Air Temperature and Air Motion

Air motion may be caused by natural convection, be mechanically forced, or be a result of the body movements of the space's occupants. The natural convection of air over our bodies dissipates body heat without added air movement. When temperatures rise, we must increase air movement to maintain thermal comfort. Insufficient

Figure 15-8 Evaporation.

air movement is perceived as stuffiness, and air stratifies, with cooler air near the floor and warmer air at the ceiling.

A noticeable amount of air movement across the body when there is perspiration on the skin is experienced as a pleasant cooling breeze. When surrounding surfaces and room air temperatures are 1.7°C (3°F) or more below the normal room temperature, we experience that same air movement as a chilly draft. Our necks, upper backs, and ankles are the most sensitive to chills. This accounts for the popularity of scarves, sweaters, and socks in the winter.

When the moving air stream is relatively cooler than the room air temperature, its velocity should be less than the speed of the other air in the room to avoid the sensation of a draft. Air velocities between 3 and 15 meters per minute or mpm (between 10 and 50 ft per minute or fpm) are generally comfortable. We sense a 2°C (1°F) for each 4.6 mpm (15 fpm) increase above a velocity of

9.2 mpm (30 fpm). Air motion is especially helpful for cooling by evaporation in hot, humid weather.

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