How to build the right band or plate heater for the application
Basic characteristics, general rules
Mica and ceramic band heaters and plates are contact resistance heating elements. The pressure on the heated component is essential for these elements and can affect the life-time of the heating element positively or negatively. It is mainly about the transfer of thermal energy from the heating element to the heated component; the heat should be transferred evenly over the area. In the case of local overheating (poor heat dissipation, higher power consumption per unit area), the resistance winding may overheat in this area and subsequently burn out. Depending on the temperature, temperature maps remain on the stainless steel casing (color change due to temperature). Band and plate heaters are divided into mica and ceramic. In most cases, it depends on the design itself, but it can be said that mica band and plate heaters are for temperatures up to +350 °C and ceramic band and plate heaters are for temperatures up to +500 °C. We know from practice that they can work beyond their temperature limits. This division is partly related to the maximum surface load, for mica band and plate heaters the surface load value is up to 3.5 W/cm2, for ceramic band and plate heaters this value is 6-7 W/cm2. These values are related to the load value of the surface of the heating wire or heating strip. Above these load values band and plate heaters are mainly produced without a guarantee against damage due to overload. Thanks to the fact that these are spare parts for already operating heating elements, there is usually no problem with the life-time even if the load exceeds the maximum recommended load values several times. These are already tested heaters that work under certain, predetermined conditions. For new applications, when the heating element is just being designed, it is more advantageous to follow the maximum recommended power input for the heating area. It is generally more advantageous for heating elements to be subject to temperature regulation and to be heated gradually. This is a resistance heating element that heats itself to the maximum, as designed.
Design of mica and ceramic band heater
The design of the band heater, whether mica or ceramic, is based on the customer's requirements and ideas, but there are limits here too. Band heaters are determined by their diameter and width. The number of clamping elements depends primarily on the width of the band heater. For mica band heaters are usually used clamping elements without springs, for ceramic band heaters are used clamping elements with a spring. The best location of the electrical box (with or without terminal block) is opposite the clamping elements (180° position). If necessary, the box can be placed in another position. It is important that the box does not interfere with other components of the band heater or that it is not too close to the edge of the heater. It is also necessary to choose cable or wires length. If the band heater have an electrical box with a ceramic terminal block the heater can be supplied without a cable. The required cross-section of the cables is determined according to the length and required power input. In the vast majority of cases, it is a nickel core, the cross-section is slightly thicker than in the case of a copper core. The nickel core can heat up slightly due to its greater resistance. This can be eliminated by an even larger cross-section of the cables. In electrical boxes with a ceramic terminal block high temperature cable lugs (nickel) are used. The size and type of the box is based on the dimensions of the band heater and the required direction of the terminals. The band heaters can be equipped with an element to support temperature sensing.
Konstrukce slídové a keramické topné desky
U topných desek je zapotřebí definovat ohřevnou plochu, nejčastěji tvaru obdélníku, ale je možné vyrobit topnou desku kruhového tvaru, případně desky tvarové, s výkusy apod. V případě, že se jedná o desky, které jsou přišroubovány k ohřívané součásti, je nutné definovat polohy a rozměry otvorů pro šroubové spoje a také je třeba definovat samotnou tloušťku topné desky. V případě, že je deska přišroubovaná k ohřívané součásti bez roznášecí (přítlačné) desky, je vhodnější tuto desku zapravit rovnou do slídové topné desky a potom místo obvyklých 4-5 mm tloušťky může mít deska 8-12 mm. Tento rozměr je závislý na rozměru samotné topné desky. Plocha cívky se úměrně zmenšuje s počtem a velikostí šroubů, které drží topnou desku. S tím je nutné počítat při návrhu topné desky. U topných desek je podobná varianta elektrického vývodu přes elektrické krabičky, ale je možné použití přímého vývodu pomocí vodičů NVS přímo z topné desky z hrany topné desky.
Lze vyrobit i keramické topné desky, může být i kruhová s tím, že je samozřejmě nutné počítat s tím, že díky keramickým nosníkům vznikají na krajích hluchá místa. Keramické desky mají stejnou konstrukci jako keramické topné pásy, v tomto případě je ale možné použít ze strany keramických nosníků plášť. Konstrukce keramických topných desek je konstrukčně složitější oproti slídovým topným deskám a využívají se poměrně vzácně.