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 in effect 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 also can 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 inline, 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 -- 30 W/in2 compared to 23 W/in2 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) and non-pressurized (open tank) systems.
Pressurized SystemsThe 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 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 many National Pipe Thread (NPT) sizes, materials, wattages and voltages, and with various sheath materials and thermostats. Applications include deionized or demineralized water, oils, hydraulic and crude oil, caustic cleaners, chemical baths, 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 3,000 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 then is inserted into the open end of the bayonet, allowing for removal of the heater without draining the tank.
Circulation or inline 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 1.25" NPT screw plug size to as large as 14" dia. Custom units have been made up to 44" nominal pipe size.
Booster heaters are a type of circulation heater. They are suitable for applications using lower wattage, including inline operations or engine preheating. Booster heaters with copper and steel sheaths are suitable for heating water and oils.
Non-Pressurized SystemsOver-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. Sheath materials, kilowatt ratings, terminal enclosures and mounting methods can be selected to match the process requirements. Over-the-side heaters are suited for heating small quantities of water, oils, solvents, salts and acids, and they often are used for freeze protection.
Selecting a HeaterMost 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.
Properties of the Material to be Heated. It is 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 heater sheath, 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; a light oil such as vegetable oil could use up to 30 to 40 W/in2. 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.
Startup and Maximum Operating Temperatures. In essence, this is the temperature differential (T) from startup to operating conditions.
Maximum Flow Rate of the Material Being Heated This is needed to determine the wattage requirements. The minimum flow rate also may 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 gases.
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.
Volume or Weight of the Heated Material. These are needed to determine kilowatt requirements for startup.
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 for 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 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.
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.
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 600 V. Consult the heater manufacturer regarding the agency approvals for heater voltage and diameter limitations. The phase is not limited by anything other than possibly the type of heater and the number of elements making up the heater assembly.
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 also are 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 typically are used when environmental conditions pose no problem.
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.
Physical Sizing and Wattage Requirements. A 10 kW heater can be a screw-plug, square flange, 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 on how long the tank is. A larger flange size may be needed due to requiring a short heater, or if there is a lot of length, a plug or square flange may be sufficient.
Installation TipsEach heater's current should be checked before installation because moisture can enter the heater element insulation during shipping 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 VDC 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 200 to 300oF (93 to 149oC) 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.
The proper temperature rating of the wire coming into the heater also is important. A minimum of 392oF (200oC) wire for process heaters is recommended although higher-rated wire may be required for some applications.
Power feed line connections must be compatible with the heater and meet 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 shut-off switches can be installed to avoid heater failure should the liquid level drop too low.
SIDEBAR: Sizing the HeaterThe 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, or Q = W x CP x T) 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).