Electric circulation heaters require proper use, maintenance
Electric circulation heaters are used in a variety of applications. They heat gases such as nitrogen, hydrogen and hydrocarbons and liquids such as water, oils and acids. They are also used in a variety of industries, including oil and gas refining, petrochemical and chemical processing. Refining often requires electric heating systems in ancillary processes such as hydrocarbon and hydrogen gas...
Electric circulation heaters are used in a variety of applications. They heat gases such as nitrogen, hydrogen and hydrocarbons and liquids such as water, oils and acids. They are also used in a variety of industries, including oil and gas refining, petrochemical and chemical processing.
Refining often requires electric heating systems in ancillary processes such as hydrocarbon and hydrogen gas heating, asphalt heating, steam superheaters and mole sieve regeneration heaters and systems. Chemical processing heating and control requires temperature measurement, process control, tank and vessel heating, duct air heating and storage tank heating.
Electric circulation heaters are frequently used as heat exchangers, which have a similar purpose but different application. Heat exchangers are also controlled using a different method. An electric heater can perform very well or very poorly, depending upon the heater design, as well as the control scheme employed in its application.
Optimization, application considerations
Figure 1 shows the construction of a typical electric circulation heater. There are similarities to a shell and tube heat exchanger that are worth noting. Both house the medium to be heated in a circulation vessel. The difference is that a heat exchanger uses tubes or passages through which a hot (or cold) medium flows. Instead of tube bundles, the circulation heater has an electrical heating element.
The 'shell side’ medium picks up heat from the higher temperature 'tube side.’ The electric heater, on the other hand, uses electric heating elements inserted into the 'shell side’ medium directly to provide the heating effect. Virtually all of the electric heat generated by the electric circulation heater is transferred into the medium. Therefore, an electric heater is virtually 100% efficient.
Maintaining electric heaters
After they are installed, effective maintenance procedures will extend the life of electric circulation heaters:
Ensure that high-limit sheath sensors are used to prevent overheating, which would damage the heater elements. If there is a flow blockage, the medium will stop flowing and heaters will begin to overheat as the outlet process temperature sensor tries to drive the outlet temperature toward a setpoint. A flow switch is often employed to ensure that the heater is immediately shut down in a no-flow situation.
Ensure that the heater is properly installed, either vertically or horizontally. The heaters are normally designed for horizontal installation. If heaters are vertically installed, internal high-limit sensors must be properly placed, and the enclosure temperatures must not get too hot because the heater is in an upright position. The goal is to keep the enclosure temperature at or below 200 F. Above this temperature, cables and wiring must be rated for higher temperature applications. Check inside the terminal housing for corrosion due to ambient conditions or loose line connections. If oxidation is present on the line connections, clean and retighten them. If moisture or fumes are present, a different terminal housing may be required. Once the maintenance is complete, thoroughly blow clean with dry, oil-free air.
Scale build-up on the sheath must be minimized. If not controlled, it will inhibit heat transfer to the liquid and possibly cause overheating and failure. If scale build-up is discovered on other tubular elements, it is important to clean those units as required. Various brands of cleaning chemicals can remove scale build-up. Water treatment companies are good sources for this information. Another way to remove scale is to periodically clean the heater. Brush the scale off with a wire brush, or clean the heater element in a mild, caustic chemical solution, using a brush and chemical that will not harm the heater sheath. A mild sandblasting of virtually any type of sheath is often very effective, but take steps to prevent damaging the heater sheath.
Coking is another problem that can lead to early heater failure. It often occurs in oil or other viscous products and increases as sheath temperatures increase. A flat tubular element’s sheath runs cooler than that of a round tubular element when operated at the same watt density. Therefore, the flat element has a lower potential for coking. The degree of coking varies greatly, depending upon the maximum operating temperature of the oil being heated.
Do not get silicone lubricant on the heated section of the heater. It will prevent 'wetting’ of the sheath by the liquid and act as an insulator, which can cause the heater to fail.
Poor wiring connections account for many problems in the field. Regularly check to ensure that electrical connections are tight. Process temperature and the electrical current going through the terminal area create heat in the terminal enclosure. As the process heats up and cools down, connections can be loosened, eventually leading to heater failure. A torque of 20 pound-inch on each heater stud is recommended. In addition, the connections should be free of oxide, dust and dirt build-up. Ensure that the interior of the terminal enclosure is clean and dry, and free of dirt, dust, oil and rust.
Thermal cycling may cause sealed joints such as flange mounting bolts to relax, resulting in leaks. Tighten threads and flange bolts.
Periodically check sensing probes (thermostat or thermocouple) to ensure they are operating properly and that the connections are all good. Also, remember to check proper grounding for safety.
<table ID = 'id3002971-0-table' CELLSPACING = '0' CELLPADDING = '2' WIDTH = '100%' BORDER = '0'><tbody ID = 'id3001529-0-tbody'><tr ID = 'id3008649-0-tr'><td ID = 'id3008651-0-td' CLASS = 'table' STYLE = 'background-color: #EEEEEE'> Author Information </td></tr><tr ID = 'id3008661-3-tr'><td ID = 'id3008663-3-td' CLASS = 'table'> Richard Hartfelder is product manager of Process Heating & Control at Watlow, St. Louis. He holds a bachelor’s degree in mechanical engineering technology from Milwaukee School of Engineering and an MBA from Webster University. He has been with Watlow for almost 20 years in a variety of roles from applications and technical support to product and sales training to industrial marketing and sales management, including several overseas assignments. </td></tr></tbody></table>