In today’s episode of the Dust Safety Science podcast, we go over the question of whether combustible dust can explode in a freezer. This Safety Share episode is based on a question that came through our HelpDesk a few years ago. A fire investigator had a number of questions, namely “Could this happen?” He came across our website and reached out to have a chat about it.
In this case, there was a stand-up freezer that exploded without any indication of having any issues beforehand. The contents were all regular food. There are no aerosols and no alcohol in the freezer itself prior to the explosion. There’s no indication that there’s an upset condition or fires or anything going on like that. It did include several bags of very fine white, all-purpose flour.
Fortunately, no one was injured, but the explosion did an impressive amount of damage. It blew off the door of the fridge, shattered windows in the room and did structural damage as well. At that time at least, they ruled out the coolant in the refrigerator. The freezer was still operational. The fan was no longer operational but the compressor and coils were still running.
The question is why? The materials in the fridge are not damaged. All that was left was flour. There were some other materials in there, but none that looked like they could be used to ignite an explosion. So that begs the question: how did the explosion happen?
There was a small hole in one of the bags of flour. To the disbelief of the investigators, the bag appeared to have been the source of the explosion. There were no other containers open in the freezer. The bag itself wasn’t ruptured; it just had a hole where flour poured out. White flour was embedded in the insulation in the crack in the door where the flour was, causing mild charring. You could see mildly burnt flour as well.
So was a dust explosion a possibility in this case? Everything seemed to point in that direction.
An explosion can occur when all five sides of the pentagon are present: fuel, oxidizer, dispersion, ignition source, and confinement. In this case, dispersion might be the most challenging. If the flour had any moisture in it, it might have frozen in chunks. But it was very dry and the hole was lower than the level of flour in the bag.
The real question is, “Can dust be ignited at this low temperature and propagate a flame?” You have to look at the minimum ignition energy at that lower temperature. Can it actually propagate a flame when you have a flame kernel develop? Are the reaction rates in that material fast enough to be able to sustain the fireball and then drive that explosion?
These are all the standard parameters that you would look at for combustible dust. There’s no reason to think they wouldn’t apply at zero degrees Celsius when we know they do apply at 20 degrees Celsius. So the best guess for the freezer is that it was slightly below freezing but at very low humidity, likely single digit. The possibility and even maybe the likelihood of having strong static sources is certainly there.
We found an article about people getting shocks from ungrounded freezers. Is that strong enough to overcome that minimum energy needed for that material? We recommended that the fire investigator reach out to a testing lab to know for sure. Again, there’s no reason to think that an explosion that would happen at 20 degrees Celsius wouldn’t happen at zero degrees Celsius. It’s just a matter of looking at these parameters and how they develop.
What about the dispersion issue? The leading hypothesis at the time was that the compressor was running, and it was actually running after the explosion as well. That vibration may have shaken the bag hard enough to have it topple over a bit or at least pour the contents out against the door, creating a cloud that led to the explosion.
Seeing a dust explosion like this in a residential area is frightening enough, but in an industrial setting, the damage can be much worse. You could have workers in the freezer at the time of ignition, or you could have that freezer located in a critical part of the facility where workers are nearby. It could also do critical structural damage when and if that explosion occurs. There are many things to consider if you’re going to store powders in either of these industrial freezers.
The other side of the coin is that it could be very easy to exclude a freezer like this from a hazard analysis. You look at it and think, “Oh, it’s cold in there and an explosion is unlikely.” But there’s no reason to think that an explosion that could happen at 20 degrees Celsius at room temperature becomes impossible if you cool the room down. You’d have to look at the parameters that are involved -the MIE, the MEC and the Kst- to determine whether or not that explosion can occur, and if it can occur, how severe it’s going to be. If it does occur and it’s severe, then what kind of controls do we need to have in place? So those are the two sides of the coin.
Conclusion
We haven’t recorded an incident that caused injury from a freezer explosion, but there’s no reason to think it can’t happen. This is a hazard scenario that could be very easy to overlook, so it should be addressed in hazard analyses at facilities where powders are stored in freezers, especially in conditions where it’s very low humidity and there’s an ignition source present. If there’s the remotest possibility that an explosion can happen, controls must be in place.
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Dust Safety Share
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