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Granted - I had this question while doing the last rap out of Heaps in the pitch black while exhausted, so please forgive me if it's ridiculous. Is there a danger from the descender overheating and causing damage to the rope? I alternated between going faster so my Totem wouldn't stay in the same place for too long and pausing to let it cool down. If there is a danger what's the best way to mitigate it?
Yeah- there is a danger. The best way to mitigate it is to get it off your rope as soon as you're down. The heat won't start to melt unless it's stopped on one place. Typically this happens when you're at the end of the rap and back on solid footing.
Probably depends on the type of rope, too. I foolishly used a polypro rope once and glazed almost the whole thing with a hot ATC. Scary low melting point.Quote:
Originally Posted by xxnitsuaxx
Kevlar? Not so much. More of a concern of damage to the rap device perhaps.
Besides going slow, I think I've heard of folks using water to cool down a hot rap device. Also, soaking the ropes in water prior to a long drop might work too.
I'd be curious to hear what other folks would offer up, though...
Stopping to let it cool down means that the rope is absorbing some of that heat all in one place.. I am sure I am an oddball but I don't like having my rope glazed all to heck and being a caver I have a rack so that is what i use.. much much less risk of overheating and much easier to change friction as you go :)Quote:
Originally Posted by xxnitsuaxx
It gets discussed now and again. There are stories of sport rappellers burning through a rope but only on really long raps, and only when deliberately going REAL fast. Google fails to find anything for me right away.
Worry about it? A little.
Polyester softens at around 240 deg C (464 deg F). Yes, when we run down the rope fast and hit a wet spot in the rope, it hisses - indicating the device is warmer than 100 deg C (212 F). But, it is a LONG way from there to 240 deg C.
There are numerous mechanisms that would tend to prevent your rappel device from heating up too far. As it gets warmer, it transfers heat to the rope, which then disperses it. Unless you stop, in which case that energy goes into one spot. Much of the heat transfer of a warm Pirana is via radiation, which goes up as the 5th power of the temperature difference, so as the Pirana gets hotter, it radiates A LOT more energy.
It certainly seems possible to singe the fuzzy fibers on the outside of a rope, though that I have usually seen from it running across a piece of textile that heats up and gives up melt to the rope (such as, if the rope was running across a sling or a pack). Having done that last rap in Heaps a dozen times or more, I have NEVER noticed a singed or melted part or segment on the rope afterward. Yes, the rope has hissed, a lot! Yes, the device is way too hot to touch. But damage an actual, real, polyester or nylon rope - seems unlikely.
YMMV. CCPD.
Tom :moses:
5th power? What is that, "Jone's Law"?Quote:
Originally Posted by ratagonia
Nat
This was discussed back on the ACA site a few years ago. Some interesting tests took place http://www.canyoneering.net/forums/s...highlight=heat
Oops, OK, my memory perfect less than is...Quote:
Originally Posted by nat
4th power, according to the Stefan-Boltzmann Law: q = ε σ (Th4 - Tc4) Ac
http://www.engineeringtoolbox.com/ra...fer-d_431.html
T :moses:
Nerd Alert!
Since we will all benefit from radiation (not radioactivity--infrared radiation) cooling our descender we will get the best results when the descenders are black in color (for example, look at the heat sink on the processor in most PC's) . It also works in our favor that the temperature for the math Tom shows is degrees Kelvin (zero degrees F= 255 degrees Kelvin). Another consideration is size. A descender with larger surface area will run cooler.
There is also the question of "if is it better to go fast or slow"? Faster builds up more heat but the contact time is less, slower creates less heat but the contact time is longer. So what's the answer--it depends. It's kind of like when I'm out for a run and need to drop a load. Should I run faster to get to the bathroom (that makes the urge worse) or run slower (I'll have to hold it longer). It all depends--do I feel like running with just one sock on or not?
I'm going to borrow the infrared camera from work and make some measurements next time I'm out practicing. I'm just curious how hot things do get.
It's been quite some time since I did my time at the Massachusetts Tool or Die Company, but...Quote:
Originally Posted by agostinone
Colleague Agostino, I expect better...
I does not matter that the Temperature is in Kelvin - the energy transferred is as the 4th power of the temperature difference - the base makes no difference. The units will be consist - metric or Anglish.
At the temps we are talking, the color in visible light makes no diff. The emissivity in the IR does, and certainly a coating could be applied that would increase the emissivity. What it would look like in the visible spectrum, if anything??? Hard to say.
The speed does make a huge difference. What we are trying to determine is the maximum temperature reached - is it enough to singe the rope? There are several ways to look at the problem, and thankfully we are blessed by an acute lack of data, therefore it is all conjecture. We could make some guesses, then calculate say, the max temperature reached at a certain rappelling speed. Or the relationship between rappelling speed and max temperature reached.
Yes, a larger device will run cooler - more mass, more surface area. But these factors might be overwhelmed by the location of heat build up in different devices. Some will transfer more of the descent energy into the rope, some will retain it for themselves.
Since we do not hear of people burning through devices on rappel, I think we can assume that the # of incidents of such is quite small. Losing control while on rappel - much bigger.
Still worried? Bring a squirt bottle. Stop every now and again and give your device a squirt.
Of course, now you are going to start worrying about annealing your device mid-rappel -- uh oh, then what happens???
Tom :moses:
I believe the only real world testing I've seen was done by ATS.
Can't find their video right now.
http://www.youtube.com/ATSadventures#p/u/3/APreETBFDHwQuote:
Originally Posted by oldno7
It wasn't much of a test. And it wasn't really very real-world.
T
Slow and steady is the best way to descend on a long free hanging rappel. The device avoids overheating and you don't stop at any spot to burn the rope.
Nerd Alert--Continued.
Sensei Tom, I too have been away from academia for a long time. I may be confused not by the heat transfer but by the algebra but I think that it may make a difference that the temperature is in Kelvin. I think the formula is ~(Th^4-Tc^4) which should yield a different answer than (Th-Tc)^4.
I to am not aware of a case of the rope actually getting scorched. I just got all excited at a chance to see how heat transfer theory applies to one of my hobbies (I think that qualifies me as a nerd--crap maybe I should have kept this to myself).
Why are you concerned with radiation? Conduction should be the issue.
Entirely correct, Grasshopper. My apologies for questioning your nerd-dom.Quote:
Originally Posted by agostinone
Tom
:amazon:Quote:
Originally Posted by ratagonia
I agree:mrgreen:
The overall question is how hot does the rappel device get? Hot enough to singe the rope or burn through it?Quote:
Originally Posted by dsr70
When we classroom that, the question becomes: what is the relationship between how fast one rappels vs. how hot the rappel device gets?
Figuring out how much energy goes into the system is easy - it is merely the potential energy converted to heat (less the change in kinetic energy (hopefully a small number)). However, figuring out WHERE the heat goes (some into the rope, some into the rappel device); and then figuring out how fast the heat dissipates off the rappel device is more challenging.
Since the singeing temperature for Polyester is fairly high (240 deg C), we will be dealing with circumstances where the rappel device is at quite a bit higher temperature than the surrounding environment (say, 37 deg C). Where there is a significant temperature difference, radiative heat loss tends to be the dominant heat transfer mechanism.
The claim here is that because the heat loss is proportionate to the difference of the 4th power of the temperatures (in Kelvin), while it may be not so hard to get the rap device up to a skin-burning temperature (60 deg C) or to the point when it hisses off water that is on the rope (100 deg C), it could be quite challenging to get it up to the much higher temperature required to soften polyester (240 deg C).
Running a spreadsheet we can see:
Temp .......................Tc.......Th.......dif 4th powers
deg C................ ......degK...deg K
60 (burn skin)...........305........335........ 3.9E+09
100 (evap H2O).........305........375........11.1E+09
200 (almost singe)....305........475.........42.3E+09
240 (soften poly) ......305........515.........61.7E+09
For the Physics 101 class, we have turned the problem around to "How fast do I need to rappel to achieve X temperature?". Since the energy input is proportional to rappel speed, I think we can see:
A. observation: we have all heated up a rappel device hot enough to burn ourselves...
B. observation: many of us have heated up a rappel device hot enough to evaporate the water on the rope with a hiss...
C. getting to 200 deg C requires rapping 3.8X faster than B.
D. getting to 240 deg C (the danger point) requires rapping 5.5X faster than B.
I am positing a very long rappel, where we are looking at the steady state - perhaps this is an oversimplification...
Of course, without actually measuring things in the field, it is quite difficult to reach any ACTUAL conclusions.
Tom :moses:
The big yellow one is the sun!
Well said Copernicus!Quote:
Originally Posted by scubabryan
Radiation would dominate in a vacuum, but in our atmosphere conduction both direct and convective exceed the radiative transfer by a good ratio unless temps are very high. The radiative constant of 10^-8 means a delta T on the order of 650 is needed for radiation to dominate.
Quick calcs:
Convective transfer: q = k A (delta T)
k for free air is about 15. A is the same for both equations. Let's assume a delta T of 200.
So q = 3000 A
Radiative transfer: q = sigma A (delta T)^4
sigma is about 5.7 * 10^-8, A is fixed, delta T is 200.
So q = 91 A
Therefore, convection alone is transferring 30 times the heat of radiation. Direct conduction is highly dependent on contact area of course, so no telling what that may be in this scenario. The A in its equation is different. Very interesting stuff and a great Q.
I see your point, but...Quote:
Originally Posted by dsr70
As our colleague Professor Agostinone pointed out, it is not the 4th power of the temp diff, it is the diff of the 4th powers of the temperature - quite a different animal. Perhaps you would re-run your claim with this distinction.
Tom
I appreciate all the responses. As a liberal arts guy\law school grad I had to resort to Googling several of your responses. The takeaway message that I'm getting is that the rope's not going to melt through but I still don't want to come to a complete stop? And that no matter how hardcore engineers might pretend to be, deep down they really just want to pick out pocket protectors and work out equations on their slide rules?
Holy cow! Yes! Not (delta T)^4 but the difference of the temps to the fourth. Mind is failing. Sad. I was a math teacher at one time, up to Calc BC, and completely misread the equation at quick glance the other day.
The factor is 1 rather than 30 in favor of convection. Throw in direct conduction and the sum total radiation contributes is probably 20% in my guess.
I'm intrigued by this all and am going to set up a spreadsheet and take some measurements of the rap device/rope interface and see what I can come up with.
perhaps a smooth, continuous, slow speed also keeps the heat downQuote:
Originally Posted by xxnitsuaxx
you weren't using a plumbers torch in Imlay?Quote:
Originally Posted by ratagonia
My thesis is that the rapid increase of radiation with temperature difference makes it very difficult to get up to a temp that will soften the polyester.Quote:
Originally Posted by dsr70
I am surprised you can go to a rate for convection without using some kind of length - as convection goes up a lot with the height available, as the chimney driving effect increases...
T
xxnitsuaxx--I must say you have this nerd (disguised as a canyoneer) pegged. I found myself getting all twitterpated when heat transfer entered into the discussion.Quote:
Originally Posted by xxnitsuaxx
Back to your original inquiry about scorching the rope, you'll should be fine. When you are at the top of the rap you poses a certain amount of potential energy (more of it the higher the rap and the bigger the beer belly). That energy gets converted to heat on your way down. How hot it gets depends on how fast you go. Here is a helpful analogy: if you have a battery and you hook up a 10W light bulb the bulb gets hot and the battery drains. Same battery hooked up to a 100W light bulb will drain the battery faster and the bulb will get a heck of a lot hotter.
Seems to me worts case would be a long free hanging rappel that is rapped really fast and then stopped just before you touch the ground. That way the rap device is as hot as it's going to get and all your weight is mashing it into the rope.
I reccommend putting your prusik/autobloc on the uphill side of the descender (use a wrap3 to the fourth power) so that when you stop to let the thingy cool down and it melts through the softened polyester at least you'll still be attached to the rope......lemme think? how many rescues or fun rides have been done with a 2.....or maybe a 3 person load on a single rope with a single descender? Maybe statistics would answer some questions as well as the pure physics? Last year a rappel from the now infamous "BIRD PERCH" with two people and all gear (on the same device simultaneously) didn't end up with a melted in the middle and oh shit and we're gonna die now situation! Things get hot....so does the desert! I worry more about the 50% reduction in the strength of a rope when it is wet and the possibility of a rock clipping it when I'm on it bouncing my merry way down! (oh yeah...and folks that don't look up while on descent and realize all the maneuvering that they do cuts the shit out of ropes rubbing on edges).
I'm curious where this figure comes from?Quote:
Originally Posted by Bo_Beck
10% to 15% seems the more reasonable figure.
Tom :moses:
Sterling Rope Co. suggests up to 30% loss of strength, but I saw a study at ITRS 2002 in Golden Co. that showed up to 50% loss of strength. I just do know that a wet rope is definately weaker than a dry rope and since I use 8mm single strands, my concern on occasion would be a falling rock clipping my rope. Had a 10.5mm on the 2nd to last pitch of Moonlight Buttress cut more than halfway through.Quote:
Originally Posted by ratagonia
Not exactly 50%, and not exactly an answer to low stretch or static lines, but interesting nonetheless! Check it out.Quote:
Originally Posted by ratagonia
http://www.singingrock.com/article.a...&nLanguageID=2
Will have to get my notebook from the ITRS 2002 I attended and see if I can find the study that showed up to 50%.
I am trying to track down sources for various numbers. Sterling has access to information from tests conducted by boating people, and it is not clear that these numbers translate over into canyoneering applications.Quote:
Originally Posted by Bo_Beck
Please note the Singing Rock article is about nylon dynamic ropes. The analysis talks about the peculiar chemical properties of nylon with water.
Tom :moses: