RADIANT HEATING WITH INFRARED - Watlow

D. The required oven length can now be determined.
Oven length x width x 20.8 W/in 2 = watts required = 42,372 watts
Oven length = 42,372 watts
Oven length = 34 in
5 ft x 12 in x 20.8W/in 2
ft
Comments/Questions
What should the heater wattage be?
Figure 18 shows that a RAYMAX 1120 panel requires approximately 17 W/in 2
to achieve 1000°F in open air. In the actual application, a slightly lower
wattage is required, since some of the radiated energy is reflected and reradiated
back into the heater. Since temperature controllers will be used to
maintain the required heater temperature, any excess power will not be used
or wasted and a higher wattage provides room for adjustment. However, a
grossly over-powered system will require unnecessarily high amperage
service, large switching devices, and large gauge lead wire.
If large amounts of water or solvent are being dried from an object, it is
necessary to vent air through the oven to carry the vapors away. The density
of flammable vapors, in particular, must be carefully controlled to prevent
explosions or fire. Substantial amounts of vented or forced air flow through the
oven will convect heat away from the heaters and product, and additional
wattage will be required.
What if the oven user could not accommodate an oven length of
35 inches?
Higher temperature and power heaters could be used to shorten the oven, but
temperature and speed control become more critical. Our calculations
indicate that almost all of the power used goes into vaporizing the water. Once
the water is gone, the temperature of the cloth will rise rapidly.
Example # 3: Heating a Semi-Transparent Plastic Sheet
A thermoforming process requires that 0.003 inch thick PVC be heated to
330°F. A 48 inch X 48 inch piece of material will be indexed between top and
bottom heater banks, spaced six inches away. Our goal is to heat the plastic
as efficiently as possible and determine the time required to do this.
1. Collect the data, make assumptions.
-∆T Plastic = 330°F - 60°F = 270°F
-Specific heat plastic* = 0.25 BTU/lb °F
-Weight of plastic* = 90lb/ft 3 X 1ft 3 /1728 in 3 X 0.003 inch
thick = 0.000156lb/in 2
-Heat up time = to be determined
In addition to the above information, a transmission spectrum for PVC is
needed.
Watlow recommends consulting
NFPA Bulletin 56A from the National
Fire Protection Association in
Batterymarch, Massachusetts, for
precautions in avoiding fire hazards,
flammability and proximity hazards,
and ventilation requirements.
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