How Emergency Thermal Blanket Works
How Emergency Thermal Blanket Works
As was written, thermal insulation is based on the use of substances with very low thermal conductivity. But sometimes the most effective way to protect persons or devices from leakage or gain of heat is to use a thermal radiation insulator. Thermal radiation does not require any medium for energy transfer. Moreover, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum.
In contrast to heat transfer by conduction or convection, which take place in the direction of decreasing temperature, thermal radiation heat transfer can occur between two bodies separated by a medium colder than both bodies. For example, solar radiation reaches the surface of the earth after passing through cold layers of atmosphere at high altitudes.
In order to insulate thermal radiation, space blankets (depending on the function, also known as a Mylar blanket, emergency thermal blanket or safety blanket) can be used. Space blankets were first developed by NASA in 1964. The highly reflective insulators are often included in emergency kits (therefore emergency thermal blankets) and are also used by long-distance runners after finishing a race to avoid a large swing in body temperature. Space blankets are designed to reflect heat back to body or deflect heat when used as a shelter from the sun, they can be used to insulate everything from the Mars rovers to marathon runners, from satellites to sun shields, and from rockets to residences. It is one of the simplest, yet most versatile spinoffs to come out of the Agency. Highly reflective foils in radiant barriers and reflective insulation systems reflect radiant heat away from persons and living spaces, making them particularly useful in very cold climates.
In case of emergency thermal blankets, the blankets are used to prevent/counter hypothermia. Their compact size before unfurling and light weight makes them ideal when space is at a premium. They may be included in first aid kits and also in camping equipment. Their design reduces the heat loss in a person’s body which would otherwise occur especially due to:
- Thermal radiation. As was written, radiation heat transfer This is achieved by minimisation of radiant heat transfer rate, q [W/m2], from a body to its surroundings is proportional to the fourth power of the absolute temperature. In this case, the emissivity of a material plays also very important role. The emissivity, ε, of the surface of a material is its effectiveness in emitting energy as thermal radiation and varies between 0.0 and 1.0. The surface of a blackbody emits thermal radiation at the rate of approximately 448 watts per square metre at room temperature (25 °C, 298.15 K). Real objects with emissivities less than 1.0 (e.g. aluminium foil) emit radiation at correspondingly lower rates (e.g. 448 x 0.07 = 31.4 W/m2). See also: Kirchhoff’s Law of thermal radiation
- Water evaporation and large scale convection. Thermal blankets are usually made from BoPET (Biaxially-oriented polyethylene terephthalate) airtight foil, causing them waterproof and windproof. This prevents large scale convection and heat losses caused by evaporation of perspiration.
The blackbody emissive power, Eb [W/m2], from a blackbody to its surroundings is proportional to the fourth power of the absolute temperature and can be expressed by the following equation:
Eb = σT4
where σ is a fundamental physical constant called the Stefan–Boltzmann constant, which is equal to 5.6697×10-8W/m2K4 and T is the absolute temperature of the surface in K.