ASHRAE Journal - May 2020 - 38

TECHNICAL FEATURE

effectiveness is limited by the humidity
reduction the system is able to achieve.
Larger airplanes frequently use the crown
space as a recirculation duct, and large
amounts of cabin air pass through it. These
flows are additive to the stack pressure flows.
The CCDS must be capable of humidity
reduction down to the dew point of the outer
skin with more than 1,000 cfm (472 L/s) of
flow capacity to eliminate condensation
(Figure 5, Page 36).

Fire Stop as a Flow Blocker

FIGURE 6 Cabin wall "fire stop" between frames showing the air leakage gaps that allow air to flow
through it.

Sidewall Liner
Additional
Installation
Outer Skin
Stringers

Cabin
Flow
Location of
Frames

Gaps

Frames

Inboard

Forward

Airplanes have a flow blocker (fire stop) that
is typically located between the winFIGURE 7 A simplified example of bypass ventilation flow loss for basement and airplane cabin.
dows. The purpose is to prevent smoke
Wall Zone Porous Wall
Wall Zone Porous Wall
and fire from rising up the envelope
Envelope
during an emergency. The fire stop
Envelope
Mixed Zone
Mixed Zone
Flow
Flow
requirement may be met by sealing
Envelope Zone
Breathing Zone
Envelope
Breathing
Zone
with additional insulation (Figure 6).
Zone
Although the blockage is more
than sufficient for fire or smoke, it
does allow some flow of air to pass
through it and also the envelope over
Exit
Airplane Cabin
Basement
the cold outer skin and structure of
effluent levels in the room, also depending on the locathe airplane.
tion of the sources and the circulation pattern in the
Percent Ventilation Air Passenger Bypass
room.
Examples of Envelope Air Ingestion
The envelope zone is bound by the cold outer wall
In Figure 7 a simplified diagram is used to convey
and a porous inner wall (and/or ceiling). The cold outer
the physical aspects of the ventilation zones that best
wall removes heat from the zone, lowering its temperadescribe a room with a sealed wall (concrete wall of a
ture relative to the room. Buoyancy effects lower the
basement) and a room with a porous ceiling and wall
pressure within the zone, drawing air from the room
with a floor exit (airplane cabin). The inlets are not
through the porous wall. The zone contains insulation
shown because the human effluent (humidity, particuthat impedes the flow somewhat. Unlike a basement
late and gaseous contaminant) levels are supply location masonry foundation wall through which outside water
dependent.
passes via capillary action as well as any cracks, the
There is no bypass ventilation flow for the basement,
airplane outer wall is impervious to outside moisture.
but odors and water vapor will have the same effect on
If the envelope is filled with air that is cleaner than
condensation within the envelope as in the airplane.
the air in the mixed zone (also the exit), outdoor air is
The breathing zone will have the highest water vapor
bypassing the room.
concentration if the inlet circulates the ventilation air
The following equation is used to quantify the amount
around the occupants rather than over them.
of ventilation air bypass loss. The rise in CO2 above that
The mixed zone represents the concentrations of efflu- of ambient outdoor air, which is assumed to be at 400
ppm, rather than the CO2 level is used to keep the calcuents that are expected in a well-mixed system.
lation compatible with other human effluents like water
The wall zone extends over the room side and ceilvapor, germs, etc.
ing surface of the wall. This zone could have the lowest
38

ASHRAE JOURNAL

ashrae.org

M AY 2020

https://www.ashrae.org/

ASHRAE Journal - May 2020

Table of Contents for the Digital Edition of ASHRAE Journal - May 2020