Extinguishing Agents Study Guide for McGraw-Hill's Firefighter Exams (page 2)
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.
Mechanical foam used today is a mixture of air, water, and liquid foam concentrate (protein- or synthetic-based). The combination of these three ingredients produces a bubble blanket solution that will flow over and around combustible solids and liquids. Unlike water, foam is lighter than flammable liquids and will float on the surface of the liquid. Foam is used on both Class A and Class B fires. It will conduct electricity, however, and should not be used on energized electrical equipment. Foam extinguishes fires in four ways: cooling; smothering; suppressing vaporization, and separating the flames from the fuel surface, thereby reducing radiated heat feedback.
Carbon dioxide (CO2) is a relatively nonreactive gas that penetrates and spreads throughout the area of the fire. It is used primarily to extinguish Class B and Class C fires because it is nonconductive. CO2 extinguishes fires by displacing the air (oxygen) content that is reacting with the fuel. It also has a cooling effect on the fire, which aids in lowering the temperature of the fuel below its ignition temperature.
The dry chemical ammonium phosphate extinguishing agent is used on Class A-, Class B-, and Class C-type fires, whereas the dry chemicals sodium bicarbonate or potassium bicarbonate are used to extinguish Class B- and Class C-type fires only. Dry chemical agents are still being used to extinguish fires in animal-based, saturated fat cooking oils. These agents leave a residue that is nonflammable on the extinguished material, which reduces the likelihood of reignition. Dry chemical agents extinguish fires by cooling, smothering, and reducing radiated heat feedback, and most importantly, by inhibiting the chemical chain reaction of the oxidation process.
The wet chemical extinguishing agent contains a blend of potassium acetate, citrate, and water (40 to 60 percent). It is a low pH (acidic) agent that was originally developed for preengineered cooking equipment fire extinguishing systems for use on today's unsaturated fat vegetable cooking oils (Class K) that rapidly burn very hot and are difficult to extinguish. This agent is applied as a fine mist to help prevent grease splash and fire reflash while cooling the appliance.
Clean agents are gases, gas mixtures, and vaporized liquids that are replacements for Halon gas extinguishing agents that are rapidly being phased out for being destructive to the Earth's ozone layer. These agents are nonconductive and noncorrosive, and they leave no residue and are therefore especially valuable in protecting telecommunication, electrical, and computerized equipment. Clean agents extinguish fire by interfering with the chemical chain reaction between oxygen and the fuel vapors, by absorbing heat, and by displacing the air (oxygen) content inside a room or space. Examples of clean agents include halocarbon gases, inert gases, powdered aerosols, and water mist. They are utilized to extinguish fires in Class A-, Class B-, and Class C-type materials.
Dry powder extinguishing agents are used to extinguish fires in combustible metals and combustible metal alloy (Class D) materials. They are generally in the form of a salt (sodium chloride) or powdered copper metal and graphite-based powder. Fire in a combustible metal causes the salt to cake and form a crust over the burning material. This transformation acts to smother the metal, thereby excluding oxygen and suppressing vaporization. Salts are preferred for the extinguishment of magnesium, uranium, and powdered aluminum, sodium, potassium, and sodium/potassium alloys. Powdered copper metal and graphite-based powder are preferred for fires involving lithium and lithium alloys.
Labeling and Rating of Portable Fire Extinguishers
Fire extinguishers are labeled for quick identification of which classification of fire they will be effective on. The label shows both recommended and unacceptable uses for the extinguisher.
Also found on the label is the Underwriters Laboratories (UL) numerical rating. The system is divided into Class A and Class B ratings only. The rating system helps the user determine and compare the relative extinguishing effectiveness of portable fire extinguishers. A basic breakdown of the rating system is as follows.
The A rating is a water equivalent rating. The number 1 preceding the letter A is equal to 1.25 gallons of water. Numbers greater than 1 are multiplied by 1.25. For example, a 2A rating is equal to an extinguishing capacity of 2.5 gallons of water.
The B rating is an area of coverage equivalent rating. The number 1 preceding the letter B is equivalent to 1 square foot of coverage. Numbers greater than 1 are multiplied by 1. For example, a 20B rating is equivalent to 20 square feet of coverage.
Note: There is no numerical rating for Class C, Class D, and Class K fire extinguishers.
Washington Virtual Academies
Tuition-free online school for Washington students.
- Coats and Car Seats: A Lethal Combination?
- Kindergarten Sight Words List
- Signs Your Child Might Have Asperger's Syndrome
- Child Development Theories
- 10 Fun Activities for Children with Autism
- Social Cognitive Theory
- Why is Play Important? Social and Emotional Development, Physical Development, Creative Development
- GED Math Practice Test 1
- Problems With Standardized Testing
- The Homework Debate