Air movement and thermal comfort: The new ASHRAE Standard 55 provides information on appropriate indoor air velocities for occupant comfort

Air movement and thermal comfort The new ASHRAE Standard 55 provides) information on appropriate indoor air velocities for occupant comfort By Marc E. Fountain and Edward A. Arens, Ph.D. Associate Member ASH RAE Member ASHRAE ecent I-{VAC design innovations, energy conservation concerns and new laboratory data on drafts have brought substantial attention to the issue of acceptable levels of air movement in office environments. Air movement may provide desirable cooling in warm conditions, but it may also increase the risk of unacceptably cool drafts. Detectable air movement may be perceived by the occupants as provid- ing freshness and pleasantness to the breathing air, yet it may also be perceived as annoying. Clearly, a specific air speed has many possible physiological and subjective con- sequences. These range from a pleasant sense of coolness to an unpleasant sense of draft, depending on the air temperature, mean radiant temperature, humidity, cloth- ing, metabolic rate and air movement preference of the occupant. Since the turn of the century, ASH- RAE and themial comfort researchers have worked to define levels of air movement that are acceptable to the widest possible group of individuals within an evolving architectural setting, and to incorporate these results into an indoor environmental standard. This article outlines the current state of this discussion. Reference is also made to research investigating the effect of air movement on thermal comfort and the development of air velocity limits in the latest AS}-IRAE thermal comfort standard. Why is air velocity important? HVAC engineers design systems to move energy and ventilating air through buildings. Many, if not most, commercial buildings built since the middle of this century use air distribution systems to deliver heated and/or cooled air to occu- pied spaces. Accordingly, ASHRAE and other organizations have produced standards and guidelines for distributing this air. Included in these documents are specifics such as: volume of air per unit time, percentage of outdoor air, and type and location of duct outlets. In general, design recommendations have favored specifying delivered cfm per square foot of occupied space rather than specifying air velocity for achieving thermal comfort. However, the desired end-product of HVAC systems is not cfm per square foot, a cooled building interior or air move- ment per se; it is the comfort, health and satisfaction of building occupants. Beyond special cases such as laborato- ries and clean rooms, efforts in I-{VAC are primarily directed at producing thermal comfort and air quality that are acceptable for breathing. The focus of this article is the influence o-f the air movement (created by an HVAC system) on thermal comfort. Air velocity is one of six main varia- bles affecting human themtal comfort. The other five include three physical variables (air temperature, mean radiant temperature and relative humidity) and two behaviorally regulated variables, (metabolic rate and clothing insulation). In humans, the thermoregulatory sys- tem is responsible for maintaining the heat balance of the body using a core setpoint of 98.6 °F (37 °C) within the constraints of the _ six variables given above. This system con- trols the release of metabolic heat by regulating skin temperature, primarily by varying skin blood supply and sweating at the skin surface. Convective heat transfer at the skin varies with surface temperature and local air motion across the skin surface. Exten- sive laboratory studies have shown that thermal sensation vote (an important method for measuring thermal comfort) is closely related to skin temperature in cool and comfortable conditions. In warm and warm-humid conditions, moisture on the skin has a strong effect on thermal sensa- tion, particularly after sweating mechan- isms have been triggered. About the authors Marc E. Fountain is a PhD candidate at the University of Califomia—Berkeley (UCB). He received his BA in physics from UCB. ASHRAE it 2.1 (Physiology and Human ‘ Environment) and has been active in ther- mal comfort research since 1986 including contributions to ASH RAE Research Projects RP-462 and RP-702. Edward A. Arens is a professor in the Department of Architecture at the Univer- sity of California-—Berkeley and director of the Center for Environmental Design Research. He received his PhD in archi- tectural science from the University of Edinburgh and his undergraduate and masters degrees from Yale University. Arens is serving on the SPC 55-92R Standards Project Committee, and is a corresponding member of TC 2.1 and a past-chairman of TC 25 (Air Flow Around Buildings). Fountain is a corresponding member of l .4 SHR.-1 E Journal August 1993