How to Select a CBRN filtration system for your shelter?

The CBRN filtration system for shelters is a unique product. Unlike many products which should work until they break, the CBRN filtration system is intended to remain in standby for decades (hopefully forever) but to be at the top of its performance when you need it, even after a decade or two.

This is why the first step to do is to evaluate the manufacturer, are they new in the business?, how many systems do they produce a year?, do they offer a wide range of sizes? (to fit to your needs), what is their quality assurance protocols? Is the product certified?, and most important, when was it certified and by whom? (if a product was tested 10 years ago, it may not be the same product by now. The raw materials, the design, and the manufacturing method\equipment) may have changed and may affect the quality of the product).

The second step would be to define the specific system\airflow which is required.

There are 3 main parameters to consider (and select the highest value) when choosing the right airflow:

1. Number of people in the shelter multiply by airflow per person- Define the airflow per person and multiply it by the number of persons. Common standards use airflows between 3 to 6m3h per person. We normally consider 6m3h for Civil Defence or private shelters as it is also good for warm\hot temperatures (and our world is not getting cooler…). Military airflow requirement may be even as high as 17m3h/person, but that is mainly because they are not just resting but active combat units. Although this is the first factor normally considered, some doctrines calculate it backwards by defining first the airflow in the shelter and then defining the maximum number of persons allowed (since you are very likely to bring in also your next-door neighbor or your cousins, if they are lack of shelter).
2. ACH (Air Changes per Hour)- To define the airflow based on ACH, you multiply the shelter’s volume by the selected number of ACH. Recommended ACH is 2-6ACH with 1ACH considered as a minimum and approved mainly for private shelters or shelters with very high volume and very little number of persons.

3. Leakage\exhaust- A CBRN shelter must be able to maintain overpressure (compared to the outside) in order to prevent any contaminants from entering the shelter (other than through the CBRN filter). To achieve the overpressure (100-250Pa is the common overpressure) the airflow introduced by the filtration system should be greater than any leakage and exhaust combined. Exhaust airflow is the sum of all exhaust points in the shelter (from the airlocks, toilets, kitchen, etc.). As the shelter should be gastight, it is recommended to make a tightness\leakage test prior to the installation and define the leakage rate. If such rate is not available, it is common to add a safety factor of 25% to the airflow. Some doctrines may add the 25% safety margin to compensate for the shelter and seals deterioration after years (and also to ensure the pressure will raise above 100Pa).

Calculation\selection example (random values):

1. 50 persons X 6m3h = 300m3h.
2. Shelter’s volume is 200m3 X 2ACH = 400m3h.
3. The leakage & exhaust rates are calculated as 440m3h + 25% safety margin = 550m3h.

550m3h is the highest value, so we should select the next higher system, for example the ARC600 system which is providing 600m3h.

Remember that since you are planning to use the shelter after many years and while explosions may occur outside, the conditions may change (like the leakage rate) so always add safety margins and choose the higher value.