Aircraft Ventilation System Design Challenges and Solutions

Copyright Indoor Air Technologies Inc. Canada & USA 1-800-558-5892

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Douglas S Walkinshaw, PhD., P.E.

Passenger aircraft ventilation system design challenges and

solutions

April 2005


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Passenger Aircraft Ventilation System Design Challenges

Figure 1. A passenger aircraft ventilation system must cope with higher occupancy densities, a wider range of occupant ages, health conditions and activities, more unusual ventilation air contaminants, lower ventilation air moisture content and more severe fire safety challenges than any other public space ventilation system.


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Current ventilation norms

Building offices – 7 persons/10,000 ft3, Aircraft cabins – 230 persons/10,000 ft3

Building offices – > 18 L/s/person filtered low outdoor air VOCsAircraft cabins – > 3.75 L/s/p unfiltered bleed air VOCs, ozone

Building offices – 50 L/s/person (100 CFM/p)Aircraft cabins – 7.5-10 L/s/person (15-20 CFM/p)

Building offices – Fleecy chairs, carpets with low trafficAircraft cabins – Fleecy chairs, carpets with high traffic

Building offices – 20-65%Aircraft cabins – 5-25%

Building offices – copier, humanAircraft cabins – human, combustion, anti-corrosion treatment

Building offices – escape in minutesAircraft cabins – escape often not possible

Building offices – air pressure = 0.9 to 1 atm; BLOC = 95-100%Aircraft cabins – air pressure = 0.75 atm, BLOC = 85-90%

Occupant contagion

Indoor/outdoor air exchange rate

Air flow rate (outdoor + recirculation air)

Dust and allergens

Relative humidity

Volatile organic compounds

Fire, biological/chemical agent release

Blood oxygen content (BLOC)

Buildings: - 20F to +110FAircraft : - 55F to +110FOutdoor temperature environment

Table 1. Current ventilation norms: aircraft passenger cabins versus buildings


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Six ventilation design challenges addressed in this presentation

Table 2. Aircraft ventilation design challenges and a set of solutions

Challenge # Aircraft Ventilation Challenge

1 Unusually contaminated ventilation air.

2 Exposure to occupant-generated pathogens and VOCs.

3 Limited biological/chemical agent release, fire fighting and smoke control capability

4 Low occupied space humidity and fuselage condensation.

5 Fuselage offgasing on the ground.

6Passenger cabin thermal discomfort due to warm fuselage on the ground on sunny days, and cold fuselage during cruising flight.


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ENGINE BLEED AIR CONTAMINATION SOURCESa. Ingestion of other aircraft engine fume exhaustb. Bearing oil leaksc. Deicer ingestiond. Oil coated ventilation ducts which sorb and later

desorb these contaminants

AIRCRAFT VENTILATION DESIGN CHALLENGE #1Contaminated ventilation (engine bleed) air

Table 3. Aircraft ventilation air contaminants


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Figure 2. Volatile organic compound chromatogram for bleed air during flight, TVOC = 270 ug/m3. The dominant branched alkanes are typically associated with fuels and solvents. Their origin could have been the oil coating the ducts acting as a sorbent of, for example, earlier ingested engine exhaust fumes.



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Bleed air study by

Fox

Bleed air study by Nagda

AM SD

Ethanol 36.0 118.7 0.0 0.0Acetone 9.7 18.7 3.0 2.7Acetaldehyde 8.8 15.6 1.0 1.2Toluene 5.5 8.6 3.8 5.9Propionaldehyde 4.0 7.4 0.8 1.3Methylene chloride 1.6 6.7 1.8 2.5m/p-Xylene 3.4 5.6 0.5 0.5o-Xylene 1.6 3.9 0.0 0.0Tetrachloroethylene 1.5 2.2 0.0 0.0Benzene 0.0 0.0 1.0 1.0

Compound name

Aircraft ventilation air ug/m3 (ref 2,3)

Air outside office buildings, ug/m3 (ref 1)


Table 3. Some volatile organic compound concentrations found in cabin ventilation air versus averages for the same VOCs in the air found outside office buildings.


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a. Aircraft ventilation and air circulation per person rates are lower than in any other public indoor environment.

b. High aircraft cabin occupancy density creates peak occupant generated pathogen and VOC exposures sooner than in any other public indoor environment.

c. Ceiling supply air diffusers are remote from perimeter seats, exacerbating the pathogen and VOC exposure loads in near the cabin liner.

AIRCRAFT VENTILATION DESIGN CHALLENGE #2Occupant-generated pathogen & volatile organic compounds (VOCs)

Table 4. Reasons for high occupant generated pathogens and VOCs in commercial aircraft passenger cabins.


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Table 5. Three dominant volatile organic compounds found in aircraft cabin air versus their concentration averages in some office buildings.

Cabin air low GM

Cabin air high GM AM SD

Ethanol 324.0 1,116.0 81.1 82.3

Toluene 6.6 68.0 11.0 7.5

Acetone 40.8 58.9 26.3 8.9

Aircraft cabin air, ug/m3 (ref 2,3)

Office buildings, ug/m3 (ref 1)Compound

name


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0

0.05

0.1

0.15

0.2

0.25

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Time after occupancy, hours

Rela

tive b

ioeff

luen

t (g

ases,

pat

co

ncen

trati

on

s

V= 5CFM/p SLE,occupancy=230 p/10,000ft3V= 5CFM/p SLE,occupancy=25 p/10,000ft3V= 5 CFM/SLE,occupancy = 7p/10,000ft3

Equilibrium concentrationFor all ODs

Office OD

School OD

Aircraft cabin OD

Figure 3. The impact of occupancy density on occupant-generated pathogen and VOC exposures. Aircraft ODs are higher and movement more limited than in any other common publicspace, creating higher occupant-generated VOC and pathogen exposures in aircraft cabins than in any other public space.



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a. Fire in the cavity behind the liner currently cannot be extinguished using Halon as phosgene will be produced which could enter the cabin and kill the occupants.

b. Smoke in the cavity behind the liner can enter into the cabin.

c. Smoke, biological and chemical agents released in the occupied space currently can only be exhausted at the floor in most aircraft. This is inefficient as convection currents tend to move these agents in the opposite direction.

AIRCRAFT VENTILATION DESIGN CHALLENGE #3Limited firefighting, smoke, biological and chemical agent control

capability

Table 6. Problems with current emergency air contaminant removal measures in aircraft passenger cabins.


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The generally accepted minimum relative humidity for comfort is 20% to 65%. The optimal relative humidity range for health is 40-50%. Typically neither of these ranges are met in aircraft for two reasons:

a. The low moisture content of the ventilation air creates aircraft occupied space humidities of 10% or lower, during international flights. Such low occupied space humidity can cause respiratory and eye discomfort. For some with respiratory problems, it can result in acute health problems.

b. Aircraft occupied spaces generally are not humidified. This is because humid cabin air passes through openings in the occupied space liner onto the cold fuselage behind. Here the moisture condenses creating a number of problems. These include added dead weight, deterioration of the insulation, wiring and fuselage, microbial growth and drips through the liner onto the occupants and furnishings below.

AIRCRAFT VENTILATION DESIGN CHALLENGE #4Occupied space humidity and fuselage condensation

Table 7. Fuselage condensation – the reason why aircraft passenger occupied spaces are not humidified


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Stack Pressure = Air density *(Ti-To) * H/2

Figure 4. STACK PRESSURES create air circulation flows between the occupied space and the fuselage cavity behind the liner. This air circulation deposits occupied space air humidity as condensate on the cold fuselage. Stack pressures are created by the thermal gradient between the occupied space and the outdoor air, which can exceed 70 Celsius degrees during cruise.

AIRCRAFT VENTILATION DESIGN CHALLENGE #4Occupied space humidity and fuselage condensation


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AIRCRAFT VENTILATION DESIGN CHALLENGE #5Fuselage offgasing

Figure 5. Volatile organic compound chromatogram for fuselage (envelope) VOCs at take-off, TVOC = 22,000 ug/m3. VOCs primarily originated with anti-corrosion treatment oil. These VOC concentrations are highest before take-off with the fuselage heated in a summer sun. Other fuselage VOC sources include wet insulation and microbial growth.


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0

100

200

300

400

500

600

ug/m3

World indoornorm

Canadaindoor norm

Gasper Envelope

Figure 6. Envelope and gasper microbial volatile organic compounds (MVOCs). Samples taken while on the runway. MVOCs were higher than indoor air norms in the fuselage envelope behind the occupied space liner, and lower in the gasper bleed air. The MVOC primary origin in the envelope likely was the insulation.

AIRCRAFT VENTILATION DESIGN CHALLENGE #5Fuselage offgasing and microbial growth


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AIRCRAFT VENTILATION DESIGN CHALLENGE #6Thermal discomfort

a. On the ground, during boarding and prior to take-off, aircraft occupied spaces can be uncomfortably warm. In part this is because the aluminum skin and cavity behind the liner heat rapidly in the sun to temperatures well in excess of 100F. The forced air circulation cannot keep up with this thermal load.

b. In the air during cruise, the cavity behind the liner cools under external temperatures in excess of –50 F, reaching its cold soak condition in about an hour. This can make for cool occupied spaces, particularly under low occupancies and during sleeping periods on international flights.

Table 8. Occupied space thermal discomfort is caused in part by high fuselage external thermal loads.


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A ventilation technology used in industrial environments

ECHO Air*

is being developed for aircraft to address these six ventilation design challenges.

*USA patent 6,491,254 B1Patents pending in Canada, Germany, France, Spain, Sweden & United Kingdom


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Six Aircraft Ventilation Design Challenges & The ECHO Air Solution

Table 9. Aircraft occupied space ventilation challenges problems and ECHO Air measures

Challenge # Aircraft Ventilation Challenge ECHO Air Solution

1 Unusually contaminated ventilation air.Pass bleed air through cavity behind liner to filter particles and sorb VOCs before entering the occupied space as cleaned ventilation air.

2 Exposure to occupant-generated pathogens and VOCs.

Improve ventilation effectiveness by supplying ventilation air through the liner as well as via ceiling diffusers.

3 Limited biological/chemical agent release, fire fighting and smoke control capability

Exhaust smoke from the occupied space through the liner and from there directly to the outdoors, keeping the liner at negative pressure relative to the occupied space. If there is a fire behind the liner, inject Halon into this cavity while keeping this

4 Low occupied space humidity and fuselage condensation.

Pressure the cavity behind the liner to prevent humid cabin air entry to the fuselage.

5 Fuselage offgasing on the ground. Exhaust and depressurize the cavity behind the liner during runway taxi and ascent.

6Passenger cabin thermal discomfort due to warm fuselage on the ground on sunny days, and cold fuselage during cruising flight.

Cool and heat the cavity behind the liner as required using appropriately tempered bleed air.


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Occupied space liner and the fuselage envelope

The ‘envelope’ is the section between the cabin liner and the plane aluminum skin.

Figure 7. Currently air movement in the aircraft envelope is uncontrolled. This uncontrolled movement creates fire hazard, air quality and moisture problems. The cabin envelope is located between the cabin liner and the aircraft aluminum skin. It contains the structural frame, insulation wiring and ducts.


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• The envelope is pressurized relative to the occupied space by offsetting stack pressures which would otherwise force occupied space air into the envelope. This pressurization is achieved byinjecting engine bleed air or another dry air source, at a measured rate and temperature, assisted where required by flow blockers and liner sealing reducing injection rate requirements. This injected air subsequently enters the occupied space through liner leakage.

• Pressure difference barrier required across the liner: >1 Pa at all locations at all times. (1 Pa = 0.004 in. w.g.; 1 Pa= 0.00015 psi)

ECHO Air Aircraft Ventilation System

“envelope air pressurization mode”

Table 10. ECHO Air envelope pressurization relative to the occupied space during normal cruising flight.


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Bleed air supply to envelope is filtered, pressurizes and conditions the envelope and then it enters cabin.


“envelope air pressurization mode”

Envelope flow blockers reduce stack pressures and airflow requirements.

Figure 8. ECHO Air envelope pressurization relative to the occupied space during normal cruising flight.

Drawing courtesy of Air Data Inc., Canada.


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“envelope air depressurization mode”

• The envelope is depressurized relative to the occupied space by offsetting stack pressures which would otherwise force envelope air into the occupied space. This depressurization is achieved by exhausting air from the cavity at a measured rate, assisted where required by flow blockers which reduce stack pressures, and liner sealing reducing exhaust flow requirements.

Table 11. ECHO Air envelope depressurization relative to the occupied space during taxi, ascent and emergency situations


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Envelope depressurization traps envelope contaminants & draws in cabin air contaminants

Direct envelope exhaust eliminates envelope air cont-aminants

Envelope tubes inject fire suppressant

Envelope smoke sensors identify and locate an envelope fire

Envelope flow blockers limit envelope fire spread.


“envelope air depressurization mode”

Figure 9. ECHO Air envelope depressurization relative to the occupied space during taxi, ascent and emergency situations.

Drawing courtesy of Air Data Inc., Canada.


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ECHO Air Ventilation Advantages Over Current Aircraft Ventilation Designs

Envelope Pressurization Mode

a. Filters engine bleed air particles and sorbs gases before this air enters the occupied space as ventilation air, reducing and eliminating associated health and comfort risks.

b. Improves ventilation effectiveness through better occupied space air circulation, reducing VOC and pathogen exposure.

c. Prevents humidity condensation behind the liner and the associated fuselage structural and insulation damage while allowing occupied space relative humidity to be safely increased to comfortable and healthy levels.

d. Improves occupant thermal comfort through cabin liner radiant heating and cooling.

Envelope Depressurization Mode

a. Quickly vents smoke from electrical fires behind the cabin liner without allowing its entry into the occupied space, preventing potential injuries and death.

b. Enables the safe use of Halon to suppress fires behind the cabin liner.

c. Accelerates the clearing of bioterrorism agents and of smoke, preventing potential injuries, illness and death.

d. Removes humidity and volatile organic compounds from the cavity behind the liner during taxi and take-off before they can freeze in the envelope or enter the occupied space.

Table 12. ECHO Air ventilation advantages over current aircraft ventilation system designs

Documents

Aircraft Ventilation System Design Challenges and Solutions