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Application Guide
Electric Heaters
Power Calculations— Radiant heating
When the primary mode of heat trans­fer is radiation, we add a step after Equation 5.
Equation 6 is used to calculate the net radiant heat transfer between two bodies. We use this to calculate either the radiant heater temperature required or (if we know the heater temperature, but not the power required) the maximum power which can be transfered to the load.
Equation 6—Radiation Heat Transfer Between Infinite Size Parallel Surface
(Tt-
T2)
Pr A
Pr
A
S
Ti(°R) T2(°R) et F
S
F
e
o
o
(144 in2/ft2) (3.412 Btu/Wh)
Power Absorbed by the Load (watts) - from Equation 4 or 5
Area of Heater (in2) - known or assumed
Stephan Boltzman Constant
0.1714 • 10-8(Btu/Hr. Sq. Ft. °R4)
Emitter Temperature (°F + 460)
Load Temperature (°F + 460)
Emissivity Correction Factor - see below
Shape Factor (0 to 1.0) - from Reference 139, page 155
CD B>
CD
S
es = Heater Emissivity (from Material Emissivity Tables)
eL = Load Emissivity (from Material Emissivity Tables)
Ds = Heater Diameter
DL = Load Diameter
Emissivity Correction Factor (ef)
■1 + 1 -es e|_
Plane Surfaces
Concentric Cylinders Inner Radiating Outward
Concentric Cylinders Outer Radiating Inward
et
et :
es Dl
et = — + f es
Power Evaluation
After calculating the start-up and operating power requirements, a comparison must be made and various options evaluated.
Shown in Reference 1 are the start-up and operating watts displayed in a graphic format to help you see how power requirements add up.
With this graphic aid in mind, the following evaluations are possible:
• Compare start-up watts to operating watts.
• Evaluate effects of lengthening start-up time such that start-up watts equals operating watts (use timer to start system before shift).
Comparison of Start-Up and Operating Power Requirements Ref. 1
10 -
9 -8 -7 -6 -5 -4 -
10% Safety Factor
Losses Conduction 2/3 of {Convection Radiation
i0% Satety Factor
S.F
(Ql)
2"
ÌT
Operating Losses Conduction Convection Radiation
Initial Heat to Melt or Vaporize
(Ql)
S. 4-
(Qc)
3
Process Heat—Melting or Vaporizing
(Qd)
Initial Heat to Raise Equipment and Materials
Operating Process Heat to Raise Product from T1 to T2
(Qb)
(Qa)
Start-Up
Process
19
Time (Hours)
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Pages:
1-50
51-100
101-150
151-200
201-216
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