Direct Electric Heat Exchangers, aka circulation heaters, are used in a variety of applications. They are used to heat gases like nitrogen, hydrogen and hydrocarbons, as well as liquids such as water, oils, acids, solutions and other fluids.
DEHE’s are frequently used in similar applications as shell and tube heat exchangers, which have the same purpose but don’t have a direct electric heat source. Shell and tube type heat exchangers have a heat source that is external to the pressure vessel. This article will discuss the considerations that help optimize the application of electric process heaters.
Figure 1 shows the construction of a typical DEHE, and Figure 2 shows a typical shell and tube heat exchanger. Notice the similarities and the differences between both types of heat exchangers. Both contain the medium to be heated in a pressure vessel. The difference is that a shell and tube heat exchanger uses tubes or passages through which a hot (or cold) medium is flowing. This is the energy source for the exchanger. The “shell side” medium thus
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Optimized heater performance is achieved by determining the heat transfer rate, and selecting a heater sheath watt density that assures the fluid is not degraded and the heater sheath temperature is acceptable for durability.
The typical application information requested when specifying a DEHE includes: Description of the application, electric supply voltage and phase, medium name. i.e. hydrogen, methone, etc., flow rate (either mass flow rate or volumetric flow rate at standard pressure/temperature conditions), nozzle size connections, process sensor and/or high limit sheath sensor and type desired and any physical size constraints, among other considerations.
Reputable suppliers of DEHE’s always validate at least heat load duty requirements (kw-h) to see if it matches customer calculated values. This assures that the heater solution offered will provide enough power for proper heating and processing. A safety factor of approximately 10 percent of calculated power requirements is often in order to account for variances is flow, fluid thermal properties, pressure, voltage and resistance. In situations where the potential for large variances exist, 20 percent is often added to provide an additional margin for error.
Materials
Normal practice is to choose the heater element sheath and vessel materials that will minimize corrosive attack by the medium, provide a long service life, yet minimize cost. This is based on the corrosiveness of the materials being heated, as well as service temperatures.
In some cases, the heater element sheath material is a higher grade material than the rest of the circulation heater. This is done for many reasons. Alloy 800 sheaths may be used to heat process water, whereas the vessel material could be carbon steel. The Alloy 800 material is much more expensive than using carbon steel sheath. However, this conservative design approach prevents corrosive attack by water contaminants such as chlorides that could corrode the relatively thin wall of a tubular heater element, before the desired end of life. Using 304 SS (normally a lower cost alternative to Alloy 800), however, might subject the sheath to ion stress corrosion cracking in water heating applications that contain chlorides. In gas heating applications, the sheath temperature of the heater element often operates at much higher temperatures than the surrounding gases. This could lead to corrosive attack of lower grade sheath materials. A reputable DEHE manufacturer will select the best value-added material solution that meets the life and cost goals of the customer.
