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.
Depending on the design, heat exchangers have an efficiency ranging from 70% to 90%. And we must consider that heat exchangers require a primary heat source, the one that delivers the hot service fluid. This operating fluid is supplied by various sources, often steam boilers or thermal oil heaters, both of which are fueled by fossil fuels. Since the overall efficiency of the heating process depends on the heat exchanger and its heat source, it would go too far at this point to list every possible combination and the associated efficiency. The effective efficiencies are well known by everyone in the business.
It may sound exaggerated to say that electric heaters are 100% efficient and this is often contested by those who are less familiar with electric heating. It is not just a simple statement, but this efficiency corresponds to the facts if one considers only the heating element itself. Einstein and the immersion heater send greetings from physics lessons. No energy is lost. All electric energy is converted into heat. Of course, here too, the entire chain, including the generation of electric energy, must be considered. But if renewable energy can be used, then the subject is very interesting (see Power-to-Heat).
Almost universal application of electric heaters
Electric heaters can be used for almost every application where heating of stationary or flowing fluids is required, as long as it remains within the range of the temperatures already mentioned. For heating liquids such as water, oils of all kinds, acids or alkalis, as well as gaseous media such as air, natural gas, methane or nitrogen, just to name a few, electric heaters can be a very good, compact and usually also cost-effective solution. It is nearly impossible to list all known applications for electric heaters.
Electric heating as a primary source of heat is a clean process that does not generate combustion gases through open flames. Electric heaters can be installed directly into the process line where heat is needed without the need for additional steam or hot oil piping. No specialized staff is needed for the operation of an electric heater and they require almost no maintenance during normal operation. The system is controlled by contactors or thyristors, by means of which the temperature can be controlled very quickly and accurately.
Electric heaters, depending on the fluid to be heated and the final process temperature, can be built in a relatively compact design. If water is to be heated, high watt densities (W/cm²) can be applied to the heating elements. Schniewindt has built a 10 MW flanged immersion heater mounted in a circulation heater with a DN 800 (32”) nominal diameter and a total length of 3.000 mm. Of course, such a compact design can only be applied to a water heater. When liquids with high viscosity or low thermal conductivity must be heated or high temperatures are to be reached, such compact solutions are not always possible. But even in these cases, the dimensions are still comparable to those of conventional heat exchangers.
Electric heaters can also be installed in potentially explosive atmospheres, allowing their use in the chemical and petrochemical industries, as well as in refineries and offshore platforms. Here, the areas of zone 1 & 2 as well as the temperature classes T1 to T6 are covered.
But electric heaters also have their limits as far as the application is concerned. Not that electric heaters could not heat everything up. However, there are some areas for which any responsible manufacturer of electric heating systems will advise against using his products directly in the process.
This means that electric heaters are not always suitable for some particular fluids, as they would have to be designed in a way to ensure a high level of operational safety for the protection of the fluid to be heated, causing the price of this heater being beyond economic rationality.
Whenever heat exchangers are indispensable for heating processes, electric heaters are a very interesting alternative to provide the required hot service fluid. While oil- or gas-fired heat sources must be installed in separate rooms or areas, electric heaters can be mounted directly next to the heat exchangers. This not only reduces the unnecessary need for long piping with avoidable heat losses during transport of the service liquid but increases the overall efficiency of a heating process by the additional replacement of the less efficient fired heat source.
In summary, electric heaters are a good alternative to heating fluids, but they cannot replace conventional heat exchangers in all processes. However, they are also a serious option for new investments as a replacement for fired heat sources to achieve both higher overall efficiency and compliance with environmental regulations. The topic of CO2 savings (CO2 certificates) through the use of renewable energies should also be or become very interesting for many users and operators.
Schniewindt, founded in 1829, is a performance-oriented, independent family business based in Neuenrade, Germany. The heating technology sector of Schniewindt covers the development, production and marketing of electric heating elements and equipment for heating gases, liquids and solids of any kind, including outdoor installation or in areas classified as potentially dangerous (hazardous areas). Schniewindt’s high-end heating products are available through representatives and distributors in many countries worldwide.