This is an interesting thread, and right up my alley. I’ve been involved in the design of fire detection and suppression systems in the oil industry in Alaska for the past 15 years. I’ve dealt with, designed or specified Halon 1301, FM 200, Novec 1230, Dry Chemical, Water Mist, CO2 and AFFF systems mostly in hydrocarbon handling facilities. I won’t even claim to be an expert, but have a pretty good grasp on the various systems, strengths, weaknesses and issues that most sales reps don’t have the technical savvy to be aware of.
I’m sure most people are aware of the fire triangle, i.e. you need fuel, air and heat to start and maintain a fire. Fire suppression looks at breaking that triangle, and different agents attack the triangle in different ways and sometimes in a combination of ways. It’s important to understand that the Halocarbon and Fluorocarbon agents i.e. what are often referred to as clean agents act not by breaking the triangle in the conventional manner, rather than interact with the chemical reaction of pyrolysis. I.e. as the heat of the fire causes the clean agent gas to break down, the fuel would rather chemically react with the byproducts of the breaking down clean agent system than with the oxygen in air. If you’ve ever had a chance to use a Halon 1211 hand held fire extinguisher on a fire you’d agree that it seems like magic how it puts out the fire.
Looking at the various agents with a brief discussion of pros and cons, and issues with the agents:
Clean agents. Essentially they all work via the same mechanism. The reason they were so popular is they you could discharge a system into an area and unlike CO2, you wouldn’t kill the occupants, and there would be nothing to clean up after the discharge. That is all good and fine provided there is no fire in the enclosure. If there is a fire, the agent breaks down, the byproducts of the agent breaking down are Fluoric and Bromic acid and breathing those acids will ruin your lungs. Also those acids would ruin expensive equipment which is what lead to the development of water mist for turbine enclosures. As far as what agent is best, the newer agents do a wonderful job of putting out fires, really no different than Halon 1301. In general their weakness in comparison to Halon 1301 is that they need to be applied in concentrations that are very close to the concentration where they will cause health effects. Clean agents work well for class A, B and C fires and in three dimensional fires i.e. both pool fires, and spray or jet fires. For use in automobiles, they are probably the best choice of a suppression agent provided you can contain the agent for a sufficient amount of time to put out the fire. If you’re going to discharge into the cockpit, you really need to do some concentration analysis to make sure you aren’t putting yourself at risk. On the plus side, it is an extremely fast acting agent and a properly designed system will have the fire out in seconds, but if you have hot surfaces that could cause fuel or oil to reignite, you’ll need to keep the concentration of the agent in the engine bay long enough to prevent re-ignition or to allow you to bring the vehicle to a stop and exit it.
Water mist looked like the wonder replacement for clean agents. It uses water but in extremely small quantities by generating essentially a fog. There are no nasty byproducts to wreck your lungs should you be in a fire and a water mist system discharges. The down side is you’ll be in an extremely dense fog where you’ll have difficulty seeing. The real downside of water mist is that it works best when you have a large fire in a small enclosure where the fire is actively respirating and draws the water mist to the fire where the water droplets flash to steam and both reduce the oxygen and cool the fire. The downside is that it is a relatively slow acting agent. I’m not aware of water mist systems sized for cars, and as you’d need both a water bottle and a nitrogen bottle to pressurize the water and drive it through the discharge piping it would take up a lot of space. In general I’m not a fan of water mist except for very specialized applications and I’ve seen it used in many applications where I consider it not the best choice.
Someone mentioned AFFF. I’ll try to keep it brief. With a low expansion system you’re limited to pool fires as the purpose of the agent is to smother and cool the pool of hydrocarbons. Really not appropriate for 3 dimensional fires in an engine bay and you’d need water, a means of pressurizing the water, and the AFF agent, even bulkier than water mist and I’m not aware of a system sized for an engine bay. High expansion would solve the issue of a 3 dimensional fire. Either system discharged in an engine bay would make for a serious road hazard for anyone driving behind you.
The last system to look at would be dry chemical. It’s still not well understood how exactly dry chemical stops a fire, it sort of works like a clean agent, and sort of works to break the oxygen chain. Anyhow, it is very effective on hydrocarbon fires. The upside compared to a clean agent is that you don’t have the same issue of keeping the agent contained in the area with the fire, but you do have to be concerned that your system discharges to where the fire is located. As mentioned previously it is somewhat corrosive and cleanup can be a pita. Personally I think a hand held dry chem extinguisher would be well paired to a clean agent bottle for the engine bay.
I hope I wasn’t too verbose.
I’m sure most people are aware of the fire triangle, i.e. you need fuel, air and heat to start and maintain a fire. Fire suppression looks at breaking that triangle, and different agents attack the triangle in different ways and sometimes in a combination of ways. It’s important to understand that the Halocarbon and Fluorocarbon agents i.e. what are often referred to as clean agents act not by breaking the triangle in the conventional manner, rather than interact with the chemical reaction of pyrolysis. I.e. as the heat of the fire causes the clean agent gas to break down, the fuel would rather chemically react with the byproducts of the breaking down clean agent system than with the oxygen in air. If you’ve ever had a chance to use a Halon 1211 hand held fire extinguisher on a fire you’d agree that it seems like magic how it puts out the fire.
Looking at the various agents with a brief discussion of pros and cons, and issues with the agents:
Clean agents. Essentially they all work via the same mechanism. The reason they were so popular is they you could discharge a system into an area and unlike CO2, you wouldn’t kill the occupants, and there would be nothing to clean up after the discharge. That is all good and fine provided there is no fire in the enclosure. If there is a fire, the agent breaks down, the byproducts of the agent breaking down are Fluoric and Bromic acid and breathing those acids will ruin your lungs. Also those acids would ruin expensive equipment which is what lead to the development of water mist for turbine enclosures. As far as what agent is best, the newer agents do a wonderful job of putting out fires, really no different than Halon 1301. In general their weakness in comparison to Halon 1301 is that they need to be applied in concentrations that are very close to the concentration where they will cause health effects. Clean agents work well for class A, B and C fires and in three dimensional fires i.e. both pool fires, and spray or jet fires. For use in automobiles, they are probably the best choice of a suppression agent provided you can contain the agent for a sufficient amount of time to put out the fire. If you’re going to discharge into the cockpit, you really need to do some concentration analysis to make sure you aren’t putting yourself at risk. On the plus side, it is an extremely fast acting agent and a properly designed system will have the fire out in seconds, but if you have hot surfaces that could cause fuel or oil to reignite, you’ll need to keep the concentration of the agent in the engine bay long enough to prevent re-ignition or to allow you to bring the vehicle to a stop and exit it.
Water mist looked like the wonder replacement for clean agents. It uses water but in extremely small quantities by generating essentially a fog. There are no nasty byproducts to wreck your lungs should you be in a fire and a water mist system discharges. The down side is you’ll be in an extremely dense fog where you’ll have difficulty seeing. The real downside of water mist is that it works best when you have a large fire in a small enclosure where the fire is actively respirating and draws the water mist to the fire where the water droplets flash to steam and both reduce the oxygen and cool the fire. The downside is that it is a relatively slow acting agent. I’m not aware of water mist systems sized for cars, and as you’d need both a water bottle and a nitrogen bottle to pressurize the water and drive it through the discharge piping it would take up a lot of space. In general I’m not a fan of water mist except for very specialized applications and I’ve seen it used in many applications where I consider it not the best choice.
Someone mentioned AFFF. I’ll try to keep it brief. With a low expansion system you’re limited to pool fires as the purpose of the agent is to smother and cool the pool of hydrocarbons. Really not appropriate for 3 dimensional fires in an engine bay and you’d need water, a means of pressurizing the water, and the AFF agent, even bulkier than water mist and I’m not aware of a system sized for an engine bay. High expansion would solve the issue of a 3 dimensional fire. Either system discharged in an engine bay would make for a serious road hazard for anyone driving behind you.
The last system to look at would be dry chemical. It’s still not well understood how exactly dry chemical stops a fire, it sort of works like a clean agent, and sort of works to break the oxygen chain. Anyhow, it is very effective on hydrocarbon fires. The upside compared to a clean agent is that you don’t have the same issue of keeping the agent contained in the area with the fire, but you do have to be concerned that your system discharges to where the fire is located. As mentioned previously it is somewhat corrosive and cleanup can be a pita. Personally I think a hand held dry chem extinguisher would be well paired to a clean agent bottle for the engine bay.
I hope I wasn’t too verbose.
