Operative temperature
Operative temperature (previously known as resultant temperature or dry resultant temperature, but renamed to align with ASHRAE and ISO standards) is a simplified measure of human thermal comfort derived from air temperature, mean radiant temperature and air speed. It can be useful in assessing the likely thermal comfort of the occupants of a building.
Actual thermal comfort is dependent on environmental factors, such as air temperature, air velocity, relative humidity and the uniformity of conditions, as well as personal factors such as clothing, metabolic heat, acclimatisation, state of health, expectations, and even access to food and drink. However as empirical fits to these variables are very complex (see predicted mean vote), a simpler measure can be more useful in practice.
Operative temperature is defined as:
Operative temperature = (tr + (ta x √10v)) / (1+√10v)
Where
ta = air temperature
v = air speed (m/s)
Or:
Operative temperature = (( hr x tr) + (hc x ta )) / ( hr + hc )
Where
hc = convective heat transfer coefficient
hr = radiative heat transfer coefficient
Where the air speed is less than 0.1m/s, (as is typical in buildings) radiative and convective heat transfers may be similar, and so the equation can be simplified to:
Operative temperature = (ta + tr)/2
In many spaces, with low air velocity and where air temperature and mean radiant temperature may be similar, air temperature alone can be a reasonable indicator of thermal comfort. However, in spaces where surfaces may be heated or cooled, where there is significant thermal mass, or where solar radiation is present, air and radiant temperatures may be very different and so it is necessary to take account of radiant temperatures in assessing thermal comfort...
NB Illustrated Guide to Mechanical Cooling (BG 1/2010), written by Kevin Pennycook and published by BSRIA in 2010, defines resultant temperature as: ‘A temperature often used to specify a design condition for a space, it combines air temperature, surface temperature and air velocity in a single index.’
Thermal Comfort (TG 22/2023) published by BSRIA in 2023 defines operative temperature as: ‘….a measure of thermal comfort which takes into account air temperature, mean radiant temperature and air speed. The term dry resultant temperature was formerly used for this. Operative temperature is a better measure than dry bulb temperature for specifying setpoints within many buildings as it considers the radiative component (from equipment such as chilled ceilings and fabric components such as windows) and air movement in the space.’
[edit] Related articles on Designing Buildings
- BREEAM Thermal comfort.
- Dry-bulb temperature.
- Globe temperature.
- Humidity.
- Maximum and minimum workplace temperatures.
- Mean radiant temperature.
- Predicted mean vote.
- Psychometric chart.
- Running mean temperature.
- Sling psychrometer.
- Thermal comfort.
- Thermal indices.
- Temperature.
- Wet-bulb temperature.
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