Building-Envelope and Ventilation Considerations

Insulation should ensure that the internal surface temperature of walls and ceilings is less than 9°F above indoor-air temperature. In hot climates, a barrier should be provided on the underside of a roof to avoid radiant gain from the roof to occupants. With appropriate ventilation of the space, indoor-air temperature will be equal to outdoor-air temperature.

Comfort in Air-Conditioned Spaces

Consider an air-conditioned space in which, during summer:

According to ASHRAE Standard 55-2004, the range of temperatures to satisfy 80 percent of the occupants in such a space is 76.5°F to 81.3°F (a difference of 4.8°F).

Different Comfort Expectations

It is important to appreciate the difference between the comfort expectations of occupants in air-conditioned spaces and the comfort expectations of occupants in naturally conditioned spaces. In Houston in August, the comfort expectation for an air-conditioned space is 76.5°F to 81.3°F, while for a naturally conditioned space it is 73.2°F to 85.8°F.

Temperature Offsets

With Thermal Comfort Tool software, the relevant output for air-conditioned spaces is predicted mean vote (PMV). The PMV needed to satisfy 80 percent of the occupants in an air-conditioned space is between -0.5 and +0.5.

Temperature offsets from elevated air speeds can be obtained by setting air temperature at 77°F. The other reference settings for Thermal Comfort Tool are:

The dry-bulb-air-temperature setting, then, is increased until the PMV is +0.5, the upper limit of the comfort zone to satisfy 80 percent of occupants. The difference between 77°F and the temperature setting at a PMV of +0.5 is the temperature offset.

Estimating Cooling-Energy Savings From Temperature Offsets

Facility-maintenance engineers at Travis Air Force Base in California claim that turning up the thermostat on air-conditioning equipment 2°F reduces cooling operating costs by 8 percent (4 percent per degree Fahrenheit).

In spaces characterized by non-sedentary activity, such as gymnasiums, elevated air speeds of up to 315 fpm can provide temperature offsets of up to 6.9°F. The potential energy reduction from a 6.9°F temperature offset using a savings rate of 3 percent per degree Fahrenheit is 21 percent.

Destratification Savings During Cooler Months

When a space is heated during cooler months, hotter air rises toward the ceiling, stratifying with a significant temperature gradient, 0.75°F per foot from floor to ceiling. In a space over 10-ft high, heat energy above head height is wasted, as it does not contribute to occupant thermal comfort in the occupied zone. Thorough mixing of this hotter air with cooler air (destratification) results in a uniform air temperature throughout the space. Typically, 10 percent of heating energy is saved for each 10 ft of floor-to-ceiling height.

Approximately half of the air in a space needs to be circulated every hour for effective destratification. Twenty-four-foot-diameter circulator fans at low speed (6.9 rpm) offer much greater aerodynamic efficiency (up to 669 cfm per watt) than smaller fans, delivering 98,940 cfm.

Destratification in Air-Conditioned Spaces

HVAC engineers often have difficulty balancing air-conditioning systems. Large circulator ceiling fans can mix the air in air-conditioned spaces to a uniform temperature and eliminate system-balancing problems. Large circulator ceiling fans also can serve as the first method of cooling in milder climates, delaying the start-up of an HVAC system for substantial energy savings.

Conclusion

Energy consumption will be a concern of business owners as long as heating and cooling costs continue to fluctuate. The need to control costs, however, must be balanced with the need to maintain comfort, thus, ensuring high productivity and occupant satisfaction. With large circulator fans, higher rates of efficiency can be achieved than with HVAC systems alone. In some cases, large circulator fans even can eliminate the need for air conditioning. When air conditioning is required, however, large circulator fans can reduce the load on an HVAC system and eliminate the need for ductwork. With proper air movement, desired occupant comfort and reduced energy consumption can be achieved for any space.

Source: HPAC

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