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Thread: Thermal Balance

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    MembersZone Subscriber E229Lt's Avatar
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    Default Thermal Balance

    The term gets thrown around a lot but how many different views are out there as to what THERMAL BALANCE really is?

    A layer, a current...what's your definition?


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    Correctly or not, it seems like this term and thermal layering are used interchangeably.

    I think of it as higher = hotter and lower = cooler.

    If you spray water toward the ceiling for too long (especially in a fog pattern) it moves the hotter air/gases down toward the floor and brings the room "out of balance" and cooks you.
    FTM-PTB-DTRT

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    Forum Member DeputyChiefGonzo's Avatar
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    It's the temperature gradient between the different layers of air in a room, much like the nautical term...thermocline, which decribes the difference in the temperature in layers of water.

    The difference, of course, is that while water is heated from above, ie, the sun, the air in a building that is on fire is heated from the fuel that is burninmg

    Working support for the Recuit program at the Massachusetts Fire Academy, I get to see "thermal balance" every burn day.
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    Forum Member ThNozzleman's Avatar
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    Default

    It is also confused with "thermal boundary" which is a weather related term.

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    Senior Member FFMcDonald's Avatar
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    Question

    Trying to look at this from a completely non-firematic point of view.

    A "Balance" is something that is in equilibrium. Not too heavy - not too light. Equal. Balanced.


    And from dictionary.com....
    2. A single thickness of a material covering a surface or forming an overlying part or segment: a layer of dust on the windowsill; a cake with four layers.
    3. A usually horizontal deposit or expanse; a stratum:layers of sedimentary rock; a layer of warm air.
    4. A depth or level: a poem with several layers of meaning.
    5. An item of clothing worn over or under another


    In my meager humble opinion - I think that the term 'Thermal Balance' is used incorrectly - and that along the same lines of thinking as WTFD10, and Captain Gonzo - I have to go with the thermal layering.
    Marc

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    I think that in this case "balance" and "layering" are somewhat interchangable. Thermal Balance is what keep our workspace relatively clear, while the ceiling and upper areas are not. There is a "balance" between the relatively cool "layer" and the heated "layer".

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    MembersZone Subscriber E229Lt's Avatar
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    Next question. Why all the talk of not "upsetting" the Thermal Balance? Sometimes it is said like "don't look into the sun". Exactly why should we be over concerned with the Thermal Balance as we begin our attack? Exactly what are the negatives here?

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    Forum Member DeputyChiefGonzo's Avatar
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    Originally posted by E229Lt
    Next question. Why all the talk of not "upsetting" the Thermal Balance? Sometimes it is said like "don't look into the sun". Exactly why should we be over concerned with the Thermal Balance as we begin our attack? Exactly what are the negatives here?
    The negatives...

    1. Survivability of victims! Using the wrong type of stream or the incorrect method of attack with reported people trapped in the building will give them a death sentence.

    2. Finding the fire. Upsetting the thermal balance/layer makes it that much harder to find the location of the fire and/or victims.

    While having a thermal imager on the initial fire attack is nice... I am a firm beliver that Murphy lurks on the fireground... upsetting the balance and having the imager go kaput is not a good thing!
    ‎"The education of a firefighter and the continued education of a firefighter is what makes "real" firefighters. Continuous skill development is the core of progressive firefighting. We learn by doing and doing it again and again, both on the training ground and the fireground."
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    Senior Member FFMcDonald's Avatar
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    If I take the stone from your hand - may I leave??

    Are you asking this as a question? Or are you posing a scencario - only to tell us the true correct answer...


    Anyway - I'll give this a shot.

    Why all the talk of not "upsetting" the Thermal Balance?
    Personally, I feel you shouldn't upset the thermal balance for two reasons.
    1. You still have some visibility and that can aid in your advance to the seat of the fire, search and rescue, etc...

    2. There may be some breathable air left in the area if there is a victim.

    If there isn't fire - and you are only surrounded by smoke - you shouldn't have the knob flowing. Water doesn't put out smoke.

    Exactly why should we be over concerned with the Thermal Balance as we begin our attack? Why upset it and drop the smoke to the floor and loose what visibility is left? If the smoke is going to bank down to the floor - let it -- Hopefully the truckies are up on the roof cutting a vent to make it easier for the engine to advance. If they are- then your smoke will rise anyway - and your visibility will improve.

    Exactly what are the negatives here?

    I don't follow what you mean completely - but if you hit the superheated air with the handline - then you steam the room. Making it 'rather uncomfortable' for you and your crew- and completely untenable for a possible victim.

    Hope I am not "too wrong" --

    E229LT - let us know what you want - where this is going....
    Marc

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    -- The opinions presented here are my own; and are not those of any organization that I belong to, or work for.

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    This is straight out of the Delmar Thompson Learning Firefighter's Handbook-Essentials of Firefighting and Emergency Response; Chapter 4-Fire Behavior; Thermal Balance and Imbalance. It says:
    When combustion occurs, heat is liberated as part of the oxidation process. The properties of molecules are such that as heat is applied or absorbed, the molecular makeup becomes agitated. The more agitation, the more collisions that occur among molecules. These collisions cause the outermost molecules to push into places where they are less likely to bump into others. Simply, that means that the outermost boundary of the molecular collection will push outwardly and expand. In a given volume of gas or liquid, this means that the weight of the mass will decrease as the number of molecules separate from one another and push out. An illustration of this would be the expansion of a hot air balloon. As more molecules (air) are put into the given volume, the boundary of the space expands. It stands to reason that the less weight that a given volume occupies, the lighter it will be. The lighter any fluid-like substance is, the greater the tendency to gravitate to the upper levels. Conversely, the heavier a volume is, the greater its tendency to drop to the bottom. Looking at the anatomy of heat given off by a fire, one can visualize the lighter heated air moving upwardly and the cooler air dropping to the lower levels. These characteristics become significant when fighting a fire. Knowing how and why heat moves will enable the firefighter to employ tactics that will limit extension, confine the fire and extinguish it.
    Again using a balloon as an illustration, we know that the contents of a lighter than air container (i.e., a hot air balloon) will cause it to rise. It rises because of an imbalance in the given volume of the container compared to the surrounding atmosphere. The air inside the balloon is lighter and therefore causes it to rise. It rises because of the thermal imbalance with the surrounding atmosphere. When the balloon reaches the point where it is no longer lighter than the surrounding air, but is equal in weight, then a balance has been achieved. While we have been using a balloon to visualize the concept of the hot air, hot air itself acts in the same way. Hot air rises from a fire and will continue to rise until it reaches equilibrium with its surrounding atmosphere. When confined to a structure, the hotter air will accumulate on the ceiling of the room and bank down until it can find an escape route. Sometimes it will bank down right to the floor. Watching the smoke from a fire will give us an indicator as to what is happening with the heated air. Because the unburned particles of fuel found in smoke are affected by hot air, tracking the smoke will expose the path taken by the hot air.
    It is the physical characteristics of thermal balance and imbalance that cause smoke to column and mushroom. The heated air and smoke rise. The hotter the air, the faster and more violent the ascent. When the thermal balance of the air has been reached, the ascent ceases and establishes equilibrium with the surrounding atmosphere. The smoke then stratifies and begins to move horizontally in all directions from the central thermal column. The result is a form that looks like a mushroom. However, in a structure, the mushrooming of smoke and heat occur for different reasons. In a structure, the heated air meets an obstruction in the ceiling and, being unable to rise further, spreads out horizontally seeking another vertical artery. If unable to find that artery, it banks down and compresses the volume of air in the structure. This compression causes air pressure within the confined vessel (i.e., the structure) and any opening will show smoke and possibly fire being violently expelled with such force that it causes the smoke to roll and billow.
    Hope this helps.
    CR
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    Forum Member Weruj1's Avatar
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    that Artie is such a smooooooth poster........I am in agreeance with WTFD10 and Capt Gonzo.
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    We used to teach recruits about thermal balance through the use of thermocouples installed in our burn building. The thermocouples were hooked up to instrumentation on the outside of the building that provided temperature readings at three different levels. The temperature at each level varied considerably until the suppression team applied water. At that point, temperatures in the upper point of the room would drop dramatically, but the temperature at the floor (where the firefighters and victims were) would increase. Once a team saw the impact on thermal balance, the next step was to practice using supression tactics that resulted in the least disruption of the thermal layers. The system also allowed an instructor to monitor conditions in the room take action if conditions started to deteriorate beyond what we considered acceptable.

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    Default A question from the peanut gallery. . .

    If you were to go into a fire compartment and open up your straight stream - or solid, if you prefer - toward the ceiling, does this action (besides steaming you and your crew like clams):

    A) Form a true temperature inversion where a cooler layer exists over a hotter layer, pushing the hotter layer downward.

    B) Create a more-or-less uniform temperature throughout the compartment, though much hotter at floor level than before and much cooler at the ceiling.

    Are these effects primarily driven by temperature differentials, or is turbulence caused by steam expansion the main player here?
    ullrichk
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    Originally posted by E229Lt
    Next question. Why all the talk of not "upsetting" the Thermal Balance? Sometimes it is said like "don't look into the sun". Exactly why should we be over concerned with the Thermal Balance as we begin our attack? Exactly what are the negatives here?
    At some point you have to upset the Thermal Balance....otherwise there would be a lot of cellar holes around.

    Until you find the seat of the fire, and can conduct a primary search of that area....upsetting the thermal balance will make our job harder. Of course the other factor is that steam seems to find places to burn you that normal radiant heat doesn't. End up looking like a Fenway Frank...

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    MembersZone Subscriber E229Lt's Avatar
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    Are you asking this as a question? Or are you posing a scencario - only to tell us the true correct answer...
    Knowing my own history here I can understand the question. In fact I really never researched this whole issue very closely and I'm wondering if I should. I look at the Thermal Balance as another technical issue which delays my getting started on what needs to be done. I am willing to grow, if needed, but for now, I know at some point in my attack I'm going to make the room get pretty snotty and, for now, am more inclined to just get on with it.

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    Forum Member Lewiston2FF's Avatar
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    My understanding as a Chemist and a FF of thermal balance is along the same lines as what Gonzo said. It would be the stratification of heated air. I think alot of FFs in my Co would agree with you, but with the type of nozzles we use, it would prove disasterous for any victims.
    It also can make life pretty uncomfortable for the guys upsetting that thermal balance. I think if you wait control by pulsing until some ventilation can be provided, a fire can be put out with minimal upset to the thermal balance.

    I am willing to grow, if needed, but for now, I know at some point in my attack I'm going to make the room get pretty snotty and, for now, am more inclined to just get on with it.
    I will agree with you to a point, how snotty that room gets is up to you. You gonna wait it out until the smoke and heat have somewhere to go or you wanna put it out right away and have to suck it up?
    Personally I like my ears the way they are.
    Shawn M. Cecula
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    IACOJ Division of Fire and EMS

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