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Watt density determination
Industrial heater manufacturers have developed standard watt density values for heater elements used in a variety of circulation heater applications. Watt density is simply a measure of the amount of wattage generated per unit of surface area on the sheath of the electric heater element. The watt density is one of many specifications and features that affect the sheath temperature of the heater and the internal resistance wire temperature. The goal is to keep the resistance wire as low in temperature as possible and to avoid thermal cycling extremes. This promotes a very long heater life. Another goal is also not to get the external heater surface too hot, since it might damage the medium being heated.
The advantage, however, of using a higher watt density is that manufacturers can pack more power into a smaller package and reduce the overall size and cost of the heater.
Compare the watt density ratings for process water, heat transfer oil and heavy fuel oil. Notice the dramatic differences in “allowable”
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Materials like lubricating oil are more viscous than water, so the convective heat transfer coefficients will also be lower compared to water. This means the heater has difficulty in “pushing” its heat into the oil medium. Keep watt densities lower to prevent carbonization or coking of the oil.
If the temperature is too hot, it will cause a layer of “insulation” to form on the heater sheath, causing the heater to operate at a higher temperature, aka fouling. When operated in a hydrocarbon, the layer of carbon deposits increases, and the heater temperature rises to compensate for the inability of the medium to pull heat away from the sheath. This results in higher internal wire temperatures and shorter element life.
In a DEHE, methods are often employed to try to boost the watt density with the objective of keeping heater costs down. One common method is to use segmental baffles. Segmental baffles are essentially alternating segments or plates in a circulation heater that force the gas or liquid to flow across (cross flow) the heater elements instead of parallel to the elements. This increases turbulence and the “wiping action” of the heater elements and allows the medium to more effectively absorb heat energy. Using baffles can boost the watt density employed by anywhere from 10-25 percent.
The potential drawback of using baffles is that they increase pressure drop across the circulation heater. Pressure drop can be a larger issue with compressible fluids such as gases vs. liquids, which are essentially non-compressible. Pressure drop issues can often be overcome through well engineered baffle design, nozzle size, nozzle orientation and/or sometimes vessel size.
Well equipped DEHE manufacturers use CFD and/or FEA thermal modeling to verify that standard watt densities may or may not be used. The flow characteristics of the fluid through each heater will vary based on a whole host of factors. The methods of heat transfer going on inside the heater also are very complex. That is why it is so critical to get a complete set of process and application information in order to provide the best, most cost effective heater solution for the customer.
DEHS’s routinely need ASME (American Society of Mechanical Engineers) or other types of third-party code pressure vessel stamping. The term “code stamping” refers to the practice of third party firms (called authorized inspectors) validating design calculations, proper material usage and test methods for pressure retaining bodies. When in compliance, the inspector “stamps” a seal of approval on the finished pressure vessel or vessel part, which certifies that the vessel and the heater bundle flange are ASME code compliant. Statutory and regulatory laws and directives determine the need for ASME or similar stamps. The location of installation normally determines what stamp is applied to the DEHE. These stamps help assure the safety and insurability of the facility and workers in that facility.
