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Big Ass Fans Reduce Temperature Stratification and Winter Heating Costs in Hangars
The addition of HVLS fans is a cost-effective method to reduce stratification in large, open spaces such as aviation hangars. The decreased temperature differential minimizes heat loss through the building envelope, reduces equipment run time, and reduces gas usage. This report summarizes findings of a test completed at a 15,000 square-foot hangar in Central Kentucky.
Hangars present a tremendous air destratification energy savings opportunity because many have large, open spaces with high ceilings and lower insulation values than other more commonly occupied spaces. Heating hangars to meet comfortable temperature requirements often consumes large amounts of energy and produces an excessive amount of emissions. Like many other areas of industry, airports in Central Kentucky are now seeking energy-efficient systems to comply with increasingly stringent regulations without sacrificing occupant thermal comfort, all in an effort to reduce operating costs and reduce carbon footprint. BACKGROUND:
Cross section through space showing temperature with heaters on and fan off (top) versus fan on (bottom).
PROJECT SCOPE: A 15,000 square foot hangar with a 40 foot peak and 20 foot eaves was used as a test facility to validate the use of HVLS fans as a practical method for reducing thermal air stratification and HVAC usage in large spaces. A 20’ Powerfoil X3.0 fan was placed in the center of the facility with the blades approximately 27 feet above the floor. The space temperature control (a thermostat on an interior wall) was set at 65 degrees Fahrenheit (F). Temperature and humidity loggers were used to monitor the temperature along the back wall at 5 feet, 15 feet, and 35 feet above the floor. The HVLS fan was operated on an alternating weekly schedule. When the fan was turned on, the speed was maintained at 25% of the maximum operating speed for 24 hours a day to provide continuous air mixing without inducing draft effects to the occupants below.
Table 1 shows the average daily temperature at the three measurement locations and the differences in observed temperatures at the 35 foot elevation compared to the 5 foot and 15 foot elevations respectively when the HVLS fan was not in use. Table 2 shows similar data for the week the fan was operational. Over the week the fan was off, the highest temperatures were consistently recorded at the 35 foot elevation, while the closest temperatures remained at the 5 foot elevation, proving the existence of a very real stratification issue. The collected data showed a maximum daily average temperature gradient of 8.5 degrees F. The maximum gradient occurred while the outdoor temperature was one of the lowest recorded during the week. This time period is when the heating equipment would be working the hardest to maintain indoor temperature and links a higher magnitude of stratification to the heating system operation. When the fan was on, the indoor temperatures across all elevations were more uniform. The maximum daily average recorded temperature gradient while the fan was on was 1.3 degrees F (a 7.2 degree F improvement). Unlike the fan off configuration, the reported averages were not dependent on the heater operation when the fan was on. That is, when the fan was mixing air in the hangar, temperatures were uniform regardless of the HVAC operating mode. KEY DATA AND OUTCOMES:
TABLE 1 - Stratification results while fan was “off” showing daily average temperatures (°F) from 5 ft, 15 ft, 35 ft (1.5 m, 4.6 m, 10.7 m) away from floor with outside air conditions (0A) for comparison. Temperature differentials (Δ T ) from 35 ft (10.7 m) and 15 ft (4.6 m), and 35 ft (10.7 m) and 5 ft (1.5 m) are displayed. FAN OFF 35 FT 15 FT 5 FT 0A ΔT(°F) DAILY AVERAGES TEMP. (°F) TEMP. (°F) TEMP. (°F) TEMP. (°F) 35-5 FT 35-15 FT
TABLE 2 - Stratification results while fan was operational showing daily average temperatures from 5 ft, 15 ft, and 35 ft (1.5 m, 4.6 m, and 10.7 m) away from floor with outside air conditions for comparison. Temperature differentials from 35 ft (10.7 m) and 15 ft (4.6 m), and 35 ft (10.7 m) and 5 ft (1.5 m) are shown. FAN ON 35 FT 15 FT 5 FT 0A ΔT(°F) DAILY AVERAGES TEMP. (°F) TEMP. (°F) TEMP. (°F) TEMP. (°F) 35-5 FT 35-15 FT
Nov. 2
Nov. 10
69.5
66.9
64.2
57.7
5.2 2.6
67.8
67.4
67.2
59.1
0.6 0.4
Nov. 3
Nov. 11
68.2
66.9
63.3
50.1
4.9
1.4
66.4
66.2
66.0
54.7
0.4
0.2
Nov. 4
Nov. 12
69.2
67.4
62.5
47.4
6.8
1.8
65.9
65.7
65.1
50.0 0.8 0.2
Nov. 5
Nov. 13
70.8
69.0
64.1
48.8
6.6
1.8
66.6
66.5
65.5
50.9
1.1
0.1
Nov. 6
Nov. 14
72.6
70.8
64.0
47.6 8.5
1.7
66.8
66.9
66.5
52.8
0.3
-0.1
Nov. 7
Nov. 15
71.9
70.6
66.8
58.9 5.2
1.3
67.7
67.8
67.4
55.3
0.3
-0.1
Nov. 8
Nov. 16
72.7
71.2
67.8
60.3
4.9
1.5
67.7
67.4
66.4
52.6
1.3
0.3
Nov. 9
70.7
68.9
65.1
51.2
5.7
1.8
Overall Average
67.0
66.8
66.3
53.6
0.7
0.2
Overall Average
70.7
69.0
64.7
52.8
6.0
1.7
Maxiumum Value
67.8
67.8
67.4
59.1
1.3
0.4
Maxiumum Value
72.7
71.2
67.8
60.3
8.5 2.6
The figures below compare the time required for the indoor air temperature to reach a near uniform profile across space heights and the time it took for the air to restratify after the fan was turned off. When the fan was turned on, it took only 10 minutes of mixing for the temperature difference to fall within 1 degree F. When the fan was turned off, the air began to restratify in the facility to a temperature difference of 3 degrees F between the 5 foot and 35 foot temperature loggers. The amount of time observed for the air to restratify was 10 minutes. The rapid return to stratified air indicates fans need to be operated continuously during the cold weather season.
The addition of the HVLS fan reduced stratification and achieved near uniform temperature conditions within 10 minutes of operation. Once the fan turned off, stratification of the air started to reappear in as little as 15 minutes. This indicates that continuous or nearly continuous operation of the fan may be required to minimize heat loss through the envelope. The HVLS fan also reduced the normalized gas use by 29%, which consequently translated to significantly lower winter energy usage. Using HVLS fans in hangars is an energy-efficient method of creating a more uniform thermal environment due to the fan’s low power requirements and high potential for utility savings in the cold weather season. CONCLUSIONS: Gas heater use was also monitored during the same weeks. The HVLS fan created indoor air temperature uniformity in the facility, reduced heat loss from the building, and reduced heating system operating hours, consuming less natural gas. When the fan was operational, a 29% decrease in the gas usage was experienced. For Frankfort, Ky., gas savings during the heating season would be approximately $4,150 and the energy cost for operating the fan would be $50. The simple payback for the project would be approximately 2 years. While heating fuel savings and energy cost of operating fans will vary by building and locale, any building with tall heated spaces in a climate with heating demand should stand to benefit from the utilization of HVLS fans for destratification.
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