How is the heat transfer coefficient of an electric heater calculated? !

Regarding what you mentioned, I’ve asked many manufacturers, and the response I got in most cases was that they don’t have any mathematical formulas for this; it’s all based on experience – taking into account factors such as the capacity of the heater and the type of heat exchange medium, all determined through years of experience. It’s also a rather large project; I look forward to receiving guidance from those with more experience!

That’s mysterious. Electric heaters aren’t either anything fancy. Can’t calculate this? !

The problem isn’t even clearly stated. . . The K value of an electric heater ultimately depends on the heat transfer coefficient on the fluid side. . Electric heating elements, whether they are rod-shaped, sheet-shaped, or plate-shaped, all share the characteristic of having a high power per unit area on their surface. They are somewhat similar to a computer’s CPU, and can be used for frying eggs. Taking a computer CPU as an example, the heat generated on its surface needs to be dissipated by fans located on the cooling fins; all you need to do is calculate the area of those fins along with the heat transfer coefficient associated with the fan’s airflow. The heat transfer coefficient of a CPU is extremely high – a quite astonishing value. It basically depends only on the thickness of the heating element (the heat transfer coefficient is equal to the thermal conductivity divided by thickness). Since this value is very high, far exceeding the heat transfer coefficient resulting from convection in the fluid side, the overall heat transfer coefficient is determined by the fluid side. For instance, if you calculate that the heat transfer coefficient for air cooling a CPU is 100, then your overall heat transfer coefficient will also be 100; by then calculating the relevant area, you’ll be pretty much on the right track. This is also why electric heater manufacturers generally do not carry out such calculations – they don’t need to, as they only have approximate empirical data regarding the area. The actual calculations are done by design firms and engineering companies; you just need to calculate the convective heat transfer coefficient on the fluid side.

Here is an engineering example for the original poster: as shown on the left side of the figure below, the gas flows from bottom to top through a steel cylinder composed of electric heating elements (each element has a power of 10 kW, with a total of 12 such elements). Due to limitations in the number of heating elements that can be used, they cannot be arranged radially (as that would require too many elements); instead, they must be arranged vertically along the direction of the gas flow. Within 1 hour of operation on-site, the electric heating element overheated, forcing us to reduce the current and operate at reduced capacity; later, it was modified to match the design shown on the right side. . Everything is OK. . . Compare them and you’ll understand what’s going on. . . It’s supplied as a complete set by the electric heater manufacturer; the manufacturer just makes things up without any calculation, and as a result, they get it wrong. . .

Okay, thank you so much! I’ve figured out a problem that I hadn’t been able to solve for a long time.

It’s really great; the explanation is very clear. I used to think only about the total power required for electric heating, but it turns out that just like with heat exchangers, not only must the total power be sufficient, but heat transfer considerations also need to be taken into account.