The Air Subsystem stores and maintains the system's atmospheric gases, including pressure control, overall composition, and trace constituents. The Air Subsystem is also responsible for fire detection and suppression and vacuum services.
Interfaces: Biomass, Food, Thermal, Waste, Water, Crew, EVA Support, Human Accommodations, In-Situ Resource Utilization, Integrated Control, Power
Air regeneration is one of the more time-critical life support functions. The air mix we have evolved into while living on Earth is comprised of the following gases:
78% nitrogen (N2)
21% oxygen (O2)
1% argon (Ar)
0.03% carbon dioxide (CO2)
Nitrogen is largely inert, but with the help of bacteria or fertilizer manufacturing can play a part in the Nitrogen Cycle.
Oxygen is the most important component for humans and combined with CO2, forms the basis of the carbon cycle.
Humidity
In addition to the above components, humans are accustomed to relative humidity (RH) of 40-70%. Plants typically prefer a higher humidity level than humans, so many terresetrial CELSS designs have a separate habitat (hab) for humans. It is possible that at a reduced total pressure, a higher RH would be preferred to maintain human comfort (for cough production, membrane diffusion, etc), and reduce flammability in the the habitat.
Pressure
Humans are adapted to breathing this gas mixture at a pressure of 40-200 kPa, but since O2 is the active component (as far as animals are concerned), lower pressures can conceivable be tolerated if the partial pressure of O2 content is maintained (percentage increased as p(N2) decreased). Pressure differentials between inside the CELSS and outside environment impacts structural integrity considerations, requiring airlocks, and other pressure regulation measures.
Typical control (steady state) values are given in the table below. Some generally prefer to use normal sea-level pressure because that is the condition under which most known data were collected and because people can live satisfactorily for extended periods under these conditions. Because the space shuttle and Soyuz were designed for this, the ISS utilizes standard atmosphere. Others, however, prefer lower pressures, to reduce the mass of required gas, and to reduce the pressure differential of the surrounding environment. Current EMUs / spacesuits are typically maintained at lower pressures (30-50 kPa). The US Apollo program in the 1960-70's, utilized pure O2 at 30-50 kPa in the lander, and 10-20 kPa in the suits. For NASA Advanced Life Support Baseline Values and Assumptions Document, a nominal pressure of 70.3 kPa is assumed based on Lin (1997).
The tolerable partial pressure of carbon dioxide, p(CO2) for humans is higher than the currently acceptable plant nominal. The generally accepted optimum for flowering plants is 0.120 kPa, but the practical upper limit on carbon dioxide for plant chambers is currently unestablished. Separate atmospheric concentrations could be used for crew compartments and plant chambers by regulating inter-chamber gas transfer rates. Earth normal p(CO2) is 0.035 kPa to 0.040 kPa, but humans can tolerate more than 10 kPa (provided pO2 is above minimum).
'Typical Assumed Steady-State Values for Vehicle Atmospheres'
(adapted from NASA Advanced Life Support Baseline Values and Assumptions Document)
| Parameter | Units | Lower | Nominal | Upper |
|---|---|---|---|---|
| Carbon Dioxide Generated | kg/CM-d | 0.466 | 0.998 | 2.241 |
| Oxygen Consumed | kg/CM-d | 0.385 | 0.835 | 1.852 |
| p(CO2) for Crew | kPa | 0.031 | 0.4 | 0.71 |
| p(CO2) for Plants | kPa | 0.04 | 0.12 | TBD |
| p(O2) for Crew | kPa | 18.0 | 18.0 - 23.1 | 23.1 |
| Total Cabin Pressure | kPa | 48.0 | 70.3 | 102.7 |
| Temperature | K | 291.5 | 295.2 | 299.8 |
| Relative Humidity | % | 25 | 60 | 70 |
| Perspired Water Vapor | kg/CM-d | 0.036 | 0.699 | 1.973 |
| Respired Water Vapor | kg/CM-d | 0.803 | 0.885 | 0.975 |
| Leakage Rate (spaceflight) | %/d | 0 | 0.05 | 0.14 |
| Leakage Rate (test bed) | %/d | 1 | 5 | 10 |
In addition to the carbon dioxide load noted above, human beings also emit volatile compounds, products of metabolic processes, on a per crewmember per diem basis. Many materials used in habitat design off gas as well, emissions on a per mass of equipment per diem basis listed here.
These compounds are historically removed by the trace contaminant control technologies.
For conservative designs, the maximum design loading case should be no more than the mean rate plus 1.6 standard deviations.
Air Purification
Air Flow
CELSS: AIR & WATER FLOW - SUBSYSTEMS
I. DEHUMIDIFIER/atmospheric moisture condenser
A. Inputs
1. evaporation from bio-bed surface
2. transpiration from bio-bed plants
3. fan forced warm oxygen-rich air from bio-bed plants
4. fan forced cool condenser coil air from air well
B. Outputs
1. clear water to clear water tank
2. dry oxygen-rich air to HUM
3. heated condenser coil air to composter/drier
II. AIR WELL
A. Input: warm moist air from greenhouse
B. Outputs:
1. cool dry air to dehumidifier condenser coils
2. Condensed moisture to CWT