Application Guide

Electric Heaters

Equation 1

Power Calculations—Conduction and Convection Heating

Q
A orQ
B =w
• C
p • ∆ T3.412Q
A =Heat Required to Raise Temperature of Materials During Heat-Up (Wh)Q Equation 1—Absorbed Energy, Heat Required to Raise theTemperature of a Material
B =Heat Required to Raise Temperature of Materials Processed in Working Cycle (Wh)w=Weight of Material (lb)C Because substances all heat differ-ently, different amounts of heat are required in making a temperature change. The specific heat capacity of a substance is the quantity of heat needed to raise the temperature of aunit quantity of the substance by one degree. Calling the amount ofheat added Q, which will cause achange in temperature
p =Specific Heat of Material (Btu/Ib
• °F) ∆ T to a weightof substance W, at a specific heat of material C ∆ T=Temperature Rise of Material (T
Final - T
Initial )(°F)This equation should be applied to all materials absorbing heat in theapplication. Heated media, work being processed, vessels, racks, belts, and ventilation air should be included. Example: How much heat energy is needed to change the temperature of 50 lbs of copper from 10°F to 70°F?Q=w
• C
p • ∆ T=(50 lbs)
• (0.10 Btu/Ib
• °F)
• (60°F) = 88 (Wh)3.412
p , then Q =w
• C
p • ∆ T.Since all calculations are in watts, an additional conversion of 3.412 Btu = 1 Wh is introduced yielding: Equation 2 Q Equation 2—Heat Required to Melt or Vaporize a Material
C or Q
D =w
• H
f • H
v In considering adding heat to asubstance, it is also necessary toanticipate changes in state that might occur during this heating such asmelting and vaporizing. The heatneeded to melt a material is known as the ORw 3.4123.412Q latent heat of fusion andrepresented by H
C =Heat Required to Melt/Vaporize Materials During Heat-Up (Wh)Q
D =Heat Required to Melt/Vaporize Materials Processed in Working Cycle (Wh)w=Weight of Material (lb)H
f . Another statechange is involved in vaporization and condensation. The latent heat ofvaporization
f =Latent Heat of Fusion (Btu/Ib)H H
v =Latent Heat of Vaporization (Btu/lb)
v of the substance isthe energy required to change a sub- stance from a liquid to a vapor. Thissame amount of energy is released asthe vapor condenses back to a liquid. Example: How much energy is required to melt 50 lbs of lead?Q = w
• H
f Q= (50 lbs)
• (9.8 Btu/Ib) = 144 (Wh)3.412 Btu/(Wh)Changing state (melting and vaporizing) is a constant temperature process.The C
p value (from Equation 1) of a material also changes with a change instate. Separate calculations are thus required using Equation 1 for the material below and above the phase change temperature. 16