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  • positive pressure ventilation

    i'm interested in finding out the pros and cons concerning PPV.i 've heard alot of talk about it and was wondering does it work,in what situations can it be applied,and also the initial evolution for butting it to work.also some of the do's and dont's concerning its uses,i.e. what types of structures warrant this evolution and which ones dont.thanks for your comments mike m also sorry for the add on,Can anyone explain in brief Mittendorf's theory of Ventilation

    [This message has been edited by mike m (edited 04-25-2001).]

  • #2
    Positive pressure ventilation does indeed work in many different types of structures, but it does require the structure to be intact to do so. Some of the many Pros of PPV are: Forced ventilation can be set up without entering the smoke filled area; PPV can be used in either vertical or horizontal ventilation efforts; removal of smoke and heat is faster; fans are not hanging in the doorways or windows blocking ingress/egress; PPV will remove smoke and heat from large, high ceiling areas that are difficult to clear with negative pressure ventilation. On the down side, interior carbon monoxide levels can sometimes be increased when using PPV fans, and there is also the possibility that hidden fires can be extended. When performing PPV, you must first secure an outlet for the air that will be pushed into the structure. Failure to do so will result in heated air/smoke/fire blowing back out the point of entry. As with other types of ventilation, attack teams should be ready to go before ventilation is started, as you are about to really put the fan to the flames.

    Mike

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    • #3
      Pyro said it right, but I want to and one more thing...

      It is ideal to use a PPV fan in a flashover situation b/c it helps remove the heated gases, and may prevent the flashover.

      However in a backdraft situation , you might want to leave it on the truck, blowing in the structure with the PPV fan will give the fire the air it needs to breath and will make the situation worse.

      A good size up by command will help you decide whether or not to use a fan.

      pat

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      • #4
        PPV is a great tool, but you should be careful to prevent it from becoming your only ventilation technique. In some cases, especially with limited manpower, it's easier and faster to break windows. We have run into the problem of relying too heavily on PPV. You need to understand that the insurance co. is going to pay to replace the windows, broken or not, if the building sustained a good fire. (I install replacement windows on the side-no I'm not trying to get extra work) I've seen guys overhauling in thick smoke and heat for 20 minutes in a room with a full wall of windows because they were using PPV to clear it.
        Also, don't forget about hydraulic ventilation-it also has it's place.

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        • #5
          I agree with some of what has been said, however one comment I feel is contradictory to one of the "advantages" mentioned.

          It was said that one could set up PPV from the outside without entering the smoke filled area, but in the disadvantages it was mentioned that one had to secure an outlet thru a structure.

          To me Securing an outlet means not only taking out a window (which hopefully is the room of origin) but also making sure there are no obstructions like closed doors between the fan and the exhaust point. My question is how is this done without entering and checking to see there is a clear path for the heat and Gasses?

          This leads to the question how safe is it to enter a building without proper venting?

          If this isn't done as already mentioned that fire and heat will chase its way back to the fan (source of O2) and potentially cut off the nozzle team.

          I have also been wondering if it really creates "positive pressure" in a structure or if it is more like just creating "forced" horizontal ventilation. Has anyone seen any empirical studies using Barometers or anything?

          As for some other counter indications here are some I have been taught.

          -Fire in concealed spaces.
          -Fire in an attic or Cockloft
          -Potential Victims trapped (what happens if you break the window from the outside for an exhaust point and there is a victim in that room??? Where will the fire and heat theoretically go?)
          -Don't fight the wind, if you can't set it up to work with the natural wind don't do it...you will loose against Mother Nature.

          Those are a whole lot of variables that you have relatively little control over or insightful knowledge about early on in a fire. Perhaps putting the fans in place and not using them until knockdown would be a smarter and safer practice.

          Two cents from a fireman.

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          • #6
            To address some of Fred's concerns, PPV is good to use when you have a very clear idea on where the fire is.

            If you have a house charged with smoke, but no clear signs of where the seat of the fire is it's probably not the time for PPV until you have manually vented at gotten an idea for the location of the fire.

            But take a simple floor plan house -- one room shows signs of fire, and there's no counter-indications that the fire may have taken hold of the basement or attic, you vent the room and set up PPV.

            Either the door to the fire room is open, and the PPV will push the products of combustion back through the door and out the vented window. Or the door to the fire room is closed, the crew still advances in a relatively clear environment, then they open the door when ready to flow, and the PPV helps push the heat and unburned fire gases out of the building.

            It is not for every building or situation, but when conditions are correct for it, PPV is a very effective in helping an attack and clearing the building of smoke.

            ------
            As for "positive" pressure, while I don't have imperical readings, I have felt the difference in opening doors under "postive" and "negative pressure situations. Don't forget if a door is 3 feet wide and 7 feet tall, that's over 3000 square inches of surface, so even a 1/30th of a PSI difference can make that door feel a hundred pounds harder to open. And when we're using PPV fans in drills around the station, you find it's harder to open a door against the PPV charged part of the station. On the negative ventilation side, I doubt fire service fans could generate the force to feel it, but our apparatus bay is vented by a 24" fan that sucks out diesel fumes, and on the odd occassion it's stuck on with the apparatus doors closed, it's noticeably harder to open a door against it, and you can actually hear the building creak. So I don't doubt for a second that there is indeed a "positive" pressure developed when PPV is applied relatively close to accepted practices.

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