Posted by byager | Fire and Rescue, General, Health, PPE, Performance, Safety
Thursday, July 1st, 2010 7:07 am

How fire affects protective clothing

Fires are dangerous and unpredictable. Temperatures can reach 2000° F in a matter of seconds. The following describes the various heat conditions present in a fire and their effect on protective clothing.

Types of heat transfer: There are three types of heat transfer in a fire that could cause burns: conduction, convection and thermal radiation. Conduction is the direct transfer of heat through contact with the hot object. Convection is the transfer of heat through a medium; for example, air. Thermal radiation is the transfer of heat in the form of light energy. Fire fighters experience all three types of heat in a fire.

Conduction: Wet or compressed protective clothing increases the risk of being burned by conductive heat. Water is a very poor insulator. It can create a conductive bond between surfaces that might not otherwise touch. This increases the potential of heat conduction by displacing the insulating air between and within the layers of clothing. Water conducts heat with dangerous and unpredictable efficiency. Conductive heat transfer burns can be caused by contacting heated surfaces or objects. Serious conductive heat burns can result by compressing parts of your protective clothing and exposing yourself to too much heat. Compression brings surfaces closer together and displaces air. This results in the transfer of heat between outside surfaces and clothing layers. An example of this type of injury is the blistering that occurs on knees while crawling on hot surfaces, or where the SCBA straps have squeezed the surrounding fabric against the skin. Another common compression injury occurs (even without contacting a hot object) when the fire fighter’s forearm is extended toward the heat source while holding a hose.

Convection: Convected heat travels through the air, even if there is no immediate appearance of fire or the fire seems distant. Convected heat can elevate the temperature of your protective clothing to a point at which conductive heat burns can easily occur, particularly if your protective clothing is wet or damp.

Thermal radiation: Thermal radiation is the transfer of heat in the form of light energy into a material, directly from flames or reflected from hot objects. Factors that affect the speed of radiant heat transfer include the temperature difference between two surfaces, their distance from each other, and the reflectiveness of each surface. Radiant heat becomes stored energy as it strikes the surfaces of your gear. As it grows in intensity, it transfers heat inward from the outer (shell) layer. Injuries from radiant heat transfer can be experienced at temperatures as low as 180° F.

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