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Application Guide

Electric Heaters

Equation 3D—Combined Convection and Radiation Heat Losses

Power Calculations

Q
L4 = A
• F
SL Continued Q
L4 =Surface Heat Losses Combined Convection and Radiation (Wh)A=Surface Area (in Combined Convection and Radiation Heat Losses Equation 3E—Total Losses
2 )F
SL =Combined Surface Loss Factor at Surface Temperature (W/in
2 )This equation assumes a constant surface temperature. Some curves in Reference 139 (page155) combine both radiation and convection losses. This saves you from having to use both Equations3B and 3C. If only the convection component is required, then the radiation component must be determined separately and subtractedfrom the combined curve. Q Total Heat Losses The total conduction, convection andradiation heat losses are summedtogether to allow for all losses in thepower equations. Depending on the application, heat losses may make uponly a small fraction of total powerrequired... or it may be the largest portion of the total. Therefore, do not ignore heat losses unless previousexperience tells you it’s alright to do.
L = Q
L1 + Q
L2 + Q
L3 If convection and radiation losses are calculated separately. (Surfaces are not uniformly insulatedand losses must be calculated separately.)ORQ
L = Q
L1 + Q
L4 If combined radiation and convection curves are used.(Pipes, ducts, uniformly insulated bodies.) Equations 4 and 5 S tart-Upand Operating Power Required Equation 4—Start-Up Power (Watts) Both of these equations estimaterequired energy and convert it topower. Since power (watts) specifiesan energy rate, we can use power to select electric heater requirements. Both the start-up power and the operating power must be analyzedbefore heater selection can take place. P
A + Q
C s =Q +2(Q 3Q
L )
• (1 + S.F.)t
s A =Heat Absorbed by Materials During Heat-Up (Wh)Q
C =Latent Heat Absorbed During Heat-Up (Wh)Q
L =Conduction, Convection, Radiation Losses (Wh)S.F.=Safety Factor t
s =Start-Up (Heat-Up) Time Required (hr)During start-up of a system the losses are zero, and rise to 100 percent atprocess temperature. A good approximation of actual losses is obtained when heat losses (Q
L ) are multiplied by
2 �
3 . Equation 5—Operating Power (Watts) P
B + Q
D o =Q +(Q
L )
• (1 + S.F.)t
c Q
B =Heat Absorbed by Processed Materials in Working Cycle (Wh)Q
D =Latent Heat Absorbed by Materials Heated in Working Cycle (Wh)Q
L =Conduction, Convection, Radiation Losses (Wh)S.F.=Safety Factor t
c =Cycle Time Required (hr) 18