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W A T L O W

Application Guide catalog - 7347 Application Guide

Electric Heaters

Safety Factor Calculation

Review of Heater ApplicationFactors

Heaters should always be sized for ahigher value than the calculated figure, often referred to as adding in a safety factor.Generally speaking, the fewervariables and outside influences—the smaller the safety factor.Here are some general guidelines:•10 percent safety factor for largeheating systems or when there are very few unknown variables.•20 percent safety factor for small tomedium heating systems where you are not 100 percent sure you have accurate information.•20 to 35 percent for heatingsystems where you are makingmany assumptions.section of the Watlow Heaters catalog,page 262, to ensure that the sheath material and watt density ratings are compatible with the liquid being heated.Immersion heaters used in tanksshould be mounted horizontally nearthe tank bottom to maximize convec-tive circulation. However, locate the heater high enough to be above anysludge build-up in the bottom of thetank. Vertical mounting is not recommended.The entire heated length of the heater should be immersed at alltimes. Do not locate the heater in arestricted space where free boiling or a steam trap could occur.Scale build-up on the sheath andsludge on the bottom of the tank mustbe minimized. If not controlled they will inhibit heat transfer to the liquidand possibly cause overheating andfailure.Extreme caution should be taken notto get silicone lubricant on the heatedsection of the heater. The silicone willprevent the “wetting” of the sheath by the liquid, act as an insulator, andpossibly cause the heater to fail.
Electric Heaters Continued Heater Life Requirements Heaters utilizing lower temperature insulating materials (silicone rubber and mica) have life limiting factors associated with exceeding the temperature limits of the insulationand with thermal cycling. Flexibleheaters and mica strip and band heaters must be properly sized andcontrolled to minimize the temperature swings during thermal cycling of the elements. Temperature Thermal Cycling The higher the temperature, theshorter a heater’s service life. Mineral insulated heaters using traditional alloys for resistance elements are subject to the life limiting factor of wire oxidation. The winding wireoxidizes at a rate proportional to the element temperature. If the elementtemperature is known it is possible toproject a heater life as shown on the table in Reference 2. Excessive thermal cycling willaccelerate heater failure. The worst cycle rate is one which allows full expansion and full contraction of the heater at a high frequency(approximately 30 to 60 seconds onand off).Prevent excessive cycling by usingsolid state relays (SSRs) or SCRpower controllers. If using SSRs, set the temperature controller’s cycle time to one second. If using SCR power controllers (like Watlow’s DIN-A-MITE Below are the estimated life expect-ancies for mineral insulated heater types: FIREROD
® , FIREBAR
® ,Tubular, MI Cable, MI Strip, MI Band. Ref. 2
Internal Element Temperature Approximate °C (°F) Life 815(1500)3 1 â�„ 2 yrs.870*(1600)*1 yr. (2000 hrs.)925(1700)4 mos.980(1800)1 ® ), be sure to use the variabletime base, burst-firing version.
1 For Immersion Heaters
� 2 mos.1040(1900)2 wks.1095(2000)1 wk.1150(2100)2 days Use the Corrosion Guide, page144,and the Selection Guides in theTubular Elements and Assemblies
*Application charts and operating recommen-dations use maximum 870°C (1600°F) internaltemperature to insure expected heater life greater than one year. 21