Background of the Invention
TECHNICAL FIELD
The present invention relates to a method and apparatus for controlling the environment within an enclosed space. More particularly, the present invention relates to an environmental control system for providing controlled ventilation of the interior space of an aircraft body, such that interior condensation and corrosion is reduced, cabin air quality is improved, the cabin can be humidified to healthy levels without increasing condensation and associated deleterious effects, and envelope fires can be directly suppressed and vented.
BACKGROUND OF THE INVENTION
In the embodiments of the invention described below and illustrated in the appended drawings, the "body" of an aircraft is comprised entirely within the fuselage, and excludes the wings and tail surfaces, as well as those portions of the nose and tail cones which extend beyond the respective nose and tail pressure bulkheads. However, it will be understood that the present invention is equally applicable to other aircraft geometries (such as, for example flying wing and lifting body designs). Thus in general, and for the purposes of the present invention, the "body" of an aircraft will be considered to be that portion of the aircraft which is pressurized during normal cruising flight, and within which it is desirable to control the environment in order to enhance safety and comfort of passengers and crew.
For the purposes of the present invention, the body of an aircraft is considered to be divided into a cabin, one or more cargo bays, and an envelope which surrounds both the cabin and the cargo bay(s). The terms "cabin" and "aircraft cabin" shall be understood to include all portions of the interior space of the aircraft which may be occupied during normal flight operations (i.e. the passenger cabin plus the cockpit). The term "envelope" shall be understood to refer to that portion of the aircraft body between the cabin (and any cargo bays), and the exterior surface of the pressure shell (including any pressure bulkheads) of the aircraft. In a conventional jet transport aircraft, the envelope typically comprises inter alia the exterior fuselage skin; nose, tail and wing root pressure bulkheads; insulation blankets; wire bundles; structural members; ductwork and the cabin (and/or cargo bay) liner.
The term "ventilation air" is defined as outside air typically introduced as bleed air from an engine compressor. For the purposes of this invention, "ventilation air" shall be understood to be outdoor air brought into the cabin by any means, for example, engine bleed air, either with or without filtering. "Ventilation air" does not include recirculation air or cabin air, filtered or otherwise reconditioned, which is supplied back into the interior space of the aircraft. For the purposes of this invention, "recirculation air" shall be understood to comprise air drawn from the interior space of the aircraft, possibly conditioned, and then returned to the cabin.
To facilitate understanding of the present invention, the following paragraphs present an outline of condensation/corrosion, air quality, and fire problems encountered in typical jet transport aircraft, and conventional measures taken to address such problems.
Moisture Condensation Problems
Aircraft are subjected to sub-zero temperatures (e.g., - 50C) when flying at cruising altitudes. While the aircraft skin is slightly warmer than outside air due to air friction, temperatures behind and within the insulation blankets (particularly adjacent the skin) cool to 0C to -40C, depending upon flight duration and altitude. When cabin air passes behind the insulation, it can reach the temperature at which its moisture starts to condense (i.e., its dew point). Further cooling beyond this temperature will result in additional condensation (as liquid water or ice) on the skin and other cold sinks.
Cabin air circulates behind the insulation, drawn through cracks and openings by pressure differences created when the cabin is depressurized during ascent, for example, and during flight by stack pressures (buoyancy effect). Stack pressures are created by density differences between the cooler air behind the insulation and the warmer air in front of the insulation. The density difference creates a slight negative pressure in the envelope (relative to the cabin) near the ceiling of the cabin and a slight positive pressure in the envelope near the floor of the cabin.
The effects of this condensation range from a simple nuisance through increased operation costs to decreased aircraft life. The more an airplane is used, the greater its occupant density and the lower its ventilation rate per person, the higher its potential for condensation problems. Cases have been reported of water dripping from the cabin paneling. Wetting of insulation increases thermal conduction and, over time, adds weight, increasing operating costs. This condensation increases the potential for electrical failure. It can lead to the growth of bacteria and fungi. It causes corrosion, leading to earlier fatigue failure and reduced aircraft life. Some estimates place capital and maintenance costs attributable to such condensation at up to $100,000 annually for larger, heavily utilized passenger aircraft.
Conventionally, passive measures have been used to cope with the envelope moisture problem. These include anti-corrosion coatings, drainage systems, and deliberately maintaining cabin humidity well below American Society of Air-Conditioning Engineers (ASHRAE) Standard recommended levels.
United States Patent No. 5,386,952 (Nordstrom) teaches a method for preventing moisture problems by injecting dehumidified cabin air into the envelope. However, the installation of dehumidifiers, as taught by Nordstrom, increases electrical consumption, occupies additional volume, and adds dead weight. Thus in a recently published study ("Controlling Nuisance Moisture in Commercial Airplanes") Boeing Aircraft Company concluded that active dehumidification systems, such as those taught by Nordstrom, are not cost-effective, even though they can reduce moisture condensation within the envelope. Additionally, the dehumidification system taught by Nordstrom is incapable of addressing related cabin air quality issues, as described below.
Cabin Air Quality
Relative humidities above 65 percent, which commonly occur in aircraft envelopes for even relatively low cabin humidities, can support microbial growth under appropriate temperature conditions. Such growth can include Gram-negative bacteria, yeasts and fungi. Where sludge builds up, anaerobic bacteria may grow, producing foul smelling metabolites. Saprophytic microorganisms provide nutriment for Protozoa. Exposure to aerosols and volatile organic compounds (VOCs) from such microbial growth can result in allergenic reactions and illness.
The relative humidity of outside air at typical cruising altitudes is frequently less than 1-2% when heated and pressurized to cabin conditions. Consequently, since cabin air normally is not humidified, on longer flights some passengers may experience dryness and irritation of the skin, eyes and respiratory system, while asthmatics may suffer incidences of bronchoconstriction. High air circulation velocities compound this problem. While humidification of the cabin air during flight would alleviate the "dryness" problem, it would also exacerbate the potential for microbial growth and damp material off-gassing in the envelope.
Thus, although it would be of benefit for health purposes to maintain higher cabin air relative humidities which are within the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Standard, this is made impracticable by the envelope condensation problem.
Other air contaminants in aircraft causing sensory irritation and other health effects can originate from ventilation air, passengers, materials, food, envelope anti-corrosion treatments, envelope microbial growth, etc. Ventilation air contaminants originate outdoors and within the engine (when bleed air is used). Potential contaminant gases and particulate aerosols include:
- combusted, partially combusted and uncombusted hydrocarbons (alkanes, aromatics, polycyclic aromatics, aldehydes, ketones);
- deicing fluids;
- ozone, possibly ingested during the cruise portion of the flight cycle; and
- hydraulic fluids and lubricating oils, possibly originating from seal leakage within the engine.
Gas chromatography/mass spectrometry (GC/MS) head space analyses of engine lubricating oil (Figure 9a), jet fuel (Figure 9b), and hydraulic fluid (Figure 9c) indicate some of the potential VOCs that might be found in aircraft ventilation air.
Figure 8a shows a GC/MS plot of a ventilation air sample taken in a jet passenger aircraft during the cruise portion of the flight cycle (28000 ft and -34C). The total concentration was 0.27 mg/m3 at a cabin pressure altitude of approximately 8000 ft. For comparison, ventilation air VOC concentrations for downtown buildings typically are less than a third of this concentration. VOCs identified include 3-methyl pentane, hexane, 3-methyl hexane, toluene, hexanal, xylene, and many C9-C12 alkanes. Additional compounds reported by other researchers include formaldehyde, benzene and ethyl benzene. Many of the compounds in the jet fuel (Figure 9b) can be seen in this ventilation air sample. The total VOC (TVOC) concentration was 0.27 mg/m3 at a cabin pressure altitude of approximately 8000 ft. Of this some 0.23 mg/m3 could have a petroleum (combustion source). The TVOC concentration is equivalent to a TOC exposure of 0.36 mg/m3 at sea level. In comparison, urban residential ventilation air TVOC concentrations are typically less than one-third this aircraft ventilation air concentration (i.e., <0.03 mg/m3), and building room air TVOC concentrations typically are less than 0.5 mg/m3. One postulate for the high VOC concentrations found in aircraft is that periodic incidents of lubricating oil leakage produce aerosols which enter the ventilation system and progressively coat the interior surfaces of the supply ducts. This coating, in turn, could sorb VOC's ingested during taxi from the exhaust of other aircraft. These VOC's may subsequently be released into the cabin during flight.
Contaminated ventilation air increases ventilation rate requirements to achieve any particular space concentration target. For example, a ventilation rate with TVOCs=0.36 mg/m3 must be three times higher than one with TVOCs = 0.036 mg/m3 to maintain a room TVOC concentration of 0.5 mg/m3.
Cabin air contaminants can originate from materials and, possibly, microbial growth in the envelope as well as from cabin furnishings, food and passengers. Contaminants in the envelope enter the cabin when cabin air is circulated behind the insulation, drawn there by envelope stack pressures and by decreasing cabin pressures (for example, during ascent).
Figure 8b shows a GC/MS plot of envelope air in an aircraft parked when the temperature in the air space between the skin and insulation was approximately 35C. The total (TVOC) concentration was 22 mg/m3. Of this, some 21 mg/m3 had a petroleum source and 0.6 mg/m3 could have had a microbial source. VOCs from one source of these envelope contaminants, an anti-corrosion treatment, is illustrated in Figure 9e. This head space sample was taken at -5C, a temperature representative of the temperature behind the insulation during the early portions of cruising flight. This anti-corrosion treatment emitted many of the compounds seen in the envelope and the ventilation air, plus a number of cycloalkanes and aliphatics not seen in the other samples. Figure 9d shows the head space GC/MS plot of a general purpose cleaner (2-butanone or methyl ethyl ketone) used on this aircraft. This compound was also identified in the envelope, engine oil, ventilation air and anti-corrosion treatment samples.
When the envelope is cooled in flight or warmed on the ground, envelope material off-gassing and sorption of contaminant gases change. For example, under ideal conditions, the deposition of VOCs of interest behind the insulation could increase a hundred-fold for temperature decreases over the typical flight cycle temperature range.
Condensation of higher molecular weight compounds at higher concentrations may occur when the envelope is cooled. For example, the maximum concentration of dodecane (a compound found in the ventilation air and anti-corrosion treatment samples), at -40C is 0.26 mg/m3.
One implication of the above is that during the ascent and the early portions of the cruise flight cycle while the envelope is still relatively warm, envelope VOCs could pose an air quality problem for passengers. Another implication is that cabin air VOCs will be deposited (sorbed) in the envelope when it is cold, particularly during later stages of the cruise portion of the flight cycle. For example, both ventilation air VOCs (Figure 8a) and the cabin cleaner VOC (Figure 9d) can be found in the envelope air sample (Figure 8b).
Some aircraft have high efficiency particulate filters (HEPA) filters which will remove human microbial aerosols that enter the circulation system. Some have catalytic converters to remove ozone. Very few have sorbent air cleaners to remove ventilation-air and cabin VOCs.
Fire and/or Pyrolysis in the Envelope
In the case of a fire, thermal and electrical insulation systems in the envelope as well as other materials in the cabin can undergo pyrolysis and burning, generating toxic smoke and combustion products. Conventionally, this problem is addressed by employing fewer combustible materials, and using hand-held containers with non-toxic fire suppressants. Currently, insulation is under review in this regard with a prevention program potentially involving more than 12,000 commercial aircraft.
Under any cabin fire emergency, the objective is to exhaust the smoke from the cabin while suppressing the fire. There is currently no method in place to directly suppress or extinguish fire and/or pyrolysis within the envelope. Nor is there any effective means of preventing smoke within the envelope from penetrating into the cabin. Furthermore, exhaustion of air from the cabin is usually via grilles at the floor, which undesirably enhances smoke circulation throughout the cabin.
United States Patent No. 4,726,426 (Miller) teaches a method of fire extinguishment in aircraft cabins using ventilation ducts in communication with the cargo fire extinguishment system. However, this system does not address envelope fires and/or pyrolysis, or the health and safety problems associated with exposing passengers to potentially lethal combinations of fire suppressants and their combustion products in combination with fire and smoke.
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