Elevated air speed and thermal comfort

How air speed meets thermal comfort goals

ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy (2020) identifies six factors that affect thermal comfort, including clothing insulation, metabolic rate, air temperature, mean radiant temperature, relative humidity, and air speed. Increasing air speed enhances heat transfer via convection and evaporation, which provides a cooling sensation named “Cooling Effect”. It allows the body to maintain thermal comfort at higher air temperatures than what would be comfortable in still air. Figure T1 shows how the air speed is associated with cooling effect. More importantly, this cooling effect is instantaneous and beneficial during the transitional moments (i.e., stepping into the building from a hot and humid outdoor environment). The ASHRAE Standard 55 (2020) and the CBE Thermal Comfort Tool help design how much air movement is needed for thermal comfort and occupant satisfaction.

Figure T1. Cooling effect of increased air speed for a ‘typical’ office worker in cooling conditions (operative temperature of 24.4 °C [76 °F], 50% relative humidity, 0.6 clo, 1.13 met).

Thermal comfort calculation with elevated air speed

ASHRAE Standard 55 (2020) provides a method called The Elevated Air Speed Comfort to calculate thermal comfort in situations of elevated air speed. This method uses a combination of the Analytical Comfort Zone Method combined with the Standard Effective Temperature (SET) method. The SET output translates the six thermal comfort factors (from above) into a single temperature equivalent that can be compared across a variety of comfort conditions. In addition, the cooling effect initiated by increased air speed is also used to calculate the Cooling Fan Efficiency (CFE). CFE is defined in ASHRAE Standard 216 (2020) as the ratio of the cooling effect to the input power of the fan. CFE gives people a standardized way to compare how much cooling a fan provides when consuming the same energy.

Human response to air movement

Do the occupants prefer an environment with a cooler temperature or an increased air speed and slightly higher temperature? Research has shown that people prefer an environment where they can use fans with a slightly higher than usual temperature (i.e., 26 °C [79 °F]) compared with the typical air conditioning temperature setpoint (i.e., 23 °C [73 °F]) without fans (Schiavon et al., 2016). Studies conducted in office buildings showed that the results were aligned with the findings obtained from laboratory experiments (Lipczynska et al., 2018). In addition, using a human heat balance model, a study found that fans can be used to cool occupants even if the ambient temperature exceeds normal skin temperature (i.e., 35 °C [95 °F]) (Tartarini et al., 2021).