The Airliner Cabin Environment and the Health of Passengers and Crew (2002) / Chapter Skim
Currently Skimming:
2 Environmental Control
2 Environmental Control
Pages 33-72
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 33... ...
For the purposes of this report, each component or subsystem that is integral in providing the necessary environmental conditions in the aircraft cabin is considered to be part of the ECS, even if it is technically part of another aircraft system. This chapter first describes the important functions ofthe ECS, including background information on principles of ventilation, temperature control, and humidity control.
|
From page 34... ...
Association of Flight Attendants, AFL-CIO, Washington, DC. tPublic presentation to the NRC Committee on Air Quality in Passenger Cabins of Commercial Aircraft, by J
|
From page 35... ...
35 m l .0 Ace O O O O O O O ~ N N ~ (D ~ (Oo)
|
From page 36... ...
Thus, the minimal PO2 allowed in the aircraft cabin at the maxima] allowed cabin pressure altitude of 2,440 m (8,000 ft)
|
From page 38... ...
As described below, the rate of flow of outside air has a substantial and direct impact on the concentration of contaminants in the cabin air. The flow rate has a negligible effect on the PO2, in that only a tiny portion ofthe oxygen in this air is consumed by the aircraft occupants.
|
From page 39... ...
Cc is volume fraction of contaminant in cabin air, CO is volume fraction of contaminant in outside air used for ventilation, S is strength of contaminant source, kg/s (Ibis) , ma is ventilation rate of outside air, kg/s (Ib/s)
|
From page 40... ...
. Some reduction in concentrations of such cabin air contaminants can be achieved by using the minimal practical flow of outside air and increasing the flow of recirculated air if the recirculation filters are effective at removing the contaminants in question (see recirculation section later in this chapter)
|
From page 41... ...
The outside air at or near quite pure and requires no a naW1devarletyofcontaminantsfromindusground level' however' can Cd°dinttal to outside air contaminants, leaking hydrautrial and urban sources n kc fluid, spilled fuel, or deicing fluid can be entrained in the air supply systems, h cleaning systems to remove any of these contamifew, if any, aircraft eve nants. Transient Response of Cabin Environmental Conditions nations 2- through 2-3 describe contaminant concentrations under steady-state conditions Contaminant concentrat ons in the cabin do not
|
From page 42... ...
As with transient responses discussed previously, residence time is proportional to the quantity derived by dividing the volume ofthe space by the ventilation flow rate, so residence time in aircraft is typically much shorter than in buildings. Residence time is particularly important for O3 and its byproducts (see Chapter 3 for additional discussion)
|
From page 43... ...
, ms is the flow rate of conditioned air supplied to the cabin, kg/s (Ibis) , and cp is the specific heat of the air, ~000 J/kg-°C, (0.25 Btu/Ib °F)
|
From page 44... ...
Although the air mixes locally in the cabin, the air supply and air exhaust flow rates are matched along the length of the cabin as much as possible to minimize net flows along the length of the aircraft. Distribution of the air to the cabin occurs through diffusers located in the center ofthe ceiling ofthe aisles, above the windows, or along the overhead baggage compartments.
|
From page 45... ...
2001~. The need for circulation to provide thermal uniformity generally sets the requirement for the total flow rate of air that must be supplied to the cabin.
|
From page 46... ...
If the conclitioned air is first mixed with recirculated air, Ti will be greater than Ts, and the total flow rate of air will be greater than the flow rate of conditioned air required according to Equation 2-4. Recirculation Aircraft designers have four important factors that must be considered when determining flow rate requirements for the ECS: the flow rate of outside air required to remove contaminants from the cabin, the flow rate of conditioned air required to remove heat from the cabin, the totalflow rate of air required to provide adequate circulation in the cabin, and the flow rate of outside air required to pressurize the aircraft.
|
From page 47... ...
Recirculation air flow t_ ~ Conditioned air flow 1~: ~ Outside air flow FIGURE 2-4 Cabin air supply without recirculation and with recirculation: (A) without recirculation- all flow of air to cabin comes from outside air, and flow rate of outside air must be large enough to meet greatest needs for cabin air circulation, temperature control, and contaminant control; (B)
|
From page 48... ...
48 ca Cal ;^ En o · _ ._ C)
|
From page 49... ...
49 Go Go Go ~ ~ Go Go ~ ~ ~ ~ _ Go Do Go Do Do Do ~ ~ ~ ~ ~ ~ ~ red 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ _ ~ ~ ~ ~ .
|
From page 50... ...
Consequently, the conditioned air can be supplied at a much lower temperature (Ts in Equation 2-4) without causing discomfort from cold drafts, and the maximal flow rates of conditioned air required for temperature control can be reduced to match those required for contaminant control more closely.
|
From page 51... ...
It shows that most aircraft use bleed air rather than ram air as the source of cabin air, and it demonstrates that as aircraft have moved from 100% bleed air to recirculated air, the amount of outdoor air has decreased. The use of recirculation has been common in the design of building environmental control systems for many years.
|
From page 52... ...
Air supplied to the cockpit is humidified on a small fraction ofthe current aircraft fleet, but on most aircraft the cockpit is normally drier than the passenger cabin air because of the higher ventilation rates in the cockpit. Humidity that is too low canbe avoided to some extent by using the lowest feasible flow rate of outside air at cruise altitudes.
|
From page 53... ...
Whether the FAA minimal design flow rate of outside airis appropriate for ventilation might be controversial. However, any attempt to increase cabin humidity by decreasing the ventilation flow rate will increase contaminant concentrations in the cabin air, and any attempt to reduce contaminant concentrations in the cabin by increasing the ventilation flow rate above the FAA minimum reduces humidity in the cabin further.
|
From page 54... ...
( ~ Aircraft pressure hull Low-pressure cold outside air (20 kPa, -60°C) At , ~ I ' \ I ' Mixing manifold Cabin air | ~ \ Pa, 22°C)
|
From page 55... ...
At a given stage, the pressure increases as the speed increases. Bleed air is compressed air that is extracted from the engine compressor.
|
From page 56... ...
to coot the bleed air. The temperature of the bleed air supplied to the aircraft is regulated by controlling the flow of fan air through the heat exchanger.
|
From page 57... ...
At cruise altitudes, the air can contain excessive O3. The bleed-air systems of some aircraft are equipped with O3 converters to reduce the O3 in the bleed air to acceptable concentrations before it is supplied to the ECS.
|
From page 58... ...
Air-conditioning packs are used to cool and, if necessary, dehumidify the bleed air from the engines or APU before it is supplied to the aircraft cabin. Commercial passenger aircraft typically have two to four air-conditioning pecks.
|
From page 59... ...
~ | To trim air Engine 2 bleed air _ ~ ) 1 t ompressor Turbine / ' 1 Ran 1 ~ Fan air Primal I ~ ~ ~ | | | - heat exchanger Secondaly Wheat exchanger To cabin FIGURE 2-8 Common configuration of air-conditioning packs.
|
From page 60... ...
Air Distribution System Figure 2-9 shows a typical air distribution system for an aircraft passenger cabin. Of all the systems described in this chapter, the cabin air distnbution system probably varies most from aircrafito aircraft, so We specific details of the system described here do not necessarily apply to every aircraft.
|
From page 61... ...
Tr~m-air valve zone can be separately controlled by means of a thermostat that governs the amount of hot trim air mixed with the bleed air. Additional thee Cal control is available to each passenger on some aircraft through gaspers.
|
From page 62... ...
Airlines also have the option of supplying air to the aircraft downstream of the packs by using a low-pressure ground cart, which is essentially a conventional air conditioner on a mobile cart. With this method, air is supplied only to the cabin air supply system, not to the pneumatic system, and no filter is used (see Figure 2-9~.
|
From page 63... ...
Just as the engine compressors have the potential for contaminating the supply air, any alternative means of compressing the air also has the potential for contamination, although the nature ofthe potential contamination can be different. Thus, using an alternative to the bleed air as a source of outside air does not automatically ensure uncontaminated air.
|
From page 64... ...
Although some APUs provide compressed air in much the same manner that the main propulsion engines provide bleed air, by extracting compressed air from the turbine-engine compressor, some APUs use a compressor.that is independent ofthe engine compressor, as descnbed previously. Definitive data are not available, but the committee saw no evidence that contamination from the APU or brought into the APU is any less likely to affect cabin air adversely than is bleed air.
|
From page 65... ...
. As with all ASHRAE standards, itis a voluntary consensus guideline andisnotlegally birding unless adopted by the applicable regulatory body for a particular application.
|
From page 66... ...
However, a provision in Section 6.1.3.4 of Standard 62- 1999 might allow the outdoor air flow rate to be as little as half that listed above for some flights less than 3 h Tong. The provision is meant to reflect the fact that human-generated contaminants in the indoor air take some time to rise in response to occupancy.
|
From page 67... ...
It is not possible to compare the above requirements from ASHRAE Standard 62-1999 with those inFAR25 directly; because the FAR25 requirements are for cabin air and the ASHRAE requirements are for inlet air. 03, in particular, will continue to react after the air is in the cabin.
|
From page 68... ...
, includes a table from AIR4766, Air Quality for Aircraft Cabins that specifies the maximal concentrations of contaminants in engine bleed air. Those limits are presented in Table 2-3.
|
From page 69... ...
· The flow of outside air must tee adequate for contaminant control in the cabin, whether or not recirculation is used. As long as the outside-air flow rate is appropriate for contaminant control in the cabin and the recirculation system is properly designed, operated, and maintained, recirculation does not normally affect cabin air quality adversely.
|
From page 70... ...
Consequently, it is important that the ECS not be shut down for a long period when the aircraft is occupied except in the case of an emergency, because excessive contaminant concentrations and uncomfortably high temperatures can occur quickly. RECOMMENDATIONS · FAA should rigorously demonstrate in public reports the adequacy of current and proposed FARs related to cabin air quality and should provide quantitative evidence end rationales to support sections ofthe FARs that establish air-quality-related design and operational standards for aircraft (standards forCO, C02, 03, ventilation, end cabinpressure)
|
From page 71... ...
l 999b. ASHRAE Standard Ventilation for Acceptable Indoor Air Quality.
|
From page 72... ...
2001. Determine Aircraft Supply Air Contaminants in the Engine Bleed Air Supply System on Commercial Aircraft.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.