Electric heating can be any process where electrical energy is converted to heat. A perfect example is an electric heater. It has various applications that include water and oil heating, cooking and space heating, and it is widely used in industrial processes. An electric heater works on the principle of Joule heating, which is a process where an electric current, when passed through a conductor, gets converted into heat.

Tubular electric heater technology has been in practice for more than 30 years, but more recently, its use in chemical and petrochemical industries has

increased. Improvements in safety features, control schemes and product design have given this technology an advantage over other means of heat.

A basic tubular heating element consists of nickel-chromium (Ni-Cr) wire that provides resistance to electricity generating heat. Compact magnesium oxide (MgO) insulation and a metal sheath surround the Ni-Cr wire (figure 1). Cold pins, or metal conductors, are used to make electrical connections to the resistance wire, and electric termination can be made in various forms. Because MgO is hydroscopic, it is sealed to prevent moisture from entering.

The tubular elements are welded into a flange, making an immersion heater assembly. Normally, an immersion heater assembly consists of heating elements, flange or tube sheet, thermocouples and its housing and bussing for element circuit. To heat a fluid or gas directly, this heater assembly can be bolted into the tank or mounted into a pressure vessel.

Detailed information about the application is required for most applications in the petrochemical industry to ensure successful heater performance. Some of the information required includes:

• Medium to be heated.
• Inlet and outlet temperature.
• Operating pressure.
• Flow rate or tank size.
• Design temperature and pressure.
• Area of use (i.e., indoor, outdoor).
• Hazardous location (If yes, state Class, Group and Division).
• Allowable pressure drop.
• Required heatup time.
• Inlet and outlet pipe connection sizes.
• Voltage available.
• Accuracy of temperature control required.

Proper sheath and vessel material selection are important. Some of the factors that affect material selection include desired design and sheath temperature, design pressure and the corrosive nature of the medium to be heated.

Heater Types and Applications

Immersion heaters are widely used in a range of applications in the chemical processing industry (CPI). The heaters are used in viscous materials, molten materials and gases, water, oils, solvents and process solutions. They are available in different characteristic choices such as size, termination connections, sheath materials and accessories, kilowatt ratings (power) and voltages (electrical potential). Due to the complete transmission of heat within the liquid or gas, immersion heaters are energy efficient.

Square-flange heaters can be found in applications such as storage tanks and industrial water boilers that hold fuel oils, caustic solutions, degreasing solvents and heat transfer fluids. Screw-plug heaters are commonly used in applications like demineralized and process waters, caustic cleaners, antifreeze solutions (glycol), industrial and clean-water rinse tanks, deionized water, liquid paraffin, hydraulic and crude oils and chemical baths. Through-the-side immersion heaters generally are used in high-pressure applications such as superheated and compressed gas tanks, but they also find use in non-pressurized tanks.

Commonly, steel flanged immersion heaters are used for low-flow gas heating, heavy and light oils, and lubricating oils. They also are used in deionized and demineralized water, detergent solutions, process water and soap. There are many advantages of using steel for the heater such as minimized heat loss, corrosion resistance and extended life.

Stainless steel flanged heating elements are more suited with mild-to-severe corrosive solutions. For sanitary purposes, they are also used in the food industry.

Common Uses for Electric Heaters

In recent years, there has been a depletion of gas reserves in the oil and gas industry with a constant increase in demand. The rising cost of natural gas has created a need for an alternate source for preheating produced fluids in heater treaters in oil fields. Electric immersion heaters provided a safe, efficient, reliable way to fulfill this need when compared to other heating techniques.

Electric heating also is used in the petrochemical industry to provide freeze protection and process maintenance on the piping systems. Advantages in this particular application include minimum installation cost, lower operational cost and better control of heat.

Electric resistance heating also can act as a substitute for conventional steam and direct-fired heating in the petroleum, petro-chemical and chemical industries.

Removing sand from crude oil is an expensive process, and the efficiency of separation is minimized due to the high oil viscosity in many oil fields. To help make the oils more viscous, electric heating has been used by oil companies in oil fields to address the problem of low efficiency with high viscosity. Electric heating also has other applications in oil fields related to oil sands separation technology, including separation of oil sands of aqueous air, cleaner ultrasonic degreasing of oil sands, and application of micro-emulsion in oil sands lotion.

An example is in heavy oil desanding of offshore platform, where a special copper powder is used as the conducting material for the electric heater, which is placed between the tubing and the heating coils. This powder has high resistivity and a transient heating effect that helps achieve the temperature requirements in a shorter period of time. Results include high thermal efficiency, ease of installation, low maintenance and stable and reliable performance.

Selecting the right heater based on the application required is important and depends on the characteristics or requirements of the application. Usually, heat

required for the job is determined, which then is converted to the required electrical power, and a heater is selected accordingly. The method for determining the power required is the same for heating liquids, solids or gases.

Properties of the material to be heated also play an important role in the selection of a heater. For example, if the liquid is crude petroleum oil, which usually is thick and viscous it requires a very low watt density; whereas, vegetable oil, which is very light, could only use up only 30 to 40 watt/in2. Watt density depends on thermal conductivity, viscosity and specific heat of the oil. Estimating proper watt density protects the heater against coking.

Coking is a deposit usually formed on the heater’s sheath due to chemical breakdown of the material being heated. The amount of coking depends on the maximum operating temperature of the material being heated. It usually occurs in petroleum products, which causes the life of the heater to deteriorate and leads to early failure.

Heater design plays a vital role in preventing or minimizing coking. For example, the sheath of a flat tubular element is cooler than that of a round tubular element when operated at the same watt density; hence, the flat element has a lower risk for coking. So the potential for coking should be a factor to consider when designing the heating system.

At the end, it can be said that electric heater applications range from heating various liquids and gases to extremely high temperatures, steam superheating, heat transfer fluids, fuel oils and corrosive solutions. Electric heating technology has progressed a long way in safety, design standards, reliability and controllability.
There are no emission concerns, which helps keep the environment safe. The key to proper performance is good engineering practices and knowing as much as possible about the application upfront to ensure a good design. 

References
1. http://en.wikipedia.org/wiki/Electric_heating#cite_note-3.
2. Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition, McGraw-Hill, New York, ISBN 0-07-020974-X, pages 21-144 to 21-188, 1978.
3. http://en.wikipedia.org/wiki/Joule_heating.
4. Robert Klein, “Immersion Heaters: Selection and Implementation,” Chemical Engineering. 113.1, p. 44-48, Jan 2006.
5. Ding Feng, Nian Liu, Xiaofei Chang, Peng Wang, Chao Ruan and Hong Zhang, “The Application of Electric Heater in Heavy Oil Desanding Of Offshore Platform,” IEEE, 2011.
6. Rob Bohn, Mike Bange and Joe Foreman, “The Basics of Electric Process Heating,” IEEE, Paper No. PCIC-94-14, 1994.
7. James E. Palastak, “Use of Electric Immersion Heating Elements in Oilfield Heater-Treaters,” SPE Eastern Regional Meeting, Society of Petroleum Engineers, 4-6 November, Columbus, Ohio, 1981.
8. Jack H. Sheets, Edward J. Sand, “Development and Application of Electric Heating to Deicing Of Aircraft Propellers,” IEEE, Vol. 68, 1949.
9. C.J. Erickson, James D. Lyons, N.R. Rafferty, Chet Sandberg, “A Study of Steam vs. Electrical Pipeline Heating Costs on a Typical Petro-Chemical Plant Project,” IEEE Paper No. PCIC-90-02, 1990.
10. Anon, “Steam Substitution in Chemical Process, Petrochemical and Petroleum Industries,” Research and Development Report, Canadian Electrical Association, ISSN: 08232660, March 1987.