Elevated air speed and thermal comfort
Last updated
Last updated
In this section, we discuss the thermal comfort related effects of increased air movement using fans for cooling applications. We also discuss tools such as the CBE Thermal Comfort Tool to help determine the right air speed and other factors for optimal thermal comfort. Figure 3 shows the cooling effect – or how many degrees warmer the air temperature can be to provide the same level of thermal comfort – associated with increased air speeds. This figure also highlights that the design air speeds discussed in this guide are well below the air speeds that a person experiences every day. For example, a design speed of 0.5 m/s [100 fpm], equal to approximately 2 °C [4 °F] cooling effect, is approximately half the air speed that a person experiences just from the relative motion of walking slowly through still-air conditions.
Thermal comfort, here defined as the occupant’s satisfaction with the perceived thermal sensation, depends on how much heat is released or retained by the occupant’s body. Human thermoregulation, as depicted in Figure 4, is the heat transfer process to and from the body that occurs in four ways: radiation, convection, evaporation, and conduction.
ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy (2020) identifies six factors that affect thermal comfort. The metabolic rate is the rate of transformation of chemical energy into heat and mechanical work, based on the level of activity (e.g., walking presents a higher metabolic rate than seating). The clothing insulation is the insulating effect of clothing preventing heat loss from the body (e.g., shorts and a short-sleeve shirt have lower insulation levels than pants and a heavy sweater). The air temperature is the temperature of the air, usually measured by the thermostat. The mean radiant temperature is the average temperature of the surfaces surrounding a certain point, weighted by the view angle of each surface to that point. The air velocity or air speed at a certain point is the influential factor we explore when fans are used. And the last one is humidity, which indicates how much moisture the air contains.
Fans increase air speed and heat transfer via convection and evaporation, which provides a cooling sensation. It allows the body to maintain thermal comfort at higher air temperatures than what would be comfortable in still air. In addition to providing comfort at increased temperatures, fans are capable of providing instantaneous comfort effects that thermostat adjustments usually cannot because it controls a slower process. A thermostat and HVAC system that conditions the whole room generally takes 15 minutes or longer before the occupant can perceive a change in their thermal environment. However, when an occupant feels too warm, turning on or increasing the speed of a fan instantly provides a cooling sensation, also known as the cooling effect. Fans are also ideally suited to providing adaptive or transitional comfort for changing human comfort conditions. Adjusting fan speeds can help accommodate the natural fluctuations in body temperature and comfort preferences throughout the day. The adjustable nature of fans can provide enhanced thermal comfort during transitional moments, such as the changing comfort needs when transitioning from an active metabolic rate event (for example, after walking from a meeting in a different part of the building or arriving in to work from a morning commute) to a resting metabolic rate (such as sitting at a desk in an office), or simply due to different personal thermal comfort requirements of occupants in the same physical space. The ASHRAE Standard 55 (2020) and the CBE Thermal Comfort Tool both help to inform how much air movement is needed for thermal comfort and occupant satisfaction.
ASHRAE Standard 55 (2020) provides a method called The Elevated Air Speed Comfort (Section 5.3.3. of the standard) to calculate thermal comfort in situations of elevated air speed. This method uses a combination of the Analytical Comfort Zone Method with the Standard Effective Temperature (SET) method (Normative Appendix D of standard). Since increasing air speed has a cooling effect, the method calculates adjusted air and radiant temperatures according to how occupants are expected to feel under increased air speed conditions to calculate a new PMV value. The “Standard Effective Temperature” (SET) output translates the six thermal comfort factors (presented before) into a single temperature equivalent. The SET provides a single metric that can be compared across a variety of comfort conditions.
The cooling effect is also used to calculate the Cooling Fan Efficiency (CFE). CFE is defined in ASHRAE Standard 216 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 amount of energy.
In addition to directly cooling occupants, fans also effectively mix the air in a space, which has several applications. The most common of these applications is where the temperatures in space are unwantedly stratified, with warmer air close to the ceiling and cooler air near the floor (Figure 5). This typically occurs in spaces with high ceilings or where the heating equipment has a relatively high discharge temperature and low mixing momentum. In these conditions, ceiling fans can mix the air in the space such that the temperature at the floor (and near where the thermostat is located) is close to the average temperature in the space. This can save energy and improve occupant comfort. Ceiling fans are most effective in providing destratification, as they are located close to the ceiling and can run in either direction to achieve this mixing, although they will use more power to achieve the same mixing effect when operating in reverse (moving air upward) than forwards (moving air downward). Note that during destratification, the space is typically operating in heating mode and operating at the lower end of the range of temperatures that define the thermal comfort zone. As such, it is very important to maintain very low air speeds in the occupied zone to avoid the sensation of draft. Depending on the specific conditions, the occupant locations, and the minimum speed capabilities of the fan, running the fan forward or reverse may be better to achieve this goal.
In theory, elevated air movement accelerates heat loss from occupants via convection and evaporation in a warm environment. The question is, do occupants appreciate elevated air movement? Or do the occupants prefer an environment with cooler temperatures than increased air speed? An experiment conducted in a climatic chamber in Singapore investigated occupants’ thermal acceptability under different temperature setpoints with and without fan operation. Figure 6 shows that occupants were thermally more acceptable at 26 °C [79 °F] with higher air movement from personally controlled fans when compared with the condition at 23 °C [73 °F] without a fan. Surprisingly, it also reported that the subjects found themselves thermally more acceptable with temperature at 29 °C [84 °F] with a fan than at 23 °C [73 °F] without a fan. These results revealed that the cooling strategy of only lowering the temperature setpoint might not sufficiently satisfy the occupants, while elevated air movement within the space can play a role in increasing thermal acceptability at higher temperature setpoints.
To validate the results from the experiment in a climatic chamber, a similar experiment has been conducted in a Singapore office building studying the occupants’ thermal acceptability and thermal preference responses under three conditions (23 °C [73 °F] without a fan, and 26/27 °C [79/81 °F] with fan). The study found similar higher thermal acceptability among the occupants for the environment at 26 and 27 °C [79 and 81 °F] with elevated air speed than the condition at 23 °C [73 °F] without fan operation. Besides, the occupants were overcooled and reported a preference for a warmer environment at 23 °C [73 °F]. These results from a real building aligned with the findings obtained in the climatic chamber experiment, where both studies suggested that occupants in tropical climate regions preferred a warmer temperature with elevated air speed over an environment with a cooler temperature without sufficient air movement.
Some health guidelines actively advise not to use a fan when indoor air temperatures exceed the skin temperature (~ 35 °C [95 °F]). Is it true that a fan shall not be used under heat-stress conditions? Using the human heat balance model, a study found that fans could potentially be used by a healthy young adult even if air temperature exceeds 35 °C [95 °F], because elevated air speed through the human body increases sweat evaporation from the skin (Tartarini et al., 2022). Figure 7 demonstrates under which conditions the fan use could be beneficial. The green zone shows the environmental conditions in which the fan operating at V = 0.8 m/s [160 fpm] is beneficial (i.e., providing additional cooling to the human body). The dark green zone represents the conditions where fan usage is still beneficial, but people are likely to suffer from heat strain. The red zone indicates the conditions under which fans are not beneficial and should not be used. The red line shows the temperature limit from WHO, which the usage of fans is not recommended. These findings indicate that fans could be used in some conditions to cool people even when the air temperature exceeds skin temperature. More human experiments and tests with non-healthy groups should be performed. If available and affordable, air conditioning would guarantee safer conditions than just fans.
