Designing a Fire hose nozzles

The fundamental mechanism of any fire control technique involves one or more of the following strategies:

  1. Reduce ambient temperature
  2. Dilute amount of oxygen
  3. Introduce radical quenchers in the system
  4. Remove the flammable material

In US, a variety of ground and aerial equipment are used to effectively implement a combination of 1 and 2 above. In some cases, chemicals that can quench the radicals can be sprayed over the affected area, however use of such chemicals can cause toxicity concerns. Therefore, till date the most prominent technique for fire control is use of water jets.

Water is an excellent fire control material due to its thermal, physical and chemical characteristics. When water is introduced in a fire, two key fire control effects occur:

  1. Cooling effect: Water has a heat capacity of 4.2 J/g.K and a latent heat of vaporization of 2442 J/g. What this means is that if say a gallon of water, at 20⁰C in poured on a burning fire and all the water evaporates, then total heat that water will extract from the fire is given by the following equations:

Weight in gms of 1 Gallon of water = 3780 gms

Heat absorbed in going from 20⁰C to 100⁰C = 4.2 x 3780 x (100⁰ - 20⁰C) = 1.27MJ

Heat absorbed due to evaporation = 2442 x 3780 = 9.2 MJ

Total heat absorbed = 9.2 + 1.3 = 10.5MJ

However, in real life, not all the water may evaporate. Some water may fall and get absorbed by the porous surface. This is especially true in case of wild wires. Water that is evaporating is extracting heat from the fire plumes and water that ends up falling on the surface cools the fuel surface.

It is noteworthy that depending on the initial temperature of water, heat absorbed via evaporation is significantly higher than heat absorbed via specific heat.

  1. Dilution effect: In addition to cooling effects, evaporating water helps in diluting oxygen and fuel vapors. For a large number of hydrocarbon fuels, Limiting Oxygen Concentration (LOC) is around 12% in air. What this means is that if oxygen percentage in air drops below 12%, the flame (oxidative reaction) will extinguish. As water evaporates, its volume increases by almost 1700 times. This means that 1 gallon of water will evaporate in over 1700 gallons of steam. Even though this effect is more effective in a closed environment, but this can be significant in wild fire situations as well.

Even though water has such superior fire extinguishing capabilities, some other key aspects need to be considered to maximize efficiency:

  1. Reach and penetration: Given the wild fire geometry and intensity, the line of attack need to be as far as possible. This means water jet need to have maximum velocity that is allowable with given equipment. Another key aspect to consider is penetration of water jets. In wild fire situations, the hot gases maybe flowing at significant velocity. Any droplets that get blown away before reaching the fire lead to lower efficiency and may even pose risk to fire fighters.
  2. Water stream’s evaporation efficiency: Water stream’s evaporation efficiency is more critical in wild fires as compared to urban fires. In wild fires the burning structures include grass, trees and airborne foliage. Water that falls on the ground may seep into the porous surface and not contribute to cooling. As an example, say 1 Gallon of water is introduced in a fire area and only 0.5 Gallon of water evaporate. Other 0.5 Gallon falls on the porous ground and gets seeped in the ground. Total heat absorbed in this case would be:

Weight in gms of 1 Gallon of water = 3780 gms

Heat absorbed in going from 20⁰C to 100⁰C = 4.2 x 3780 x (100⁰ - 20⁰C) = 1.27MJ

Heat absorbed by 0.5 Gallon due to evaporation = (2442 x 3780)/2 = 4.6 MJ

Total heat absorbed = 4.6 + 1.3 = 5.9 MJ

As compared to 10.5 MJ/Gallon absorbed when 100% of water is evaporated, only 5.9 MJ/Gallon is absorbed when 50% of water is evaporated. In addition to lower cooling efficiency, lower evaporation rate would also lead to smaller degree of dilution.

  1. Geometry: Geometry of water stream can play a critical role both for fire control rate and ergonomics of the fire fighters. Ideally more lateral area covered by a water jet would allow fire fighter to cover larger area with less bodily movements. However flat sheets can tend to be unstable and convert to droplets before reaching the suitable target. To allow greater reach and spread, diverging streams are most ideally suited.