ASHRAE Journal - May 2020 - 44

TECHNICAL FEATURE

completely prevent humid cabin air entry depends on
the envelope ELA and the tightness of a flow blocker. The
injected air will leak back into the cabin above the flow
blocker and also, but to a lesser extent, through the flow
blocker as illustrated, but now the leakage into the cabin
is all in the desired direction.

FIGURE 17 Illustration of an airtight flow blocker.

Plate

Seal Plate at Perimeter
Foam

Air Barrier Over the Insulation
Similar sheets could be placed over the insulation
and taped together to form a tight air barrier inner seal
under the cabin (sidewall) liner and over exposed insulation under the floor. Join this air barrier to the crown
air barrier. This will increase liner air tightness and
decrease the amount of injection air required.

Flow Blocker

Stack Pressure
Across Insulation at
Cold Soak

Pressurizing the Sealed Insulation Envelope

The envelope should be sufficiently pressurized to ensure
outward flow over the entire inner surface of the
envelope.
If the envelope is pressurized over cabin pressures
by the air injection by 0.5 Pa (0.002 in. w.g.) or more
throughout cruise level flight, there will be no envelope
condensation during this time. The measures taken and
their impact on stack pressures and airflows are illustrated in Figure 18. The envelope is sealed on the cabin
side with polyester film sheeting.
With the three measures-crown and liner sealing,
one or more flow blockers, and envelope net pressurization with dry air of 0.5 Pa (0.002 in. w.g.)-in-flight
condensation and ventilation air bypass of occupants
will be prevented. The tighter the flow blocker(s) and
the sealing of the crown and behind the cabin liner, the
44

ASHRAE JOURNAL

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M AY 2020

Stack Pressure Across
Insulation with Polyester
Film Sheet Seal Behind Liner
And Floor Flow Blocker

(d)
Crown

Dry Air Injection
Over-Pressure

Cabin

Envelope

0.5 Pa
Cabin

Envelope

Cabin

Envelope

Cabin

Envelope

Z
There are any number of ways to create a well-sealed
flow blocker. One is to install lightweight, fire-rated
plates, which fit tightly into the
FIGURE 18 Provide sufficient envelope pressure to ensure outward flow.
frame spaces. These plates would
have cut-outs for wire bundles
(b)
(c)
(a)
Crown
Crown
Crown
and ducts. The gaps could be filled
1.5 Pa
with flexible spray foam or adhe3 Pa
2 Pa
sive. This flow blocker sealing is
Dry
Air
Moist Air
illustrated in Figure 17. This seal is
2 Pa
designed to fail at high pressures
2 Pa
1.5 Pa
during any sudden cabin depresFlow
Flow
Floor
Blocker
Blocker
surization event.

3.5 Pa
Flow
Blocker
Net Pressure Across Insulation
with Dry Air Injection, Polyester
Film Sheet Sheet Behind Liner,
and Floor Flow Blocker

smaller amount of dry injection air required, perhaps
no more than one-tenth of the amount of current CCDS
dehumidified airflow rates. Just inserting a flow blocker
at the floor could reduce condensation rates by as much
as 50% (Figure 18b). The 0.5 Pa (0.002 in. w.g.) minimum
net pressurization of the envelope relative to the cabin
is chosen to minimize any molecular back diffusion of
cabin water vapor into the envelope through small leakage openings.
Note that pressurizing the envelope with dry air is
different than dehumidifying it like a CCDS. In the
former case, humid cabin air cannot enter the envelope through leaks and openings, as dry air will be
exiting there. In the latter case, the dehumidified air
dries wet surfaces and dilutes the humid cabin air that

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ASHRAE Journal - May 2020

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