| Air movement is one of the six main variables determining human thermal comfort. The others are air temperature, relative humidity, mean radiant temperature, metabolic rate, and clothing insulation. HVAC design innovations, energy conservation concerns, and new laboratory data on fan-cooling and drafts have recently brought substantial attention to the issue of acceptable levels of air movement in office environments. International standards that specify the physical conditions for thermal comfort indoors include air movement limits that are constructed from often conflicting evidence and thus can be problematic to apply. A primary reason is that while air movement provides desirable cooling in 'warm' conditions, it can also increase the risk of unacceptably cool drafts. The transition from desirable cooling to uncomfortable draft is a complicated function of physics, physiology, and human expectation. This work focuses on air movement in the temperature range 25.5 C to 28.5 C where this transition occurs and includes an examination of moving air delivered at ambient and cooler-than-ambient temperatures.The experiment was done in four stages. The first two used a thermal manikin to quantify the physical effects of air movement cooling in a controlled way. A thermal manikin was exposed to a range of air velocities from a desk fan, a floor-mounted diffuser, and a desk-mounted diffuser at a single ambient temperature. Next, the experiment was repeated with supply temperatures from the floor and desk-mounted diffusers several degrees C cooler than the ambient temperature.In the third and fourth stages of the project, human subjects were exposed to the same physical conditions as those of the thermal manikin previously (air movement at ambient and cooler-than ambient temperatures) but given control of the air supply velocity. The subjects were asked to adjust the air movement as they pleased to make themselves comfortable. This produced subjective votes as well as preferred velocities.Following the experimental work, empirically derived functions were established for predicting preferred air velocity. A model is proposed that predicts the percent of people satisfied as a function of air temperature and air velocity. |
