Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Environmental Control and Life-Support System The environmental control and life-support system (ECLSS) is designed to control the temperature, humidity, and composition of space-station air; recover water; dispose of waste; and detect and suppress fires. ECLSS consists of six subsystems: temperature and humidity control (THC), atmosphere control and supply (ACS), atmosphere revitalization subsystem (ARS), fire detection and suppression (FDS), water recovery and management (WRM), and waste management (WM). The first three subsystems are directly related to the maintenance of cabin air quality. The THC subsystem controls cabin air temperature, humidity, and recirculation rate, as well as the exchange of air between modules and equipment air cooling. This subsystem also includes food storage and control of airborne particulates and microbes. Air temperature and humidity are controlled by a condensing heat exchanger with auto- matically controlled bypasses in each node and module of the station. From a central location, the THC subsystem manages intermodule ventilation, air exchange between pressured elements of the station to maintain the proper total pressure and O2 and CO2 partial pressures. Particulate contaminants are to be controlled by drawing the cabin atmosphere through filter elements that consist of 70-/im, hydrophilic, pleated screens followed by dimple-pleated, borosilicate-fiber HEPA filters. A protective metal grate collects much larger particles before they enter the filter element. The ACS subsystem will use stored O2 and N2 for contingency re- pressurization and compensation for normal atmospheric leakage. The ARS is intended to monitor and control the major components of air (O2, N2, H2O, and CO2) within the pressurized portions of the space station. H2, CO, and CH4 concentrations also will be monitored. The ARS functions consist of atmospheric monitoring, CO2 removal, and contaminant control. Removal of trace contaminants is to be accomp- lished by the trace contaminant control subassembly (TCCS) that con- 41
42 GUIDELINES FOR DEVELOPING SMACS sists of a fixed activated charcoal bed, a high-temperature catalytic oxidizer, and a lithium hydroxide bed. In the baseline subsystem, CO2 will be removed by a four-bed molecular sieve. CONTAMINANT CONTROL AND MONITORING The schematic in Figure 1 shows the portions of the ECLSS devoted to contaminant control for a single module. Air is to be circulated at a relatively high rate through the cabins, and large debris and small particles will be removed at each cabin's return air vent. Approxi- mately 60% of this air stream will be recirculated back to the cabin and 40% will pass through a condensing heat exchanger to remove excess moisture. The condensate will be a source of potable water. Assuming 100% removal efficiency and a module volume of 74.23 m3, the TCCS has an equivalent dilution capacity of 0.21 air changes per hour for contaminants removed by charcoal (at a charcoal-bed flow rate of 15.29 m3/hr) and 0.06 air changes per hour for contaminants removed by catalytic oxidation only (at an oxidizer flow rate of 4.25 ms/hr). The ARS is designed to maintain exposures below the 180-day SMACs for normal rates of contaminant generation when the station is "permanently manned," and below the 30-day SMACs when the station is only "man-tended." In addition, means are being devised to reduce contaminant levels following unanticipated releases to meet the 1 - and 24-hr SMACs, but these additional safety measures are not part of the baseline design. Air monitoring is critical to the functioning of the ACS subsystem and the ARS (Humphries et al., 1990), not to mention to the astro- nauts' health and safety. The ARS includes the major constituent analyzer (MCA). This will be a mass spectrometer for analyzing cabin air for O2, N2, H2, CO2, H2O, and CH4 in all pressurized areas of the habitation and laboratory modules. Air samples will be drawn to the MCA for analysis from seven locations sequentially. Each analysis will require about 1 min. Thus, measurements will be made at each location approximately once every 8 min. A separate instrument will monitor CO by nondispersive infrared spectroscopy. The ARS includes the atmospheric composition monitor assembly (ACMA), which will measure trace contaminants, CO, total par- ticulates, and the major atmospheric constituents listed above. Trace
43 i i 1 Return to ^ c O LJ T3 CD CO 2 co O 1 -5 ^ f k 1 ^ o ol Subassemb .0 i Assembly CO t Exchanger C>2 Removal WAIUI^BI 1 CO CO 85 1 ^N | « ⢠1 and ARS. oc a? Q V S./2 b I tn co⢠-^\ w ^ O c o O t 1 i â¢Â§ °° co" w CO co b o o 8 o â¢* CO CO o o M c â¢c 1 CO m S⢠m 1 3 9 CO o § § o O 75 1 d 0 0 1 r 8 o â¢s ^ i b* co £ B | CO CO c O I1 CD i c i ^ X T3 0) i_ (D u_ m £ ® CD EQJ «-⢠T d |l i 1 ^J I CO m o E co⢠& SSI 5 3. ^, 1$$^ =*⢠=L £& L 8? >88t ° S VSr W ^A/ 50< ° CJ j ^ Intermodule 8 ⢠» Ventilation c> â i o t jQ _c _c 3 a i i
44 GUIDELINES FOR DEVELOPING SMACS contaminants will be analyzed by gas chromatography/mass spectros- copy. As in the MCA, the ACMA will bring air samples from pres- surized areas to the central ECLSS analytical assembly. Each analysis will require approximately 30 min, resulting in a monitoring frequen- cy at each location of about once every 4 hr. In addition to the analytical instrumentation in the ECLSS, the toxicology subsystem of the environmental health system for crew health care has extensive air-monitoring capability and also includes dedicated continuous-monitoring instruments for total hydrocarbons, hydrazine, HC1, HF, HCN, CO, and 30 targeted volatile organic compounds. RECOMMENDATIONS Sources Criteria for permitting materials on the space station should be developed based on their physical, chemical, and toxicological proper- ties and the ability of the ARS to limit their concentrations. If some experiments utilizing substances that do not meet these criteria are judged to be of such high priority that they should be performed on the space station, the experimental modules should be subjected to extensive testing, preferably in microgravity, to ensure complete con- tainment. The subcommittee recommends development of instruments for the continuous monitoring of contaminants of concern. The subcommittee strongly recommends that the space-station de- sign include the capability of isolating the laboratory modules from other portions of the space station in the event of severe con- tamination. Good design practices for laboratories on earth usually include restricting air flow from the laboratory to adjoining non- laboratory spaces by keeping the laboratory under negative pressure with respect to those adjoining spaces. On the space station, however, air will be flowing, by design, from the laboratory module to adjacent nodes to maintain CO2 levels and temperature and humidity control. An additional consideration is that keeping the hatches open between modules and nodes facilitates astronaut movement, creating more efficient working conditions. Thus, the laboratory modules are not isolated from the crew living quarters under normal conditions. Be-
ENVIRONMENTAL CONTROL AND LIFE-SUPPORT SYSTEM 45 cause of the forced air exchange and open hatches, accidental release of an acutely toxic material in the laboratory module could contamin- ate air throughout the space station before measures could be taken to close off the laboratory area. Some toxicants work so rapidly that those exposed at high levels are incapable of donning their emergency breathing apparatus. In the current design, CO2 is to be used as a fire suppressant. Halon 1301, an alternative agent used on the space shuttle, presents little problem when air is only partially recirculated. It is not recommended for complete recirculation because the ARS will remove chlorofluoro- carbons only very slowly and may generate more toxic airborne con- taminants. The subcommittee is concerned also that some products generated by fires and by chemical fire extinguishers could rapidly and severely compromise performance and impair the health of the astronauts. Rates of emission of contaminants have been estimated for category 1 sources, i.e., those that emit contaminants continuously or routinely. For sources in categories 2 and 3 (failure events and ac- cidental releases from experimental modules), reliability analysis should be performed to estimate the likelihood of release. For exam- ple, overheating of an electrical component can lead to a high rate of emission from electrical insulation. The probability of such events should be estimated and the potential contaminants and release rates determined. This information may then be incorporated in the risk assessment. Appropriate countermeasures should be developed for such events if the baseline ARS is shown to be inadequate. Environmental Control and Life-Support System The design criteria for the ARS have been driven largely by the need for the control of contaminants from category 1 sources. The primary concern has been to keep contaminants that are released con- tinuously or frequently at acceptable levels. More consideration needs to be given to control of contaminants from accidental releases. Based on the volume of Lab-A module (74.23 m3), Figure 1 shows that about one-fifth of the module's atmosphere passes through the TCCS in 1 hr. Assuming 100% removal efficiency, a module volume of 74.23 m3, and the flow rates given in Figure 1, the time required to achieve
46 GUIDELINES FOR DEVELOPING SMACS various fractional reductions in the concentrations after a sudden release is shown below for contaminants removed by the charcoal bed and for contaminants removed only by the catalytic oxidizer. Removal Time fhr) Fractional Reduction (C/Co) Charcoal Catalytic Oxidizer 0.5 3.36 12.1 0.1 11.2 40.2 0.01 22.4 80.4 Thus, the TCCS seems to be inadequate to respond to an accidental release of an acutely toxic chemical. NASA intends to develop meas- ures to respond to sudden releases, but hardware for this purpose is not part of the baseline station configuration. Consideration also should be given to the alternative of increasing the capacity of the TCCS as a means of responding to accidental releases. The design criteria should take into account category 2 sources and the 1- and 24-hr SMACs of contaminants released from these sources. Another concern regarding the design of the ARS is the relative locations of the dehumidification unit and the TCCS. Condensate from the cabin atmosphere, although vented overboard during the man-tended phase, will be used after treatment as potable water in the permanently manned configuration. Air filters for removal of particulate matter will be located upstream of the dehumidifier. Nevertheless, the cabin air condensate is expected to contain numerous volatile organic compounds that are either present as vapor in the return air stream or vaporize from particulate material collected on the filters. Condensate from space-shuttle missions has been shown to contain a wide variety of organic contaminants. Air Monitoring Because of the high rate of air circulation, it is unlikely that personal (breathing zone) monitoring will yield much better estimates of exposure than area monitoring, with the possible exception of exposures experienced by those working very close to a contaminant
ENVIRONMENTAL CONTROL AND LIFE-SUPPORT SYSTEM 47 source. For those personnel, the use of passive air sampling or biological monitoring or both should be considered. General The dynamic mass-balance model of contaminant emission and removal rates available at NASA should be used to predict the tem- poral contaminant concentration profiles associated with all reasonably probable emission scenarios. (Probable is defined by the Federal Aviation Administration as an event having a probability of at least 10~5/flight-hr(FAA, 1988).) These scenarios should include low-level continuous emissions, periodic releases, and abrupt high-level releases. The concentration maximum and the 1-hr, 24-hr, and 180-day time- weighted averages could then be calculated from the hypothetical concentration-time profiles and compared with the SMAC values. This model should include humans as both sources and sinks. Results of these tests could be used to identify the contaminants of greatest concern and to take appropriate precautions. Continuous monitoring, alarm systems, and emergency response procedures would be required if exceeding a short-term limit (1 -hr SMAC) is reasonably probable. Periodic air monitoring or biological monitoring or both as well as deliberate response procedures would be sufficient for most other contaminants. In addition, the use of the model in this manner would help identify gaps in the current understanding of system performance. The overall plan for monitoring air contaminants, biological monitoring of the crew, and complying with the SMAC values should be developed by a team including industrial hygienists, physicians, toxicologists, behavioral toxicologists, and health physicists, as well as chemists and engineers.