ASHRAE Journal - May 2020 - 45

TECHNICAL FEATURE

has entered the envelope. The former case is
obviously the preferable one in that it uses
10 times less dry air, and it keeps the insulation envelope dry both in the sidewalls and
at the crown. Similarly sized sealing building envelopes can overcome winter stack
pressures with injection ﬂows of 15 cfm to
30 cfm (7 L/s to 14 L/s) (total for the building) for 2 Pa (0.008 in. w.g.) stack pressures.
As aircraft cabin's stack pressures will be 4
Pa (0.02 in. w.g.) at cold soak, perhaps a 60
cfm (28 L/s) (total for the airplane) injection
is an achievable target for aircraft envelope
pressurization.

Dry Air Source

FIGURE 19 Sealed envelope dry air injection.

Outer Skin
Main Duct
Ribs

Liner

Main
Cabin Air
Duct

Fl ow

Bl o c

Fl ow

ACU

ker

Bl o c

ker

Piccolo Tubes

Outdoor Air

Airﬂow Controller
Airﬂow Direction
While dehumidiﬁers can be used to produce
Envelope
the dry injection air, engine bleed air or any
Injection Air
outdoor ventilation air can also be used without dehumidiﬁcation since ambient air is dry
entering the cabin during take-off and ascent while the
at cruise altitudes. Using engine bleed air rather than
dehumidiﬁers saves in weight and energy consumption, envelope air is still warm. It could also be used for venting the envelope while the plane is on the ground, with a
as the majority of it will circulate back into the cabin as
hot air supply for example.
useful ventilation air.

Examples of Dry Air Injection and Flow Blocker Options

Summary

The dry air injection airﬂows into the envelope that
pressurize it are illustrated in Figure 19 with two longitudinal ﬂow blockers, one at the ﬂoor and one above the
windows.
While circumferential piccolo injection air tubes are
shown between each set of ribs, injection could simply
be longitudinally at the crown using the main tubing and by tubing below the blocker at the top of the
windows. Having a ﬂow blocker above the windows
will enable the crown injection air to enter the cabin
above the passengers and be used by them as ventilation air. If there is a ﬂow blocker only at the ﬂoor, then
only crown injection is required, and in this case the
injection air will enter the cabin above the ﬂoor and be
drawn into the cargo space below. In this case only the
portion of the air recirculated from the cargo space will
be used as a component of the cabin ventilation air.
The envelope can also be depressurized using this
system by exhausting it to the outdoors. This could be
advantageous to prevent any air contaminant and moisture accumulation while the plane is on the ground from

The envelope around the airplane cabin has varying
degrees of ﬂow porosity, depending upon how tightly
ﬁtting the walls, ceiling and bins are and the amount
and packing of the insulation. The tightest insulation
packing occurs at the ﬂow blockers, which are intended
to limit vertical ﬂow through the envelope. The size of
the gaps in the blockers affects how much ﬂow passes
through the envelope.
The cabin has a slightly elevated pressure relative to
the cargo space, which forces ﬂow through the return
air grilles to the underﬂoor recirculation system during
thermally neutral conditions (i.e., outer skin temperature equals cabin air temperature). The stack effect created by cold walls in ﬂight or on cold days on the ground
causes some ﬂow to bypass the cabin. The stack pressures can be much larger than the pressure drop across
the return air grilles.
Depending on the ﬂow pattern relative to the location
of the efﬂuent sources (passengers), the air that bypasses
the cabin through the envelope may increase or decrease
the efﬂuent levels in the cabin.
M AY 2020

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