How to properly employ immersion heaters in chemical process applications
Immersion heaters have a wide variety of applications in the chemical process industries. Knowing which ones to specify and how to properly install them can make a manufacturing process more cost-efficient. This article describes the different types of immersion heaters and their typical applications, explains how to select, size, and specify immersion heaters, and provides guidance on installing and using the heaters.
Immersion heaters, as the name implies, are directly immersed in water, oils, viscous materials, solvents, process solutions, molten materials, and gases. By generating all the heat within the liquid or process, these heaters are virtually 100 percent energy efficient. Many designs are available from heater suppliers, as stock items and are offered in numerous sizes, kilowatt ratings, and voltages, and with a variety of termination connections, sheath materials and accessories. Manufacturers can also custom engineer an immersion heater or system for a specific application.
The basic types of immersion heaters are the screw plug, flange, pipe insert or bayonet, circulation or in-line, booster, over-the-side, and vertical loop. They are usually available in either a round tubular design or a flat tubular design. Flat tubular immersion heaters can typically run at a higher watt density – 30W/in.2 compared to 23 W/in.2 for a round tubular heater in a typical application such as heating low viscosity lubricating oil – while not having a higher sheath temperature. Heaters are also grouped into two categories – pressurized (closed) systems and non-pressurized (open tank) 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 either 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 it with another heater, which minimizes extensive equipment downtime.
The screw plug heater is inserted into a threaded opening in a tank wall or into a mating full or half coupling. Screw plug immersion heaters are available in a variety of National Pipe Thread (NFT) sizes, materials, wattages, and voltages, and with various sheath materials and thermostats. Applications include de-ionized or de-mineralized water, oils, hydraulic and crude oil, caustic cleaners, chemical baths, antifreeze (glycol) solutions, liquid paraffin, process water, and industrial and clean-water rinse tanks.
ANSI flange heaters are through-the-side heaters for liquid immersion applications requiring high wattage in large tanks – up to 3000 kW or higher. Applications are similar to those of screw plug heaters, but ANSI flange heaters are used in higher pressure applications (up to 3,000 PSI), such as tanks of superheated steam, compressed gases, or liquids.
Pipe insert, or bayonet, heaters are used for heating liquids in huge (millions of gallons) storage tanks. The heater is mounted inside a pressure-tight bayonet pipe that mates to a flange connection on the side of a storage tank, thus supplying the pressure boundary. The heater is then inserted into the open end of the bayonet, allowing for 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 comes out the end, it is heated to the proper temperature. This design has a fast response and an even heat distribution. Heaters can be as small as a 1 1/4inch NPT screw plug size to as large as 14 inch in diameter. Custom units have been made up to 44-inch nominal pipe size.
Booster heaters are a type of circulation heater. They are ideal for applications using lower wattage, including in-line operations or engine preheating. Booster heaters with copper and steel sheaths are ideal for heating water and oils.
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.
Over-the-side heaters are formed into L and O shapes and are installed in the top of a tank, with the heated portion directly immersed along the side or at the bottom. Over-the-side heaters evenly distribute heat to liquids and viscous solutions. They are 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 ideal for heating small quantities of water, oils, solvents, salts and acids. They are often used for freeze protection.
The thin-profile vertical loop heater is available in a round tubular heater design and hangs over the side of an open tank. Another over-the-side type is the drum heater, which easily fits into the bung hole of a 55-gallon drum. It typically melts heat-sensitive materials, such as paraffin (wax), lard, grease, and coconut oil. A pre-wired thermostat protects the material from being overheated.
Many heater choices are limited by specific characteristics or requirements of the application. 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 determining the heat required to do the job. The heat requirement is converted to electrical power, and the most practical heater can then be selected for the job. Whether the problem is heating solids, liquids or gases, the method for determining the power requirement is the same.
In defining the problem, the following factors should be considered:
1 Properties of the material to be heated. It is very important to know the type and quality of the fluid being heated. For example, if the fluid is rinse water for parts, is the water clean or is it contaminated with traces of acids or alkalis, which are often left behind when rinsing parts? Acids cause corrosion and buildup on the sheath of the heater, which can act as an insulator and causes the heater coil to fail prematurely due to overheating. If the fluid is an oil, what type of oil is it? For instance, a crude oil is very thick and viscous and requires a very low watt density, whereas a very light oil such as vegetable oil could use up to 30-40 w/in.2. The watt density depends on the viscosity, specific heat, and thermal conductivity of the oil. Choosing the proper watt density ensures that coking does not occur.
2. Startup and maximum operating temperatures. In essence this is the delta T or temperature change from startup to operating conditions.
3. Maximum flow rate of the material being heated. This is needed to determine the wattage requirements. The minimum flow rate may also be required to help determine the watt density requirements. If the flow rate is too low and the watt density too high, excessive coking can occur in oils or excessive sheath temperature can occur in air and gasses.
4. Required time for startup heating and process cycle times. The longer the startup time allowed, the lower the kilowatt requirement. This is because the kilowatt requirement is inversely related to the time needed to heat up the medium to operating temperature.
5. Volume or weight of the heated material. These are needed to determine kilowatt requirements for startup.
6. Characteristics of the containing vessel. The weight is used to determine the kilowatt requirement for startup. The dimensions of the vessel are required to determine heat losses in the initial startup equations and to determine kilowatt requirements maintaining the operating temperature. The vessel’s material of construction could affect the type of heater chosen and the way it is supported in the tank, especially if it is a plastic container. Factors involved in material selection include the threat of galvanic corrosion, wattage requirements, and structural support. Also, whether the vessel has an open top or closed top will greatly affect heat loss – a closed top will significantly reduce the kilowatt requirement to heat up and maintain the process. For pressurized containing vessels, the requirements of a pressure vessel code, such as ASME, may be applied to the manufacturing of the heater.
7. Vessel insulation. If any insulation is present, its thickness and thermal properties will affect the heat loss from the vessel. Heat loss on connecting piping is normally compensated for by alternative heating, such as mineral insulated cable or heat-tracing cable.
8. Temperature monitoring and control. Sensing and control methods and locations vary greatly depending upon the precision requirements for the process and heater sheath temperatures. For example, a simple freeze protection application may only require the use of a mechanical bulb and capillary type thermostat, which is most economical, to monitor the process. For more precise measurement and control, a thermocouple or resistance detector (RTD) sensor may be used in conjunction with a microprocessor-based controller. A high-limit sensor located on the sheath prevents overheating, which could lead to premature
failure or accelerated buildup of contaminants. The temperature sensor should be located at the point where the process temperature is most critical. For instance, in a circulation application, the sensor should be located in or nearest to the outlet nozzle of the vessel. In an open tank, the sensor should be positioned high enough to avoid contamination from sludge and low enough to receive maximum natural fluid convection without obstructing the operation of the system.
9. Electrical requirements. Voltage and phase are governed primarily by independent agencies, such as
Underwriters Laboratories (UL), National Electrical Code (NEC) and the Canadian Standards Association (CSA). Voltage is limited due to the dielectric properties of the heater. For example, the maximum voltage capacity of most heaters, depending upon the diameter, would not exceed 600V. Consult the heater manufacturer regarding the agency approvals for heater voltage and diameter limitations. Phase is not limited by anything other than certain heater designs. The phase is not limited by anything other than possibly the type of heater and the number of elements making up the heater assembly.
10. Electrical limitations. The biggest limitation is a maximum of 600V because of dielectric capabilities of the heater. Resistance limitations are also encountered when there are voltage and wattage extremes, either too low or too high. For example, if the voltage is too high and wattage too low, the resistance on the heater coil would be so high that the thin-gauge wire typically used would be too fine. The reverse, high wattage and low voltage, creates a need for a wire of such heavy gauge that it is impractical to manufacture the heater. Consult suppliers for their various manufacturing capabilities. Always consider UL and CSA agency approvals.
11. Environmental conditions. The ambient temperature and wind conditions can affect heat loss and should be taken into consideration when calculating kilowatt requirements. Hazardous environments, such as corrosive and explosive situations, are also important considerations. For example, a stainless steel enclosure is resistant to corrosive processes. 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 wash-or rinse-down cleaning. Often, a combination NEMA 4 and 7 rating is required. General-purpose NEMA 1 enclosures are typically used when environmental conditions pose no problem.
12. Contingencies. Because the thermal system design may not take into account all the possible or unforeseen heating requirements, a safety or contingency factor that increases heater capacity beyond calculated requirements is applied. A factor of 10 percent is typically used. However, when there are many variables and some unknowns, safety factors up to 20 percent may be considered.
13. Physical sizing and wattage requirements. A 10kW heater can be a screw plug, square flange or ANSI flange – or even an over-the-side heater. But why choose one over another? That decision may be based on the openings available in the tank or simply based on how long the tank is. A larger flange size may be needed due to needing a short heater, or if there is a lot of length, a plug or square flange may be sufficient.
Sizing the heater
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 the basic heat-transfer equation (heat equals mass times heat capacity times temperature change: Q=WC^7) 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. Thermal system heat losses. The heat losses are calculated by multiplying the exposed surface area, the startup time and a surface loss factor.
6. Total startup power requirements. The results of Steps 1 and 3 are added together and an appropriate safety factor (typically 10 percent) 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. The total wattage is divided by the active heater surface area. The latter is calculated based on the length of heater element immersed in the fluid, the surface area per linear inch, and the total number of heater element lengths (two lengths element multiplied by the number of elements).
Heating water – a shortcut
For many immersion heating applications, some relatively simple formulas are available for quickly estimating kilowatt requirements. Table 2 can be used for water heating applications. To use the chart, find the amount of liquid in gallons on the left and the desired temperature rise at the top. Read from the chart the wattage needed to heat the water in one hour.
Alternatively, one of the following equations could be used, where Qs=flo rate in gal/min, ∆Tf=temperature rise in ºF,Q l = flow rate in L/min, ∆Tс = temperature rise in °C, and t is the heating time in h.
For heating flowing water:
kW = Qs x∆Tf x0.16 (1a)
kW = QL x ∆Tc x 0.076 (1b)
For heating water in tanks:
kW = (Qs x ∆Tf)/(375 x f↑) (2a)
kW = (Ql x ∆Tс)/(790 x t↑) (2b)
Each heater’s current should be checked before installation, since during shipping and storage moisture can enter the heater element insulation and affect the heater’s performance. The same problem may occur if the heater has been idle for a week or more.
Each circuit should be checked using a 500-V d.c. megohm meter, and the reading should be at least 10 M∩. Lower values may be acceptable, but the supplier should be consulted for more information.
A low current reading does not mean the heater is bad and has to be returned. There are several ways to increase the megohm level. One is to put the heater in an oven at 200ºF-300ºF and leave it overnight or until the megohm readings are acceptable. The second way is to energize the heater at no greater than 50 percent of the rated voltage until the megohm reading reaches its proper specification. Consult the heater manufacturer for more details.
The proper temperature rating of the wire coming into the heater is also important. A minimum of 200ºC wire for process heaters is recommended, although higher-rated wire may be required for some applications. It is best to consult the supplier for the proper wiring information.
Power feed line connections must be compatible with the heater and meet National Electric Code (NEC) specifications. All installation wiring should be done 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 allow convective circulation. They must be located high enough to be above any scale or sludge buildup on the bottom of the tank.
The entire heated length of the heater should be immersed at all times. Do not locate the heater in a restricted space where free boiling or a steam buildup could occur. Low-level shutoff switches can be installed to avoid heater failure should the liquid level drop too low.
Maintenance to maximize performance
To increase the life and performance of an immersion heater, follow these tips:
Make sure the sheath material and watt density ratings are compatible with the liquid being heated. Application and specification guides supplied by manufacturers provide a complete listing of materials along with maximum temperatures and watt density recommendations.
For circulation or in-line heaters, make sure there is adequate flow. If the flow rate is too low, the heater will overheat and fail prematurely. A flow switch can be used to monitor or shut off the heater if a blockage occurs in the system. Alarms are often used in conjunction with them.
Corrosion of the heater can lead to problems ranging from equipment downtime to serious safety hazards. Because sheath temperature plays such an important role in the corrosion process, it is important to accurately monitor the heater during operation. Place temperature sensors on the areas of 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.
Also 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 buildup on the sheath and sludge on the bottom of the tank must be minimized. If not controlled, they will inhibit heat transfer to the liquid and possibly cause overheating and failure.
The flat tubular heater design prevents the buildup of scale in water immersion applications. Because of its unique geometry the heater “breathes” breaking scale and deposits off its sheath. If scale buildup is discovered on other tubular elements, it is important to clean the units as required.
There are various brands of cleaning chemicals that can remove scale buildup. 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.
Extreme caution should be taken not to get silicone lubricant on the heated section of the heater. The silicone will prevent “wetting” of the sheath by the liquid and act as an insulator, possibly causing the heater to fail.
In addition to coking and scaling, poor wiring connections account for a large percentage of problems in the field. Make sure that the electrical connections are tight. This should be done on a regular basis because process temperature, as well as the amperage going through the terminal area, creates heat in the terminal enclosure. When the process heats up and cools down, the connections can be loosened, which eventually leads to heater failure. A torque of 20 in.″ lb. on each heater stud is recommended. In addition, the connections should be free of oxide, dust and dirt buildup.
Make sure the interior of the terminal enclosure is clean and dry, and free of dirt, dust, oil and rust.
Thermal cycling may also cause sealed joints, such as flange mounting bolts, to relax, resulting in leaks. Tighten threads and flange bolts.
Periodically check the sensing probes (thermostat or thermocouple) to make sure they are operating properly and that the connections are all good. Check proper grounding for safety.
Make sure the power is turned off before doing any maintenance procedures.
Robert C. Klein is a Key Accounts Manager, and has been with Watlow for more than 30 years. Watlow is a leading designer and manufacturer of heaters, controllers and temperature sensors.