The purpose of heat exchangers and electric heaters, whatever their design, is to change the temperature of liquids or gases. However, there are some differences between them. Probably the two most notable differences are: Electric heaters, as the name suggests, are used solely for heating. It is possible to design heat exchangers both for heating and cooling.
With heat exchangers, a service fluid heats a process fluid without the two having direct contact, but with electric heaters only the fluid to be heated is in the process. Heat is generated by tubular heating elements immersed directly in the process fluid. A tubular heater consists of a coiled resistance wire centred in a tube and electrically isolated from the tube wall with highly compacted magnesium oxide, providing excellent heat exchange between the wire and the tube and the fluid being heated.
This seems very interesting at first glance. Instead of heating a service fluid that then heats the process fluid, the required amount of heat is transferred directly from electric heating elements to the process fluid. Reduced heat losses through reduced piping, elimination of double temperature control (for process and service fluids) and maintenance-prone valves.
Here, at least, a very relevant question arises: Are electric heaters intended to replace traditional heat exchangers? The answer is no. Each heating system has its own advantages. However, they both have their technical limitations, so they cannot be applied to every heating process. Because of this, a comparison between both methods is worthwhile.
Heating processes requiring high final temperatures place heat exchangers at a disadvantage compared to electric heaters. Regardless of their efficiency, heat exchangers can heat the process liquids to just below the maximum temperature of the service fluid. Hot water (under pressure up to 150°C), steam (up to 375°C at 221 bar), and mineral or synthetic heat transfer oils (up to 400°C) can be used as operating fluids. Thus, the final temperatures of the process liquids are always slightly lower than those of the operating fluids.
In contrast, electric heaters can heat liquids to 625°C (molten salt storage) and gases to 750°C at pressures ranging from a few millibars to a few hundred bars. The process temperatures for air and gas are limited by the maximum surface temperature at which the desired alloy of the tubular heater can function. This is typically in the range of 450°F – 450°C for high-performance alloys. 900°C, which guarantees good operational safety for air and gas temperatures of up to 750°C.
900°C, ensuring good operational safety at which the liquid can be exposed without altering its properties. One example is the decomposition of liquid petroleum products at critical temperatures. It is necessary to regulate not only the process temperature, but also the surface temperature of the tubular heating elements in direct contact with the fluid being heated.
By designing the correct watt density (W/cm2), the surface temperature of an electric tubular heater is limited. This depends on the properties of the medium to be heated, the flow rate, and the temperatures to be achieved. The surface temperature of heating elements can be predicted with absolute precision by thermodynamic calculations. HeatR, a specially developed and verified software, is used at Schniewindt for this purpose
Heat exchangers have an efficiency ranging from 70% to 90%, depending on their design. It is also important to remember that heat exchangers require a primary heat source, the one that delivers the hot fluid. Steam boilers and thermal oil heaters, both of which are fueled by fossil fuels, supply this operating fluid. The efficiency of the heating process depends on the heat exchanger and the heat source, so listing every possible combination and the associated efficiency would be too much at this point. The actual efficiencies of the heating process are well known.
People who are less familiar with electric heating may find it exaggerated to claim that electric heaters are 100% efficient. If one considers only the heating element itself, this efficiency does not just appear to be a simple statement. Greetings from the physics class, Einstein. Nothing is lost. Electrical energy is transformed into heat. In this case too, the entire chain must be considered, including the generation of electricity. But if renewable energy can be used, then the subject becomes very interesting (see Power-to-Heat).
Electric heaters are almost universally used
As long as the temperature remains within the range of the temperatures already described, electric heaters can be used in almost every application where heating of stationary or flowing fluids is needed. Electric heaters can be an excellent solution 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, to name just a few. It is nearly impossible to list all the applications for electric heaters.
The use of electric heating as a primary source of heat produces no combustion gases through open flames. Electric heaters can be installed directly into a process line without the need for additional steam or hot oil piping. For the normal operation of an electric heater, no specialised staff is needed, and they require almost no maintenance. The system is controlled by contactors or thyristors, which can vary the temperature very rapidly and accurately.
Depending on the fluid to be heated and the final process temperature, electric heaters can be built in a relatively compact design. To heat water, high watt densities (W/cm2) can be applied to the heating elements. Schniewindt has manufactured a 10 MW flanged immersion heater mounted in a circulation heater with a nominal diameter of DN 800 (32″) and a length of 3.000 mm. A compact design like this can only be applied to a water heater. When liquids with high viscosity or low thermal conductivity must be heated or high temperatures must be reached, compact solutions are not always feasible. However, even in these cases, the dimensions are comparable to those of conventional heat exchangers.
Moreover, electric heaters can be used in potentially explosive environments, such as chemical and petrochemical plants, refineries, and offshore platforms. Zones 1 and 2 as well as temperature classes T1 to T6 are covered here.
As far as their application is concerned, electric heaters have their limitations as well. However, they are capable of heating any surface. Despite this, there are some areas for which electric heating system manufacturers will advise against using their products directly in the process.
It is for this reason that electric heaters are not always suitable for some fluids, as they would need to be designed in a way to ensure a high level of operational safety for the protection of the fluid to be heated, which would make the cost of this heater uneconomical.
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