Hdb Env Chem Vol. 4, Part H (2005): 261–x DOI 10.1007/b107248 © Springer-Verlag Berlin Heidelberg 2005 Published online: 8 August 2005
Aircraft Cabin Air Filtration and Related Technologies: Requirements, Present Practice and Prospects S. Michaelis1 (u) · T. Loraine2 1 School
Of Safety Sciences, UNSW, 2052 Sydney, Australia
[email protected] 2 The British Air Line Pilots Association (BALPA), 81 New Road, Harlington, Hayes UB3 5BG, UK
[email protected] Susan Michaelis is a former BAE 146 pilot and now a part time masters student at UNSW. Tristan Loraine is Chairman of the BALPA Cabin Air Quality Task Group which was set up by BALPA to explore all issues of the cabin environment and to make recommendations. His input to this article represents emerging thinking from the task group. Echo-Air diagrams courtesy of Indoor Air Technologies Inc, Canada and USA. 1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
262
2
Recirculated Air Composition . . . . . . . . . . . . . . . . . . . . . . . . .
263
3 3.1 3.2 3.3
Recirculated Air Filtration Methods Direct Interception . . . . . . . . . Diffusional Interception . . . . . . . Inertial Impaction . . . . . . . . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
263 264 265 265
4 4.1 4.2 4.3
High Efficiency Particulate Air Filters . . . . . . . . . . Number of Recirculated Air Filters per Aircraft Type . Cost Saving of Recirculated Air . . . . . . . . . . . . . Operational Effect of Unserviceable Recirculation Fans
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
265 267 268 269
5 5.1 5.2 5.2.1 5.2.2 5.2.3 5.3 5.4 5.5 5.6 5.6.1 5.6.2
Bleed Air Filtration . . . . . . . . . . . . . . . Non Regenerative Chemical Filtration System . Regenerative Chemical Filtration Systems . . . Temperature Swing Adsorption . . . . . . . . Pressure Swing Adsorption . . . . . . . . . . . Pressure Temperature Swing Adsorption . . . Plasma . . . . . . . . . . . . . . . . . . . . . . Ultraviolet Light . . . . . . . . . . . . . . . . . Nanocrystalline Materials . . . . . . . . . . . . Catalytic Converters . . . . . . . . . . . . . . . The Reduction Catalyst . . . . . . . . . . . . . The Oxidation Catalyst . . . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
269 270 271 271 272 272 272 273 273 274 274 274
6
ECHO-Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
275
7
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
279
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
281
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . . . . . . . . . .
. . . .
. . . . . . . . . . . .
. . . .
. . . . . . . . . . . .
. . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
262
S. Michaelis · T. Loraine
Abstract Aircraft cabin air being supplied from the engines or APU is known to occasionally be contaminated with hydraulic fluids, engine oils, and pyrolysis products of these which need to be removed to ensure that the crew and passengers are not exposed to any contaminants. One way of achieving this is to filter these contaminants out of the outside air before it reaches the crew and passengers. Additionally, some aircraft cabin air is recirculated and this also needs to be filtered to remove bacteria and viruses. This chapter reviews a number of catalytic, physical, and ventilation system alternatives to simple filtration that could help to eliminate the risk of contaminated outside air or recirculated air from entering the passenger cabin. Keywords Air quality · HEPA · Bleed air · Cabin air · TCP · Bacteria · Virus · Aircraft cabin fumes · Contaminated air · Engine oils · Hydraulic fluids Abbreviations APU Auxiliary Power Unit ASHRAE American Society of Heating, Refrigeration and Air-conditioning Engineers ASTM American Society for Testing and Materials FAR Federal Aviation Regulations HEPA High Efficiency Particulate Air Filter JAR Joint Aviation Requirements SIL Service Information Leaflet TCP tri-cresyl-phosphate TOCP tri-ortho-cresyl-phosphate DOCP di-ortho-cresyl-phosphate MOCP mono-ortho-cresyl-phosphate VOC volatile organic compound
1 Introduction Over the last few decades, aircraft manufacturers have sought ways to make aircraft engines more efficient and burn less fuel to make them more economical. An aircraft engine as well as producing thrust to propel the aircraft has other demands put on it. These include hydraulic pumps, electrical generators and provision of cabin air. Some of the air which could be used for thrust is “bled” off and passed through air-conditioning packs where it is cooled and supplied to the aircraft cabin. This air pressurizes the aircraft cabin and provides air to allow crews and passengers to survive whilst the aircraft often flies in extreme conditions such as pressures as low as 0.2 atmospheres and temperatures as low as – 60 ◦ C. In an effort to reduce operating costs many aircraft and engine manufacturers reduce the “bleed air” requirement by recirculating some of the cabin air and therefore put less demand on the engine for bleed air. Recirculation was commonplace before the jet age began. For example, the Boeing Stra-
Aircraft Cabin Air Filtration and Related Technologies
263
tocruiser of the late 1940s was equipped with an air recirculation system. In jet aircraft, filtered/recirculated air combined with outside air came into use principally with the introduction of high-bypass-ratio fan engines. At Boeing, this began with the 747 in 1970 [1]. This idea has evolved over thirty years until today when about 50% of the cabin air is recirculated air and 50% is fresh bleed air [2]. It is estimated that in so doing, an average airliner will save over US$ 60 000 per annum [3] compared to airliners not recirculating cabin air. Aircraft cabin air may contain numerous bacteria and viruses and these need to be filtered out if any cabin air is to be “recirculated”. Additionally, the air coming from the engines and APU is known to occasionally be contaminated with hydraulic fluids, engine oils, and pyrolysis products of these which also need to be removed. This chapter looks at aircraft filtration options and provides an introduction into their capabilities and the part they play or could play in commercial aviation.
2 Recirculated Air Composition Bleed air is outside air and should be fresh and clean, unless it becomes contaminated with aerosol droplets or vapors of engine oil, hydraulic fluids, other organic vapors, carbon dioxide or other by-products of combustion as they enter the aircraft. Recirculated air is not fresh air and given the large numbers of passengers on an aircraft, there are high concentrations of particulates (fibres, dust, skin particles), bacteria (up to 30 000 bacteria per minute per passenger can be released into the cabin environment from skin scales) [4], other micro-organisms as well as odors. These contaminants are all a potential risk to passengers and crews. Bacteria thrive in high humidity, and viruses in low humidity. Both conditions are found on commercial aircraft. In addition passengers will be more vulnerable to infection during a flight compared with normal non flight conditions. This is due to the closed conditions of the aircraft cabin environment, the small amount of available airspace per passenger, air continually being blown over the head area, and contact with people from diverse backgrounds.
3 Recirculated Air Filtration Methods Recirculated air-filtration systems have been designed to enhance passenger and crew health and comfort by controlling bacteria and viruses. Bleed air from the engine is cooled in an “Air Conditioning Pack” before going to a “Mix Manifold” where it is mixed with air from the cabin that is be-
264
S. Michaelis · T. Loraine
Fig. 1 Flow Chart of Recirculated Air
ing recirculated to provide crews and passengers with a mix of fresh air and recirculated air. The recirculated air-filtration systems are placed beside the “Mix Manifold” so that the cabin air that is to be recirculated passes through the airfiltration system as it enters the “Mix Manifold”. Filters used in this application should be able to remove particles down to the size of viruses (0.01 µm (micron) in diameter and below [4]), as well as bacteria and other particulate matter up to 10 µm in diameter. The 0.3 micron benchmark is used in efficiency ratings because it approximates the most difficult particle size for a filter to capture [5]. Table 1 Some typical dimensional comparisons Item
Diameter (µm)
Typical associated illness
Human hair Red blood cell Mycobacterium bacteria Pneumococci bacteria Influenza virus Rhinovirus virus
≈ 30 – 50 ≈ 8.0 0.2 – 1.0 0.5 0.1 0.03
Tuberculosis Pneumonia Flu, Croup, Pneumonia Common Cold
To enable filters to remove particles efficiently over a large dimensional window, they use several mechanisms of filtration which combine all of these mechanisms to a varying degree. The mechanisms used are: 3.1 Direct Interception Filters are made up of matrices with a well-defined pore size. If the particles are larger than the pores, they are captured by direct interception on the sur-
Aircraft Cabin Air Filtration and Related Technologies
265
face of the filter element. This mechanism can be likened to the wire screens used to separate gravel from sand. 3.2 Diffusional Interception Very small particles like viruses could pass through the empty spaces of a filter but they are influenced by Brownian motion. This is caused by the collision of rapidly moving gas molecules with the aerosol size range particles and droplets. A simple example of the three dimensional Brownian motion could be described in two dimensions as a “drunken man wandering around the square” [4]. The zig-zag movement of the microscopic particles caused by these collisions substantially increases the probability of collision with a fibre within a thick filter element, such as is normally employed for HEPA levels of efficiency. The Brownian motion causes small particles to be collected on the individual fibres and pore walls of the filter. Particles of about 0.1 µm diameter and below are captured using this principle. 3.3 Inertial Impaction Particles that have a higher density than air deviate from the air flow as it passes through the filter and impact on the surfaces or walls of the pores where they are captured. Inertial impaction works best for particles in the range 0.3 to 10 µm.
4 High Efficiency Particulate Air Filters The first HEPA filter was designed in the 1940s by the research and development firm Arthur D. Little under a classified government contract as part of the Manhattan Project, where the first atomic bomb was developed during World War II. A major advancement in air filtration technology, the filter solved a critical need to control very small particles which had become contaminated by nuclear radioactive sources [6]. Considering the condensation nuclei of radioactive iodine to be most harmful, researchers focused on the ability to capture solid particles that were created through the condensation of gases and liquid aerosols into solid matter. Having identified 0.3 micron particles as the most penetrating size and representative of the particle of concern, 0.3 microns was established as the particle size fraction at which to determine filter efficiency performance [6]. HEPA filters used in the aerospace industry are made of micro glass fibres and
266
S. Michaelis · T. Loraine
are similar to those used in hospitals. However, in the critical areas of hospitals where these are used, they filter outside air for removal of particulates and aerosols, not to recirculate potentially infectious air. Their performance or efficiency is normally reported as the capture percentage for 0.3 micron particles and to be meaningful, the reported efficiency must relate to particle size and flow velocity. According to the European air filter efficiency classification, a HEPA filter can be any filter element rated between 85% and 99.995% removal efficiency for 0.3 micron particles. However, for aircraft cabin air recirculation systems, this definition has been tightened and the current aerospace industry standard is 99.99% minimum removal efficiency by sodium flame test to British Standard BS.3928 or 99.97% minimum removal efficiency by di-octyl phthalate (DOP) test according to ASTM publication D 2986-95 [7]. This is the efficiency standard now specified by Airbus and Boeing for their new generation aircraft. The sodium chloride test consists of challenging the filter with an aerosol mist of sodium chloride (NaCl) particles, with a mean particle size of 0.58 micrometers. The DOP test consists of challenging the filter with an aerosol mist of di-octyl phthalate oil droplets, with a mean size of 0.3 micrometers. The removal efficiency, or penetration, is calculated as a percentage by measuring the aerosol concentrations upstream and downstream of the filter element under test. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) in their ASHRAE STANDARD 161 - Air Quality Within Commercial Aircraft Committee Review Draft, state in relationship to HEPA filters and recirculated air: “All air that is recirculated through the aircraft systems shall pass through a high efficiency particulate air (HEPA) filter before it is supplied to the cabin. HEPA filters used for this purpose shall meet or exceed the requirements of Institute of Environmental Science and Technology (IEST) Filter Type “B”, MERV 17 or H13 according to EN 1822-1 and shall provide 99.97% collection efficiency for 0.3 micron particles. The leak tests conducted to meet this requirement shall be on the standard aircraft holding frame used for the filter in accordance with IEST-RP-CC0034.1 or EN1822-4. These filters and their mountings shall be designed, installed and maintained as per manufacturer recommendations to prevent bypassing of unfiltered air due to media failure, improper installation, or other causes. Air used for recirculation should be extracted from the cabin at locations where the air is expected to be the least contaminated.” [8] Significant differences between microbes and chemical compounds (such as DOP and NaCl) makes the use of chemicals unsuitable for rating the microbial removal efficiency of air filters. Hence, HEPA filters on transport-category aircraft remove particles with an efficiency higher than 99.97% at 0.3 micron, significantly reducing the level
Aircraft Cabin Air Filtration and Related Technologies
267
of airborne-particulate contamination. Using the definition of a HEPA filter as either 99.99% sodium flame or 99.97% DOP, the first Airbus HEPA filters were introduced in 1994. The Airbus SIL, (ref. Airbus SIL21-065) [5] states “the existing filter element of 98% efficiency with a mean particle size of 8 microns (when tested with gravimetric method) has been replaced in production by a new filter element with a 99.99% efficiency with a particle size of about 0.3 microns (when tested by sodium flame test)”. The first HEPA filters (99.97% DOP) for Boeing aircraft were introduced on the B747-400 in 1998 (ref. Boeing SIL 747-SL-21-52-A) [5]. In 1999, United Airlines became the first major airline to install HEPA filters throughout the airline’s fleet [9]. Other airlines also have been specifying HEPA filters for new aircraft in recent years and retrofitting some aircraft. HEPA filters typically are disposable and roughly about 3 feet by 2 feet by 8 inches thick. This thickness accommodates the pleated structure of the filter medium which substantially increases the available filter area within the 3 by 2 foot frame, which reduces the pressure required to force the air through the filter. They have about a 4500 hour service life (although no regulation currently enforces change of used filters) and weigh about 5.5 lbs each [10]. Additionally, combined particulate and odor removal cabin filters are being offered by filter manufacturers such as Pall Aerospace in conjunction with Airbus [11]. These have an odor absorber fitted in series with the HEPA filter to remove odors and Volatile Organic Compounds (VOCs). The combined filters are directly interchangeable with the existing particulate cabin air filter elements. They weigh about twice the weight of a HEPA filter and Design Service Life is given as: “To be determined on in-service experience” [11]. Airlines so far have generally selected the HEPA only filter due to the financial penalties associated with the lower service life of the combined filters which need replacing more frequently. However, combined filters allow the operator to produce air that is closer to the quality of the outside air, which it is supposed to replace, so perhaps this should be regulated. 4.1 Number of Recirculated Air Filters per Aircraft Type The number of filters present on a commercial aircraft to filter recirculated air depends on aircraft types. Some examples are given in Table 2. Newer aircraft appear to have an increasing number of filters in an attempt to improve the quality of the recirculated air. In a drive to make engines more efficient and use less fuel, industry argues that by removing contaminants to such high levels there is justification to reduce further the amount of fresh air supplied from the engines from current industry practices. This would potentially enable industry greater cost savings but this is a much debated issue currently being evaluated by organi-
268
S. Michaelis · T. Loraine
Table 2 Relationship of filter numbers to maximum passenger carrying capability Aircraft type
Maximum number of passengers [12, 13]
Airbus A300-600/A310 Airbus A318/A319/ A320/A321 Airbus A330/A340
298/247
3
117/134/164/199
2
Boeing 737 -300, -500, -600, -700 Boeing 737 -400, -800, -900 Boeing 747-400 Boeing 757 Boeing 767 Boeing 777 DC-10/ MD-11
From 293 for A330-200 up to 419 for A340-600 Up to 149
Up to 189
Up to 524 in 2 class layout 228 on the -200 280 on the -300 Up to 375
No of filters [14]
4 (8 on extended range ones) 1
2
10 2 2
Up to 550
8
Up to 410
1
zations such as the ASHRAE in the USA and by the European Association of Aerospace Industries (AECMA) in Europe. 4.2 Cost Saving of Recirculated Air In the early days all passengers were supplied with 100% unrecirculated air. An important question is whether the financial savings made by the airline industry by recirculating air are really what the consumer wants, and whether this additional exposure risk is warranted. It has been estimated that the introduction of air recirculation saves airlines an average of US$ 60 000 per aircraft [4], per year. For 300 trips per year of a 200-seat aircraft this amounts to one dollar saved (by the airline) per passenger trip. This saving rises only to about two dollars per passenger trip for half the size of the aircraft, or for a 50% load factor for the 200-seat aircraft. The 15-cent per passenger hour fuel
Aircraft Cabin Air Filtration and Related Technologies
269
cost estimate for a 50% increase in the fresh air ventilation rate for a DC10-10 given in another survey by Nagda et al. is consistent with this. “For a 6-hour flight, this would calculate to a cost of about 90-cents per passenger trip. If only one more passenger became ill on a flight that used air recirculation compared with a flight without it, the cost to that individual and to society would far exceed the saving by the airline to use recirculation” [15]. 4.3 Operational Effect of Unserviceable Recirculation Fans A typical aircraft Minimum Equipment List (Dispatch Deviation Manual) which tells pilots and engineers what minimum equipment needs to be functioning prior to departure makes the following statements in relation to Recirculation Fans: Boeing 747-400
Number of recirculation fans fitted = 4
∗ With
one fan failed, increase flight planning fuel by 0.3%
∗ With
two or more fans failed, increase flight planning fuel by 0.8% [16] A Boeing 747-400 doing a typical long range flight from London to Singapore will burn 150 000 kg of fuel. Therefore, 0.3% will be an extra 450 kg, 0.8% will be 1200 kg of extra fuel needed. This shows that the amount of extra fuel that would be used in exchange for not having recycled air, a cost that perhaps passengers would be prepared to pay if they were asked to make an informed choice.
5 Bleed Air Filtration So far, we have only looked at filtering recirculated air which is very different from filtering the engine bleed air or APU ventilation air, which sometimes become contaminated with hydraulic fluids, engine oils or pyrolysis products of these not intended to ever be in the cabin air. Hydraulic fluids and engine oils usually contain a selection of toxic ingredients which include N-phenyl-1-naphthylamine (a skin sensitiser) and the organophosphate Tricresyl phosphate (TCP) [17]. The bleed air is known to become contaminated sometimes [18, 19], which prompts the question of whether this can be filtered to remove these contaminants, and what system could be used to do it. If the filters were placed in the correct location, i.e. at the pack outlet, then perhaps some of the issues could have been addressed many years ago. In particular Donaldson Company, Inc., have chemical adsorptive filters which been
270
S. Michaelis · T. Loraine
produced for various Boeing commercial airplanes for over 20 years and these filters are claimed to remove VOCs including organophosphates such as TCP from the recirculated cabin air [34]. Bleed air coming off the engines is very hot pressurized air and if filtered at source it would need a different filtration device from those designed for the filtration of microscopic particles and droplets from recirculated air which is at room temperature and cabin pressure. The next section explores the techniques to better filter bleed air which include filtering “hot” bleed air or filtering the bleed air at less extreme temperatures and pressures as it leaves the air conditioning packs after it has been cooled, before it reaches the mix manifold or aircraft cabin. 5.1 Non Regenerative Chemical Filtration System Commercial aircraft could use cold “bleed air” filtration to remove engine oils and hydraulic fluid contaminants that could be based around a popular adsorbent such as activated carbon. This was highlighted at a Cabin Air Quality Seminar in 1991 [20]. AMETEK Aircontrol Technologies in Middlesex, England have produced activated carbon “Odor Removal Filters” for the BAe 146 for some 10 years [21]. The activated carbon is a different product from the “carbon absorbent beds” offered by other filter manufacturers to remove odors. AMETEK Aircontrol Technologies use an activated carbon cloth about which the suppliers state that: “The activated carbon cloth (ACC) used in the Ametek flight deck and cabin filter are of the same specification to those currently being supplied for some military NBC applications. Our ACC was originally developed in conjunction with CDE Porton Down. Extensive testing has shown the size and distribution of the pore structure in these materials make them very good at adsorption of nerve gases which are organophosphorus compounds. Tri cresyl phosphate (TCP) is also an organophosphorus compound and therefore we see no reason why it should not be equally well adsorbed” [22]. As with all filters, it must be remembered that they are only effective if regularly replaced before they become saturated. Activated carbon is the main component found in standard NBC (Nuclear, Chemical & Biological) gas masks and protective suits. Coal becomes activated charcoal when it has been heated with steam or carbon dioxide, and in the absence of air. This process opens up millions of very small pores between the carbon atoms, resulting in highly porous charcoals that have surface areas of 300–2000 square meters per gram. These so-called active, or activated, charcoals are widely used to adsorb odorous or colored substances from gases or liquids, that attach to it by physical attraction. The huge surface area of activated charcoal gives it a large number of bonding sites. When certain chemicals pass next to the carbon surface, they attach to the surface and
Aircraft Cabin Air Filtration and Related Technologies
271
are captured. Activated charcoal is good at trapping carbon-based impurities (“organic” chemicals), as well as gases like chlorine. The more absorption sites become saturated, the less the filter will work. Therefore, the filter life will be dependent on the concentration of contaminants and the amount of contaminated air it is exposed to, as any particulate filter eventually clogs. This is also true, of course, for the HEPA filters used for recirculation of cabin air, which must be changed regularly to remain effective. Air purification technologies used on NASA spacecraft are worth looking at as they detail how far back technology was available to protect crew members when this was given a priority over cost issues. The early space programs, Mercury, Gemini, and Apollo, employed equipment that relied heavily upon physical and chemical adsorption and coarse particulate matter filtration to address these challenges. These used activated carbon to remove trace contaminants. Trace chemical contamination control still relied upon expendable adsorption beds. Little change was realized with the development of the Space Shuttle. Air purification systems used on board the Shuttle Orbiter actually reverted to systems similar to those used before Skylab. Expendable chemical and physical adsorption systems have been the rule. As a result, mission duration is limited to 15 days or less [23]. Skylab, America’s first space station, employed a similar approach for cabin air purification with the exception that carbon dioxide partial pressure control was provided by a pressure swing adsorption system. 5.2 Regenerative Chemical Filtration Systems To increase mission life or time between filter replacements a Regenerative Chemical Filtration System would be more advantageous. Sorbtion technologies capable of being regenerated can be classified by the method used to drive off the gases adsorbed, these are: • Temperature Swing Adsorption (TSA) • Pressure Swing Adsorption (PSA) • Pressure and Temperature Adsorption (PTSA) All these systems usually use a minimum of two filter beds, one adsorbing while the other is “regenerating” by the removal of previously adsorbed challenges. Following regeneration, the incoming air is diverted to the cleaned bed and the bed previously “on line” commences its regeneration cycle. 5.2.1 Temperature Swing Adsorption A TSA system is similar to current charcoal systems in that it removes gases at low (ambient) pressure, but is then regenerated by heating the regenerat-
272
S. Michaelis · T. Loraine
ing bed to temperatures in the order of 170 ◦ C (338 ◦ F). The regenerated bed must be fully cooled before it can be used on line and, unlike current systems, the adsorbent material cannot be treated to improve the range of chemicals adsorbed as such treatments would be destroyed during the regenerative heat cycle. 5.2.2 Pressure Swing Adsorption In the case of a PSA system, a technology which was first developed in 1956 [24], the on line bed operates at an elevated pressure and the off line bed regenerates at low pressure. The beds are designed to adsorb gases at elevated pressure, and release the gases when the pressure is removed. 5.2.3 Pressure Temperature Swing Adsorption PTSA systems combine the characteristics of both TSA and PSA systems in that adsorption takes place at elevated pressure and desorption is achieved by removal of the pressure and heating of the bed. PSA systems appear to offer the most solutions to commercial aviation if a will existed to do so. Pall Aerospace has integrated PSA protection into numerous military systems, including the Apache, Cobra and Comanche helicopters, and an advanced armored test bed vehicle for the army. A PSA system has even been installed in the personal limousine of the Head of State of a friendly foreign nation [24]. PSA uses in commercial aircraft filtration have been discussed since at least 1991 [20]. Experience gained by NASA with the Skylab project of the 1970s demonstrates how long the technology has been available. 5.3 Plasma Plasma technology is likened to cold combustion. Instead of using heat to break up contaminants, the plasma cells destroy molecules using highly reactive strongly oxidizing free radicals – atoms or molecules that have unpaired electrons. From the chemical point of view, destruction reaction rates normally associated with temperatures of 10 000 to 100 000 K can be realized with the gas at near ambient temperature [25]. Nonthermal Plasma Systems in combination with a particulate capture filter may be able to remove particulates and decompose chemicals and biocontaminants with lower than current energy and maintenance costs. The questions that need to be addressed before such technology could be adapted for use in commercial aviation would be:
Aircraft Cabin Air Filtration and Related Technologies
273
• What are the effects of air flow rate/residency time on efficacy? • What are the effects of plasma depth/throw (i.e. what is the relationship between the plasma size and the volume of air that can be filtered?) • What is the range of contaminants that can be decomposed? • When bleed air is contaminated with engine oil, hydraulic fluid, or pyrolysis products of these, what reaction products are produced and how safe are they? 5.4 Ultraviolet Light The key to achieving the cleanest indoor air is a multistage air purifier. Ultraviolet light systems can be used to supplement other filtration techniques. There are four principal wavelengths in the ultraviolet spectrum that lend themselves to specific applications: Photochemical UV-A, Erythemal UV-B, Germicidal UV-C, and extreme shortwave UV Energy, which generates ozone [26]. The destruction of germs and bacteria by germicidal ultraviolet light is accomplished quickly and effectively. The UV-C rays strike the various microorganisms whether they are bacteria, virus, yeast, mold or algae, and they break through the outer membrane. The radiation reaches the heart of the organisms (commonly known as the DNA) where it causes abrupt modifications. The modified DNA transmits incorrect codes or messages, which brings about destruction of the microorganisms. 5.5 Nanocrystalline Materials Nanotechnology refers to the emerging set of tools, techniques and unique applications involving the structure and composition of materials at the nearatomic, or nanometer level. A nanometer (nm) is a billionth of a meter, or a millionth of a millimeter. Although the original suggestions date back to about 1970, systematic large scale research on nanocrystalline materials only began around 1990 [27]. Nanocrystalline materials exhibit a wide array of remarkable chemical and physical properties, and can be considered as new materials that bridge molecular and condensed matter. One of their remarkable properties is enhanced surface chemical reactivity (normalized for surface area) toward incoming adsorbates [28]. As a result of their high surface areas and their enhanced surface reactivity, nanocrystalline MgO, CaO and Al2 O3 have shown remarkably high capacities to chemically adsorb organic compounds and substantially outperform the activated carbon samples that are normally employed for such purposes [29]. The literature suggests that such materials are good candidates for adsorbents of toxic industrial chemicals including metals [30]. Toxic chemicals that have been demonstrated to be chemically decomposed by nanoactive
274
S. Michaelis · T. Loraine
metal oxides at room temperature include but are not limited to the chemical warfare agents GD, VX and HD [31] and the organophosphates dimethylmethyl phosphonate (DMMP), paraoxon, parathion, diisopropylfluorophosphate (DFP) [32]. Nanocrystalline materials are now part of the filtration world and are worthy of further investigation to see how they could be used in commercial aircraft filtration systems. 5.6 Catalytic Converters To prevent oil breakdown products from entering the cabin air, catalytic converters have been used to clean the cabin air [33]. Donaldson was part of the development team for a catalytic bleed air purification unit for a military airplane cabin air application in the 1950s and is still producing that product today [34]. Catalytic converters used in cars have been around since the early 1970s and are now included in virtually every car sold in the United States. They usually use two different types of catalysts, a reduction catalyst and an oxidation catalyst. Both types consist usually of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required (to reduce cost). There are two main types of structures used in catalytic converters – honeycomb and ceramic beads. Most cars today use a honeycomb structure. 5.6.1 The Reduction Catalyst The reduction catalyst is the first stage of the catalytic converter. It typically uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2 . The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2 . For example: 2NO ⇒ N2 + O2
or 2NO2 ⇒ N2 + 2O2
5.6.2 The Oxidation Catalyst The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction
Aircraft Cabin Air Filtration and Related Technologies
275
of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example: 2CO + O2 ⇒ 2CO2 The efficiency of a catalytic converter depends on the catalyst used, pressure, temperature and residency time. A catalytic converter may be set up to function with a preset amount of contamination in mind but if this changes and the converter becomes overloaded they may then produce significant amounts of contaminants. These could potentially include elevated carbon monoxide and carbon dioxide levels, as well as the presence of unconverted, or semi-converted, hydrocarbon oil constituents and a reduction in the oxygen concentration in the air being delivered to the cabin [33]. Catalytic converters introduced on the BAE 146 did not achieve the desired efficiency and were removed by some operators for this reason [35]. As of December 2004 filtration companies such as Pall Aerospace were re-examining catalytic converters in the search for economical solutions to ongoing contaminated bleed air problems. Additionally Englelhard, who were pioneers in automobile catalytic converters, recently commercialized a combined VOC and ozone converter for use in the Airbus A 320 [36].
6 ECHO-Air The current ventilation system in commercial aircraft is based on an idea which is many decades old and widely used. A newly developed alternative ventilation/filtration system called ECHO-Air has been designed to try to improve overall system performance [37]. A prototype of the concept involved in this system has just been tested in a Boeing 737 [38]. The ECHO Air system is designed to eliminate or significantly reduce aircraft envelope condensation which can promote undesirable and potentially hazardous effects such as “rain in the plane”, microbial growth, electrical system deterioration, fuselage corrosion, and dead weight accumulation. By pressurizing the envelope with a portion of the dry ventilation air prior to its entry into the cabin, the system provides a dynamic barrier that prevents cabin air infiltration into the envelope through thermal-gradient induced “stack” pressures. Coincidentally, passing ventilation air through the envelope improves cabin air quality through absorption and filtering of such contaminants as ozone, oil aerosols and combustion VOCs without the high pressure drops associated with standard filtration systems. An airflow controller driven by an electronic control unit is used to control the envelope ventilation either positively or negatively with respect to the cabin, in relation to the phase of the flight (ground, take-off, ascent, cruise, descent, landing and taxiing). The ECU also monitors the system operation.
276
S. Michaelis · T. Loraine
On demand from a cockpit switch, the envelope pressure can be set positive or negative with respect to the cabin, giving the pilot full control to act in the case of smoke in the cabin [39]. The main idea is to divide the air circulation into two separate flows of air: one that goes in to the cabin, and one that goes in to the “envelope” (the space between the cabin liner and the fuselage) (Fig. 2). Envelope tubing (plastic, or metal for the fire suppressant injection version) is used to allow a controlled flow of air inside the envelope, both to and from it. Flow blockers are used to reduce stack pressures, and to control the air distribution within the envelope. Stack pressure is the pressure differential that exists across the liner due to the buoyancy effect of the air inside the envelope and the difference in air pressures caused by the extreme cold temperature of the envelope air near the fuselage during flight in comparison with the air temperature inside the cabin. The system uses an airflow controller to pressurize the envelope, either positively or negatively with respect to the cabin, so as to offset at least stack pressures and upstream molecular diffusion. Upstream molecular diffusion occurs across the liner coming from the lower pressure air stream. Methods of sealing and openings in the cabin liner, as well as defined pressure differences across the liner, are used to control the flow of air through the liner. Leakage dimensions (areas and thickness) in the liner are set together with air velocities to limit upstream molecular diffusion of specific gases and vapors of concern, and to control cabin air circulation and exhaust. When depressurizing the envelope relative to the cabin, a secondary air return path (additional to the cabin floor openings) is provided across the liner through the envelope. This depressurization is used to provide a more direct
Fig. 2 Illustration of the main features of the echo-air system
Aircraft Cabin Air Filtration and Related Technologies
277
Fig. 3 Exhaust flows during the exhaust mode of operation of the echo-air system. (Illustrations courtesy of Indoor Air Technologies Inc, Canada and USA)
278
S. Michaelis · T. Loraine
and effective path for venting cabin air contaminants, to exhaust envelope air pollutants including smoke and fire suppressants in the event of a fire, and to exhaust envelope moisture (Fig. 2). When pressurizing the envelope positively relative to the cabin, the air supply is of low humidity and thermally conditioned to assist with cabin thermal conditioning and maintain a dry envelope preventing moist cabin air entry to the envelope. On the ground this dry air is supplied by the aircraft APU or by a ground-based air conditioning unit. In the air, its source is a mixture of the bleed trim and pack air and is also used to ventilate the cabin. Bleed air is dry at altitude and the use of it for envelope supply keeps the envelope dry by preventing moist cabin air from entering the envelope, while entering the cabin itself to mix with cabin circulation air. The ECHO-Air system has several advantages over current cabin ventilation designs used in the industry, as summarized below: When pressurizing the envelope negatively with respect to the cabin: • It allows reduction in pathogen spread within the cabin by providing more direct exhausting of air contaminated by the passengers or as a result of a terrorist gas or aerosol release, and taking advantage of thermal plumes that normally rise to the ceiling; • It facilitates the reduction of volatile organic compounds in the cabin air by exhausting directly through the envelope of gases formed when the envelope is warmed during taxiing or on ground; • It accelerates the clearing of smoke in the cabin in case of fire in the cabin through additional exhausting from the envelope; • It allows suppression of fire or pyrolysis in the envelope by the direct injection of a fire suppressant without impacting cabin air, and it prevents envelope smoke from entering the cabin by venting it directly to the outdoors. If the envelope is initially under positive pressure (see below), such an event will be detected as normal by smoke in the cabin. At such a time, the pilot switches ECHO Air to envelope depressurization mode for an envelope fire hazard. If the envelope is already under negative pressure at the time of such an event, a smoke sensor in the envelope air exhaust will detect this. During such a hazard mitigation period, ECHO Air envelope ventilation is set at minimum depressurization exhaust rate, so as to maintain the envelope under a negative pressure while minimizing both air supply to the fire and dilution of the fire suppressant being injected. When pressurizing the envelope positively with respect to the cabin: • It filters all or a portion of the cabin ventilation air before it enters the cabin. This filtration reduces incidents of passenger exposure to combustion products ingested when taxiing behind other planes, for example, and to bleed air oil aerosols if an upstream engine lubrication bearing seal fails;
Aircraft Cabin Air Filtration and Related Technologies
279
• It provides an additional airway path thereby enabling an increase in the total ventilation/recirculation rate of the cabin air through envelope ventilation, thereby reducing pathogen spread through higher rates of air dilution and air circulation through pathogen filters and sterilizers; • It prevents entry of humid cabin air both on the ground using ground air conditioners, and in the air using engine bleed air, thus reducing moisture accumulation in the envelope, improving thermal and acoustic performance of the insulation material in the envelope, and reducing fuselage corrosion inside the aircraft; • It allows humidification of the cabin air without the drawbacks of increased accumulation of moisture inside the envelope; • It enables cabin heating and cooling via the envelope, thereby providing a more comfortable cabin temperature with smaller thermal gradients and cold drafts to passengers.
7 Conclusions There are currently no airworthiness standards or regulations which specify the level of filtration removal efficiency which must be used on board aircraft. The quality of the air, if regulations are enforced, should be regulated to some extent by FAR/JAR 25.831 which states: • “Each crew compartment must have enough fresh air (but not less than 10 cubic feet per minute per crewmember) to enable crewmembers to perform their duties without undue discomfort or fatigue.” • “Crew and passenger compartment air must be free from harmful or hazardous concentrations of gases or vapors.” The majority of modern, large, commercial aircraft, which use a recirculation type of cabin air system, utilize fine HEPA filtration, (99.99% minimum sodium flame efficiency). A small number of aircraft types have filters with lower efficiencies. Some older aircraft have either total outside air ventilation, or a small amount of unfiltered recirculation combined with the outside air. HEPA filters will only work properly if properly installed (“HEPA filters shall be installed in order to minimize the recirculation of bacteria and viruses in the air distribution system but there is no definitive time interval for replacing a cabin air filter element. HEPA filters shall be maintained according to “best practices” manufacturers’ specifications” [40]). The time interval varies between aircraft types. Manufacturers recommend that airlines follow the guidelines provided by the manufacturers in the aircraft maintenance manuals. Pall Aerospace states that: “It is often the case that airlines will choose to replace cabin air filter elements at regular “hard time” intervals to fit in with routine scheduled maintenance periods, such as a C-Check. The definition
280
S. Michaelis · T. Loraine
of a C-Check varies between aircraft models and operators” [5]. Donaldson Company also states: “Recommended service interval is one C-Check for most operators” [41]. The AMETEK Aircontrol Technologies BAE 146 filters previously discussed show that technology exists to help protect passengers and crews but regulators have to date not made these mandatory, simply optional or for information. Activated carbon technology has existed for many years to protect military personnel from potential exposures to organophosphate based nerve gases, such as Sarin. The failure of regulators to enforce protective measures may be driven by economics, as filters must be regularly replaced before they become saturated and ineffective [42]. To ensure that crews and passengers are safe from the organophosphates and other contaminants present in the contaminated engine bleed air, appropriate prevention measures by way of filtration safety options should be mandated by the regulators. In an industry so focused on flight safety with numerous back up systems it seems inconceivable that no protection systems are in place to protect crew and passengers from bleed air malfunctions. The potentially serious health problems to exposed passengers and crew from this source deserve to be given a higher priority than seems to be directed to this problem at the moment. (One particular aircraft type has over 200 Service Bulletins, Service Information Leaflets, etc. referring to contaminated air on this aircraft model since its introduction in the 1980s [42]). It seems wiser and safer to have in place safety systems that cater for all mechanical failures, poor designs or malfunctions in relationship to bleed air contamination rather than argue that bleed air contamination events are statistically a rare event and not worthy of the proper independent research and investment. If appropriate prevention measures cannot effectively protect crews and passengers then the only option is to return to the original bleed air philosophy of early jet aircraft and not allow the bleed air from the engine to directly enter the aircraft cabin. Interestingly, the latest Boeing 787 has been designed to use “bleed free” engines, where the air supply for the aircraft is not directly taken from the engine air in the traditional manner but reported to be supplied from separate compressors. “The engines in development for the Boeing 787, The General Electric GENX (General Electric Next-generation) and the Rolls-Royce Trent 1000 will eliminate the use of bleed air” [43]. Some might argue that perhaps “bleed free” engines are the safest option. The British Airline Pilots Association (BALPA) invited every leading airline, aircraft manufacturer, engine manufacturer, lubricant manufacturer, regulators and every person who has ever had an input into the issues of contaminated air, whether independent or from industry to make a presentation at the “Contaminated Air Protection Conference” in April 2005 and the conclusions were very clear, alarming, simple and included:
Aircraft Cabin Air Filtration and Related Technologies
281
• There is a workplace problem resulting in chronic and acute illness amongst flight crew (both pilots and cabin crew). • Further, we are concerned the passengers may also be suffering from similar symptoms to those exhibited by flight crew. An absurd situation often exists where those who regulate the industry often have no direct responsibility towards passenger and crew health and seemingly no urgency to enforce such protective measures. “CAA’s prime responsibilities for passengers are to regulate for their safety. It has no direct responsibilities for passenger health or comfort” [44] “Thus, HSE has no active responsibilities in relation to the health of airline passengers or crew” [45]. Filtration companies have had the technology for many years to decrease the effect of contaminated bleed air. Regrettably, their main clients, the airlines and manufacturers, have not had the will, nor see the need to invest in the technology.
References 1. Boeing (2005) Cabin air quality. http://www.boeing.com/commercial/cabinair/facts. html. Cited 2005 2. Finnair (2005) http://www.finnair.com/web/finnair/scripts/template_2level_white. jsp?pageid=-13038. Cited 2005 3. Pall Aerospace (2005) Boeing and McDonnell Douglas data. In: Cabin air filtration. Pall Aerospace, East Hills, NY 4. Pall Aerospace (2005) Cabin air filtration. APME – 500a COD/2m/5/92 5. Available at: http://www.donaldson.com/en/aircraft/cabinair/index.html 6. Available at: http://www.airtesters.com/HEPA_filters.cfm 7. Pall Aerospace FAQs for BALPA AETG website campaign August 2004. Available at: http://www.balpa.org 8. ASHRAE Standard 161 (2004) Air quality within commercial aircraft committee review craft, January 2004. In: Sections: 6.3.1 Recirculated Air Quality 9. INDA (2000) INDA e-FILTER newsletter. http://www.inda.org/period/enews/jan00. html. Cited 3 Jan 2000 10. Pall Aerospace Technical Data Sheet APME P/N QA06423-01. Not dated. 11. Airbus Publicity Card by Pall Aerospace “Combined Particulate & Odour Removal Cabin Air Filters” APM528/BP/2M/0402 April 2001 12. Available at http://www.airbus.com assuming 32 inch seat pitch. A330–200 assumes 30B at 40 in + 263Y at 32 in pitch and A340-600 assumes 36B at 40 in + 383Y at 32 in pitch 13. Available at: http://www.boeing.com 14. Data taken from applicable aircraft specific maintenance manuals 15. Hocking MB (1998) Indoor Air Quality: Recommendations Relevant to Aircraft Passenger Cabins. Am Ind Hyg Assoc J 59:446–454 16. Boeing 747-400 Minimum Equipment List. Last updated 2004 17. Winder C, Balouet JC (2002) The Toxicity of Commercial Jet Oils. Environ Res Section A 89:146–164
282
S. Michaelis · T. Loraine
18. Michaelis S (2003) A Survey of Health Symptoms in BALPA Boeing 757 Pilots. J Occupational Health & Safety, Aust & NZ 19(3):253-261 19. Cox L, Michaelis S (2002) A survey of symptoms in BAe 146 aircrew. J Occupational Health and Safety – Australia and New Zealand 18:305–312 20. Needelman WM (1991) New Technologies for Airliner Cabin Air Contamination Control. “Adsorption & Chemical Methods For Gaseous Pollutants”. Needelman WM Associate Director, Scientific and Laboratory Services Department, Pall Corporation. Presentation at the International Seminar on Cabin Air Quality in Commercial Airliners, Paris, 19 June 1991 21. Available at: http://www.aircontroltechnologies.co.uk 22. Letter from Giles M of Vapour Management Systems Ltd to Loraine T of British Airline Pilots Association AETG, Cited 4 October 2004 23. Perry JL, LeVan D Air Purification In Closed Environments: Overview Of Spacecraft Systems. Available at: http://www.natick.army.mil/soldier/jocotas/ColPro_Papers/ Perry-LeVan.pdf 24. Available at: http://www.pall.com/Aerospace_2947.asp 25. Golkoski C, Hedge A (2003) Nonthermal plasma air filtration technology. Super Pulse & Dept. Design & Environmental Analysis, Ithaca, NY 26. Available at: http://www.purennatural.com/fs.php?center=airpurifiers%2 Fultravioletairpurifiers.php 27. Available at: http://physics.umbc.edu/ ∼takacs/nano.html 28. Klabunde KJ, Stark JV, Koper O, Mohs C, Park DG, Decker S, Jiang Y, Lagadic I, Zhang D (1996) Nanocrystals as stoichiometric reagents with unique surface chemistry. J Phys Chem B 100:12142–12153 29. Khaleel A, Kapoor PN, Klabunde KJ (1999) Nanocrystalline metal oxides as new absorbents for air purification. Nanostructured Mater 11(4):459–468 30. Decker SP, Klabunde JS, Khaleel A, Klabunde KJ (2002) Catalyzed destructive adsorption of environmental toxins with nanocrystalline metal oxides: fluoro-, chloro-, bromocarbons, sulfur, and organophosphorous compounds. Env Sci Technol 36(4):762– 768 31. Wagner GW, Procell LR, O’Connor RJ, Munavali CL, Carnes CL, Kapoor P, Klabunde KJ (2001) Reactions of VX, GB, GD, and HD with nanosize Al2 O3 : formation of aluminophosphonates. J Am Chem Soc 123:1636–1644 32. Rajagopalan S, Koper O, Decker S, Klabunde KJ (2000) Nanocrystalline metal oxides as destructive adsorbents for organophosphorus compounds at ambient temperatures. Chem Eur J 8:2602–2607 33. Van Netten C, Leung V (2000) Comparison of the constituents of two jet engine lubricating oils and their volatile pyrolytic degradation products. Appl Occupational Environ Hyg 15(3):277–283 34. Verbrugge K (2004) Sales Engineer, Donaldson Europe personal communication. September 2004 35. BA6 36-10-11: Ansett internal engineering release on work undertaken on BAe modifications. Remove engine/APU catalytic converters. Ansett work completed 7/95 36. Air Transportation Center of Excellence (2004) Proposal to FAA by air transportation center of excellence for airliner cabin environment research, vol 1 37. Walkinshaw DS, Mitalas GP, McNeil CS, US Patent 6,491,254 (Dec 10, 2002) 38. Indoor Air Technologies Inc. Media release available at: http://www.cyberus.ca/∼dsw/ iat/echoairboing.html 39. Interview with leader of Echo Air Project, Mr Doug Walkinshaw. http://www. indoorair.ca/iat/echoairfaq.html
Aircraft Cabin Air Filtration and Related Technologies
283
40. ASHRAE Standard 161 (2004) Air quality within commercial aircraft committee review draft, January 2004. Section: B. Maintenance 41. Donaldson (2005) BIOAdvantage aircraft cabin air HEPA filter with antimicrobial protection, product datasheet. http://www.donaldson.com 42. Available at: http://www.aopis.org 43. Available at: http://encyclopedia.thefreedictionary.com/Bleed%20air and http://www. balpa.org/intranet/BALPA-Camp/The-Aircra/The-Aircraft-Environment.pdf 44. House of Lords Select Committee on Science and Technology Air Travel and HealthHL Paper 121-I. (2000) 3.19 CAA lays down aviation safety standards in areas broadly similar to those of ICAO and JAA, and sets them out in regulations made under the Air Navigation Order (ANO) 45. House of Lords Select Committee on Science and Technology Air Travel and HealthHL Paper 121-I. (2000) 3.21 The Health and Safety at Work etc. Act 1974 applies to aircraft in and over Great Britain but has no role outside the airspace above Great Britain. The Executive (HSE) set up under the Act seeks to avoid duplicating the activities of other regulatory bodies associated with health and safety. Its interface with CAA is the subject of a Memorandum of Understanding. Aircraft have been exempted from many regulations made under the governing Act (p 1)
