ASHRAE Journal - April 2020 - 32

TECHNICAL FEATURE

heating degree days4 for Frankfort, Ky., gas savings during the heating season would be approximately $4,150,
while the energy cost for operating the fan would be $50.
Simple payback on the project would occur in approximately two years.

TABLE 3 Summary of gas heater use

TABLE 4 Summary of gas heater use

while fan was off.

while fan was on.

HEATER USE TOTALS-FAN OFF

Total Use

35,958

HEATER USE TOTALS-FAN ON

ft3

Total Use

24,881 ft3

Total Cost

$276.52

Total Cost

$191.33

Normalized Use

465 ft3 /HDD

Normalized Use

330 ft3 /HDD

CFD Models

Computational ﬂuid dynamics models were used to
simulate the stratiﬁcation of air caused by the gas heating units. The hangar facility was modeled based on the
actual dimensions and heat sources that matched the
manufacturer's published performance data for the unit
heaters added to the space.
Using software, a steady-state analysis was set up and
run until convergence was attained. The fan geometry
was meshed to an acceptable resolution and quality
(maximum element size was #, and maximum skewness
values were less than 0.95), and the fan ﬂuid zone was
assigned an angular velocity of 17.5 rpm. A realizable
k-epsilon turbulence model was used, as was a coupled
pressure equation solver model.
The hangar temperature was set at 40°F (4.4°C) to start
the simulation, and the heaters were run for a period of
three minutes without the HVLS fan to allow air to stratify within the space. This is shown in the left image of
Figure 8. The fan was then turned on to 25% of maximum
speed. After 11 minutes of fan operation, the air within
the space was fully destratiﬁed (as seen in the right
image of Figure 8); this closely aligned with the measured, collected data at the hangar via installed loggers.

Conclusion
Evaluating the impact of HVLS fans in an airplane
hangar proved air stratiﬁcation can be minimized from
ﬂoor to ceiling while also providing signiﬁcant winter
heating savings. During the ﬁrst week, when the HVLS
fan was not in use, the average recorded temperature
gradient from ﬂoor to ceiling was 6.0°F (3.3°C). The
following week, while the fan was in use, an average
ﬂoor-to-ceiling temperature difference of 0.7°F (0.4°C)
was observed. The HVLS fan effectively mixed the stratiﬁed air to produce more uniform temperature conditions despite the size of the hangar. The 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, which indicates that continuous
32

ASHRAE JOURNAL

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APRI L 2020

FIGURE 8 Cross section through space showing temperature with heaters on and
fan off (Top) versus fan on (Bottom).

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 heating costs. Using HVLS fans in airplane
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.

References

1. Armstrong, M., B. Chihata, R. MacDonald. 2009. "Cold
weather destratiﬁcation energy savings of a warehousing facility."
ASHRAE Transactions 115(2):513 - 518.
2. Pignet, T., U. Saxena. 2002. "Estimation of energy savings due
to destratiﬁcation of air in plants." Energy Engineering 99(1):69 - 73.
3. Baxter, G., P. Srisaeng, G. Wild. 2018. "An
assessment of airport sustainability, part 2-
energy management at Copenhagen Airport."
Resources 7(2):32.
4. 2017 ASHRAE Handbook-Fundamentals,
Chapter 14.

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ASHRAE Journal - April 2020

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