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IFE Unit 1: Fire Engineering Science > Heat Transmission > Flashcards

Flashcards in Heat Transmission Deck (4)
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1

Conduction

Mostly present in solids (can occur in all states).
Heat energy is passed through molecules from their nearest neighbours with heat flowing away from source to low temp areas.
Molecules vibrate about a position and pass energy by collisions.
Thermal conductivity varies between materials.
Best conductors are silver and copper.
Non metallic solids are poor conductors.
Good conductors of heat are usually good conductors of electricity too.
Thermal conductivity is measured experimentally and denoted by symbol K.
Flow of heat is measured in Joules per Second (J/s) and the unit is Watt therefore 1J/s is 1 Watt.
Steel girders can cause heat and fire travel between compartments.
But wooden doors are good barriers as although it may burn, it is a poor conductor of heat.

2

Convection

This only occurs in liquids and gases e.g. when a pan of water is heated e.g. it is heated from the bottom on a gas ring and as the water warms up, it expands and becomes less dense and so the given volume is lighter.

The heated liquid is buoyant it rises, and colder liquid takes it place at the bottom. This then becomes heated and moves to the top. Heat energy is carried by the molecules as they move until all of the water is the same temp. In comparison to conduction where the molecules do not move from their position too much. In convection it is the movement of the molecules that causes the heat transmission.

Convection is used in domestic hot water systems and in heating systems using radiators. Most heat from radiators is carried away using convection.

3

Convection Continued

Convection also causes the updraft in chimneys.

If hot gases leave from the top of a house, cool air must enter from the bottom to replace them which will aid in the combustion process.

4

Convective Heat Transfer

The transfer of heat between a fluid (liquid or gas) to a solid.
e.g. a hot object in air loses heat partly by convection. The layer of air next to the hot surface becomes heated and therefore buoyant in comparison to the surrounding cooler air. It rises and carries the heat away and this hot air is replaced by cooler air.
This then becomes heated, the heat moves away and a convection current is set up.