Thermal runaway


Thermal runaway

Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature leading to a destructive result. It is a kind of positive feedback.

Chemical engineering

In chemical engineering, thermal runaway is a process by which an exothermic reaction goes out of control, often resulting in an explosion. Also known as a "Runaway Reaction" in organic chemistry.

Thermal runaway occurs when the reaction rate increases due to an increase in temperature, causing a further increase in temperature and hence a further increase in the reaction rate. Exothermic side reactions and combustion begins at higher temperatures, accelerating the thermal runaway. It has contributed to industrial chemical accidents, most notably the 1984 explosion of a Union Carbide plant in Bhopal, India that produced methyl isocyanate. Thermal runaway is also a concern in hydrocracking, an oil refinery process.

Thermal runaway is most often caused by failure of the reactor vessel's cooling system. Failure of the mixer can result in localized heating, which initiates thermal runaway. Wrongful component installation is also a common cause. Many chemical production facilities are designed with high-volume emergency venting to limit the extent of injury and property damage when such accidents occur.

Electronics

Bipolar transistors

Some bipolar transistors (notably germanium based bipolar transistors) increase significantly in leakage current as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through the transistor and with it the power dissipation. This causes a further increase in C-E current. This is frequently seen in a push-pull stage of a class AB amplifier. If the transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not made temperature-dependent, as the temperature rises, both transistors will be increasingly turned on, causing current and power to further increase, eventually destroying one or both devices.

If multiple bipolar transistors are connected in parallel (which is typical in high current applications) one device will enter thermal runaway first, taking the current which originally was distributed across all the devices and exacerbating the problem. This effect is called "current hogging". Eventually one of two things will happen, either the circuit will stabilize or the transistor in thermal runaway will be destroyed by the heat. Hence current hogging term is related to thermal runaway.

Power MOSFETs

Power MOSFETs display increase of the on-resistance with temperature. Power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop. (However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel and current hogging does not occur). If the transistor produces more heat than the heatsink can dissipate, the thermal runaway happens and destroys the transistor. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. See also Thermal Design Power.
* [http://www.ipes.ethz.ch/ipes/2002thermal/runaway/runaway.html Java applet demo of MOSFET thermal runaway]

Microwave heating

Microwaves are used for heating of various materials in cooking and various industrial processes. The rate of heating of the material depends on the energy absorption, which depends on the dielectric constant of the material. The dependence of dielectric constant on temperature varies for different materials; some materials display significant increase with increasing temperature. This behavior, when the material gets exposed to microwaves, leads to selective local overheating, as the warmer areas are better able to accept further energy than the colder areas - potentially dangerous especially for thermal insulators, where the heat exchange between the hot spots and the rest of the material is slow. These materials are called "thermal runaway materials". This phenomenon occurs in some ceramics.

Batteries

When handled improperly, some rechargeable batteries can experience thermal runaway, resulting in overheating. Sealed cells will sometimes explode. Especially prone to thermal runaway are lithium-ion batteries. A report of an exploding cellphone occasionally appears in newspapers. Laptop batteries from Dell were recalled because of fire and explosions. [ [http://www.theinquirer.net/default.aspx?article=32550 Dell laptop battery fires] ] The Pipeline and Hazardous Materials Safety Administration (PHMSA) of the U.S. Department of Transportation recently established regulations regarding the carrying of certain types of batteries on airplanes because of their instability in certain situations. This action was partially inspired by a fire on a UPS airplane [ [http://hazmat.dot.gov/regs/ntsb/av/AAR0707.pdf PHMSA article on the UPS airplane fire] ]

Electronics and tropical environments

Many electronic circuits contain provisions against thermal runaway. This is most often seen in transistor biasing arrangements. However when equipment is used above its designed ambient temperature, thermal runaway can in some cases still occur. This occasionally causes equipment failures in tropical countries, and when vents are blocked.

Digital electronics

The leakage currents of transistors increase with temperature. In rare instances, this may lead to thermal runaway in digital electronics. This is not a common problem, since leakage currents make up a small portion of overall power consumption, so the increase in power is fairly modest - for an Athlon 64, the power dissipation increases by about 10% for every 30 degrees Celsius [ [http://www.lostcircuits.com/cpu/amd_venice/ LostCircuits, CPU Guide ] ] . For a device with a TDP of 100 W, for thermal runaway to occur, the heat sink would have to have a thermal resistivity of over 3 K/W (kelvins per watt), which is about 6 times worse than a stock Athlon 64 heat sink [A stock Athlon 64 heat sink is rated at 0.34 K/W, although the actual thermal resistance to the environment is somewhat higher, due to the thermal boundary between processor and heatsink, rising temperatures in the case, and other thermal resistances] . A heat sink with a thermal resistance of over 0.5 to 1 K/W would result in the destruction of a 100 W device even without thermal runaway effects.

References

External links

* [http://www.safetycenter.navy.mil/media/mech/issues/summer03/thermalrunaway.htm Safetycenter.navy.mil: Thermal runaway]
* [http://www.findarticles.com/p/articles/mi_m0FKE/is_9_48/ai_110618550 Thermal runaway of batteries in an airplane]
* [http://www.crhf.org.uk/ UK Chemical Reaction Hazard Forum] - Over 150 previously undocumented reaction incidents, many involving thermal runaway.


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