Heat and Temperature Study Guide for McGraw-Hill's Firefighter Exams

Celsius Degree

The Celsius (C) degree is a metric unit of temperature measurement. It is named for the Swedish professor Anders Celsius, who invented the Centigrade temperature scale in the 1720s using the freezing point of water as 0 degrees and the boiling point of water as 100 degrees. This unit is approved by the SI.

To convert (approximately) normal body temperature on the Centigrade scale to the Fahrenheit scale, first multiply the Celsius temperature by 1.8, or 9/5, and then add 32.

Rankine Degree

The Rankine (R) degree is a traditional unit of absolute temperature. The temperature units for Rankine and Fahrenheit are equal (1 degree Rankine represents the same temperature difference as 1 degree Fahrenheit), but the zero points differ. The zero point on the Rankine scale is set at absolute zero, which is –457.6 degrees, the hypothetical point at which all molecular movement ceases. The unit is named for British physicist and engineer William Rankine (1820–1872).

To convert degree units from the Rankine scale to the Fahrenheit scale and the Fahrenheit scale to the Rankine scale use the following formulas:

F = R – 457 and R = F + 457

Kelvin Degree

The Kelvin degree (K) is equal to the Celsius degree, but the Kelvin scale has its zero point set at absolute zero, which is –273.1. This unit is approved by the SI. The Kelvin degree is named for British inventor and scientist William Thompson, who was knighted by Queen Victoria in 1866 and named Baron Kelvin of Largs in 1892.

To convert degree units from the Kelvin scale to the Centigrade scale and the Centigrade scale to the Kelvin scale, use the following formulas:

C = K – 273 and K = C + 273

Heat Transfer

Heat can be transferred to other materials through conduction, convection, radiation, and direct flame contact.

Conduction

Conduction is the transfer of heat energy through a medium (usually a solid). Heat causes molecules within the material to move at a faster rate and transmit their energy to neighboring molecules. The heat of conduction can also be transferred from one material to another via direct contact in the same fashion as internal molecular movement. The amount of heat transferred and rate of travel is dependent on the thermal conductivity of the material. Dense materials (metals) are good conductors of heat energy. Fibrous materials (wood, paper, cloth) and air are poor conductors. In a fire situation, heat can be conducted via steel columns and girders to abutting wood floor joists causing them to smolder and eventually ignite.

Convection

Convection is the transfer of heat energy through a circulating medium (liquids and gases). During firefighting operations, hot air expands and rises, as do the products of incomplete combustion. Fire spread by convection is mostly in an upward and outward direction through corridors, stairwells, and shafts from floor to floor via hot air currents.

Radiation

Radiation is the transfer of heat via infrared or ultraviolet waves or rays. These heat waves travel in a straight line through space at the speed of light in all directions and are not affected by the wind. Objects exposed to radiated heat will absorb and reflect a certain amount of heat energy, depending on certain factors. The darker and duller the object, the more heat it will absorb and the greater chance it will reach its ignition temperature and burst into flames. Light-colored, shiny objects tend to reflect radiated heat, absorb less energy, and are less likely to reach their ignition temperature. Radiated heat waves will travel through space until they are absorbed by an opaque object. These waves will pass through air, glass, transparent plastics, and water. Large amounts of radiated heat can travel large distances (50–100 feet) to ignite nearby buildings and structures.

Direct Flame Contact

Direct flame contact is the transfer of heat energy via direct flame impingement or auto-exposure, such as occurs with a flame traveling upward and outward from a roof, window, or doorway to a neighboring building or exposure.