Upload
snoshoesam
View
5.862
Download
0
Tags:
Embed Size (px)
DESCRIPTION
• Explain the theories underlying combustion processes• Describe how fire researchers have identified combustion processes using a variety of different classifications• Provide a description of the stages and events of fire as it progresses from the initial stage to its final stage• Explain the causes of flame over, flashover, and backdraft and review the procedures to prevent and protect against such events• Describe the various methods by which heat and unburned gases move in a confined environment• Define the five classes of fires and explain how they are classified
Citation preview
© 2009 Delmar, Cengage Learning
Combustion Processes
Chapter 3
© 2009 Delmar, Cengage Learning
Objectives
• Explain the theories underlying combustion processes
• Describe how fire researchers have identified combustion processes using a variety of different classifications
• Provide a description of the stages and events of fire as it progresses from the initial stage to its final stage
© 2009 Delmar, Cengage Learning
Objectives (cont’d.)
• Explain the causes of flame over, flashover, and backdraft and review the procedures to prevent and protect against such events
• Describe the various methods by which heat and unburned gases move in a confined environment
• Define the five classes of fires and explain how they are classified
© 2009 Delmar, Cengage Learning
Introduction
• This chapter:– Considers physical and chemical process involved in
fire combustion and relates them to procedures of fire services to confine, control, and extinguish uncontrolled fires
– Emphasizes combustion processes as fires progress– Reviews fire classification methods, fire extinguishing
agents, and their advantages and disadvantages
© 2009 Delmar, Cengage Learning
What is Combustion?
• Planned and controlled, self-sustaining chemical reaction between fuel and oxygen with evolution of heat and light
• Differs from fire• Represented by fire tetrahedron
– Heat– Fuel– Oxygen– Chemical reaction
© 2009 Delmar, Cengage Learning
What is Combustion (cont’d.)
Figure 3-2 The new fire tetrahedron
© 2009 Delmar, Cengage Learning
Spontaneous Combustion
• Does not require independent ignition source• Material heats to piloted ignition temperature • After ignition, flames spread• Coal is an example of a porous solid material
that when heated, eventually reaches ignition temperature and combustion begins
© 2009 Delmar, Cengage Learning
Methods of Fire Classification
• Type of combustion• Rate of fire growth• Available ventilation• Type of materials that are burning• Stages or phases of a fire
© 2009 Delmar, Cengage Learning
Types of Combustion
• Three stages– Pre-combustion
• Fuel heated to ignition point
• Particulates released
• Entrainment gathers additional oxygen
• Heat energy radiated back into fuel
– Smoldering combustion– Flaming combustion
© 2009 Delmar, Cengage Learning
Smoldering Combustion
• Absence of flame• Presence of hot materials on surface where
oxygen diffuses into fuel• Two phases
– Solid– Gas
• Incompleteness creates very high levels of carbon monoxide
© 2009 Delmar, Cengage Learning
Flaming Combustion
• Encountered in most emergency incidents• Presence of flames• Gas or vapor has to be burning• Two categories
– Gaseous fuel premixed with air before ignition– Diffusive flaming
• Flames are generally yellow due to incomplete burning process
• Light and heat also emitted
© 2009 Delmar, Cengage Learning
Fire Classification by Type of Substance Burning
• Class A– Fires involving combustion of ordinary cellulosic
materials
• Class B– Fires involving flammable liquids
• Class C– Fires involving energized electrical equipment or
wires
© 2009 Delmar, Cengage Learning
Fire Classification by Type of Substance Burning (cont’d.)
• Class D– Fires involving combustible metals
• Class K– Fires involving cooking oils
• Saponification: process of chemically converting the fatty acid contained in a cooking medium (oil or grease) to soap or foam
© 2009 Delmar, Cengage Learning
Fire Classification by Stages and Events
• Fire stages:– Ignition stage– Growth stage– Fully developed stage– Decay stage
• Fire events:– Flameover or rollover– Flash over– Backdraft
© 2009 Delmar, Cengage Learning
Figure 3-7 Temperatures associated with the stages of fire and the unique fire events
© 2009 Delmar, Cengage Learning
Flame Over
• Flames travel through or across unburned gases in upper portions of confined area during fire development
Figure 3-8 Flame over/rollover
© 2009 Delmar, Cengage Learning
Flashover
• When heating is enough to bring other materials in room to ignition temperature, igniting all fuel materials in the room into flaming combustion
Figure 3-9 Flashover
© 2009 Delmar, Cengage Learning
Backdraft
• Additional oxygen entering the compartment is heated and expands
• Increased pressure inside room• Windows, walls, and weak points in the building
suddenly pushed outward• Firefighters caught in the sudden, explosive rush
of fire can be killed instantly
© 2009 Delmar, Cengage Learning
Building Construction and Fire Spread
• Efficiency declines if fires move vertically through buildings or bypass horizontal construction barriers– Pre-WWII concrete construction inhibited vertical
movement
• Post-WWII drywall spreads fires to other areas of building quickly
• Compartmentation is safe areas in high-rises
© 2009 Delmar, Cengage Learning
Fire Rating of Materials
• Building’s ability to withstand a fire differ because of:– Variations in workmanship– Methods of installation– Different sets of test methods– Sizes of test specimens
• Rated fire resistance of construction has some but not a substantial impact on the spread of fire
© 2009 Delmar, Cengage Learning
Weather Conditions
• Impact the burning characteristics of inside building fires and outside fires
• Stack effect: temperature difference between the outside temperature of building and temperature inside the building
• Windy conditions outside can impact horizontal ventilation activities
© 2009 Delmar, Cengage Learning
Relative Humidity
• Moisture in the form of water vapor• Always present• Affects amount of moisture in fuel• Impacts direction of fire gas movement
© 2009 Delmar, Cengage Learning
Mass/Drying Time
• Impacts how long it will take source of ignition to raise material to ignition temperature
• Thicker or heavier mass will take longer to raise the temperature of the material
• Law of latent heat of vaporization: heat absorbed when 1 gram of liquid transformed into vapor at boiling point under 1 atmosphere of pressure– Result in BTUs per pound or calories per gram
© 2009 Delmar, Cengage Learning
Heat Measurement
• Heat always flows from higher temperature materials to lower temperature materials
• Four temperature scales– Kelvin– Rankin– Celsius– Fahrenheit
© 2009 Delmar, Cengage Learning
Figure 3-11 Relationship among temperature scales
© 2009 Delmar, Cengage Learning
Heat Transfer
• Important in all aspects of combustion process• Responsible for continuance of combustion
process• Four methods of transfer:
– Conduction– Convection– Radiation– Direct flame impingement
© 2009 Delmar, Cengage Learning
Conduction
• Transfer of heat energy from hot to cold side of medium by means of energy transfer from molecule to adjacent molecule or atom to atom
Figure 3-12 Conduction is the transfer of heat energy from a material by direct contact between the movements of molecules of another higher energy material
© 2009 Delmar, Cengage Learning
Convection
• Movement of heat energy by agitation of air molecules– Reduces density of
molecules, making heated air lighter than cooler air
Figure 3-13Convection involvesthe transfer of heat by circulating currents
© 2009 Delmar, Cengage Learning
Radiation
Figure 3-14 Radiation is energy that travels across a space and does not need an intervening medium, such as a solid or a fluid
© 2009 Delmar, Cengage Learning
Direct Flame Impingement
Figure 3-15 Flamesdirectly impinging upon the materials transfer the heat, raising theirtemperature to thepoint where combustion occurs
© 2009 Delmar, Cengage Learning
Summary
• Combustion process defined by type, rate of fire growth, amount of ventilation, and type of substance that burns
• Classifications of fires: Class A, B, C, D, and K • Physical and chemical properties of fuels
feeding fires affect how a fire will burn, spread, and quickness of burning rate