Thermal runaway
From Wikipedia, the free encyclopedia
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.
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[edit] Chemical engineering
In chemical engineering, thermal runaway is a process by which an exothermic reaction goes out of control, often resulting in an explosion.
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. 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. 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.
[edit] Electronics
[edit] Bipolar transistors
Bipolar transistors (notably germanium based bipolar transistors) increase significantly in current gain as they increase in temperature. Depending on the design of the circuit, this increase in current gain can increase the current flowing through the transistor and with it the power dissipation. This causes a further increase in current gain. 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.
[edit] 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.
- Java applet demo of MOSFET thermal runaway.contact [pradeep_c07@yahoo.co.in]
[edit] 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.
[edit] 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
[edit] Electronics and tropical environments
Thermal runaway in electronic equipment is not a design factor for most electronics manufacturers and engineers, but it really exists and can cause a lot of trouble and malfunctions in tropical countries. It is seen in satellite TV receivers, portable computers or industrial programmable controllers. When the air temperature that surrounds the equipment is over a threshold level, the temperature starts a heat run, a thermal runaway that makes the electronics fail, and they then need to be cooled down and reset to start to work properly again. Basically this problem is caused by the high frequency chopped power supplies, where MOSFET transistors are used.
