bomb shelter airlock

protect your shelter like a hospital isolation room

Use the airflow from your NBC air filtration system to safely enter and exit your protected space

Bomb Shelter Airlock – published on July 16, 2020

The key factors for your bomb shelter airlock:

Your shelter needs an airlock for safe re-entry, but the protocols you use will determine how effectively it isolates your shelter from what’s in the air outside.
You must be able to calculate the air changes per hour in order to understand how long to stay in the airlock before entering the shelter.

What is an airlock?

A bomb shelter airlock is an anteroom in a shelter that has filtered air flowing one way (outward) through it. There is a door at each end of the airlock. One door opens to the outside, the other opens into the shelter. One of these doors must remain latched and sealed at all times. When you enter your the airlock from the outside, it is flushed with filtered air from the NBC air filtration system before the inner door is opened. Airlocks allow you to enter your shelter without introducing large quantities of unfiltered air into the protected space of your shelter.

Airlock types

There are stand alone dual stage airlocks used for high traffic egress and ingress applications that use a dedicated NBC filtration system to purge the chambers rapidly. They are not cost and space efficient for a single family shelter that already has a filtration system. This article describes an integral single stage bomb shelter airlock. It is built into the shelter layout (integral), has only one chamber (single stage), and is continuously purged by the filtered air from the NBC air filtration system. This sample bomb shelter layout shows the filtration system in the top left corner, the airflow through the shelter, and the airlock in the bottom right corner:

Overpressure

Bomb shelters and safe rooms are sealed environments that require ventilation to sustain life. The NBC air filtration system will create positive relative pressure inside the shelter. Compared to the outside pressure, the shelter has more air pressure. This will ensure that if there are any small leaks, the filtered air inside will flow outward and prevent unfiltered outside air from migrating inside. Without this “overpressure,” the seals on the doors and hatches must be air tight. With overpressure, they can leak a little without exposing the shelter to unfiltered (toxic) outside air. This is the driving principal behind a collective protection spaces like military bomb shelters, HAZMAT tents, and military vehicles. Modern tanks and infantry fighting vehicles have crew compartments that are overpressured with nuclear, biological, and chemical air filtration systems, just like your shelter. To verify that you have the minimum recommended overpressure of 0.3 inches (7.62 mm) of overpressure, a differential pressure gauge is used. Analog gauges are recommended over digital instruments in a shelter since they do not require batteries and have no settings. They just continually read the pressure differential. Dwyer makes the best mechanical gauge and their Magnehelic line is large and easy to read. Directly below are links to a gauge and a mounting kit that reads zero to 1 inch of water gauge. This a good range for a fast visual reading.

Full disclosure: as an Amazon Associate I earn from qualifying purchases. The convenient links below will take you directly to the items on Amazon and help support this website. Thank you very much!

Calculating the shelter air exchange rate

One air change is when your NBC air filtration system introduces enough air to fill your shelter one time. The air change rate is how many times this amount of air is moved through your shelter in one hour. We will first calculate the air change rate for the entire shelter, then for just the airlock. The formula for calculating how many air changes is the airflow rate of the NBC air filtration system divided by the internal shelter volume:

airflow / shelter volume = air exchange rate

Note that the units of measure must be consistent. If you measure your airflow in cubic feet, you must also use cubic feet for the shelter volume. The same for metric: cubic meters of airflow and cubic meters of shelter volume. And, since we are looking for air changes per hour, we must convert any airflow rating given in minutes to hours. Below is an example of how the air changes are calculated. It’s all simple arithmetic, just follow along and you will understand how to calculate the air change rate in your shelter.

Where do these numbers come from?

The internal dimensions of the shelter will be shown on the drawings of your shelter. If there are several rooms, you will need to calculate the volume of each room and add them together. Right now, we are looking at the change rate of the entire shelter. The airflow rating of the NBC air filtration system will be on a placard on the filter housing or blower (fan). It should also be advertised and supplied in the specifications from the manufacturer. Note that some NBC air filtration system manufacturers and dealers will advertise the “fresh” air rating. This is the system output rating of the blower without the air resistance of the filter cartridge. It’s a larger number – usually about double the “filtered” airflow rating is. For example, the Andair VA150 has a fresh air rating of 176.6 CFM, but it’s rated at 88.2 CFM for filtered air using the same blower. The difference in airflow is due to the resistance of the filter cartridge. This cartridge is bypassed when it’s in fresh air mode. Be sure to do all air change calculations with the filtered airflow rating. In the world of NBC air filters, fresh = dirty and filtered = clean.

Airflow

For this example, we’ll use an NBC air filter with 100 cubic feet per minute (CFM) rating for the filtered airflow. Since this airflow is in cubic feet per minute and we are trying to find the air exchanges per hour, we need to multiply the airflow by 60 minutes to get an hourly rate: in one hour (60 minutes), we get 6,000 cubic feet of filtered air (60 minutes x 100 CFM = 6,000 cubic feet).

Shelter volume

Our example shelter has internal dimensions of 10 feet wide by 20 feet long by 8 feet high. The volume of this shelter is 1,600 cubic feet (10 x 20 x 8 = 1,600).

Air changes per hour

To determine how many air changes per hour this will yield, we need to divide the airflow rate by the shelter volume: 6,000 cubic feet per hour divided by 1,600 cubic feet of shelter volume to give us 3.75 air changes per hour. This is the air change rate for the entire shelter.

The metric system

For NBC air filtration systems that are provided with a metric airflow rating in cubic meters per hour, it’s easier to convert the volume of the shelter to meters instead of converting the system airflow rating to feet. Here is the same airflow and shelter size from above converted to the metric system:

Airflow conversion

100 CFM (cubic feet per minute) airflow = 169.901 CMH (cubic meters per hour)

Shelter volume conversion

10 feet = 3.048 meters, 20 feet = 6.096 meters, 8 feet = 2.4384 meters. The shelter is 45.3096 cubic meters in volume (3.048 x 6.096 x 2.4384 = 45.3096).

Air changes per hour

Since the airflow rating is in cubic meters and we converted the room dimensions to meters, we have the same units of measure. All we have to do now is to divide the airflow rate by the shelter volume. 169.901 cubic meters per hour divided by 45.3096 cubic feet of shelter volume is 3.749 air changes per hour. The slight difference is due to the conversion from standard units of measure to metric. It’s the same room size and airflow, just calculated with the metric system to a ridiculous four places to the right of the decimal so it would come out almost the same. You should have enough of a safety factor built into your airflow calculations that you can round off these numbers to a more reasonable tenths or hundredths (one or two places) before performing these calculations.

Calculating the air changes in the airlock

We’ll use the numbers from the example above and calculate the air changes for a 6 foot by 6 foot (2 meter by 2 meter) airlock.

Airflow

The same as above: 100 cubic feet per minute = 6,000 cubic feet per hour.

Airlock volume

The volume of this sample airlock is 288 cubic feet (6 x 6 x 8 = 288).

Air changes per hour

6,000 divided by 288 = 20.83 air changes per hour.

Purge time

This is how long it takes for one complete air change in the airlock. Since the number above is in hours, we need to divide 60 minutes by 20.83 air changes to get 2.88 minutes per air change. This is the purge time of the airlock at the rated airflow of the NBC air filtration system.

Dwell time

This is how long a person should wait in the airlock when entering the shelter. There are ways to calculate the exact dwell time to get a 3 log (99.9%) reduction in contaminants, but a simple rule is to allow for three air changes in the airlock before entering the shelter. In our example, this puts the purge time at 8.64 minutes (2.88 x 3 = 8.64). Let’s round it up to 10 minutes for a 10+ percent safety factor and put it on a placard inside the airlock next to a mechanical countdown timer so it’s convenient and accurate to implement the protocol dwell time.

At Amazon: KeeQii kitchen countdown timer 60 minute

While we are equipping this airlock, let’s also install a differential pressure gauge (see the previous Amazon links above) so that the overpressure can be verified while counting down the dwell time.

Protocols for using a bomb shelter airlock

The protocols you use will determine how effectively it isolates your shelter from what’s in the air outside.

This is the protocol for egress (exiting the shelter) from the inside:

Open the inner door and enter the airlock.
Securely latch and seal the inner door.
Ensure there is filtered air purging the airlock by checking the differential pressure gauge reads a minimum of 0.3 inches (7.62 mm) of water gauge.*
Donn your personal protective equipment. Ensure that everything is properly fitted and functioning correctly.
Open the outer door, check that the area is clear of threats, exit the shelter, and secure the outer door shut.

*If there is a situation outside the shelter that only becomes apparent right as or after you open the outer door, the airlock must be purged prior to opening the inner door and re-entering the shelter. Structural fires, incoming rounds, or observation of an intruder on the property are several examples.

This is the protocol for ingress (entering the shelter) from the outside:

Open the outer door and enter the airlock.
Ensure that the inner door is securely latched and sealed.
Securely latch and seal the outer door.
Ensure that there is filtered air purging the airlock by checking the differential pressure gauge reads a minimum of 0.3 inches (7.62 mm) of water gauge. Overpressure = airflow.
Start the timer.
Remain in the airlock for the placarded dwell time while performing a personal decontamination procedure.*

*Please see the article on decontaminating radioactive fallout for the personal decontamination protocol.

Removing the toxins from your protective equipment

Besides the placard, the timer, and the differential pressure gauge mentioned above, airlocks should have a way of decontaminating and storing personal protective equipment. In our example shown above, the airlock is also the shower for the shelter. Using this shower to rinse off the outer layer of your personal protection suite (Tyvek^TM or chemical suit) will remove radioactive fallout, chemicals, and/or biological agents. We want these toxins removed from the shelter by going down the drain. The airlock should also have places to store your suit and mask. Showers have a lot of moisture, but as long as the filtration system is on we’re moving a significant amount of air through it and it will dry rapidly. The protocols for donning, doffing, and decontaminating suits and masks will be covered more thoroughly in an upcoming article. See the links below for personal radiation protection equipment.

Observation, communication, and threat detection

Airlocks in secured facilities usually feature a window on both doors so that anyone inside or outside the shelter can observe if the airlock is occupied. These windows should be omitted if the airlock opens up to an unsecured area. Instead, the threat detection and identification system should be used to determine if the airlock is occupied. Having communication between the airlock and the shelter is crucial. The person returning to the shelter may have news of some imminent opportunity or risk that cannot wait the designated protocol airlock dwell time. A pass-through port between the shelter and airlock can be advantageous. It will allow small items like phones, pistols, ammunition, and identification to be transferred to or from the airlock. The possibility of having an unknown or malicious person in the airlock should be considered. Both doors should be able to resist breaching. If radiation or chemical detectors are going to be used in the shelter, a port for the detector probe should be incorporated into the airlock as well.

Personal radiation safety equipment at Amazon:

The take-away

If you are going to invest in a bomb shelter or a safe room, it should have a decontamination airlock. To be effective, the dwell time must be calculated and it must be properly equipped and placarded. You should practice the protocols and develop a habit of following them.

Next article: Where to build your shelter – basement, backyard, or bugout destination?

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