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You may also be interested in
Temperature limiter, Heater, Cartridge heater, Coil heater, Temperature controller
Text version of the page
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Application Guide
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Electric Heaters
Power Calculations
Continued
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Equation 3A—Heat Required to Replace Conduction Losses
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QL1
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k • A • AT • te
3.412 • L
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Conduction Heat Losses
Heat transfer by conduction is the contact exchange of heat from one body at a higher temperature to another body at a lower temperature, or between portions of the same body at different temperatures.
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QL1= Conduction Heat Losses (Wh) k = Thermal Conductivity
(Btu • in./ft2-°F • hour) A = Heat Transfer Surface Area (ft2) L = Thickness of Material (in.) AT = Temperature Difference Across Material
(T2-T1) °F te = Exposure Time (hr)
This expression can be used to calculate losses through insulated walls of containers or other plane surfaces where the temperature of both surfaces can be determined or estimated. Tabulated values of thermal conductivity are included in the Reference Data section (begins on page 134).
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X
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Convection Heat Losses
Convection is a special case of conduction. Convection is defined as the transfer of heat from a high temperature region in a gas or liquid as a result of movement of the masses of the fluid. The Reference Data section (page 127) includes graphs and charts showing natural and forced convection losses under various conditions.
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Equation 3B—Convection Losses
Ql2 = A ^Fsl^Cf
QL2= Convection Heat Losses (Wh)
A = Surface Area (in2)
FSL= Vertical Surface Convection Loss Factor (W/in2) Evaluated at Surface Temperature (See Ref. 9, page 26)
CF = Surface Orientation Factor
Heated surface faces up horizontally Vertical
Heated surface faces down horizontally
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1.29
1.00 0.63
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Radiation Heat Losses
For the purposes of this section, graphs are used to estimate radiation losses. Charts in the Reference Data section (page 127) give emissivity values for various materials. Radiation losses are not dependent on orientation of the surface. Emissivity is used to adjust for a material's ability to radiate heat energy.
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Equation 3C—Radiation Losses
Ql3 =A ^ Fsl ^ e
QL3 = Radiation Heat Losses (Wh) A = Surface Area (in2)
FSL = Blackbody Radiation Loss Factor at Surface Temperature (W/in2) e = Emissivity Correction Factor of Material Surface
Example:
Using Reference 139, page 155, we find that a blackbody radiator (perfect radiator) at 500°F, has heat losses of 2.5 W/in2.
Polished aluminum, in contrast, (e = 0.09) only has heat losses of 0.22 W/in2 at the same temperature (2.5 W/in2 • 0.09 = 0.22 W/in2).
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