Consider a typical jar candle with a nice flame. The lid is closed, and the flame goes out, because there is no oxygen to sustain combustion.
At approximately what Oxygen level, does the flame go out ? Air has 21% oxygen, so presumably the flame goes out at an Oxygen level between 0% and 21%. I am looking for an approximate number : like 10%, 5%, Less than 1% …
Fires require around 16% O2 to sustain ignition.
Indeed fire suppression systems that rely on displacing the oxygen concentration in a volume (ie a room) generally target a concentration of 14% O2. This is sufficient to interrupt combustion yet allow human life
Is a CO2 Fire Suppression System Dangerous?
At 7.5% concentration, CO2 can cause asphyxiation to humans. Most CO2 fire suppression systems are designed to have 34% CO2 concentrations for a total flood of the protected space. Due to the obvious dangers of CO2, the CO2 fire suppression systems are required to have certain life safety devices to protect personnel around or near the protected spaces. One of the life safety devices is a pneumatic siren that warns people around the area that the CO2 will be discharging from the suppression system. It is important to properly train all personnel on the dangers of the CO2 fire suppression system and how to evacuate safely if the system is preparing to dump the CO2 to suppress the fire.
From the EPA:
At the minimum design concentration (34 percent) for its use as a total flooding fire suppressant, carbon dioxide is lethal.
Yes absolutely C02 is bad. It’s very rarely installed in new facilities (maybe never?) and the majority of legacy installations are being replaced with clean agent systems.
I’d say, of all the related installations of systems, 95% or more are clean agent based with the remainder being C02 or fancy things like water misting.
Clean agent systems use a variety of products, such as FM200, Novec 1230, etc. Some more detail:
Why not nitrogen? Or nitrogen mixed with a small amount of either CO2 or a oderant, so people will get out?
And is it concentration of oxygen that matters, or partial pressure (for both flames and human survival)? A system that’s designed for sea-level total pressure might not work right in Denver.
Pure nitrogen is used often. seemingly more frequently in SE Asia than in the west, I’m unsure why.
Chemical Clean agents (FM200, novec) have some specific advantages over pure gaseous clean agents like nitrogen or nitrogen/argon blends. They provide a cooling effect on discharge for one and also require less physical infrastructure (compressed bottles and piping and associated valving).
There are correction factors required for installing at altitude but I’m unclear on the physics behind it
According to the EPA site I linked to:
Flame extinguishment by carbon dioxide is predominantly by a thermophysical mechanism in which reacting gases are prevented from achieving a temperature high enough to maintain the free radical population necessary for sustaining the flame chemistry. For inert gases presently used as fire suppression agents (argon, nitrogen, carbon dioxide, and mixtures of these), the extinguishing concentration (As measured by the cup burner method (NFPA 2001)) is observed to be linearly related to the heat capacity of the agent-air mixture (Senecal 1999).
Although of minor importance in accomplishing fire suppression, carbon dioxide also dilutes the concentration of the reacting species in the flame, thereby reducing collision frequency of the reacting molecular species and slowing the rate of heat release (Senecal 1999).
Short version: CO2 has a higher volumetric heat capacity than N2, making it better at interrupting the flame chemistry. The importance of the diluent’s heat capacity shows that O2 partial pressure is not the only factor in the performance of a diluent-based fire suppression system.
For inert diluent gases, human survival is a function of O2 partial pressure. But CO2 isn’t an inert asphyxiant, it’s acutely toxic. In small concentrations the effects take time, but at large concentrations, you can pass out pretty damn quick.
But in concentrations much lower than the toxic levels, carbon dioxide also has the effect of making humans feel short of breath. That’s why nitrogen leaks are dangerous, because they can displace too much oxygen, but without ever triggering that short-of-breath feeling, so people stay in the area until they pass out. That’s why I was suggesting mixing in a small amount of carbon dioxide (i.e., just enough that in operation of the system, humans in the area would feel short of breath, but not so much that they’d be poisoned).
Mixing suppressant gases adds cost without significant benefit. Areas that use CO2/N2 fire suppression systems have an ear-piercing alarm and strobes that operate for something like 30 seconds before dispensing. And when it dispenses, it’s a violent event that you definitely won’t fail to notice (see first video at 0:45, second video at 1:35):
I like the idea Chronos. I am not sure though, that you will be able to establish a statistically valid safe (and defensible) CO2 concentration in N2. And OSHA and other regulatory agencies will not get behind it unless you have data. And data will require experiments on humans.
We use Oxygen monitors that alarm on low O2 concentrations and it works fairly well.
OK, yeah, that’s not going to go unnoticed.
Years ago, many big mainframe computer rooms were protected by Halon fire suppression systems. When triggered, they would flood the computer room with Halon gas, which would put out the fire. But without water damage, etc. to the very expensive mainframe computers.
These systems were set to produce a 5%-7% Halon in the computer room, while humans don’t lose consciousness until 10%+. Even so, there were loud alarms & strobes going off, and the switch to trigger the Halon system was usually right beside the exit door, to be hit as you exited the room.
I believe new Halon systems are very restricted now, because it is an ozone-depleting gas – an environmental problem.
I don’t know about restrictions, but yes they can be pretty bad for the environment. FM200 has a GWP of 3220. There is an alternative called Novec that has a GWP of 1, no different from CO2.
FM200 and Novec are both pretty expensive, though. For a typical engine test cell ( 25’ x 25’), refilling the tanks costs about $60,000. OTOH, they’re much better than CO2 at not killing room occupants.