Mean radiant temperature

The Mean Radiant Temperature (MRT) is defined as the uniform temperature of an imaginary enclosure in which radiant heat transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure.[1] 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.

MRT is the most important parameter governing human energy balance, especially on hot sunny days. MRT also has a strong influence on thermophysiological comfort indexes such as PET (Physiological Equivalent Temperature) or PMV (Predicted Mean Vote).[2]

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 performances. As the quality of wall increases, the wall is also warmer and therefore higher the mean radiant temperature. The higher the mean radiant temperature the less discomfort in the space. Maintaining a balance between the operative temperature and the mean radiant temperature can create a more comfortable space.[3] This is done with effective design of the building, interior and with the use of high temperature radiant cooling and low temperature radiant heating.[4]

The MRT can be measured using a black-globe thermometer. The black-globe thermometer consists of a black globe in the centre of which is placed a temperature sensor such as the bulb of a mercury thermometer, a thermocouple or a resistance probe. The globe can in theory have any diameter but as the formulae used in the calculation of the mean radiant temperature depend on the diameter of the globe, a diameter of 0.15 metres (5.9 in), specified for use with these formulae, is generally recommended. It should be noted that the smaller the diameter of the globe, the greater the effect of the air temperature and air velocity, thus causing a reduction in the accuracy of the measurement of the mean radiant temperature. So that the external surface of the globe absorbs the radiation from the walls of the enclosure, the surface of the globe shall be darkened, either by the means of an electro-chemical coating or, more generally, by means of a layer of matt black paint. [1] MRT is sometimes referred to as globe temperature (GT, or T_g) 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 T_g typically lies between the air temperature T_a and the true mean radiant temperature T_r. The faster the air moves over the globe thermometer the closer T_g approaches T_a. If there is zero air movement, T_g = T_r.

See also

External links

References

  1. ^ a b ISO 7726. Ergonomics of the thermal environment - Instrument for measuring physical quantities. Geneva, Switzerland: International Organization for Standardization. 11 1998. 
  2. ^ Fanger, P.O. (1970). Thermal Comfort: Analysis and Applications in Environmental Engineering. New York: McGraw Hill. 
  3. ^ Matzarakis, Andreas. Estimation and Calculation of the Mean Radiant Temperature within Urban Structures. 
  4. ^ Mclntyre and Griffiths, D.A. and I.D. (1972). Subject Response to Radiant and Convective Environments.