Ashley Hinojosa, Lead Project Engineer, Engineering Services
Process electric heaters are an intriguing alternative to fuel burning heaters like the shell-and-tube exchangers commonly used in industrial plant settings. Electric heating elements, rather than heated fluid in tubes, provide constant heat flux. Electric heaters can thus control duty more precisely and produce higher heat fluxes than steam condensers. Since these units have no potential for stream cross contamination, they may pose less of an operational risk compared to shell-and-tube exchangers.
The disadvantages of electric heaters often center around relative cost. Local prices of electricity vs. natural gas or steam, as well as the cost effectiveness for larger duty applications, may play a major role in the adoption (or not) of electric heaters.
All electric heaters share several attributes, including a terminal enclosure, an unheated length, and an element bundle. However, no industry-wide standard exists for the comprehensive design of electric heaters. The finer points of design, such as element details, supports, and layout arrangement, can vary greatly depending on the unit's manufacturer, size, and service.
The thermal engineer, especially those new to electric heater specification or design, must keep in mind a key difference between the “traditional” and “electric” exchangers. In shell-and-tube exchangers, heat flux varies with process conditions and selected geometry, and the wall temperature is limited to the hot fluid temperature. In contrast, electric heaters are designed to provide a constant heat flux targeted through control of the power input. As a result, the thermal engineer must design these units to reduce the variations and excursions in sheath temperature to avoid or minimize localized temperature increases (or hot spots).
Xchanger Suite® can help users check rate vendor designs of electric heaters with the four new electric service types introduced in Xist® 9.1. To properly define an electric heater design, users can enter values using new inputs such as an unheated length value, return bend diameter of the element, and distance from U-bend tangent to shell end. Xist models electric heaters using a constant heat flux method where it iterates on a calculated sheath temperature (and MTD) until a uniform heat flux is satisfied. While it cannot currently predict the precise location and temperature of hot spots, Xist checks for conditions in which they may develop. Use of Xist's average velocities and temperatures generally predicts the overall thermal performance of an electric heater design well.
Industry interest in electric heaters continues to heat up as decarbonization efforts expand, and HTRI aims to help our members stay on top of the conversation.