Warm Up – clear and simple explanation of technical terms

These form the heart of almost all our electric heaters. Tubular heaters consist of stainless steel tubes, in the center of which an electric heating wire is inserted. Its ohmic resistance defines its electrical heating power. This heating wire is encased in magnesium oxide, which electrically separates the current-carrying wire from the metallic outer body. Magnesium oxide is an extremely good heat conductor: heat generated in the heating wire is transferred to the outer sheath in contact with the medium with virtually no loss. Siekerkotte offers tubular heaters in different lengths and shapes with diameters of 6.5, 8.5, 10, 11.5 as well as 16 mm.

The most important parameter for dimensioning electric heating elements is the surface load, often also referred to as “area-related power”. It is an important measure of the stress and service life of a heating element. The surface load is expressed in watts per cm². It is calculated according to the following formula: Surface load is the power (P) divided by the heated surface area of the tubular heater (so in this case, the shell area of a cylinder): Surface load = P/ (2πrh) [W/cm²].
It should be noted that the unheated length of the tubular heater must be subtracted from the height h.
The maximum permissible surface load differs for each medium; the medium can only absorb limited thermal energy depending on its own heat capacity. The following limits (max. guide values) should not be exceeded:
Air: 4 W/cm²
Water: 10 W/cm²
Oils: 2 W/cm²

Ohm’s law (or more precisely, Ohm's rule) states that the strength of the electric current flowing through an object is proportional to the applied electric voltage. The applicable formula is U = R x I. With known voltage and current, it is quite easy to calculate electrical resistance R [Ohm].

When different resistors are arranged in parallel, the reciprocal values (also called conductance S = 1/R [Siemens]) are added. 1/Rg= 1/R1+ 1/R2+ 1/R3+ … + 1/Rn. When only two resistors are connected in parallel, the formula for calculating total resistance is simplified: Rg= R1 R2 /(R1 + R2). Voltage across each resistor connected in parallel remains the same; individual currents flowing through each resistor add up to the total current. It can be said that the current conductivity of the overall system increases with each additional resistor connected in parallel.

In the field of heating heat transfer oils in particular, film temperature plays an important role. Liquids move significantly less when directly at the boundary layer between the medium to be heated and the hot surface of tubular heaters than further away from it. The medium can only pass on absorbed thermal energy in the form of heat conduction. To prevent cracking and the resulting formation of deposits on the tubular heaters, it is important not to exceed the maximum permissible film temperature. This is regulated in the standard DIN 4754. For mineral heat transfer oils, these values lie between 260 and 350 °C, for synthetic oils up to 430 °C, and for water usually between 150 and 180 °C.

The thermal capacity C of a body represents the ratio of the heat supplied to it to the temperature increase (ΔT) caused by it. The thermal capacity of homogeneous bodies can be calculated as the product of its molar heat capacity and its amount of substance. Both values can be found in corresponding technical lists.

In heat convection, heat is transported by gas or fluid flows created by local heating or temperature differences. This means that the kinetic energy of the gas molecules is increased. Energy is transported on a molecular level.