Mean radiant temperature
Encyclopedia
The Mean Radiant Temperature (MRT
) is a concept arising from the fact that the net exchange of radiant energy between two objects is approximately proportional to their temperature difference multiplied by their ability to emit and absorb heat (emissivity). Mean radiant temperature (MRT) is simply the area weighted mean temperature of all the objects surrounding the body. This is valid as long as the absolute temperatures of objects in question are large compared to the temperature differences, allowing linearization
of the Stefan-Boltzmann Law in the relevant temperature range.
Technically, MRT is defined as the uniform temperature of a surrounding surface giving off blackbody radiation (emissivity
= 1) which results in the same radiation energy gain on a human body as the prevailing radiation fluxes which are usually very varied under open space conditions.
MRT is the most important parameter governing human energy balance, especially on hot sunny days. MRT also has the strongest influence on thermophysiological comfort indexes such as PET (Physiological Equivalent Temperature) or PMV (Predicted Mean Vote) which are derived from heat exchange models. Physiologists have discovered that living human skin has extraordinarily high absorptivity and emissivity (
= 
= 0.98), greater than almost any other known substance, matte-black metals included Consequently, we are highly responsive to changes in mean radiant temperature. The MRT also has a strong influence on our thermophysiological comfort indexes including the PET (Physiological Equivalent Temperature) or PMV ( Predicted Mean Vote).
What we experience and feel relating to thermal comfort in the building is related to the influence of surface temperature and the dry air temperature in the space we are in. The mean radiant temperature is expressed as the surface temperature. The mean radiant temperature is controlled by enclosure perfomrances. As the quality of wall increases, the wall is also warmer and therefore higher the mean radiant temperature. The higher the mean radiant termperature the less discomfort in the space. Maintaining a balance between the operative temperature and the mean radian temperature can create a more comfortable space. This is done with effective design of the building, interior and with the use of high temperature radiant cooling and low temperature radiant heating.
Thermal bridging can affect the amount of energy needed to sustain an acceptable mean radiant temperature in walls. A high quality wall require less energy and a low quality wall would require more energy.
Radiant temperature is usually measured with what it known as a globe thermometer. This is simply a normal dry bulb thermometer encased in a 150mm diameter matte-black copper sphere whose absorptivity approaches that of the skin. Hence MRT is sometimes referred to as globe temperature (GT, or
) as they are nearly equivalent.
The measured globe temperature depends on both convection and radiation transfer, however by effectively increasing the size of the thermometer bulb the convection transfer coefficient is reduced and the effect of radiation is proportionally increased. Because of local convective air currents the globe temperature
typically lies between the air temperature 
and the true mean radiant temperature 
. The faster the air moves over the globe thermometer the closer 
approaches 
. If there is zero air movement, 
= 
.
See also
MRT
- Rapid transit systems :* Jakarta Mass Rapid Transit* Kaohsiung Mass Rapid Transit system in Kaohsiung, Taiwan* Manila Metro Rail Transit System* Mass Rapid Transit system in Singapore* MRT , officially named Mass Rapid Transit...
) is a concept arising from the fact that the net exchange of radiant energy between two objects is approximately proportional to their temperature difference multiplied by their ability to emit and absorb heat (emissivity). Mean radiant temperature (MRT) is simply the area weighted mean temperature of all the objects surrounding the body. This is valid as long as the absolute temperatures of objects in question are large compared to the temperature differences, allowing linearization
Linearization
In mathematics and its applications, linearization refers to finding the linear approximation to a function at a given point. In the study of dynamical systems, linearization is a method for assessing the local stability of an equilibrium point of a system of nonlinear differential equations or...
of the Stefan-Boltzmann Law in the relevant temperature range.
Technically, MRT is defined as the uniform temperature of a surrounding surface giving off blackbody radiation (emissivity
= 1) which results in the same radiation energy gain on a human body as the prevailing radiation fluxes which are usually very varied under open space conditions.MRT is the most important parameter governing human energy balance, especially on hot sunny days. MRT also has the strongest influence on thermophysiological comfort indexes such as PET (Physiological Equivalent Temperature) or PMV (Predicted Mean Vote) which are derived from heat exchange models. Physiologists have discovered that living human skin has extraordinarily high absorptivity and emissivity (
= = 0.98), greater than almost any other known substance, matte-black metals included Consequently, we are highly responsive to changes in mean radiant temperature. The MRT also has a strong influence on our thermophysiological comfort indexes including the PET (Physiological Equivalent Temperature) or PMV ( Predicted Mean Vote).What we experience and feel relating to thermal comfort in the building is related to the influence of surface temperature and the dry air temperature in the space we are in. The mean radiant temperature is expressed as the surface temperature. The mean radiant temperature is controlled by enclosure perfomrances. As the quality of wall increases, the wall is also warmer and therefore higher the mean radiant temperature. The higher the mean radiant termperature the less discomfort in the space. Maintaining a balance between the operative temperature and the mean radian temperature can create a more comfortable space. This is done with effective design of the building, interior and with the use of high temperature radiant cooling and low temperature radiant heating.
Thermal bridging can affect the amount of energy needed to sustain an acceptable mean radiant temperature in walls. A high quality wall require less energy and a low quality wall would require more energy.
Radiant temperature is usually measured with what it known as a globe thermometer. This is simply a normal dry bulb thermometer encased in a 150mm diameter matte-black copper sphere whose absorptivity approaches that of the skin. Hence MRT is sometimes referred to as globe temperature (GT, or
) as they are nearly equivalent.The measured globe temperature depends on both convection and radiation transfer, however by effectively increasing the size of the thermometer bulb the convection transfer coefficient is reduced and the effect of radiation is proportionally increased. Because of local convective air currents the globe temperature
typically lies between the air temperature and the true mean radiant temperature . The faster the air moves over the globe thermometer the closer approaches . If there is zero air movement, = .See also
- Thermal comfortThermal comfortThermal comfort is a term used by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, an international body. It is defined as the state of mind in humans that expresses satisfaction with the surrounding environment...
- http://ottp.fme.vutbr.cz/predmety/EnvEng/EnvironmentalEngineering.pdf
- http://ergo.human.cornell.edu/DEA3500allnotes.html