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Considerations for Optimizing the Application of Electric Process Heaters

Mon, 07/14/2008 - 11:44am

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Heaters can be tailored to your operation - just know what to look for and what questions to ask
By Richard Hartfelder
Product Manager - Process Heating & Control - Watlow
Electric circulation heaters are used in a variety of applications, from heating gases like nitrogen, hydrogen, and hydrocarbons to heating liquids such as water, oils, acids and other fluids. They are also used in a wide variety of industries, including the pharmaceutical industry.

Electric circulation heaters are frequently used in similar applications as heat exchangers, which have a similar purpose, but are applied differently. 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. This article will discuss the considerations that help optimize the application of electric process heaters.

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. Virtually all of the electric heat generated by the electric circulation heater is transferred into the medium, thus an electric heater is virtually 100 percent efficient. Optimized heater performance is achieved when finding balance between life considerations and economy of design.

You have heard it before, "garbage in, garbage out." The same goes for wanting to apply circulation heaters to a particular process. You must gather all of the application information and provide it to the vendor. A sample of application information needed:

- Description of the Application.
- Electric Supply Voltage and Phase.
- KW rating if already known.
- Medium name.
- Thermodynamic properties (If not a common medium like water, air, oil, nitrogen, etc.).
- Flow rate (either mass flow rate or volumetric flow rate at standard pressure / temperature conditions).
- Inlet temperature of the medium.
- Outlet temperature (Need this to calculate the desired temperature rise).
- Design temperature (Need this to determine vessel and plug or flanging material pressure ratings).
- Operating pressure.
- Design pressure (Need this to determine vessel and plug or flanging material pressure ratings).
- Process sensor and/or high limit sheath sensor and type desired.
- Any constraints (length limits, footprint limits, height limits, amperage limits, etc.)
- Any preferences (many customers may stipulate a certain material to be used, certain inlet / outlet nozzle sizes, maximum watt densities on the heater elements, etc.).
- Hazardous Location, Gas Group and Temperature Rating Designations (Per NEC, IEC or other codes. This affects the type of electrical termination enclosure needed on the circulation heater).
- Thermal insulation (if desired and also if an insulation thickness is known).
- Any other special requirements of note.

- Analysis to determine appropriate materials, heater watt densities and heater configuration.?The first thing a supplier of electric circulation heaters should do is to check if the wattage provided by the customer is correct, based on the application information provided. This ensures that the heater solution offered will have enough power available for proper heating and processing. A safety factor of approximately 10 percent of calculated power requirements is needed in order to make up for potential heat losses and reserve should a system upset occur, which requires additional power for a short time. When application information is uncertain, 20 percent if often added to provide 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. Some general material guidelines are provided below for some sample fluids:

(Click image for larger version.)


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. For example, Alloy 800 sheath is used to heat process water, whereas the vessel material is carbon steel. The Alloy 800 material is much more expensive than using carbon steel sheathing. However, this conservative design approach prevents corrosive attack by water contaminants such as acids, caustics, etc. that would corrode the relatively thin wall of a tubular heater element made of carbon steel or other lower alloy materials. 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. Also, 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. Again, a sound reason for using Alloy 800. Thus, the conservative design approach will increase the price of the circulation heater, but the Alloy 800 will ensure a far longer service life.
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. It not only indicates how "hot" the heater element will be operating on the outside, but also provides an indication of how "hot" the resistance wire is operating on the inside. 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 as it may 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" maximum watt density maximums. Water is a great absorber and transferrer of heat energy from a heating element, therefore the watt density can be higher. But driving watt density too high will cause film boiling - essentially the heater element operates in a "bubble" of steam. Now instead of water there to pull heat away from the heater element, the bubble forms a layer of insulation and causes the temperature of the heater in that area to dramatically increase in temperature and cause early heater failure.

Now consider oil. Oil is more viscous than water, so the convective heat transfer coefficients will also be lower. This means the heater has difficulty in "pushing" its heat into the oil medium. Also, we need to keep watt densities lower to prevent carbonization or coking of the oil. If the temperature is too hot, it will cause a layer of carbon "insulation" to form on the heater sheath causing the heater to operate at a higher temperature. As the layer of carbon deposits increases, 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 circulation heater, 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 baffles. Baffles are essentially alternating segments or plates in a circulation heater that force the gas or liquid to flow across the heater elements instead of parallel to the elements. This greatly 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 to 25 percent.

The drawback of using baffles is that they increase pressure drop across the circulation heater. Pressure drop, if too large enough, can cause the flow rate of the entire system to slow. Pressure drop is normally a larger issue with gases than with liquids as liquids are not very compressible. Increasing the inlet and outlet nozzle sizes as well as increasing the heater vessel diameter all help reduce the effects of pressure drop.

Regarding watt density, higher-end circulation heater manufacturers use 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 are also 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.

One aspect of applying electric circulation heaters is oftentimes the need for ASME (American Society of Mechanical Engineers) or other type of third-party code stamping. The term "code stamping" refers to the practice of 3rd 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. There are no laws that govern the application of ASME ,unless written into local ordinance regulations. Normally a decision is made by process, mechanical and/or quality compliance engineers as to whether or not ASME code stamping is required. In the end, the decision revolves around safety and insurability of the facility and workers in that facility.

Should AMSE stamping of an electric circulation heater be required, suppliers must employ the relevant portions of the code. Just as with heat exchangers, the entire assembly of the electric circulation heater must be viewed as the complete pressure retaining vessel. And, just as with the pressure headers on heat exchangers, the pressure retaining flange of the immersion heater bundle must go through specific design calculations to ensure structural integrity at design temperatures and pressures. Specifiers and purchasers of ASME code stamped electric circulation heaters (and immersion heater bundles meant for installation in ASME code stamped vessels), must make certain that the immersion heater bundle flange be designed and 'U-Part' Code Stamped in accordance with ASME Section VIII, Division 1 provisions UG-39 and UG-44 per ASME Interpretations VIII, 1-95-128 and VII, 1-98-95.

In European Union and also in other countries, there are specific laws or directives which guide the manufacture and use of pressure retaining equipment. These laws also apply to the use of pressurized electric circulation heaters. For example, in Europe, PED is the pressure vessel directive used. It states that below 0.5 Bars of pressure (around 7 psig) no code compliance is required. Above 0.5 Bars, depending on the combination of pressure, vessel volume and type of medium heated, the level of code compliance increases proportionately from standard engineering practices up through categories I through IV. Post installation, there are several maintenance procedures that will extend the life of the electric circulation heater.

* Ensure that high limit sheath sensors are used to prevent overheating and damage to the heater elements. If there is a flow blockage, the medium will stop flowing and heaters will begin getting hot as the outlet process temperature sensor tries to drive the outlet temperature toward set point. 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 will be vertically installed then care must be taken that internal high limit sensors are properly placed as well as ensuring enclosure temperatures to not get too hot when in upright positions. Normally, the goal should be to keep the enclosure temperature at or below 200°F. Above this temperature, care must be taken to use cables and wiring that are rated for higher temperature applications as well as potentially de-rating the wiring due to high temperature operation. 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, they 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. There are various brands of cleaning chemicals that can remove scale build-up. Water treatment companies are a good source 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, although one must take great care to not damage 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, so 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 since process temperature, as well as the amperage going through the terminal area, creates 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 lbf-in. on each heater stud is recommended. In addition, the connections should be free of oxide, dust and dirt build-up. Make sure, too, 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 make sure they are operating properly and that the connections are all good. Also, remember to check proper grounding for safety.
About the author: Rich Hartfelder holds a Bachelors degree in Mechanical Engineering Technology from Milwaukee School of Engineering as well as 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. For more information, visit www.watlow.com.
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