1) Polycarbonate visors can melt at temperatures lower than their melting point due to radiant heat transfer. While the air temperature inside a hot fire container may be 300°C, radiant heat from hot surfaces transfers more energy and causes higher surface temperatures on the visor than measured by thermometers. 2) Radiant heat is infrared energy that cannot be measured by thermometers and adds heat beyond what is transferred by convection from hot air alone. Even when air temperature is below the melting point, radiant heat from very hot surfaces inside containers can raise visor surface temperatures high enough to melt polycarbonate. 3) Proper functioning of a DV helps cool visors by convection from

1. Heat

2. Radiant Heat
Scott User Group 2010

3. A Most Common Question
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.

4. Various Types of Heat
• Convected heat
• Conducted heat
• Radiant heat
Convection 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

5. In a Hot Fire Container
• 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.

6. Measurement of Radiant Heat
• 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).

7. Radiant Energy
Radiant energy that strikes a
surface can be
1) Reflected
2) Absorbed
3) Transmitted
(Semitransparent material)

8. • 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.

9. • 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.

10. Radiation Intensity Level of Damage
(kW/m2)
37.5 Sufficient to cause damage to process equipment
Minimum energy required to ignite wood at indefinitely long
25
exposure
Minimum energy required for piloted ignition of wood, and
12.5 melting of plastic tubing. This value is typically used as a
fatality number
Sufficient to cause pain in 8 seconds and 2nd degree burns in
9.5
20 seconds
Sufficient to cause pain in 20 seconds. 2nd degree burns are
5 possible. 0 percent fatality. This values often used as an
injury threshold.
1.6 Discomfort for long exposures

11. Thermal Dose Unit
• 1 TDU = 1 (kW/m2)4/3s.
Level of Exposure
Result
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

12. A Most Common Question
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.

13. In a Hot Fire Container
• 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.

14. 3 States of Matter
Gas = X atoms per Cm3
Liquid = 1000X atoms cm3
Solid = 2000X atoms Cm3
Ratio of 1:1000:2000

15. Air Temperature
• 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

16. Visors
Normal Cold Hot (maybe Warm Warm with
melting) (not melting) very hot
surface
Heat Transfer Mechanisms
Cold
Convected Radiant Heat
Air
heat from
from
surroundings
DV

17. Normal
Inside mask Outside mask

18. DV Working - Normal
Outside Temp
Cold Air
from DV

19. DV Not Working - Heat
(Visors Heats Up)
Convected
heat from
surroundings
(ie 300oC air)

20. Question 1
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.

21. DV Working - Heat
(Visor heats up but
DV air also cools
visor)
Convected
Cold Air heat from
from DV surroundings

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

23. DV Working - Heat
(Visor heats up but DV air also
cools visor)
Radiant heat added which
increases visor surface
temperature dramatically