Air Supply Principles for Civil Defense Shelters and Protective Structures
Ventilation, CBRN Filtration & Complete "Close-Up" Modes
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Have you thought about how the air supply system works in closed spaces?
Rather this is a civil defense shelter shelter, spaceship, or submarine the principles for creating a safe and suitable physical environment are very similar.
Underground shelter equipped with CO2 Scrubbers, Oxygen Supply and other Life-Support Systems
As we approach the design of such complex systems, we must take into consideration many different issues, one of them being how to create the conditions that will allow people to function and live inside this closed space for relatively long periods of time.
Whether it is in a submarine, spaceship, or a protective shelter, the structure walls are usually serving to separate people from the environmental or hostile conditions outside the isolated area. But when we create such an isolated area, we need to take into consideration how to maintain the basic need of people to continue functioning like in “normal” conditions.
First let’s talk why the full Isolation "close-up" mode is needed in the shelter?
This operation mode is commonly used in special purpose shelters or shelters that located in zones with high risk of fire, potentially dangerous industries, and areas with high potential of natural disasters like the danger of flooding. In all these shelters the close-up mode is needed to protect people until they can be evacuated, or the danger will pass.
Fires or Heavy Contaminated Environment may create a need for Lock-Up (Isolation) mode
Without entering all technical details and how the systems are working, in general such shelters can be operated in the following modes:
1. Natural ventilation mode
This mode usually used in normal time, the doors and hatches are open, and air circulates without additional help from air supply systems
2. Ventilation mode
This mode is used when the shelter in operated but there is no threat of CBRN contamination outside the doors and hatches are closed, and air supply controlled by the air supply system, that pushes the air into the shelter creating air circulation in the shelter. In this mode, the air is usually filtered only by coarse dust filters (pre-filters).
3. CBRN filtration mode
When there is a risk of CBRN event or contamination of the air outside, the shelter doors and hatches are closed and sealed, and the air pulled from outside go through the special CBRN filters. The shelter must be airtight, and the air supply system designed in such way that positive air pressure (overpressure) is created inside which helps to prevent the contaminated air from entering the shelter. The air is constantly changed in the shelter and “normal” conditions are created.
4. Full close-up mode
This mode is usually used in the beginning of an incident, when it is not fully known what the status outside is, and later in critical situations for example when the concentration of contaminants outside or their composition can’t be effectively treated by the air filtration system. Other scenarios can include flooding by water or strong fire outside. In this mode the air supply and the overpressure need to be maintained by other means as there is no possibility to provide fresh air from outside.
Let’s go deeper into the air supply process in the full close-up mode:
As we know, the breathing process has two main stages, in simplified example people inhale oxygen rich air and exhale air with higher content of carbon dioxide (CO2). Today in normal environment outdoor CO2 levels tend to average around 400 ppm and the levels indoors can increase far beyond that, upwards of 1000 ppm or even 2000 ppm but in high levels of over 5000 ppm can immediately become dangerous to health and life, and anyway deteriorate operative ability of persons inside the shelter.
In the normal conditions the levels of oxygen and carbon dioxide are stable, but problem starts when we need to maintain such conditions in airtight spaces with no external air supply. Immediately after switching the shelter into a close-up mode the level of oxygen will be getting lower and the level of carbon dioxide inside the shelter starts to rise because the air filtration is not used, and no fresh air is entering the shelter. In addition, the shelter overpressure shall be maintained, what’s creates the need to compensate the loss of air volume.
Some of the currently used solutions are presented below:
Oxygen Supply / Addition:
1. Oxygen (or compressed air) stored in special cylinders
It can be maintained in a gas form under pressure or as liquid oxygen and released from these tanks when needed
2. Oxygen generator
Can separate oxygen from compressed air stored in the tanks or additional solutions for adding O2.
3. Oxygen Candles: Providing Emergency Air
Oxygen candles are used as a backup source of breathable air in an emergency. In addition, they supply an on-demand source of oxygen, utilized in airplanes, space stations, and other applications such as shelters, safe havens and refuge chambers.
Carbon dioxide CO2 removal:
1. Carbon Dioxide Scrubbers
that are based on soda lime or lithium hydroxide material. Soda lime is a mixture of chemicals (sodium hydroxide and calcium hydroxide) that’s commonly used to reduce the carbon dioxide concentration in closed spaces. The main disadvantages of such systems can be creation of heat and moisture in the process and large space needed for storage of the filters or the material.
2. Regenerative carbon-dioxide removal system
Usually based on solid amine material and used on spacecrafts and submarines and may have advantages in operation and space requirement. The main disadvantage is a relatedly high energy consumption because of the processes requires for their operation as well as a high cost if compared to standard CO2 scrubbers.
For a conclusion:
The carbon-dioxide removal system is not a standalone system and has to be integrated as part of the general solution, combined with Oxygen supply system for keeping the safe levels and maintain the overpressure level inside the shelter.
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