The objective of Electricfor’s constructive thermal classes is to define a standard construction of heating elements depending on the sheath material, its maximum permissible temperature and the type of seal. These three characteristics (specially the sheath material) should be always chosen taking into account the working environment.
AISI 304: Austenitic stainless steel. It has an optimum weldability and good resistance to corrosion at room temperature. It has also a satisfactory service at high temperature (from 800ºC to 900ºC) and good mechanical properties. Besides, this kind of steel contains a low carbon percentage which avoids the precipitation of carbides during prolonged time at high temperature. It also has an excellent behaviour for most welding applications because of its maximum carbon percentage (0, 08%).
AISI 304L: Austenitic stainless steel. It has an optimum weldability and good resistance to corrosion at room temperature. If it is kept for some time in a critical temperature range (from 450ºC to 800ºC), an inter-crystalline precipitation of chrome carbide may occur and as a consequence, an inter-granular corrosion.
AISI 321: A specific amount of titanium is added to the AISI-304 components in order to avoid the formation of chrome carbide. This fact prevents the phenomenon of inter-granular corrosion, making this material suitable to be used over prolonged periods of time in a critical range of temperature. It also has good resistance to the formation of cinders (up to 800ºC).
AISI 316L: It contains an addition of molybdenum (2÷3%), which gives it more resistance to corrosion by pitting and better comportment than previous steels against stress corrosion. It’s carbon content is lower than 0,03% which hinders the chrome carbide formation, thus increasing its resistance to intergranular corrosion.
INCOLOY® 800L: Refractory stainless steel with high nickel and chrome content. Good resistance to formation of cinders up to 1,100ºC. It presents high tensile strength and good corrosion resistance at high temperatures.
INCOLOY® 825: This is a nickel-iron-chrome alloy with additions of molybdenum and copper. It offers good resistance to both reducing and rusting acids, to stress corrosion, to pitting and to interstitial corrosion. ® Brand-name registered by "The International Nickel Co."
COPPER (SF-Cu DIN 1787): Semi-noble metal and highly resistant to water corrosion by nature.
TITANIUM: Titanium is a metallic element that presents a compact hexagonal structure; it is hard, refractory and good electrical and heat conductor. It presents high corrosion resistance because of the passivation phenomenon that it undergoes (it forms an overlying oxide).
TYPES OF SEAL
Airtight seal: This seal does not allow moisture penetration inside the heating element, maintaining insulation values for more than five years. Sheath and seal temperature should not exceed 300 ºC and 150 ºC respectively. If the sheath temperature exceeds 150 ºC or the seal temperature exceeds 150 ºC, insulation values decrease rapidly and leaks appear shortly.
Extra airtight seal: This seal does not allow moisture penetration inside the heating element, maintaining insulation values for more than ten years. In this case, the sheath temperature can reach up to 600ºC. If the sheath temperature exceeds 600ºC or the seal temperature exceeds 150ºC / 250ºC (according to thermal class), insulating values decrease rapidly and leaks appear shortly.
Transpirable seal: When an element works over 600ºC, it consumes oxygen. If the seal does not allow the oxygen to enter, the element life is reduced to 100 hours. Therefore, a seal has been designed to allow the proper amount of air entrance in order to achieve the suitable respiration of the heating element. However, to achieve a proper respiration, it must be verified that the element is switched-off at intervals of 5 minutes, at least a 15% of the time and that it doesn’t work without stopping more than 8 ours in a row. Stoppages can easily be those originated by media control, such as thermostats, etc. in a usual way in most applications.
Due to the special conditions of this seal, allowing air to entry, it also allows the entry of moisture. Therefore, isolation can decrease to values close to 1MW and, at the time of connecting the heating element after a prolonged shutdown, transient leaks of current may occur. These leaks can reach 5mA per kilowatt during this period, and then drop to values of 0,2mA per kilowatt until the temperature is established. To make sure that leaks disappear before the temperature stabilization and thus comply with UNE-EN-60335, it must be ensured that the seal temperature is at least 110ºC.
LIMITS OF USE OF THE HEATING ELEMENTS
The tubular sheath temperature is measured with a thermocouple of thin and low-mass wire welded to the heating zone of the tube’s sheath.
(*) In breathable seals, leaks can reach 5mA per kilowatt during heating; however they remain within the limit of the values imposed by the norm when the regular working temperature is reached and as long as the temperature of the seal reaches a maximum of 110ºC. For this reason, we recommend that you do not opt for such a seal unless absolutely necessary due to the temperature that the seal or the tube will have to reach.
(**) The maximum temperature of the seal coincides with the maximum temperature permitted in the tube’s sheath. Whatever, when installing, other temperature limitations should be kept in mind, such as the maximum temperature in the supply conductors, terminals, flanges, etc.
While designing the heater in which the resistors will be included, it has to be taken into account that the temperature reached by the resistors should not dangerously affect the other parts of the heater.
In order to ensure a reasonable life expectancy, you should check the capacity of the protecting material in resisting corrosion in the real conditions created by the machine in operation. We remind you that even stainless steel in drinking water may show signs of corrosion. For more information, see our technical sheet NTT-4101 or contact our Technical-Commercial department.