small material samples.

 

In 1982 the ASTM preformed the first full scale fire room tests to determine the importance of heat release rate in a building fire.  First they needed to consider the typical conditions within fire where an occupant death occurs.  The assumptions made were:

 

-Ignition of a fuel remote to the occupant

-The fire grows and spreads throughout the structure creating smoke and toxic     gases

-These toxic gases incapacitate and untimely kill the occupant.

 

With these parameters a building fire can be broken down into four quantitative fields.

 

-speed of ignition

-rate flame spread

-heat release rate

-release rate of smoke, toxic gases and corrosive products

 

The first example was a simple case where a single upholstered chair was ignited in a room with a single door opening.  Using this as a baseline, three other version were calculated to predict the time to incapacitation and time to death of any occupants.

 

Fig 4.  Take not the drastic reduction in survivability when the HHR is doubled.

 

As we can see in Fig. 4, the only factor to directly diminish the time to death was doubling the heat release rate.  This proves how the heat release rate is the single most important variable in the development of fire.  Through this research, Dr Babrauskas has developed the three principals for this.

 

  1. Heat release rate is the driving force for fire. This is because heat release rate is an exponential phenomena. Simply heat makes more heat.  This cannot be said for other variables that contribute to the lethality of fires.  toxic gases do not create more toxic gases; smoke does not create more smoke; etc.

 

  1. Most other variables are correlated to heat release rate. As the heat release rate increases so does the products of combustion such as smoke, toxic gases, room temperatures and fire spread. This is clearly depicted with the fire development curve we are all familiar with.

 

  1. High heat release rates directly indicates a high threat to life. For example, a sheet of paper is easily ignitable and has a high flame spread rate.  This alone does not make a burning sheet of paper dangerous.  However, a high heat release rate translates to rapid temperature changes, faster flashover times and an increase in the production of the products of combustion.

 

I know the question you have is “Why is this important to me?  I am a line firefighter.  How can this help me on the street?”  To answer this question let’s learn from a tragedy that occurred in Cincinnati on March 21, 2003.

 

Oscar Armstrong III was killed in the line of duty after being caught in a flashover at a residential structure fire.   The two story house was your typical urban bungalow of ordinary construction with exterior walls constructed with brick walls and the interior with wooden members.  These members were covered with thin wooden paneling throughout the first floor.

 

The fire started in the kitchen from an unattended pot of grease cooking on the stove.  There was heavy fire on the first floor from the C side reported upon arrival.  After a mere 3 minutes 40 seconds a flashover occurred killing Armstrong and injuring 2 others.

 

Initial operations transpired as follows:

 

-Upon arrival Engine 9 arrived on the scene and stretched their 1 ¾” handline to the front entrance (Side A) which was locked.  (Estimated time: 60 seconds)

 

-They then stretched the hose to the rear of the structure. (Side C) They were then advised to return to the front and attach from the unburned side.  (Estimated time: 45 seconds)

 

-The line is then led back to the front where an axe was retrieved to force entry. (Estimated time: 30 seconds)

 

-After unkinking the line and establishing water Engine 9 enters the structure.  (Estimated time: 55 seconds)

 

-A flashover occurs and water is sent to the initial attack line but delayed due to kinks. (Estimated time: 30 seconds)

 

We learned in the previous article about flashover that our goal as an engine company is to have water on the fire within 90 seconds after making entry to prevent a ventilation induced flashover.  This is a prime example of this benchmark.  After spending over 2 minutes placing the initial attack line entry was finally made.  A mere 85 seconds later a flashover occurred killing one and injuring two others.

 

Why did this happen to fast?  High heat release rate.

 

Although the fire occurred in a legacy home, it’s contents were made of synthetic materials.  These materials have an extremely high heat release rate that caused the fire to rapidly grow and consume an enormous amount of oxygen driving the fire into a decay stage.  Although the reports of heavy fire may lead some to perceive an advanced stage fine, respect must be given to amount of energy that is released when these contents combust.  A single overstuffed chair has enough energy to drive a 10‘x10’ room to flashover.  If your like me, you not only have a chair but a couch, love seat, tv, tables, ect.  You see my point.

 

The concept of heat release rate can easily appear very intangible and irrelevant to the street firefighter.  However, when you are faced with a fire that so grows rapidly it is nearly impossible to react to changing conditions there is one single factor driving this phenomena: Heat Release Rate.  We need to repeat this phenomena knowing that it alone can place us in a precarious situation.

  1. Dr. Vytenis Babrauskas, . (1996). Heat release rate: a brief primer. Available: http://www.interfire.org/features/heat_release.asp. Last accessed 10th Dec 2011.

 

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