ASHRAE Journal - May 2020 - 34

TECHNICAL FEATURE

Flow (Percent of Conditioned Air)

Stack Pressure at Crown (Pa)

Stack Pressure at Crown (Pa)

where
FIGURE 2 Effect of a floor level envelope flow blocker on stack pressure.
Q = predicted flow rate, ft3/s (=
B. No Seal at Floor Level
A. Airtight Seal Near Floor Level
cfm/60)
11
6
Outside Wall
Cd = discharge coefficient (varOutside Wall
10
Temperature (°F)
Temperature
(°F)
ies between 0.6 for small
9
5
-60 -50
sharp-edged openings and 1
8
-60 -50
-40
for large openings).
4
7
-30
-40 -30
ELA = equivalent or effective leak6
-20
2
2
3
age area, ft (= in. /144)
5
-20
-10
-10
4
ρ	 =	 mass air density, 0.002313
10
2
10
3
3
3
slug/ft (= 0.07451 lb/ft /g) at
32
32
2
11.02 psia, −60°F
1
1
ΔPr = reference pressure differ0
0
7
8
9
10
11
12
ence (lb/ft2) or customar7
8
9
10
11
12
Crown Height (ft)
Crown Height (ft)
ily in units of Pa (= 47.88 ×
lb/ft2)
Using Equation 4, Figure 3 shows the percent of cabin air
FIGURE 3 Percent of a 20 cfm/passenger conditioned cabin air supply drawn
behind the insulation at cold soak versus ELA per passenger and top of wall stack
passing behind the insulation at stack pressures rangpressure ΔP. Cabin air pressure = 75 kPa, envelope leakage Cd = 0.6.
ing up to those at cold soak (−60°F [−51°C]) for a range
100
Top of Wall Stack Pressure 12
of envelope ELAs per passenger at ¾ atmosphere cabin
90
pressure (11.02 psia [75.98 kPa]) at Cd = 0.6. This Cd rep80
8
resents the flow coefficient for an envelope that has an
70
6
air space between the walls and the insulation.
60
4
Tests on an older B737-200 with the wall sealed at the
50
3
40
floor found a equivalent leakage area in the cabin liner
2
30
(sum of the leaks), as defined in Equation 4, of 1.65 in.2
1
20
(1065 mm2) per passenger, with a crown stack pressure
10
of 4 Pa (0.02 in. w.g.) at cold soak. This equates to 20% or
0
more of the cabin conditioned air supply passing behind
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
*
the cabin insulation at cold soak, depending on the
2
Equivalent Leakage Area (ELA) (in. /passenger)
actual envelope flow blocker leakage at the cabin floor or
between windows.
the reduction in outdoor air supply rate was introduced
For example, the envelope might have some leakage
around the edges of the flow blocker. The stack pressures so pathogen levels in theory would not be any higher. In
and floor vent pressure differential create high velocities addition, the amount of humidity condensation on the
cold fuselage behind the insulation increased dramatiaround the flow blockers (Figure 4). Note that the prescally. To address the increased humidity condensation,
sures and temperatures within the envelope are lower
the following steps were considered by manufacturers:
than in the adjacent cabin area for this example.
Controlled Drainage Path. As water freezes on the
Measures That Affect Envelope ELA
outer skin during flight, large amounts of water as ice
More than two decades ago, following the elimination
could accumulate in the insulation envelope on any part
of tobacco smoking on the airplane, the outdoor air ven- of the cold structure over many flights if the airplane
tilation rate in commercial aircraft was reduced by 50%. is operated in cold climates. As soon as the airplane
This reduction effectively doubled occupant-sourced
encounters an environment warm enough to melt the
airborne pollutants, so a HEPA filtration rate equal to
ice, this water will begin to flow downward to the cabin
*When the temperature between the insulation and the skin reaches its lowest value.
34

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