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Air

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" class="wiki wikinew text-danger tips">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 humidity levels equivalent to 1-4% water (H2O). Plants typically prefer a higher humidity level than humans, so most CELSS designs have a separate habitat (hab) for humans.

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 percentage of O2 content is increased. Pressure differences 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. Others, however, prefer lower pressures, to reduce the mass of required gas, the mass of the vehicle, and the requirement to pre-breathe with current EMUs or “spacesuits.” Reduced pressure normally entails increasing the percentage of oxygen, relative to other gases in the cabin atmosphere, which increases the risk of fire. Here, a nominal cabin 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 what is accepted as desirable for most plants. The generally accepted optimum for plants is 0.120 kPa (1,200 ppm), but the practical upper limit on carbon dioxide for plant chambers is currently unknown. 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 (350 to 400 ppm).

'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

Created by admin. Last Modification: Monday 16 of March, 2015 22:40:05 CDT by admin.