Application Guide catalog - Watlow - #19

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.

(Tt-

T2)

Pr A

Pr

A

S

Ti(°R) T2(°R) et F

S

F

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

e_s = Heater Emissivity (from Material Emissivity Tables)

e_L = Load Emissivity (from Material Emissivity Tables)

D_s = Heater Diameter

D_L = Load Diameter

■1 + ¹ -es e|_

Plane Surfaces

Concentric Cylinders Inner Radiating Outward

Concentric Cylinders Outer Radiating Inward

et

et ^:

es Dl

et = — + ^f es

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).

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)

(Qc)

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)

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