How to use immersion heaters in chemical process applications

Immersion heaters have a variety of applications in the chemical process industries. Knowing which heaters to specify and how to install them can make a manufacturing process more cost-efficient. It’s time to learn about immersion heater types; typical applications; and selecting, sizing, specifying, installing and using the heater.

Immersion heaters, as the name implies, are immersed in water, oils, solvents, process solutions, molten materials and gases, where they release all their heat within the fluid, which makes them nearly 100% energy-efficient. Immersion heaters are offered in a wide variety of sizes, kilowatt ratings, voltages, terminations, sheath materials and accessories. They are often custom engineered for a specific application.

The basic immersion heater configurations are the screw plug, flange, pipe insert or bayonet, circulation or in-line, booster, and of an over-the-side style. They’re usually available in a round tubular design or a flat tubular design. The flat variety can operate at a higher watt density without overheating the sheath. Heaters also are grouped into two categories – pressurized (closed) systems and non-pressurized (open tank) systems.

Pressurized systems

The square flange immersion heater is used in industrial water boilers and storage tanks holding degreasing solvents, fuel oils, heat-transfer fluids and caustic solutions. The assembly consists of a round or flat tubular heater brazed or welded to a four- or six-bolt flange with screw lug or threaded stud terminals for wiring connections. These heaters bolt directly to a mating companion flange that is welded to a tank wall or nozzle. Assembly change is as easy as unbolting the flange and replacing the heater, thus minimizing extensive equipment downtime.

The screw plug heater fits a threaded opening or full or half pipe coupling in a tank wall. Applications include high-purity water, oils, caustic cleaners, chemical baths, glycol solutions, liquid paraffin, process water and clean-water rinse tanks.

ANSI flange heaters are through-the-side heaters for liquid immersion applications that require high wattage in large tanks. The applications are similar to those of screw plug heaters, but ANSI flange heaters are used in high-pressure applications such as superheated steam, compressed gases or liquids. Pipe insert, or bayonet, heaters are used for heating liquids in extremely large storage tanks. The heater is mounted inside a pressure-tight bayonet pipe that mates to a flange connection on the side of the tank, thus supplying the pressure boundary that allows removal of the heater without draining the tank.

Circulation, or in-line, heaters are all-in-one units with the heater mounted inside its own insulated tank. The heater has inlet and outlet piping and the liquid or gas flows through the tank. By the time the material exits, it’s heated to the proper temperature. This design has fast response and even heat distribution. Heaters can be as small as a 1-1/4-in. NPT screw plug size to as large as 14 in. diameter. Custom units have been made up to 48 in. nominal pipe size. Booster heaters are a type of circulation heater for applications using lower wattage, including in-line operations or engine preheating. Booster heaters with copper and steel sheaths are used for heating water and oils, respectively.

An innovative circulation heater is available for applications that demand precise temperature control for gases and other fluids. Rapid response heat exchangers provide faster thermal response and higher power in a smaller footprint when compared to most other conventional circulation heaters. These typically have low wattage requirements and are single phase.

Non-pressurized systems

Over-the-side heaters are formed into L and O shapes and are installed in the top of a tank, with the heated portion immersed along the side or at the bottom. Over-the-side heaters provide even heat distribution. They’re portable, easily removed for cleaning of heaters and tanks, and provide ample working area inside the tank. A variety of optional sheath materials, kilowatt ratings, terminal enclosures and mounting methods are available. Over-the-side heaters are designed for heating of water, oils, solvents, salts and acids. Often, they’re used for freeze protection.

Two other over-the-side style heaters are the thin-profile vertical loop heater is a tubular heater design that hangs over the side of an open tank and a drum heater that easily fits into the bung hole of a 55-gallon drum and is used for melting heat-sensitive materials, such as paraffin (wax), lard, grease, various oils, and other viscous fluids. A pre-wired thermostat protects the material from overheating.

Cost comparisons

Specific application characteristics or requirements limit most heater choices. Square flanges and screw plugs are generally the most economical solution while ANSI flange heaters and circulation heaters are usually more costly as their size and power requirements are much greater.

Selecting a heater

Most electrical heating problems can be solved by converting the heat requirement to its equivalent electrical power. Whatever the application, the method for determining the power requirement considers the following.

Properties of the material to be heated: It’s important to know the type and quality of the fluid being heated. For example, if the fluid is water, is it clean or contaminated or potable?

Acids cause corrosion and insulating buildup on the heater sheath, which can cause overheating and heater failure. If the fluid is thick and viscous oil, it requires a low watt density, whereas light oil could tolerate sheath watt densities to 30 W/sq. in. to 40 W/sq. in., avoiding coking by considering the oil’s viscosity, specific heat and thermal conductivity.

Startup and maximum operating temperatures: In essence, this is related to the temperature difference (delta T) between startup and operating conditions.

Maximum flow rate of the material being heated: This factor determines the wattage required. The minimum flow rate also might be required to help determine the watt density requirements. Too low a flow rate and too high a watt density can lead to excessive coking or excessive sheath temperatures in oil applications.

Cycle time: The longer the startup time allowed, the lower the kilowatt requirement.

Mass of the heated material: This determines kilowatt requirements for startup.

Characteristics of the containing vessel: The weight determines the kilowatt requirement for startup. Vessel dimensions and presence of a cover determine heat loss. The vessel’s material of construction affects heater choice and mounting configuration. Heaters for pressurized vessels can bear the ASME code stamp.

Insulation: Insulation thickness and properties affect heat loss from the vessel. Mineral-insulated cable or heat-tracing cable compensates for losses from connecting piping.

Temperature monitoring and control: Sensing and control methods and locations depend on the precision requirements for the process and heater sheath temperatures. For example, a simple freeze protection application might require only a mechanical bulb and capillary-type thermostat. For more precise measurement and control, a thermocouple or RTD might feed a PLC. A high-limit sensor on the sheath prevents overheating and premature failure or can determine if there is excessive contaminant buildup.

Locate the temperature sensor where the process temperature is most critical. For instance, in a circulation application, locate the sensor in or nearest to the vessel’s outlet nozzle. In an open tank, position the sensor high enough to avoid contamination from sludge and low enough to receive maximum natural fluid convection without obstructing system operation.

Electrical issues: Voltage and phase are governed by independent agencies, such as Underwriters Laboratories (UL), the National Electrical Code (NEC) and the Canadian Standards Association (CSA). In most cases, the heater’s dielectric properties limit the voltage to about 600 V. The phase isn’t limited by anything other than possibly the heater type and the number of elements making up the heater assembly.

Resistance limitations are encountered only at voltage and wattage extremes. For example, if the voltage is high and wattage low, heater coil resistance would be too high to use the standard thin-gauge heater wire. High wattage and low voltage leads to a need for a wire of such heavy gauge that it’s impractical to manufacture the heater. Consult suppliers for their various manufacturing capabilities. Always consider UL and CSA agency approvals.

Environmental conditions: Ambient temperature and wind conditions can affect heat loss and should be taken into consideration when calculating kilowatt requirements. Hazardous, corrosive and explosive environments are important factors. In explosive atmospheres, a NEMA 7 explosion-resistant electrical enclosure must be used. NEMA 4 ratings are for moisture resistance and may be needed in outdoor or for rinse-down cleaning. Often, a combination NEMA 4 and 7 rating is required. General-purpose NEMA 1 enclosures typically are used when environmental conditions pose no problem.

Sizing the heater

Because the system design might not take into account all possible or unforeseen heating requirements, apply a safety or contingency factor of 10% to increase heater capacity beyond calculated requirements. However, when there are many variables and some unknowns, safety factors to 20% might be considered.

The basic steps in sizing an immersion heater involve calculating the following:

1. Power required for initial heating of the fluid and the tank. Use standard heat-transfer equations to calculate the fluid heating requirement.
2. Power required to heat the fluid during the operating cycle.
3. Heat required to melt or vaporize materials during initial heating.
4. Heat required to melt or vaporize materials during operating cycle.
5. Heat losses by standard equations.
6. Total startup power requirements. The results of steps 1 and 3 are added together and an appropriate safety factor (typically 10%) is applied.
7. Total operating power requirements. The results of Steps 2, 4 and 5 are added and the safety factor applied.
8. Watt density. Calculate the total wattage divided by the active heater surface area based on the length of heater element immersed in the fluid, the surface area per inch, and the total number of heater element lengths.

Installation tips

Check each heater’s megohm value before installation because moisture can enter the heater element insulation during shipping and storage. The same problem may occur if the heater has been idle for a week or more. Low megohm readings indicate the presence of internal moisture, which can cause Ground Fault Circuit Interrupters (GFCIs) to trip.

Check each circuit using a 500-VDC megohm meter, and the reading should be at least 10 Megohms. Lower values may be acceptable, but consult the vendor for more information. A low reading doesn’t mean the heater went bad. Increase the resistance by putting the heater in an oven at 200ºF to 300ºF overnight or until the readings are acceptable. Or, energize the heater at no greater than 50% of the rated voltage until the reading reaches its proper specification. Consult the heater manufacturer for more details.

The temperature rating of the wire coming into the heater also is important. A minimum of 200ºC wire for process heaters is recommended, although higher-rated wire may be required for some applications. It’s best to consult the supplier for the proper wiring details. Power feed line connections must be compatible with the heater and meet National Electric Code (NEC) specifications. Wiring should be installed in accordance with the NEC and other state and local codes.

Immersion heaters used in tanks should be mounted horizontally near the tank bottom to maximize convective circulation. They must be supported above any scale or sludge buildup on the tank bottom.

The unit’s entire heated length should be immersed at all times. Do not locate the heater in a restricted space where free boiling or a steam buildup could occur. Install low-level shutoff switches to avoid heater failure should the liquid level drop.

Maintenance maximizes performance

Follow these tips to increase the life and performance of an immersion heater.

• Make sure the power is turned off before doing any maintenance procedures.
• Make sure the sheath material and watt density ratings are compatible with the liquid being heated. The manufacturer’s application and specification guides provide a complete listing of materials along with maximum temperatures and watt density recommendations.
• Ensure the circulation heaters and in-line heaters have adequate flow to prevent overheating and premature failure. Use a flow switch to shut off the heater and sound an alarm if blockage occurs.
• Heater corrosion can lead to equipment downtime or serious safety hazards. Because sheath temperature plays such an important role in the corrosion process, it’s important to monitor the heater during operation. Place temperature sensors on the sheath where the highest temperatures are expected — on the top of the heater bundle in an open tank with a horizontally mounted heater or nearest the vessel outlet in a circulation heater.
• Check inside the terminal housing for corrosion and loose connections. If the line connections are oxidized, clean and retighten them. A torque of 20 in.-lb. on each heater stud is recommended. If moisture or fumes are present, a different terminal housing may be required. Once the maintenance is complete, thoroughly blow the housing clean with dry, oil-free air.
• Minimize scale buildup on the sheath and sludge on the bottom of the tank. If not controlled, they’ll inhibit heat transfer and possibly cause overheating and failure.

A flat tubular heater isn’t subject to scale buildup in water immersion applications as much as a round tubular heater. Because of its unique geometry, the heater breaks scale and deposits off its sheath. If scale buildup is discovered on other tubular elements, it’s important to clean the units. Water treatment companies are a good source for information about various brands of cleaning chemicals that can remove scale buildup. Another way to remove scale is by cleaning the heater periodically. Remove the scale with a wire brush or clean the heater element with a brush and a mild caustic that won’t harm the heater sheath.

A mild sandblasting often is very effective, but take great care not to damage the heater sheath.

• Coking can lead to early heater failure. It often occurs in oil or other viscous products and increases as sheath temperatures increase. For a given watt density in viscous fluids, a flat tubular element’s sheath runs cooler than that of a round tubular element, 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.
• Thermal cycling may also cause flange mounting bolts to relax, resulting in leaks. Tighten threads and flange bolts.
• Check the sensing probes (thermostat or thermocouple) to ensure they’re operating properly and that the connections are good. Check proper grounding for safety.

Robert C. Klein is a key accounts manager at Watlow Electric Manufacturing Co., St. Louis.