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W A T L O W

Application Guide catalog - 7347 Application Guide

Electric Heaters

Short Method

Power Calculations

Start-up watts = A + C +
2 �
3 L + Safety FactorOperating watts = B + D + L + Safety FactorSafety Factor is normally 10 percent to 35 percent based on application.A=Watts required to raise the temperature of material and equipment to the operating point, within the time desired B=Watts required to raise temperature of the material during the working cycle Calculations for Required Heat Energy
Electric Heaters When performing your own calcu-lations, refer to the Reference Data section (begins on page127) forvalues of materials covered by theseequations.The total heat energy (kWH or Btu)required to satisfy the system needs will be either of the two values shown below depending on which calculatedresult is larger.A.Heat Required for Start-Up B.Heat Required to Maintain theDesired TemperatureThe power required (kW) will be theheat energy value (kWH) divided bythe required start-up or working cycletime. The kW rating of the heater willbe the greater of these values plus asafety factor.The calculation of start-up and oper-ating requirements consist of several distinct parts that are best handledseparately. However, a short method can also be used for a quick estimateofheatenergyrequired. Bothmethodsare defined and then evaluated using the following formulas and methods: Equation for A and B (Absorbed watts-raising temperature) Specific heatWeight of material (lbs)
• of material
• temperature rise (°F)(Btu/lb
• °F)Start-up or cycle time (hrs)
• 3.412C=Watts required to melt or vaporize material during start-up periodD=Watts required to melt or vaporize material during working cycle Equation for C and D (Absorbed watts-melting or vaporizing) Weight of material (lbs)
• heat of fusion or vaporization (Btu/lb)Start-up or cycle time (hrs)
• 3.412L=Watts lost from surfaces by:• Conduction-use equation below• Radiation-use heat loss curves • Convection-use heat loss curves Equation for L (Lost conducted watts) Thermal conductivityTemp. differentialof material or insulationSurface areato ambient(Btu
• in./ft
2 • °F
• hr)(ft
2 )(°F)Thickness of material or insulation (in.)
• 3.412 15