The Debate that Really Isn’t: Part II
We discussed the basics of the fog vs smoothbore debate, but let’s get in to it, shall we. WARNING, this is going to get real nerdy.
STEAM.
This was the major argument around the development of the fog nozzle when Laymen was selling his new nozzle, “the nozzle of the future”. What is steam and how does it help us. Steam is produced when water is heated to above 212 degrees. The water molecules begin to move rapidly until they break the surface tension of the water. They begin to bubble and eventually, they will turn into a vapor. A misconception is that steam is the white cloud we often see when water is placed on fire, but this is not the case. Steam is invisible. When we see steam conversion, we associate that with the white cloud coming from the structure. The white cloud is actually the contraction of water vapors to form condensation. Steam contracts when the water vapors are cooled below 212 degrees and completed a physical state transformation once again.We will discuss contraction and condensation in a bit.
We know that water boils at 212 degrees, when this happened a change to the physical state occurs. Water goes from a liquid to a vapor when heated to its boiling point. When this occurs, steam expands and begins to expand exponentially. We know that water converts to steam and expands at 1700 times it original volume at boiling. The issue is that most super heated environments which we operate are a lot hotter than 212 degrees. Many times this can be several hundred or over a thousand degrees. Andy Frederick and William Clark makes the point that at these higher temperatures, over a thousand degrees, the steam will expand 4000 times its original volume. Another reference notes that a typical room (12x16x8, 192 Sq. ft.) has a temperature of 1,112 degrees. This is based on the work by NIST in 2009 and noted in the work of Vestal & Bridges 2011 research. At this temperature, water expands to 4200 times the original volume.
When water is turned to steam, this begins to absorb heat but also starts expanding the steam and condensation. Oxygen is pushed out and the compartment no longer has all the factors for combustion. When steam then contracts, it allows oxygen to be present and can cause fire to reignite. This is due to the contraction of the steam. Just as steam pushes oxygen out, the contraction will draw oxygen in.
Why is this important. Number one, steam burns can penetrate aspects of our turnout gear such as our hoods and around the wrists. This is due to the physical change of the water into vapor and its ability in that state to penetrate these materials due to the size of the water molecules. These are even small enough to penetrate our epidermis and be absorbed by our skin. Water conducts heat 24 times more readily than dry air. This exposes firefighters to additional injuries from steam burns especially when intensifying the steam conversion with smaller droplets of water and contraction of steam into water vapors. Secondly, this poses an additional risk for victims within the structures, adjacent to the origin room. Although steam burns for the victim may not occur as readily as we might think, the damage can be done via inhalation damage. Dry air is stopped or can only reach the upper airways, but moist air will travel deep into the lungs and sustain lung tissue injury. This causes significant issues for victims. Paramedics and doctors can intubate or conduct a cricothyroidotomy to provide ventilation to the patient when encountered with upper airway burns. When the lower airways are damaged, there is little to be done to provide adequate ventilation to these patients if the lower airways are severely damaged.
Water Droplets and Droplet Fallout Rate
Droplet size matters and as we know, the fog nozzle produces smaller droplets sizes than that of a solid stream from smoothbore nozzle. A fog nozzle will produce a droplet size around 0.33mm once the droplet has incurred the thermal degradation. The average of the fog is between (0.25-0.33mm). This has the temperature reducing power of 135 F in the compartment of 1,112F. A 0.78mm, the droplet has the ability to reduce temperature of 383 F. As we produce larger droplets of water, the ability for the temperatures to decrease is substantial. As we begin to produce these larger droplets of 1.0mm, which is typical of a SB, we see a dramatic increase in firefighting power. This is due to the Droplet Fallout Rate (DFR). This is the amount of water that is able to reach the floor of the compartment. The goal of firefighting is extinguishment through cooling and stopping the chemical reaction at the fuel-flame interface. As Andy Fredericks explained so well, suppression happens at the fuel-flame interface, not cooling the environment but the products of combustion. Our goal is to get the water droplet through the superheated gases both on its way up and on its way down.
When a nozzle is opened, the water travels high and hits the ceiling. If we have smaller droplets of water, the superheated gases will evaporate, absorb heat and/or be caught in the thermal currents and taken away via ventilation or flow path. Larger droplets of water have the ability to penetrate through thermal layers and reach the ceiling, coating the ceiling surfaces and the walls without thermal degradation or evaporation. The water hits ceiling and it will eventually fall. When the water falls, it again has to penetrate the thermal gases to reach the burning material below. When water is able to coat the burning material below, it reduces the temperature of the combustible materials in the compartment and prevents flashover by coating materials which are not yet ignited and suppresses off-gassing. Again, the goal is to place water at the fuel-flame interface and this is done by creating large droplet of water to drop to the burning material below.
When we start looking at DFR, a fog nozzle at maximum capacity can only produce a fall out rate of less than 30% to the floor, everything else is wasted in evaporation or convection. When operating smoothbore with a droplet size of 1.0mm, it has a DFR of 55% and at 2.0mm a DFR of 80% with 20% being evaporated and little to zero caught in convection.
Heat Absorption and Extinguishment Power
When looking at data in Vestal & Bridge 2011 research, they related heat absorption and stream application. At 30 feet, a fog nozzle will distribute water to 100 sq. ft. while a smoothbore will be more accurate at 2 sq. ft. A 30 degree narrow fog will produce 1.9 gpm per square feet, while a smoothbore will produce 92.5 gpm per square feet. Smoothbores will absorb 13.9 mW/ft2 while fog will absorb 0.43mW/ft2. This verifies that would ability to place water on the fuel-flame interface is improved with a solid stream, therefore producing cooling of the burning materials and other surfaces in the compartment.
Data by NIST shows the initial impact of each type of nozzle as it contributes to the temperatures of the compartment. A fog nozzle opened in the 192sq. ft space will increase temps from 400 F to 1000 F in the first 30 second, increasing the Heat Release Rate (HRR) from 12mW to 16mW. The temperatures at the ceiling will also still be capable to producing a flashover during this time and radiant heat at 3’ will be able to cause injuries and a potential for flashover. The heat flux will increase to 100kW/m2 and takes 80seconds to drop to 50kW/m2. This not only increased the temperature in the bedroom, but also in the hallways.
While using the smoothbore in these studies with 160 gpm of fireground flow, temps went from 1,112 F to less than 500 F in 30-50 seconds, dropping the HRR from 16mW to 8mW in 30 second (20 second faster than fog).
To continue this trend, Heat flux in the NIST Wind-drive fire study in 2009 also produced data to show heat flux dropped from 90 mW/m2 to 20 in one minute and 40 seconds and living room and hallway to reduce 20 kW/m2 in 45 second. After 70 seconds, ceiling temperatures decreased from 1,500 F to 200F and living room temps of 1,112 to less than 500F in 30 seconds.
From these studies, it was noted that firefighting capabilities need to be able to produce sufficient droplet size to penetrate the thermal environment and allow water to cover the burning material at the fuel-flame interface while also preventing other materials from igniting. A droplet fallout rate of 84gpm is needed to overcome the thermal environment and successfully extinguish the fire. A single line with 152 gpm of fireground flow from a smoothbore provides an adequate amount of water. To accomplish this with a fog, it would need 672 gpm due to the evaporation and wasted water.
Water dynamics is something that many people do not understand completely and in my opinion missing link in the education of firefighters. Water is our main resource, apart from firefighters, to put fires out. We need to understand water and hydraulic dynamics just as we understand fire dynamics. We can understand our enemy all we want but we also need to understand our weapon systems. Water has many different reactions to physical state changes and they impact the fire environment which we are operating in. If we are unaware of these impacts, we can place ourselves in a situation which can get us burned or severely injured.
The debate between Smoothbore vs Fog will never end. Some people get tired of the debate and that is fine, but I feel like it produces research and discovery on multiple levels for the fire service to learn. If each side is trying to prove they are right, it progresses the fire service because we are contumely experiencing new dynamics within our fire environments. This leads to new discoveries and potentially new tactics. Debate is not always bad as long as we are using the opportunity to improve our thinking and knowledge. Based on the information presented, I believe the smoothbore has a greater impact on the fire environment and a better tool for firefighters to use but I am sure the debate will continue.
***I want to encourage you all to check out the reference below. These are the source I used while writing this blog. These are great documents which you can learn a lot from.
References:
Firefighting Principle and Practices- William E. Clark
NIST 2009 Wind-Driven Fire Research
UL Fire Stream Research
The Book of Andy
A Quantitative Approach to Selecting Nozzle Flow and Stream by Jason Vestal and Eric Bridge.