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I Bernouilli effect in CO2 fire extinguishers

  1. Aug 4, 2017 #1
    My CO2 extinguisher has a restriction placed at the point in the gas horn where the CO2 enters it. Why would anyone want to reduce the gas pressure here and speed the gas up when the pressure in the CO2 reservoir is pretty high to begin with and could generate a stream that moves fast enough even without the Bernouilli mechanism?
     
    Last edited: Aug 4, 2017
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  3. Aug 4, 2017 #2

    boneh3ad

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    This is a compressible (and likely two-phase) flow application so Bernoulli doesn't apply. It's intent is likely to control mass flow rate.
     
  4. Aug 5, 2017 #3
    Thanks boneh3ad.
     
  5. Aug 5, 2017 #4

    Nidum

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    The outflow from a CO2 extinguisher is noticeably cold and in some conditions there is visible vapour . I think that the nozzle system is probably deliberately designed to cause cooling of the CO2 and so make it more effective at quenching fires .
     
  6. Aug 5, 2017 #5

    rcgldr

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    The cooling is going to happen since the pressure decreases from the tank pressure to ambient pressure. A answered by boneh3ad, it's more likely that it's just a restriction to limit the maximum mass flow rate out of the tank.
     
  7. Aug 5, 2017 #6

    boneh3ad

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    CO2 doesn't quench a fire based on temperature. It works by displacing the oxygen and depriving the fire off that critical reactant.
     
  8. Aug 6, 2017 #7
    Thanks everyone. In that case boneh3ad would you agree that it wouldn't matter where in the pipe leading from the tank to the horn at the output end the restriction was placed? The reason I'm asking about this restriction is that I want make a 1.5 m long plastic extension on the end of the application pipe on my CO2 fire extinguisher and I wanted to avoid mistakes due to misconceptions.
     
  9. Aug 6, 2017 #8

    Nidum

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  10. Aug 6, 2017 #9

    russ_watters

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    I don't know what your purpose is, but I would tend to advise against modifying a fire extinguisher that is intended to be used to extinguish fires and invite @berkeman to provide a speech on the issue...
     
  11. Aug 7, 2017 #10
    Nidum, your link doesn't explain the purpose of item 22, the nozzle. russ waters, of course it's purpose is to extinguish fires. That's what I want it for. But have you ever tried holding a 5 Kg CO2 extinguisher close enough to a fire to put it out? My purpose in extending the tube was to allow the reservoir to stand on the floor while the horn is held close enough to do the business. Have I got this right: the nozzle will have to be positioned at the small dia. end of the horn in order to create turbulence as shown in the above link and thus prevent air from being trapped in the gas flow and going to the fire?
     
  12. Aug 7, 2017 #11
    Longer hoses are available up to about a meter or so length. Ask your representative where you bought the item.
    One hand has to be on the tank control and the other on the horn.
    Correct type of horn is required so that your hand dose not freeze to it when being held - a possible solution is a grip extension back from the nozzle within the horn.
    Hose has to be able to withstand the pressure of the liquid CO2, along with connections. Doing your own questionable. Perhaps they come as a unit of tank, nozzle, hose, horn, with irreplaceable parts for performance. Ask your representative.

    What's the worst that can happen with a user modified extinguisher - it malfunctions and your house burns down.
    Since you came here to ask some questions, it might not be wise do it on your own. I would recommend expert advice from the fire-department, or the city's subcontractor consigned to do so, and for them to certify the fire-extinguisher. They may refuse with homemade alterations.
    http://smallbusiness.chron.com/fire-extinguisher-inspection-tag-requirements-40048.html

    Co2 replaces the oxygen starving the fire. The 'cold' with adequate application cools the parts so they do not flare up again. At least in theory. So keep spraying for some time even after no flames are evident.
     
  13. Aug 7, 2017 #12
    Yes, I know all this. But what is the purpose of the 1 mm dia hole just before the input to the horn? I keep asking this question and all I get is a lecture about something else, something obvious and which I already knew. Please, somebody tell me the purpose of the 1 mm dia hole. Why so small? Is it as boneh3ad said to reduce the mass flow rate? Is that the only reason? If so why can't the hole be positioned further up the tube leading to the horn?
     
  14. Aug 7, 2017 #13
    That item 22 is the nozzle at the entrance to the horn.
    I am in agreement with Boneh3ad.
     
  15. Aug 7, 2017 #14

    Nidum

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    The nozzle does control the flow rate but the nozzle and expansion cone together form a very neatly designed system which takes the mixture of liquid and gaseous CO2 from the cylinder and allow it to expand in a controlled way so that it leaves the discharge end of the expansion cone at the optimal velocity and temperature for extinguishing fires .

    If you put the nozzle at some randomly chosen location rather than the correct designed location the flow of CO2 leaving the discharge end of the expansion cone will probably be so far off the optimum condition that it becomes ineffective or useless for extinguishing fires .

    The discharged CO2 from a properly set up nozzle and expansion cone is very cold indeed - in fact so cold that it is partially CO2 snow - and this is an important factor contributing to the effectiveness of CO2 extinguishers .
     
  16. Aug 7, 2017 #15

    rcgldr

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    I'm thinking that the small hole also reduces the pressure within the hose. The pressure in the CO2 tank is probably many times greater than what the hose could tolerate.

    In this situation, the pressure in the tank is much greater than the atmospheric pressure that the flow exits into. I assume that most of the acceleration of the CO2 through the small hole is due to the pressure differential between the tank side and the hose side of that small hole, rather than Venturi effect.
     
    Last edited: Aug 8, 2017
  17. Aug 7, 2017 #16

    russ_watters

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    Generally if people disagree they will say it and if they agree - which I do - they won't necessarily chime in to just be redundant.
     
  18. Aug 8, 2017 #17

    boneh3ad

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    That doesn't make any sense, though. The throat is at the end of the hole, so that hose is going to be essentially at tank pressure throughout its length.

    Yes, and this key fact is the first clue everyone should have that this flow is going to be compressible and that Bernoulli's equation is entirely irrelevant. The "Venturi effect" is not only unlikely to be present in the traditional sense, but if the flow is at any point supersonic (and it very well might be given the shape and pressure), then the so-called Venturi effect is the exact opposite of what happens. This is why you never her "the Venturi effect" referenced in a fluid dynamics course. It gives the wrong idea that pressure and duct diameter are the important parameters, whereas in reality, pressure and velocity are the important parameters.

    I think you described the two sides of that hole you are referencing incorrectly by accident, but either way, for a subsonic flow, the Venturi effect and flow acceleration due to pressure gradient are indistinguishable phenomena. There is no one or the other. They are the same. For a compressible flow, the Venturi effect is not even a thing, as the behavior of a flowing gas changes substantially beyond Mach 1.

    I think the fundamental issue here is that we need to think for a moment about how these actually work. CO2 is stored in the bottle as a liquid and travels through the hose (at least largely) in that same state. The flow restriction here effectively functions the same way as a thermal expansion valve in a refrigeration system. The general idea is that it accelerates the flow, causing a pressure drop and at least a partial phase change. It also restricts the total mass flow rate based on its diameter.

    The second function is that, as the high pressure gas exits, it is generally going to be at a much higher pressure than the surroundings. In fact, it will be so much higher that the (mostly) gas coming out of that restriction is going to be moving at the speed of sound. As I mentioned before, when you reach Mach 1, the area-velocity relationship flips, so that expanding cone is actually still functioning as a nozzle for at least some of its length, speeding up the flow and further lowering the pressure. In effect, the whole system is one giant thermal expansion valve.

    Now, it's unlikely there is enough pressure to carry this out all the way to the end of the cone, so the flow coming out the end isn't likely to be supersonic. There is therefore likely a termination shock somewhere inside that cone to make sure the final exit pressure matches the atmosphere. It's that acceleration and pressure drop that causes the phase change, and that is a result of the restriction and cone.

    This explanation has been somewhat simplified because I've mostly ignored the multiphase nature of this flow.
     
  19. Aug 8, 2017 #18

    Nidum

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    (1) The expansion tube does not have the usual geometry where the entry is at nozzle diameter and then the diameter increases uniformly .

    What there is in a conventional design CO2 fire extinguisher is something more like a dump diffuser where the expansion tube swells rapidly in diameter near the nozzle . The remaining part of the tube going to the exit then increases further in diameter at a uniform but slower rate .

    The purpose of this geometry is to establish an annular vortex around the first part of the outflow from the nozzle . The annular vortex causes this outflow to expand rapidly - effectively making a virtual expansion cone . Any shock wave events occur somewhere in this region .

    For the remaining relatively long length of the expansion tube the flow expands at subsonic speeds .

    (2) A secondary but valuable advantage of this geometry is that it ensures that the expansion tube is completely filled with CO2 . Some other arrangements can cause the flow to detach from the tube walls which results in the final outflow of CO2 not being in the required condition .

    (3) The outflow from an extinguisher is observably diffuse and moving at a relatively low velocity .
     
  20. Aug 8, 2017 #19

    A.T.

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    True, but it makes sense to also cool the burnable material, so it doesn't ignite again once oxygen gets to it.
     
  21. Aug 8, 2017 #20
    I have a second CO2 extinguisher which has a 14 cm long metal tube between the input to the horn and the cylinder output. This is an unmodified commercial extinguisher. I don't know where the pressure reduction hole is on this one so the tube may not in fact be tolerating tank pressure but there is a 1mm dia hole seen by looking into the open end of the horn so it could be there. If it is, as I suspect, then the tube must be tolerating full tank pressure.
     
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