Extinguishing Agents Study Guide for McGraw-Hill's Firefighter Exams
Firefighters extinguish most fires using water. It is usually available in abundance at or near the fire. Water can be delivered onto the fire in a number of ways: hand lines stretched from the apparatus, hand lines stretched from a standpipe system located inside a building, sprinkler system, water mist system, master deluge nozzles, distributors, etc.
Water extinguishes fire by cooling the material (absorption of heat), smothering (steam generation), emulsification (agitation of insoluble liquids to produce a vapor inhibiting froth), and dilution (adding water to reduce the concentration of a burning soluble liquid and thereby raising its flashpoint). A review of the advantageous and disadvantageous properties of water follows.
Advantageous Properties of Water
There are several advantages to water as an extinguishing agent. Some of the characteristics of water that make it advantageous in extinguishing fires are listed below.
- Heavy, stable liquid at ordinary temperatures.
- High specific heat—The heat capacity is given in terms of the mass of the substance in pounds and is the amount of heat required to raise the temperature of 1 lb of a substance 1°F). All solids, liquids, and gases have specific heats. Only two liquids have higher specific heats than water: ammonia and ether. One BTU is required to raise the temperature of 1 lb of water 1°F. To raise the temperature of 1 lb of water from 32°F to 212°F requires 180 BTU.
- Latent heat of fusion— Melting of 1 lb of ice into water at 32°F absorbs 143.4 BTU.
- Latent heat of vaporization— The latent heat of vaporization is the conversion of 1 lb of water into steam at a constant temperature with the absorption of 970.3 BTU.
- Conversion to steam— The conversion of liquid water to steam increases its volume approximately 1,600 times, which displaces an equal volume of air, thereby reducing the volume of oxygen available for the oxidation reaction.
Disadvantageous Properties of Water
Water also has disadvantages, some of which are listed below.
- It conducts electricity.
- It has low viscosity, which means it runs off smoldering material readily.
- It has high surface tension, since it has poor penetration qualities.
- It is transparent to radiated heat.
- It freezes at relatively high temperature.
- It displaces flammable liquids.
- It reacts violently with combustible metals.
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