Interesting Design Tidbit from Utah Shelter Systems

Utah Shelter Systems may very well be the best NBC shelter manufacturer in the US. The proprietors, Sharon Packer and Paul Seyfried, have many years of experience behind them, backed up with education in various areas that pertain directly to their shelter work. They have tried hard to keep costs down while providing great builds. If you have not yet visited their web site, and are in any way interested in shelters, go over to Utah Shelter Systems and have a read-through. At the least, you will find it very instructive.

Sharon has a master’s degree in nuclear engineering, a bachelor’s in mathematics and a minor in physics. It was with Sharon that I had a recent conversation about one particular piece of equipment within their shelters. Her engineering background proved perfectly suited to answering some questions that I, and some of my readers, have had about the Andair VA-150.

Andair checical and biological filtration systemBut first, some background. The Andiar VA-150 is an air filter that is located within the hull of the shelter. It can be powered by electric motor or by human arms. Its job is to draw in outside air via a series of filter media, and to force stale air and moisture out. It is capable of removing war gases, i.e; nerve, blister, choking, blood, incapacitant and toxic gases. Shelter exposure to CO and CO2 are dealt with through the use of shut-off valves, and proper sizing of the interior to its nasty bio load – humans. (Some shelter manufacturers claim some unusually high occupancy rates, which are unrealistic in their ability to support life in a closed system, over significant durations of time.)

The questions about the VA-150 had to do with the accumulation of radioactive particles within the body of the filter unit. Over time, one might fear that an ever increasing particle load might  result in a case of severe radiation sickness. Picture a metal box, inside your shelter, glowing with the ether of death, only you can’t see it or feel it.

So, in response to questions by others and myself, I called USS and presented our questions. I was fortunate enough that Sharon herself answered the line, and even remembered me from past conversations. I told her that a few people had some concerns, and she happily explained how things work. (It’s great to get hold of an engineer that speaks English.)

I presented her with the scenario. The shelter contains a number of people anticipating the arrival of a large radioactive cloud. This cloud contains the aggregate output of several large nukes from up wind. They have buttoned-up so far in advance of the arrival that they found the need to purge the shelter of CO2 build up. They man the Andair VA-150 and begin pumping. Question: Will not the intake of radioactive fallout, which is a particulate dusting of radioactive material, build up inside the filter body and eventually cause severe medical problem for the shelter inhabitants?

Her answer, heavily paraphrased since I did not record our conversation, is like this:

Of all the things that could potentially enter the intake, the only class of them that could cause a problem within the shelter is fallout. Yet fallout doesn’t ever enter the shelter’s ventilation piping.  There is also no need for a pre-filter to assist in this. The fallout concern is dealt with via simple pneumatic flow engineering.

Fallout is particulate. It is not a gas or vapor. True, it could physically be filtered out at some point – say at the entrance to the ventilation system, but with enough of it, a filter could be clogged. That would mean a dangerous trip outside to replace it. Also, any filter in the intake is upstream of the overpressure blast valves, which slam shut when the outside air pressure rises above the inside pressure. An intake filter in that situation would be torn loose and rammed deep into the piping by the blast wave. The way that particle ingress is defeated is by the manipulation of air flow, particle mass and gravity.

The intake pipe for most USS shleters is 6″ in diameter. It’s opening faces downward, and is some distance above the ground level (or isolated within a rock crib containing stones of the proper size to slow air flow). The air entering this pipe via pumping action from the Andair unit, has to enter the opening, move upward for a distance, and then curve down again into the system.

Particulates that are heavy enough to settle to the ground in the local area can not ride the stream of air entering the intake port. They are, by their very nature, too heavy to remain within the stream, and “fall out” of it. This is because the diameter of the intake is large enough to slow the overall mass to a velocity below that required to transport debris.

The principle underlaying this is that for a given volume of air, its velocity through an opening must increase as the size of the opening decreases, and the velocity must decrease as the opening is enlarged. USS shelters use this principle to defeat fallout uptake. Simple. Clean. No pesky filter to service.

Sharon told me that a certain manufacturer (who shall  remain unnamed) uses an intake of about 2″ in diameter. This opening is located next to the door of the shelter, pointing outward. I believe the flow differential between this and the 6″ is something along the line of 8 times the flow. The velocity of the smaller port will almost certainly draw in fallout.  Even if it were larger, it would do so simply because of its orientation alone.

I asked her about debris created by a local blast. She answered that the shelter would be locked down at all points in expectation of a blast. No air would be drawn in for a period of time anyway (due to fire and smoke), so local fallout would not be an issue. As a side note, local fallout is much heavier than broadcasted fallout because it has enough mass to drop from the column quickly. This heavier particulate matter has even less of a chance to rise into the intake pipe. It is simply too heavy to be lifted.

She also explained that the rock crib design allows for diffusion of the fallout and also of the intake flow stream. For the truly paranoid, the crib can be fitted with a wash, to force fallout to the bottom of the crib. Sharon suggested that a minimum separation of crib and shelter is 10′, for mass attenuation of radiation. One happy benefit of the crib, at least for the exhaust, is that heat from the shelter is diffused through out the stones and the upper surface at ground level. This helps defeat heat detection by thermal devices.

Here is a link to their page describing filtering equipment and operations.

Her answers settled all of my concerns, and I trust they will do the same for those of you that have asked about this. If you have further questions, you can get hold of USS directly.  Contact information for Utah Shelter Systems is:
2236 So. 3270 West Unit 2; SLC, UT 84119
Mailing Address: PO Box 755; Draper, UT 84020

(801) 380-2932
(801) 816-9984 (Fax)

2 comments to Interesting Design Tidbit from Utah Shelter Systems

  • Finally someone with an authorative voice states the plain truth.



    • LP

      They are great people over there. In addition to knowing their stuff, they are just plain good people. Been in business since 1987. Sharon is the editor of the mag put out by TACDA, The American Civil Defense Association. Private effort at keeping the CD program alive in the practice of American homes.

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