Used to heat materials from wax to water, immersion heaters provide an effective means of heat transfer in liquid heating applications. To ensure that the heater specified for your application will meet your process needs, provide precise, thorough specifications to the heater manufacturer. Working together, you and the heater supplier will define the best combination of design, materials and performance ratings for your particular process.

Select a sheath material that will withstand the rigors of the chemistry. This is a critical consideration for heating equipment because the elevated temperature of the heater sheath or heat exchanger is a catalyst for accelerated chemical corrosion. Several choices often are available; however, if all other factors are equal, it is recommended that the most durable heater material be chosen. For example, if either a quartz or titanium heater will work in the application, select the titanium heater to minimize required maintenance -- it does not damage as easily as quartz. Any heater supplier can offer assistance with this selection.

An often-overlooked resource for technical assistance is the chemistry supplier. Refer to the chemical supplier's material safety data sheet (MSDS) for a breakdown of the chemistry components for heater sheath recommendation. Have this document available for the heater supplier should it be required.

Another important factor in determining heater needs is the physical layout of the tank in which the heater will be placed. The heater supplier will need to know tank dimensions, area available for heater placement and solution depth. These considerations will impact the heater's physical configuration. The ideal positioning for uniform heating would be placing the heaters evenly around the tank. Unfortunately, this usually is an impractical arrangement. In such a case, heaters should be placed in areas that promote even heating across the tank but also avoid direct contact with work in process. This will minimize the chance of the heaters experiencing physical damage.

Because heat rises, the tank's heat source should be placed near the bottom of the tank. Properly placed bottom heaters may provide more evenly heated solution than over-the-side designs. Heaters that are positioned too high in the tank can result in heat stratification problems, adversely affecting production quality.

In addition to heater positioning, the chemistry's ambient temperature and desired operating temperature must be taken into consideration. You also must determine how quickly the tank must be brought to the desired temperature. Many people are surprised at the length of time it takes to bring a tank to temperature. For example, a 500-gal tank being heated from 70 to 16oF (21 to 71oC) requires 110,000 W and 265 A service at 240 V to accomplish the task in one hour. If a six-hour heatup time is allowed instead, the same tank would require only 20 kW and approximately 50 A of power. Initial heatup time invariably is a compromise.

Some chemistries require special design considerations for heaters. For instance, electroless nickel tends to plate out more aggressively on horizontal heater surfaces, so in these applications, a heater design that maximizes vertical surfaces is preferred. Phosphate baths and caustic solutions over 20% concentration require derated heaters. A standard metal heater produces 35 to 40 W/in2 of heater surface area, and a derated heater typically is half that wattage. For the same overall heat output, the package size of the heater must be increased.

With regard to chemistries, immersion heater manufacturers publish solution guides for aqueous-based solutions that may serve as a valuable resource for determining what heater should be placed in a bath. Other types of solutions require a detailed look at the chemistry to determine a correct heater recommendation.

If large parts are run or if high throughput (lb/hr) rates are required for production, the mass of the parts going into the tank often becomes a heat calculation factor. Load factors create a heat sink effect in the solution and can dramatically draw down the tank temperature. If a lengthy initial heatup time is required, heat recovery may be insufficient when large loads are run. If the temperature of the load going into the tank and the pounds per hour of production are known, a heater supplier can calculate the requirements needed for heat recovery and compare this to the tank heatup calculation. Heaters must be sized for whichever heat requirement is greater: the initial heatup or the heat recovery needed.

Many considerations for determining tank heating are the same; however, there are additional factors to consider when sizing immersion coils.

With recent changes in ventilation requirements, surface heat losses have become a greater factor in heat calculations. Covering the tank during initial heatup will reduce surface heat loss substantially. At 160oF (71oC), an open, still tank will lose approximately 370 W of heat (almost 1,200 BTU) per square foot of solution surface per hour. With 120 ft/min of ventilation or mild agitation, the figure climbs to 650 W/ft2. With higher ventilation rates and aggressive mechanical or air agitation, the heat loss factor can increase greatly to four or five times that of a still tank. If the airflow rate of the ventilation system and volume/temperature of air agitation is known, the amount of additional heat required to offset these losses can be determined. Automatic solution replenishment also must be reviewed to determine how it impacts heater sizing. The two most important factors are incoming temperature and flow rate. Sidewall losses generally are not critical unless the tank is outdoors or in an open environment subject to cold temperatures.

Whatever the environment, a minimum 2" of solution should cover the heater's hot zone (the heated portion of the heater) at all times in the tank. Any portion of the hot zone exposed to air will cause an overtemperature condition to occur. The bottom of the tank also must be examined to ensure the heater is situated above any sludge that may accumulate (figure 1). In applications that generate high levels of solids, it is recommended that any horizontal heating surfaces be minimized. Horizontal surfaces can act as shelves and contribute to solids accumulating on the heater. Sludge and scale encrusted on heating equipment act as an insulation blanket, and the heat being produced is unable to transfer into the solution. This causes inefficiencies with the equipment and can lead to element burnout. Low solution level and the buildup of solids on heaters are two of the most common causes for premature heater failure.

Many considerations for determining tank heating are the same regardless of the heating method. However, additional factors should be considered when sizing immersion coils. Foremost, the steam pressure or, in the case of hot water systems, water temperature available at the tank, must be known. Do not assume that a pressure gauge at the boiler accurately indicates the pressure available at the tank. Line losses can be substantial and are affected by factors such as the distance to the tank, pipe size, level of insulation, and plant ambient temperature. New applications may require a review of the central heating system to ensure that the plant boiler has the necessary capacity.

Figure 1. A minimum of 2" of solution sould cover an immersion heater's hot zone at all times to avoid an overtemperature condition.

Take Measures to Ensure Safety

Electric heaters never should be installed without first taking proper precautions. For instance, electric heaters exposed to air can reach temperatures in excess of 1,300oF (~700oC). Therefore, thermal overload protection is an absolute necessity, especially in applications involving plastic tanks or other combustible materials.

Thermal protectors are factory-installed devices located at the top of the heater hot zone. In the event of low solution-level conditions, thermal protection devices minimize the potential for fire by shutting off heater power. Users should note that they are not designed to shut down power if an overtemperature situation occurs as the result of scale or sludge buildup. Both single-use and resettable devices are available. A resettable device is preferred because it alerts the operator to a tank solution-level problem with an audible or visual alarm and requires intervention to restart the system.

A liquid-level device also should be installed on the tank. Thermal protectors should not be used to alert the user to a low-level condition in a tank, but all too frequently users erroneously expect them to do so. Thermal overload switches are safety devices and should not be used as substitutes for level switches. It is vital that the user understand the correct installation of thermal protectors and that the equipment be inspected regularly. Also, only use thermal devices that are designed specifically for the heaters in use, and never mix heater and fuse manufacturers. If single-use thermal fuses are chosen, always keep spares on hand.

Most processors rely on heater suppliers for assistance. However, for suppliers to make the correct heating equipment recommendation, they need precise, accurate information from the user.