Article By Ulrich Thiele, Schniewindt, Neuenrade, Germany
While in heat exchangers a service fluid heats a process fluid without both having direct contact with each other, in electric heaters only the fluid to be heated is in the process. Here, heating is made by means of tubular heating elements immersed directly in the process fluid. Tubular heating elements consist of a coiled resistance wire centered in a tube and electrically isolated from the tube wall with highly compacted magnesium oxide, ensuring a high dielectric strength and excellent heat transfer from the wire to the tube and from the tube to the fluid being heated.
At first glance, this seems to be very interesting. Instead of heating in some way a service fluid, which then heats the process fluid, the required amount of heat is transferred from electric heating elements directly to the process fluid. Significantly lower heat losses through reduced piping, elimination of double temperature control (for service and process fluids) and maintenance-prone control valves.
At least here, a very appropriate question arises: do electric heaters intend to replace traditional heat exchangers? This question is easy to answer: NO. Each of the two heating systems offers some advantages over the other. But both also have their technical limitations, so they cannot be applied to every heating process. Therefore, it is worthwhile to make a closer comparison between both methods.
If a heating process requires very high final temperatures, heat exchangers are in disadvantage compared to electric heaters. Regardless of their efficiency, heat exchangers can heat the process liquids to just below the highest temperature of the service fluid. Operating fluids may be, as required, hot water (under pressure up to 150°C), steam (up to 375°C at 221 bar), mineral or synthetic heat transfer oils (up to 400°C). The final temperatures of the process liquids are thus always slightly lower than those of the operating fluid.
Electric heaters, on the other hand, can heat liquids up to 625°C (molten salt storage) and gases up to 750°C at process pressures from a few millibars to a few hundred bars. The process temperatures for air and gas are limited by the highest surface temperature at which the intended alloy of the tubular heater can be operated. For high-performance alloys this is in the range of 900°C, which guarantees good operational safety for air and gas temperatures of up to 750°C.
For liquids, the maximum surface temperature of the heating elements must be lower than that to which the liquid can be exposed without altering its properties. As an example, mention may be made of various liquid petroleum products which decompose at critical temperatures. This requires that not only the process temperature be controlled, but also the surface temperature of the tubular heating elements in direct contact with the fluid being heated.
In electric heating, the surface temperature of the tubular heater is limited by designing the correct watt density (W/ cm²). This is based on the properties of the medium to be heated, the flow rate and the temperatures to be achieved. The surface temperature of the heating elements can be predicted with absolute precision by thermodynamic calculation. This is done at Schniewindt with HeatR, a specially developed and verified software.