Escaping a high rise fire.

Discussion in 'Architecture & Engineering' started by kwhilborn, Nov 18, 2012.

  1. Peter Dow Registered Senior Member

    I don't think you have demonstrated an issue there. Why not slide speeds?

    Roller-coaster speeds maybe be considered and anyway roller-coaster design engineering expertise to contour the rail so as to manage centrifugal forces when navigating the corners of buildings would be employed.

    No. I don't think that's required. I was thinking of something much simpler - simple linear friction to dissipate excess gravitational potential energy on the way down, though the friction would be between the harness hook and the rail, rather than between the mat you sit on and the slide in the case of the helter-skelter.

    Yes we are agreed on that.

    To what maximum length of descent are such devices commonly available? I suggest that sky-scraper height lines are too expensive but do please link to the webpage of a commercially available product with prices which proves your point.

    Well a more parallel system has its advantages of course but I suppose there could be issues with your suggested system like getting tangled up in the lines of other users on the way down. Also if we are talking melting, I would have thought that kevlar lines would be more likely to melt than a steel rail or hook. For durability the rail and hook friction surfaces could be coated with high wear materials such as tungsten carbide.

    There is certainly a design job to be done to merge the pit stop exit rails with the main rail and ensure hooks merge without jamming.

    Well you can pay for the maintenance and testing by selling tickets to extreme sports types to take rides down the system for fun.

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  3. billvon Valued Senior Member

    Well, using kid slide speeds you could probably put someone in it once every second or so with careful planning. For a building of 20,000 people (large skyscraper) that's an evac time of over 5 hours.

    So you are talking a rail overhead plus a slide beneath? Where would the friction come from? If it's from the rail + hook, then all the person's weight has to be on the rail. What would the purpose of the slide be in that case?

    That's about half a megajoule you'd have to dissipate, so each hook would have to be designed heavy enough to dissipate that heat. (Or use an ablative material that melts/vaporizes and takes energy with it.)

    The most common ones are about 100 feet max, intended for forklift workers and high steel workers. (Rarely are they more than 100 feet from a sold surface or landing.) They run around $6K in small quantities.

    Possibly, although there is no issue with your _own_ line tangling; you'd only have to not get tangled in other lines that are below you as you are descending.

    Kevlar starts degrading at 500C; steel starts softening at around 425C. Tungsten carbide is great for durability but terrible for friction so you'd need something 'sticker.'

    Just get them to sign waivers first!
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  5. billvon Valued Senior Member

    Here's another system that's good for 1000 feet:
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  7. Peter Dow Registered Senior Member

    Nonsense. People can hook on and begin descent at every fire exit door / porch / balcony where users hook on to the pit stop exit rail. I presume there would be at least one such fire exit on every floor in the case of a one helix system and two fire exits per floor in the case of a double helix system.

    I do not see why you think people using one fire exit cannot do so simultaneously as people are using other fire exits?


    No the users' feet would dangle free. Underneath should be an emergency and maintenance staircase similar to a conventional external fire escape.


    There would be no slide to sit or lie upon. The analogy with the helter-skelter was in the helical path the rail takes and with the dissipation of energy via linear friction.

    Agreed. A 100Kg person descending 500 metres has potential gravitational energy to dissipate of

    E = m g h
    = 100 Kg x 9.81 ms-2 x 500 m = 490,500 Kg m2 s-2 or Joules

    Many are heavier than 100Kg of course but this is back of an envelope estimates.

    The specific heat capacity of steel is 0.466 J g-1 K-1

    So assuming all the energy goes to heating the hook and rail the mass temperature rise product for this 100Kg person descending 500 metres is about 1 million grams - degrees Kelvin in steel.

    In other words enough energy to heat 1 Kg by 1000 degrees K (or C) or 1000 Kg by 1 degree (K or C).

    This heating energy will not be distributed equally between rail and hook at all times because heat energy will also flow by conduction from the hotter body to the cooler. Also if there is a difference in the surface hardness between the hook and the rail, the softer surface will heat up by friction more.

    The hook will start off cold at the top of its journey and be hot at the bottom. The rail will start off cold when the first person descends and heat up gradually with each person descending.

    I would suggest air cooling helped with metal cooling fins on the hook and perhaps on the rail too would be plenty to keep temperature rises to modest levels.

    I don't think so.

    $6K for 30 metres but what is the cost for 300 metres?

    Or in front of you as you exit the fire door / porch / balcony and are obstructed with a curtain of kevlar lines blocking your exit.

    That sounds like a misleading comment. What is the specific tensile strength of Kevlar vs Steel at 425 & 500 C? That's a fairer comparison.

    Well the friction is also a function of the surface smoothness and a courser tungsten carbide surface can generate as much friction as required I suspect.

    Well the testing will be done with weighted human dummies and professional testers. If it is not safe then no-one should use it.

    So the extreme sports paying riders will not be at any more risk than the building's users who are using the fire exit in an emergency therefore the issue of insurance and waivers applies equally to all users of the building who might use this fire escape method as it does to extreme sports paying riders.

    So no, if we do not expect visitors to a business with an office in a sky-scraper to sign waivers in case they might have to use the fire escape then the building's owners would not need to demand waivers to be signed by extreme sports paying riders either. The general building insurance should cover all using the fire escape system equally.
    Last edited: Jan 5, 2013
  8. Peter Dow Registered Senior Member

  9. billvon Valued Senior Member

    From the first page:

    "A lightweight plastic reel holds the cable neatly next to the system. This
    design allows us to easily create CDD’s of any length up to 1000 feet (333
    meters) . . . "
  10. billvon Valued Senior Member

    OK. So what's your capacity? Your constraints are:

    -how far apart you want people to be
    -how accurate your sequencing/interlocking system is (i.e. if people must be 3 feet apart, and you have a 6.1 foot gap, can it "hit" that gap accurately enough with a new passenger?)
    -gotta keep speeds well below the speed at which someone hitting the ground will be injured (below about 20 fps)

    Ah, I see. (And that way I can just take the stairs!)

    Well, you're going to have a tradeoff there. The individual's hook can't absorb too much heat without melting, and the rail can't absorb too much heat from each person. One person is no problem for a long rail system but multiply that by 20,000 and you could have problems. Mass will be your friend there, so if the rail is heavy enough you could probably dump that heat.

    ?? A 300 meter line of 1000 pound test Kevlar (tradename Vectran) weighs 3 pounds. That's not going to require a heavier line to support its own weight.

    $100,000 per person? You're assuming that going 3x higher will cost 16x more? Don't see the logic in that. It certainly isn't true with any other kind of descender.

    Keep in mind that the unloaded ones will be quite easy to push through. And if there's a curtain of _loaded_ ones in front of you too dense to push through, it means there are hundreds of people below you - which means it's probably not a good idea to exit at that moment.

    No system is entirely safe. However, if you say "If it is not safe _enough_ then no-one should use it" I'd agree with that.

    In a non-litigious society I would agree - but that sure isn't what we have here.
  11. Peter Dow Registered Senior Member

    OK I see the claim now, on the Home page.

    And the demonstration videos are quite impressive actually.


    Yes these could save some lives!

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    I do like the idea as a fire safety improvement product because the residents of one flat or the business with one office in the building could install a Controlled Descent Device for their own use without waiting on the building owners to install something for everyone, a communal system such as my proposed helical rail fire escape system.

    Of course it is ideal if we can save everyone from fire but why should safety conscious families and businesses wait until everyone in the building wants to, and is willing to pay to, invest in communal safety measures? No we should not have to wait, if we can save our lives now, we should. So good product, good idea!

    On the other hand the demonstration videos seem to be limited to about 10 floors and so I remain to be convinced about about the 1000 feet maximum potential. If they claim 1000 feet then they ought to have a video demonstrating 1000 feet, right?

    Issues for sky-scraper use could be

    • Overheating - I don't see cooling built in to that particular device demonstrated in the video so at some point of continual use it is going to overheat, seize or fail. No doubt though it would be possible to produce a variation on this product with air or water cooling built in. One idea is that the energy of descent could be used to drive a fan to push air through either cooling fins or a water radiator.
    • The time - 1 meter per second is 5 minutes for 300 metres - which is fine for one person but not so good if there is a long line of people waiting to use the CDD. So if you wish to evacuate everyone in 20 minutes you would need one device per 4 people at 300 metres, (this assumes that the device can run for 20 minutes without overheating but I don't know that about this product since they don't seem to give a figure for how long it will run for), 1 device per 6 people at 200 metres height and 1 device per 12 people at 100 metres. Again though a variation on the product with computer control and effective cooling mechanism could have a higher descent rate which slows under computer control as you get near the ground for a soft landing.

    All in all an excellent product and what is more, it is available now, not just a back-of-the-envelope plan like mine!

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    Last edited: Jan 5, 2013
  12. Peter Dow Registered Senior Member

    I think people could even risk touching or bumping into each other - they do elsewhere - on stairs for example.

    Do you mean for the pit stop exit rails joining the mail helical rail? Well I said that was a design task to overcome but I am not about to design that here and now. The pay isn't good enough.

    Well the final section of rail will travel almost horizontally and people will slow down under friction. One fireman can help push people to the end of the rail if they run out of momentum before they get there.

    If the building's internal stair cases are fire and smoke free I would assume everyone, apart from extreme sports types and perhaps people who were not fit enough to walk down all those stairs, would opt to take the internal stair case. But no, the emergency external stair case under the rail is not for normal evacuation - it is for in case there is a problem with the rails - a jam or something - and firemen / firefighters /first-responders need to come along to unjam it or lower people from the rail onto the stair case and yes if you are unlucky enough for that to happen to you then you can take the emergency external fire escape stairs.

    True but if the hook has it's own sufficient-to-the-task cooling fins it will reach a maximum operating temperature where the heat being added by friction is the same as the heat lost by cooling. Think of a motor car engine - same idea.

    Well that's one friend I have got anyway.

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    You are right. I had quite forgotten the amazing strength to weight ratio of modern ropes. That's not the issue I had first assumed without checking the specifications and so I have edited the comment you quoted from my earlier reply post.

    OK. My bad.

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    And if there is a fire at your back and loaded lines blocking your path and a knife to hand ....

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    Well if someone really wants to sue you, is a waiver going to stop their lawyer anyway? Do people sign waivers when they go on a roller-coaster ride? Just make it safe, take their money and see them in court if it comes to that.
  13. Peter Dow Registered Senior Member

    Wrong assumption looks like.

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    There seem to be a number of similar devices on the market and one company, Tractel, publishes the technical sheet for their Derope emergency escape, controlled descent device.

    The Derope's technical sheet includes a table "MAXIMUM NUMBER OF CONSECUTIVE DESCENTS".

    [table="width: 800"]
    [td]Descent height[/td]
    [td]<165 ft. (50)m[/td]
    [td]<410 ft. (125m)[/td]
    [td]<650 ft. (200m)[/td]
    [td]<1,000ft. (300m)[/td]
    [td]<1,300 ft. (400m)[/td]
    [td]Capacity of 310lbs (140 Kg)[/td]
    [td]Capacity of 500lbs (225 Kg)[/td]
    So for this device, and most likely it will be similar with the High Rise Escape Systems product too, you need 1 device per 2 people to get out at 1000 feet / 300 metres.

    So the concept of controlled descent device is a good one but I think the sealed clutch friction plate idea is not an easy device to cool in time so I would like to suggest another energy dissipation device to use instead - an integral air compressor driven from the power from the descent rope and simply vent the compressed (and heated) air to the atmosphere.

    Design that right and it would be able to run continuously. Oh I am a clever boy!

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  14. billvon Valued Senior Member

    Or have the pulley drive a water pump. Dissipates the excess energy - and gets water to the sprinklers.
  15. Peter Dow Registered Senior Member

    I see a number of issues with that suggestion.

    1. That would need a water in / out supply to the CDD.

    This would mean the CDD unit had to be either

    • permanently installed in position for use and plumbed in - the demonstration videos shows the advantage and tidiness of the CDD being packed away and yet speedily hooked up from storage - and having it in place all the time would take up space which would be unsightly and inconvenient for general life but possible I suppose -

    • or if it was packed away and disconnected you'd have to waste precious escape time - connecting the supply hoses, making sure there was a good seal and the air bubbles had been flushed out - which would be dangerous and risk lives in a fire.
    - both unsatisfactory options compared to the air-compressor which requires no connections to an air supply hose or to an air receiver tank only a cool air inlet and warm air outlet in the CDD.

    2. If there is a water supply for the CDD then that same supply can probably supply the sprinklers directly without further pressure. If there's no water pressure because the sky-scraper header tanks have run dry and a power-outage is preventing the basement pumps from refilling the header tanks then a water pump high up is not going to help. Water pumps can't suck up water from below to much of a height (10 metres / 34 feet is about the maximum height a water pump can suck up) before the pressure in the input water pipeline drops and the water boils and you are pumping water vapour (very inefficiently with a water pump). So there's nothing to be gained really to help the sprinklers in most scenarios I can think of.

    If you wanted to add a function to an air-compressor CDD then the obvious function is a fire alarm. Sticking one or more air horns on the CDD air-output will probably wake or alert most of your neighbours no problems at all.

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  16. billvon Valued Senior Member

    Yes, it definitely has issues, complexity being the primary one. A big ol friction brake is a lot simpler.

    I'd say the biggest problem with the air compressor idea is that air compressors heat air, not cool it, and thus the system has to survive working at a very high temperature. And the obvious question would be - if it's designed to work at that high a temperature, why not just have it work at that temperature and dissipate the heat into the air? The higher the temperature differential the better a heat sink works.
  17. Peter Dow Registered Senior Member

    Well as we have seen the existing CDD's that are small enough for people to lift into place to use are strictly limited in continuous performance because of the issue of over-heating.

    No the temperatures don't have to be that high if the compressor compresses a high enough volume of air, the temperature rise of the air can be modest and manageable. Think of an electric hair dryer - that can consume 1 or 2 kW of energy which is about the power supplied by 100 Kg / 220 lbs descending at 1 or 2 metres per second / 3 to 6 feet per second - yet the temperature of the hot air jet from a hair-dryer is safe enough to dry hair with.

    Incidentally, that is another design idea for a CDD - rather than compress and heat the air one could heat it using an electrical heater element as per a hair dryer. So in the hair-dryer CDD the descent rope powers a dynamo / alternator and a fan to drive air over the electrical heater element. I suspect the air-compressor would turn out to be lighter for the same power consumed but it might be worth a more detailed study to check.

    "it"? Well what are you talking about now, an air-compressor CDD, a conventional sealed friction clutch plate CDD or what?

    The Derope CDD whose continuous performance statistics I quoted, does have metal cooling fins to try to help dissipate excess heat from the unit but however cool the metal fins are kept the real problem is that the heat is generated in the friction plates and the only way for the heat to escape from the friction plates of those simple units is by conduction - from the friction plates to the exterior cooling fins - and that's not really a very efficient way to cool something, as the performance statistics I quoted prove. Even if the metal cooling fins are reasonably cool if the friction plates melt then the device fails.

    With the air compressor / heater CDD the thing that gets hot is the very air you are venting from the device and the inflow of cool air helps to keep the internal compressor parts cool.
    Last edited: Jan 6, 2013

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