Archive for August, 2010

Gas Cooling: Part 2

Sunday, August 22nd, 2010

In a compartment fire, the upper layer can present significant hazards to firefighters, including potential for ignition and energy transfer). My last post, Gas Cooling, began an examination of the science behind gas cooling, application of water fog into the upper layer to reduce the potential for ignition and thermal hazards presented by the hot gases.

Figure 1. Energy Transfer Required for Cooling

With a specific heat of 4.2 kJ/kg and latent heat of vaporization of 2260 kJ/kg, it takes considerable energy to raise the temperature of water to its boiling point of 100o C and change it from liquid to gas phase steam. Smoke on the other hand has a specific heat of 1.0 kJ/kg, indicating that in comparison with water; much less energy is required to change its temperature. As explained in Gas Cooling, 11.3 MJ must be transferred from the upper layer of this compartment to water applied for cooling in order to lower the temperature of the upper layer in a compartment from 500o C to 100o C (see Figure 1). It is important to remember that the energy required to cool the upper layer is dependent on the mass of hot smoke and air in the upper layer. This value will vary with the size of the compartment and the temperature of the hot gases.

When starting out on this examination of gas cooling, we posed two questions:

• How much water is required to cool the upper layer from 500o C to 100o C?
• Why doesn’t the volume of the upper layer increase when water applied to cool the hot gases is turned to steam?

The answers to these questions are interrelated. First, let’s look at the amount of water required.

Water Required for Cooling

When water is applied for fire control and extinguishment, energy is transferred from materials that have a temperature higher than that of the water to raise the temperature of the water and to change it from liquid phase to gas phase.

The theoretical cooling capacity (TCC) of water is 2.6 MJ/kg. This value is based on heating a kilogram of water from 20o C to 100o C (0.3 MJ/kg) and vaporizing it completely into steam (2.3 MJ/kg).

Dividing the energy that must be transferred from the upper layer by the TCC calculates the amount of water that would theoretically be required to cool the upper layer from 500o C to 100o C if the energy transfer and conversion of water to steam was 100% efficient. If this was the case, the upper layer could be cooled to 100o C by applying 4.35 kg of water. Given the density of water at 20o C of approximately 1.0 kg/l, this would be a volume of approximately 4.35 liters. However, this assumes instantaneous heat transfer and 100% efficiency in conversion of water to the gas phase. Neither of which is possible in the real world!

Experimental data (Hadjisophocleous & Richardson, 2005; Särdqvist, S., 1996) has shown that the cooling efficiency of water in both laboratory experiments and actual firefighting operations ranges from 0.2 to 0.6. Särdqvist (1996) suggests that an efficiency factor of 0.2 be used for interior fog nozzles. Barnett (as cited in Grimwood, 2005) suggests that an efficiency factor of 0.5 be used for solid or straight stream application and 0.75 for fog application. In actuality, the efficiency of water application varies considerably with the design of the nozzle, skill of the nozzle operator, and a range of other factors. For our examination of gas cooling, we will use an efficiency factor of 0.6 (60%).

Multiplying the TCC of water by 0.6 adjusts the cooling capacity to account for the fact that some of the water applied into the hot gas layer will not turn to steam while passing through the hot gas layer. Some of the droplets will pass through the gas layer and vaporize on contact with hot surfaces (more on this later) and others will fall to the floor, with increased temperature, but remaining in liquid form.

Figure 2. Adjusted Cooling Capacity of Water

Dividing the 11.3 MJ of energy that must be transferred from the upper layer of the compartment by an Adjusted Cooling Capacity (ACC) of 1.56 MJ/kg determines that 7.2 kg (7.2 liters) of water are required to lower its temperature from 500o C to 100o C.

Figure 3 illustrates common flow rates from combination nozzles, Adjusted Cooling Capacity (ACC) and time required to apply the 7.2 kg of water necessary to cool the upper layer of the compartment from 500o C to 100o C.

Figure 3. Flow Rate, Adjusted Cooling Capacity, and Application Duration

As illustrated in Figure 3, if water is applied at 115 l/min (30 gal/min), several short pulses will provide sufficient water application. If the flow rate is increased to 230 l/min, a single pulse is likely to be sufficient. However, if the flow rate is increased further, it is likely that excessive water will be applied. In addition, droplet size increases with flow rate, reducing efficiency.

All Models are Wrong!

This examination of gas cooling provided a simple example of how much water is required to cool the upper layer in a given compartment. While this explanation provides a good way to understand how gas cooling works, it is incomplete. Box and Draper (1987, p. 424)observe that “all models are wrong, but some are useful”. The following factors add quite a bit of complexity to examination of gas cooling:

• The energy that must be transferred from the upper layer is dependent on the mass of the hot gases and their temperature.
• Not all of the water applied vaporizes in the upper layer (some droplets travel through the hot gases and vaporize on contact with hot surfaces and others drop to the floor without completely vaporizing).
• Temperature of the hot gases in the upper layer is not uniform (as assumed in two layer models).
• Ongoing combustion and energy transfer from hot compartment linings add energy to the hot gas layer.
• Convection and gravity current influence the movement of hot and cool gases, making conditions dynamic rather than static.

While our model of gas cooling is wrong, I believe that it is useful. Firefighters do not calculate the volume of water required to cool the hot gas layer on the fireground. However, it is important to understand how flow rate and duration impact on effectiveness and efficiency.

Important!

Remember that this example involved gas cooling in a single compartment with static conditions. The flow rate and/or duration of application for fires in larger compartments or direct attack on burning fuel may be quite different.

What’s Next?

One question remains in our examination of gas cooling. Why doesn’t the volume of the upper layer increase when water applied for gas cooling turns to steam? This will be the focus of the third post in this series.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Box, G. & Draper, R (1987). Empirical Model-Building and Response Surfaces. New York: Wiley.

Hadjisophocleous, G.V. & Richardson, J.K. (2005). Water flow demands for firefighting. Fire Technology 41, p. 173-191.

Särdqvist, S. (1996) An Engineering Approach To Fire-Fighting Tactics Sweden, Lund University, Department of Fire Safety Engineering

Svennson, S. (2002). The operational problem of fire control (Report LUTVDG/TVBB-1025-SE). Sweden, Lund University, Department of Fire Safety Engineering.

Grimwood, P. (2005). Firefighting Flow Rate: Barnett (NZ) – Grimwood (UK) Formulae. Retrieved January 26, 2008 from http://www.fire-flows.com/FLOW-RATE%20202004.pdf

Gas Cooling

Saturday, August 14th, 2010

In a compartment fire, the upper layer presents a number of hazards to firefighters including the fact that 1) Smoke is fuel, and 2) the upper layer can be extremely hot. Application of an appropriate amount of water fog into the upper layer reduces the potential for ignition and lowers the temperature of the gases (reducing thermal load on the firefighters working below). While this sounds simple, and fairly intuitive, this basic technique to control upper layer hazards is frequently misunderstood. This is the first in a series of posts that will attempt to provide a simple explanation of the science behind gas cooling as a fire control technique.

How Does it Work

When a pulse (brief application) of water fog is applied into a layer of hot smoke and gases with a temperature of 500o C (932o F) what happens? As the small droplets of water pass through the hot gas layer, energy is transferred from the hot smoke and gases to the water. If done skillfully, the upper layer will not only be cooler and lest likely to ignite, but it will contract (or at least stay the same volume) providing a safer working environment below.

As demonstrated by Superintendent Rama Krisana Subramaniam, Bomba dan Penelamat (Fire & Rescue Malaysia) a short pulse can place a large number of small water droplets in the upper layer that develops during a compartment fire (see Figure 1).

Figure 1. Short Pulse

When presenting this concept, firefighters often present me with two questions:

• Since water expands approximately 1700 times when turned to steam at 100o C, why doesn’t the upper layer drop down on top of the firefighters?
• How can such a small amount of water have such a dramatic effect on the fire environment?

Math or No Math?

Using a bit of math, there is a really good explanation as to how gas cooling really works. The best answer is a bit complex, but a good conceptual explanation can be accomplished with a minimal amount of math.

Heating the water to 100o C (212o F) and production of steam transfers a tremendous amount of energy from the hot smoke and gases to the water, reducing the temperature of the hot gases. As the temperature of the hot gases is reduced so is their volume. However, don’t forget about the steam.

When water is turned to steam, it expands. At its boiling point, water vaporized into steam will expand 1700 times. A single liter of water will produce 1700 liters (1.7 m3) of steam. The expansion ratio when water is vaporized is significant. However, due to the tremendous amount of energy required to vaporize the water (and resulting reduction in gas temperature), the final volume of the mixture of hot gases and steam is less than the original volume of hot gases within the compartment.

The Key

The temperature of the gases is lowered much more than the temperature of the water is increased. Why might this be the case? The key to this question lies in the concepts of specific heat and latent heat of vaporization. As illustrated in Figure 2, the specific heat of smoke is approximately 1.0 kJ/kg (Särdqvist, 2002; Yuen & Cheung, 1999) while the specific heat of water is 4.2 kJ/kg and even more importantly the latent heat of vaporization of water is 2260 kJ/kg. What this means is that it requires over four times the energy to raise the temperature of a kilogram of water by 1o C than it does to lower the temperature of smoke by the same amount. In addition, it requires 2260 times the energy to turn 1 kg of water to steam at 100o C than it does to lower the temperature of 1 kg of smoke by 1o C.

Figure 2. Heating and Cooling Curves of Smoke & Water

While water expand as it turns to steam, the hot gas layer will contract as it’s temperature drops. At the same pressure, change in the volume of a gas is directly proportional to the change in absolute temperature. If the initial temperature of the hot gas layer is 500o C (773 Kelvin) and its temperature is lowered to 100o C (373 Kelvin) the absolute temperature is reduced by slightly more than half (773 K-373 K=400 K). Correspondingly the volume of the hot gases will also be reduced by half.

An Example

Once the underlying concept of gas cooling has been explained, the question of how a small amount of water can have such a dramatic effect may still remain. After all, the preceding explanation compared a kilogram of water to a kilogram of air. Firefighters normally do not usually think of either of these substances in terms of mass. Water is measured in liters or gallons. If measurement of smoke and air is thought of, it would likely be in cubic meters (m3) or cubic feet (ft3). Sticking with SI units, consider the properties of water and smoke as illustrated in Figure 3:

Figure 3. Properties of Water and Smoke

While over simplified, the compartment fire environment can be considered as being comprised of two zones; a hot upper layer and a cooler lower layer, each with reasonably uniform conditions (this is the approach used by computer models such as the Consolidated Model of Fire and Smoke Transport, CFAST).

As illustrated in figure 4, our examination of gas cooling will consider a single compartment 4 meters (13’ 1”) wide and 5 meters (16’ 5”) long with a ceiling height of 3 meters (9’ 10”). The upper layer comprised of hot smoke and air is two meters deep and has an average temperature of 500o C (932o F).

Figure 4. Compartment with Two Thermal Zones

What volume of water must be applied into the upper layer to reduce its temperature from 500o C to 100o C?

Just as input of energy is required to increase temperature, energy must be transferred from a substance in order to lower its temperature. The first step in determining the water required for cooling is to calculate the energy that must be transferred from the upper layer to achieve the desired temperature reduction.

The specific heat of smoke is approximately 1.0 kJ/kg. This means that 1.0 kJ of energy must be transferred from a kilogram of smoke in order to reduce its temperature by 1o C. This requires that we determine the mass of the upper layer.

Calculation of mass involves multiplying the volume of the upper layer (40 m3) by the (physical) density of smoke (0.71 kg/m3) at the average temperature of the upper layer (500o C) as illustrated in Figure 5.

Figure 5. Mass of the Upper Layer

Specific heat is the energy required to raise the temperature of a given unit mass of a substance 1o. The same energy must be also be transferred to lower the temperature of a unit mass of a substance by 1o. As illustrated in Figure 3, the specific heat of smoke is 1.0 kJ/kg. Therefore, to lower the temperature of a single kilogram of smoke by 1o C, 1.0 kJ must be transferred from that kilogram of smoke. With an upper layer mass (Mu) of 28.24 kg, 28.24 kJ must be transferred from the upper layer to water applied for gas cooling in order to reduce its temperature by 1o C.

Reduction of upper layer temperature from 500o C to 100o C is a change of 400o. Multiplying 28.24 kJ by 400 determines the total amount of energy that must be transferred to water applied for gas cooling in order to reduce the temperature to 100o C. As illustrated in Figure 6, 11,296 kJ (11.3 MJ) must be transferred from the upper layer to the water to effect a 400o C reduction in temperature.

Figure 6. Energy Transfer Required

Now that we have determined the energy that must be transferred from the upper layer in order to lower the temperature from 500o C to 100o C, it is possible to identify how much water must be applied to accomplish this task. However, that will be the topic of my next post. In addition, I will provide an explanation as to why the volume of the upper layer does not (necessarily) increase when water applied to cool the gases turns to steam.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Särdqvist, S. (2002). Water and other extinguishing agents. Karlstad, Sweden: Räddnings Verket.

Yuen, K. & Cheung, T. (1999). Calculation of smoke filling time in a fire room – a simplified approach. Journal of Building Surveying, 1(1), p. 33-37

Sunday, August 1st, 2010

Developing and maintaining proficiency in reading the Fire using the B-SAHF (Building, Smoke, Air Track, Heat, and Flame) organizing scheme for fire behavior indicators, requires practice. This post provides an opportunity to exercise your skills using a video segment shot during a commercial fire.

Commercial Fire

This post examines fire development during a fire in an agricultural facility in Spain. First arriving firefighters observed a small amount of light gray smoke issuing from roof ventilators and doorways with low velocity.

Watch the first 50 seconds (0:50) of the video. First, describe what you observe in terms of the Building, Smoke, Air Track, Heat, and Flame Indicators; then answer the following five standard questions?

1. What additional information would you like to have? How could you obtain it?
2. What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
3. What burning regime is the fire in (fuel controlled or ventilation controlled)?
4. What conditions would you expect to find inside this building?
5. How would you expect the fire to develop over the next two to three minutes

Now watch the next 20 seconds (1:10) of the video clip and answer the following questions:

1. Did fire conditions progress as you anticipated?
2. What changes in the B-SAHF indicators did you observe?
3. How do you think that the stage(s) of fire development and burning regime will change over the next few minutes?
4. What conditions would you expect to find inside this building now?
5. How would you expect the fire to develop over the next two to three minutes

Watch the remainder of the video. If you were the Incident Commander and had crews working inside the building, what information would you communicate to them as conditions change?