Heater Catalog - Watlow

Reference DataPower CalculationsConduction and Convection HeatingTotal Heat LossesThe total conduction, convection and radiation heatlosses are summed together to allow for all losses in thepower equations. Depending on the application, heatlosses may make up only 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 is alright to do.Equation 3E—Total LossesQ L = Q L1 + Q L2 + Q L3 If convection and radiation lossesare calculated separately.(Surfaces are not uniformlyinsulated and losses must becalculated separately.)ORQ L = Q L1 + Q L4 If combined radiation andconvection curves are used.(Pipes, ducts, uniformly insulatedbodies.)Start-Up and Operating Power RequiredBoth of these equations estimate required energy andconvert it to power. Since power (watts) specifies anenergy rate, we can use power to select electric heaterrequirements. Both the start-up power and the operatingpower must be analyzed before heater selection can takeplace.Equation 4—Start-Up Power (Watts)PS = Q A + Q C 2t S 3(QL) x (1 + S.F.)Q 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 factort S = start-up (heat-up) time required (hr)During start up of a system the losses are zero, andrise to 100 percent at process temperature. A goodapproximation of actual losses is obtained when heatlosses (Q L ) are multiplied by 2 /3.Equation 5—Operating Power (Watts)Po = Q B + Q D+ (QL ) x (1 + S.F.)t CQ B = heat absorbed by processed materials in workingcycle (Wh)Q D = latent heat absorbed by materials heated inworking cycle (Wh)Q L = conduction, convection, radiation losses (Wh)S.F. = safety factort C = cycle time required (hr)546 WATLOW ®