Content:
Suspended coils
Suspended straight wires and ribbons
Open mica elements
Zigzag elements
Porcupine elements
Coils immersed in water
The wire is suspended freely between insulated points and is exposed to the mechanical stress caused by its own weight, its own spring force and in some cases also from the forces of an external spring. Kanthal® D, Kanthal® AF, Nikrothal® 80 and Nikrothal® 60 are the most suitable materials.
Permissible D/D (Coil Diameter/Wire Diameter) Ratios As A Function Of Wire Temperature In Supported Coil Elements.
Suspended coils
Characteristics
Wire coil is supported at intervals, e.g. by ceramic holders. Fiberglass cord is often placed inside coil to prevent the coil from falling down in case of element failure.
Recommended alloys
Nikrothal® 80 and Nikrothal® 60.
Kanthal® D and Kanthal® AF (mainly for wire temperatures below 600°C, where sagging is no problem).
Surface loads
Wire: 7–8 W/cm2 in forced air, 3–4 W/cm2 in natural convection.
Typical applications
Air heaters such as: laundry dryers, fan heaters, land dryers.
SUSPENDED STRAIGHT WIRES AND RIBBONS
Characteristics
Wire or ribbon may have elastic or fixed suspension
Elastic: Wire kept straight by springs when heated.
Fixed: Operating temperature is lower and low thermal expansion is advantageous.
Recommended alloys
Kanthal® A and Kanthal® AF (low thermal expansion).
Nikrothal® 80.
Surface loads
Wire: 4–12 W/cm2 (26–77 W/in2).
Typical applications
Radiators, convection heaters, hair dryers.
OPEN MICA ELEMENTS
Characteristics
Straight or corrugated heating wire or ribbon is wound on one or both sides of a mica sheet or separated mica strips. Ribbons are frequently used in this application.
Recommended alloys
Nikrothal® 80 and Nikrothal® 60.
Kanthal® D and Kanthal® AF.
Surface loads
Wire: 4–7 W/cm2 (26–45 W/in2).
For toasters: <13 W/cm2 (<84 W/in2) for wire-wound elements.
Typical applications
Toasters, convection heating, low-watt aquarium heaters.
ZIGZAG ELEMENTS
Characteristics
Deep-corrugated ribbon is supported by mica sheets. Also radial shape.
Recommended alloys
Kanthal® D and Kanthal® AF.
Nikrothal® 40.
Surface loads
Wire: 9 W/cm2 (58 W/in2).
Typical applications
Fan heaters, convection heating.
PORCUPINE ELEMENTS
Characteristics
Heating conductor consists of hairpin-shaped wire bends protruding in all directions, with holes in the center. Element is supported by a central insulated rod or insulating tube around its circumference.
Recommended alloys
Kanthal® AF.
Kanthal® AE (below 1 mm).
Nikrothal® 80.
Surface loads
Wire: 4 W/cm2 (26 W/in2) in natural convection.
For toasters: 12 W/cm2 (77 W/in2) in forced convection.
Typical applications
Hot air guns, radiators, convectors, tumble dryers, domestic ovens with forced convection.
COILS IMMERSED IN WATER
Characteristics
Wire coils supported by a plastic cage operating directly in water.
Recommended alloys
Kanthal® D and Kanthal® AF.
Nikrothal® 80.
Surface loads
Wire: Depending on water velocity, 20–60 W/cm2 (129–387 W/in2) (even higher figures occur).
Typical applications
Instantaneous tap water and shower heaters, steam generators.
