2.2 Modes of Heat Transfer

Heat Transfer Fundamentals

In a structural fire, heat is the primary driver of fire growth, structural degradation, and fire spread. Heat is defined as thermal energy in transition, moving from a region of higher temperature to a region of lower temperature. Firefighters must understand how heat moves within and between structures to predict fire behavior, protect structural integrity, and implement effective exposure protection. Heat transfer occurs through three primary mechanisms: conduction, convection, and radiation. Direct flame contact is often considered a fourth mechanism, though it is essentially a combination of conduction and convection occurring at the flame boundary.

Conduction

Conduction is the transfer of heat within a solid body or between two solid bodies in direct physical contact. At the molecular level, heat energy causes atoms to vibrate. In a conductive material, these vibrations are passed from molecule to molecule, moving heat from the hotter area to the cooler area.

The rate of conduction depends on the thermal conductivity of the material. Metals are excellent conductors of heat because their molecular structure allows rapid energy transfer. Conversely, materials like wood, masonry, concrete, and fiberglass insulation are poor conductors (insulators) because they transfer heat very slowly.

On the fireground, conduction is a common cause of hidden fire extension. Examples of conductive heat transfer include:

• Structural Steel Beams: A steel beam exposed to fire in one room can conduct heat through a masonry wall, raising the temperature of wood joists or contents in the adjacent room to their ignition point.
• Metal Plumbing Pipes: Copper or steel pipes can conduct heat from a fire room through wall assemblies, igniting wooden studs, paper backing on drywall, or insulation inside utility chases several feet away.
• Drywall Screws and Nails: Metal fasteners holding gypsum board to wooden studs can conduct heat into the wood, causing localized pyrolysis and ignition of the wall framing.

Convection

Convection is the transfer of heat energy by the movement of a heated fluid, which can be either a liquid or a gas. In structural fires, convection is the dominant mechanism of heat transfer inside the building and is responsible for the rapid vertical spread of fire and smoke.

Convection relies on the physical principle of buoyancy. As air and fire gases are heated by the fire, they expand, become less dense than the surrounding cool air, and rise. This upward movement creates a thermal column (or plume). When the rising hot gases encounter a horizontal barrier, such as a ceiling, they spread outward in all directions, creating a horizontal flow of hot gases known as a ceiling jet.

On the fireground, convection drives the movement of hot smoke and toxic gases throughout the structure:

• Vertical Shafts and Stairwells: Hot gases and smoke rise through vertical pathways like open stairwells, elevator shafts, laundry chutes, and utility pipe chases, exposing upper floors to extreme heat and initiating fire growth far above the original fire room.
• Mushrooming: When rising smoke and hot gases reach the top of a vertical shaft or the ceiling of a room and cannot escape, they spread horizontally and begin to bank down, filling the space from the ceiling downward.
• Flow Paths: Convective heat transfer is heavily influenced by ventilation. The movement of smoke and heat from high-pressure areas (near the fire) to low-pressure areas (inlet/outlet openings like doors and windows) establishes a flow path. Firefighters operating in this flow path can be exposed to rapid convective heating.

Radiation

Radiation is the transfer of heat energy through space by electromagnetic waves, such as infrared light. Unlike conduction and convection, radiation does not require a physical medium (solid, liquid, or gas) to transfer heat; electromagnetic waves can travel through a vacuum or clean air.

Radiant energy travels in straight lines until it strikes an object. When these electromagnetic waves strike a solid or liquid surface, they are absorbed, reflected, or transmitted. Absorbed radiant energy is converted back into thermal energy, raising the temperature of the target material.

The intensity of radiant heat transfer is highly dependent on:

• Temperature of the Heat Source: The rate of radiation increases by the fourth power of the absolute temperature ($T^4$) of the source. Even a small increase in fire temperature results in a massive increase in radiant heat energy emitted.
• Distance: Radiant energy decreases with the square of the distance from the source. Doubling the distance from a fire reduces the radiant heat received to one-quarter of its original intensity.
• Surface Characteristics: Dark, rough surfaces absorb radiant heat more effectively than light, shiny, or reflective surfaces.

Radiant Heat Transfer Between Non-Touching Buildings (Exposures)

On the fireground, radiation is the primary mechanism responsible for exposure fires—the spread of fire from the involved structure to neighboring, non-touching buildings across alleys, yards, or streets.

When a building is heavily involved in fire, huge amounts of radiant energy are emitted through window openings and burning walls. This energy travels across the intervening open space. If a neighboring exposure is close enough, the radiant heat raises the temperature of its exterior walls (such as vinyl or wood siding) and window frames. Vinyl siding may melt, wood may pyrolyze, and window glass may crack and fall out. Once the temperature of the exposure's combustible components reaches their autoignition temperature, or if unburned gases inside the exposure ignite, the fire extends.

Exposure Protection Tactics

Protecting adjacent exposures from radiant heat is a high-priority tactical objective. Firefighters must understand how to interrupt radiant heat transfer:

1. Direct Surface Wetting: The most effective method of exposure protection is applying water directly onto the exterior surfaces of the exposed building. Wetting the siding, window frames, and roof keeps their temperatures below their ignition points. The water absorbs the radiant energy and evaporates, shielding the structure.
2. Water Screens: Setting up master streams or high-volume handlines to spray water into the space between the burning building and the exposure. However, water curtains sprayed into thin air are relatively inefficient at blocking radiant waves; the water must form a dense, thick screen of droplets to scatter and absorb the radiation, and direct surface application should always run concurrently.
3. Closing Openings: Ensuring that doors and windows on the exposure building remain closed to prevent radiant heat from entering the interior and igniting curtains or furniture.
4. Apparatus Placement: Positioning fire apparatus and equipment outside the direct line of radiant heat to prevent damage to paint, tires, and warning lights.