Heat

Radiant HeatScott User Group 2010

1) If Polycarbonate has a melting point of 200 degrees why doesn’t it melt in a 300 degree HFC.

2) If polycarbonate has a melting point of 200 degrees why did the visor melt when my HFC was only measuring 180 degrees.

A Most Common Question

Various Types of Heat• Convected heat• Conducted heat• Radiant heatConvection occurs when air or fluid passes over a heated service

Conduction is the process of heat transfer by direct contact with another surface

Radiation is heat transfer by infrared rays

• Hot air (atmosphere) – Convected heat (generally what is measured by the thermometers)

• Hot walls – Conducted heat (ie if leant against)• Radiant heat – Infrared energy from surrounding.

In a Hot Fire Container

• Infrared energy can not be measured with a with a thermometer.

• Therefore radiant heat can only be measured using calorimetres

• Rules to remember about thermal radiation: • All objects above absolute zero (-273oC) emit infrared rays in a straight line in all

directions.• Hotter objects emit more total radiation energy per unit surface area • Hotter objects emit photons with a higher average energy (which means shorter

wavelength/higher frequency).

Measurement of Radiant Heat

Radiant energy that strikes a surface can be

1) Reflected 2) Absorbed 3) Transmitted (Semitransparent material)

Radiant Energy

• Thermal radiation, even at a single temperature, occurs at a wide range of frequencies.

• The main frequency (or colour) range of the emitted radiation includes higher and higher frequencies as the temperature increases. For example, a red hot object radiates enough in the long wavelengths (red and orange) of the visible band to see, which is why it appears red. If it heats up further, it also begins to emit discernible amounts of green and blue light, and the spread of frequencies mentioned in the first point make it appear white. We then say the object is white hot. However, even at a "white-hot" temperature of 2000 K, 99% of the energy of the radiation is still in the infrared.

• The total amount of radiation, of all frequencies, goes up very fast as the temperature rises (it grows as T4, where T is the absolute temperature of the body). An object at the temperature of a kitchen oven (about twice room temperature in absolute terms: 600 K vs. 300 K) radiates 16 times as much power per unit area. An object at the temperature of the filament in an incandescent bulb (roughly 3000 K, or 10 times room temperature) radiates 10,000 times as much per unit area.

• Radiant heat can only be measured using calorimetres

• Infrared energy can not be measured with a thermometer.

• If a hot fire container is measured at 300oC that is the air temperature and not the radiant heat.

• Depending upon the heat source there could be massive amounts of radiant heat or very little.

Radiation Intensity (kW/m2)

Level of Damage

37.5 Sufficient to cause damage to process equipment

25 Minimum energy required to ignite wood at indefinitely long exposure

12.5Minimum energy required for piloted ignition of wood, and melting of plastic tubing. This value is typically used as a fatality number

9.5 Sufficient to cause pain in 8 seconds and 2nd degree burns in 20 seconds

5Sufficient to cause pain in 20 seconds. 2nd degree burns are possible. 0 percent fatality. This values often used as an injury threshold.

1.6 Discomfort for long exposures

• 1 TDU = 1 (kW/m2)4/3s.

Level of ExposureResult

Mean Range

92 86-103 Pain

105 80-130 Threshold First Degree Burn

290 240-350 Threshold Second Degree Burn

1000 870-2600 Threshold Third Degree Burn

Thermal Dose Unit

1) If Polycarbonate has a melting point of 200 degrees why doesn’t it melt in a 300 degree HFC.

1) If polycarbonate has a melting point of 200 degrees why did the visor melt when my HFC was only measuring 180 degrees.

A Most Common Question

• Hot air (atmosphere) – Convected heat (generally what is measured by the thermometers)

• Hot walls – Conducted heat (ie if leant against)• Radiant heat – Infrared energy from surrounding.

In a Hot Fire Container

Gas = X atoms per Cm3

Liquid = 1000X atoms cm3

Solid = 2000X atoms Cm3

Ratio of 1:1000:2000

3 States of Matter

• Hot Air does not contain much stored energy compared to a solid

• Therefore can not transfer much energy into the solid as for each “hot” gas atom there are 1000+ “cold” solid atoms

Air Temperature

Heat Transfer Mechanisms

Normal Cold Hot (maybe melting)

Warm(not melting)

Warm with very hot surface

Cold Air from DV

Convected heat from surroundings

Radiant Heat

Visors

Normal

Inside mask Outside mask

Cold Air from DV

DV Working - NormalOutside Temp

Convected heat from surroundings(ie 300oC air)

DV Not Working - Heat(Visors Heats Up)

1) If Polycarbonate has a melting point of 200 degrees why doesn’t it melt in a 300 degree HFC.

1) If polycarbonate has a melting point of 200 degrees why did the visor melt when my HFC was only measuring 180 degrees.

Question 1

Convected heat from surroundings

Cold Air from DV

DV Working - Heat(Visor heats up but DV air also cools visor)

1) If polycarbonate has a melting point of 200 degrees why did the visor melt when my HFC was only measuring 180 degrees.

Question 2

DV Working - Heat(Visor heats up but DV air also cools visor)

Radiant heat added which increases visor surface temperature dramatically