dynamic model of electrical air heater

Sorry, I don't have any specific references. Depending on what aspects you need to represent, it could be a pretty straightforward thing, just a bit of dynamics between the electrical input and enthalpy increase of the stream passing by. If that's the extent of it and you can let us know, we should be able to give you a bit more assistance.

This made me recall a situation I worked on years ago. It was a small ammonia plant, the startup heater in the reactor was 3 coils of solid stainless steel rod about 20mm diameter, connected in star. The 3 ends came out through pressure seals and were powered from a big saturable reactor, can't recall the current but it was heaps. The setup was troublesome, it had to be run flat out to get the reactor to bite, and elements regularly burnt out which necessitated a total shutdown, depressure and lid lifting.

We tried to monitor the resistance of the elements from electrical measurements, but as always the situation was complex. The waveform from the reactor was anything but sinusoidal, and there was enough inductance in the coils to complicate the situation further. What killed us in the end was that the star point was floating, we couldn't get to it.