Section 2 Heating element.ppt

Section 2 Heating element 1 Introduction ① Features of heating element 1) Resistivity Dimension: L : m; A : mm2; ? : ?·mm2/m. L : cm; A : cm2; ? : ?·cm. 1 ?·cm= 104 ?·mm2/m 2) Temperature coefficient of resistance where: ρ20—resistivity at 20℃; ρt — resistivity at t ℃; α — temperature coefficient of resistivity. 3) Highest working temperature （maximum allowable temperature) 4) Chemical stability ( and physical) ② Variety and characteristics 1) Metal heating element (1) Nickel-Chromium element common brand : Cr20Ni80 , Cr15Ni60 large ? and small α. Supplementary explanations : erosion resistant layer (Q: Why Al can resist oxidation) expensive (2) Iron-chromium-aluminium element Common brand : OCr25Al5 (C0.06%), Cr13Al4 Characteristics: large ? and small α, erosion protection A) brittle after cooling; B) cheap. (3) refractory metal element Refractory metal element is made of : tungsten (W) or molybdenum (Mo) or tantalum (Ta). Characteristics: high temperature, large α, expensive, unstable Working condition: under vacuum or protection atmosphere. 2) Non-metal heating element (1) Silicon carbide (SiC) element Q: heating zone,cooling point Features: high T, used in air Aging process: A. Resistance increases with time B. The higher the T, the faster the aging process (2) Compound of molybdenum and silicon High T, used in air (oxidation protection) (3) Graphite Advantages : Very high operating temperature Easy machining Low cost Operating condition: vacuum or protection atmosphere (nitrogen, ammonia; vacuum electric furnace) 3) The shapes of heating elements for installation 1) wire The cross section is round. 2) Strip The cross section is rectangular . Installation: fastening on the side wall (lining), bottom or top. 4) Connection to power supply P 25kW : single-phase, in series or parallel 25kW P 75kW: three-phase, “Y” or