Although air is not cooled by movement alone, it can create a cooling sensation as it passes over a person's skin. The proper use of air movement, then, can increase occupant satisfaction. It also can improve energy efficiency, as substantial savings can be realized by destratifying heated air in spaces with high ceilings during cooler months.

This article discusses how large circulator fans can be used to improve comfort and save energy in indoor environments. It also discusses occupants' expectations of thermal comfort in naturally conditioned and air-conditioned spaces. Lastly, it briefly discusses the benefits of air movement in air-conditioned spaces.

OPERATIVE TEMPERATURE

Operative temperature, t_op, is defined in ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy, as the uniform temperature of an imaginary black enclosure in which an occupant exchanges the same amount of heat by radiation and convection as in an actual non-uniform environment. Standard 55-2004 provides an equation for calculating operative temperature:

t_op = (t_db + t_r) ÷ 2

where:

t_db = dry-bulb air temperature, degrees Fahrenheit

t_r = mean radiant temperature, degrees Fahrenheit

Mean radiant temperature is measured with a globe thermometer, while dry-bulb air temperature is measured with a regular thermometer.

ADAPTIVE THERMAL COMFORT

ASHRAE Standard 55-2004 includes a graph of indoor operative temperatures that would satisfy 80 percent and 90 percent of the occupants in a naturally conditioned space.

In a naturally conditioned space, air speed is used to achieve thermal comfort without refrigerative or evaporative cooling. For example, with a mean outdoor dry-bulb air temperature of 84.9°F in August, the indoor operative-temperature range for 80-percent occupant satisfaction in Houston is 74°F to 87°F (Figure 1). The average maximum dry-bulb air temperature in August in Houston is 94°F and occurs around 3 p.m., with relative humidity 65 percent.

DETERMINING THE ELEVATED AIR SPEED NEEDED TO IMPROVE COMFORT

ASHRAE Standard 55-2004 uses standard effective temperature (SET),1 calculated using Thermal Comfort Tool software (available for purchase at www.ashrae.org), to determine the elevated air speed needed to improve comfort. ASHRAE Handbook-Fundamentals2 defines SET as the air temperature of an isothermal environment at 50-percent RH in which a subject, wearing clothing standardized for the activity concerned, has the same heat stress (skin temperature) and thermoregulatory strain (skin wettedness) as in an actual environment. SET can account for:

• Air temperature.
• Mean radiant temperature.
• Humidity.
• Air movement.
• Metabolic rate.
• Clothing insulation.

The first step in determining the elevated air speed needed for thermal comfort in a naturally conditioned space in Houston in August is to determine the preferred air speed and clothing-insulation value (Clo) for 87°F, the upper limit of operative temperature for the comfort zone. From figures 2 and 3, the preferred air speed is 52 fpm, and the preferred clothing-insulation value is 0.58 Clo. (Zero Clo corresponds to a naked person; 1 Clo corresponds to a person wearing a typical business suit.)

A warehouse supervisor sitting on a stool checking paperwork on a clipboard represents a metabolic rate of 1.1 met. The clothing-insulation value of a T-shirt, men's briefs, walking shorts, ankle socks, and shoes is about 0.31. (Insulation values of typical clothing are provided in Appendix B of ASHRAE Standard 55-2004.)

Using Thermal Comfort Tool, set the operative temperature at 87°F. Then, set the typical adaptive-thermal-comfort survey reference values of 50-percent RH and 1.1 met and the preferred values of 0.58 Clo and 52-fpm air speed, and calculate SET. SET for the upper limit of the thermal-comfort zone will be 85.2°F.

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