Descender heating
When a rappeller descends, their potential energy must go somewhere. Some of that energy is used to slightly heat the rope as it is deformed around the descender. Much of the rest of that energy heats the descender.
Concerns[edit]
The primary concern with descender heating is that the descent device may become hot enough to damage the rope. In practice, this is rarely observed in canyon rappels. On longer rappels, it is not uncommon for devices to burnish the rope if they are are allowed to stay in one place for a few seconds. This wiki is not aware of any instances in which a descent device has caused a rope failure by melting through it.
Physics[edit]
The potential energy of a rappeller at the top of a drop is m*g*h where m is the mass of the rappeller and their gear, g is the acceleration of gravity (9.8 m/s/s) and h is the height of the drop. For instance, a 200 pound rappeller on a 180 ft drop has about 49,000 Watt*seconds of energy at the top of the rappel. All of this energy must be transferred somewhere by the time the rappeller has finished the rappel. If they take 60 seconds to descend, then 49,000/60 = 810 Watt*seconds are being transferred somewhere every second they descend. It takes about 910 Watt*seconds to raise the temperature of a 1 kg block of Aluminum by 1 degree C. Since a Pirana is about 0.09 kg, that means it only takes about 82 Watt*seconds to raise its temperature by 1 C. If all the energy from the rappeller's descent was used to heat the Pirana, its temperature would climb 810/82 = 10 C (18 F) every second for 60 seconds. In reality, some of the 810 Watt*seconds are transferred to places other than the Pirana so the descender does not heat up nearly this much
Where the energy goes[edit]
Although the breakdown varies with many factors and is not well-studied, the energy from a typical rappel may transfer heat to these places:
- Rope, via internal friction (strands rub against each other when the rope bends)
- Rope, via conduction (the warm descender dissipates heat to the rope via conduction)
- Air (the warm descender dissipates heat to air via convection)
- Environment (the warm descender radiates heat to the environment)
- Descender and Carabiner (the metal of the descender and carabiner get hotter)
- Water the rope has absorbed
- Rappeller's brake hand
- Rappeller's legs and the ground (when he stops the descent)
External links[edit]
- Canyon Collective discussion
- Bogley discussion
- ACA forums discussion
- ATS investigation
- Zion Adventure Co. Investigation
- Black Diamond QC Labs: Can a Hot Belay Device Melt My Rappel Slings?
- Attaway, Stephen W. The Mechanics of Friction in Rope Rescue
- Titt, Jim. Belay Device Theory, Testing and Practice
- Paul Stovall's rack heating analysis