Electric immersion heaters are an efficient and economical method of heating process solutions. When properly selected and installed, they can provide years of safe, reliable service. Here are some principles of operation and selection criteria to help you avoid overheating your elements and possibly damaging your products.
All heaters operate on the principle of heat exchange via temperature differential. With an electric heater, the operating temperature of its resistance wire is about 1,292oF (700oC). This internal temperature vs. the temperature of the fluid to be heated is the differential, or driving force, that heats the fluid.
A fact often overlooked is that the heater's surface temperature can approach the wire temperature (1,292oF) if heat is not readily removed from the heater surface, as may occur with insulating materials. Some typical insulators are air, sludge, solids buildup on the heater surface (encrustation), and fluids with low thermal conductivity.
Air. Air is an all too common problem -- not because the user wants his tank heater to heat it, but because sufficient fluid levels above the heater are absent. Evaporative and drag-out losses must be considered when selecting and placing a heater. Install the heater so that at least 2" (50 mm) of solution remains above the elements' hot zone at all times. Failing to fully submerge the hot zone can damage the heater, the tank and its contents as well as pose a substantial fire risk. Liquid-level controls and thermal protection devices always should be employed to prevent the possibility of flammable materials igniting.
Encrustation. Solids buildup, or fouling of the heater surface, is influenced by the heater surface material and the solubility of certain ions present in the solution. In addition, in every solution heating application, there is a film boundary between the heater surface and the solution being heated. This boundary layer causes the heater's surface temperature to be 20 to more than 100oF (11 to 56oC) higher than the solution temperature. Although agitation across the heater surface can reduce these gradients, it usually is impractical to provide in-tank agitation sufficient to cover the entire heater surface area. Hot spots can develop at these stagnation zones.
One way to minimize the chance of encrustation is to reduce the heater's watt density. Often called de-rating, this will reduce element surface temperatures. De-rating usually is achieved by increasing the element's surface area or lowering the internal wire temperature for a given wattage heater. Published data recommends watt density ranges of 38 W/in2 (6 W/cm2) for alkaline baths, 16 W/in2 (2.5 W/cm2) for dilute acids, and 6.5 W/in2 (1 W/cm2) for phosphatizing baths. Individual experience may provide a more effective starting point.
Fluids with Low Thermal Conductivity. Similar to the result of solids buildup, fluids with low thermal conductivity will cause an increase in heater surface temperature and localized overheating of solutions in contact with the element. Care should be exercised to keep temperature-sensitive components from these areas or to provide additional mixing to decrease the thermal gradients.
After heater sizing and watt density determination, due consideration must be given to materials of construction. Corrosion guides are as numerous as corrosives. Most guides do not cover all the factors affecting rate of corrosion (e.g., aeration, presence of halogen ions, applied plating voltage, etc.). Your process supplier knows most of these factors, and his recommendation is essential.
Heater configuration is another area either neglected or not given proper consideration. For example, if you expect a foot of sludge in the bottom of your tank, you should not install a bottom heater just because it is a convenient location. If you install side-mount heaters, install them on a side least likely to be hit during the placement and removal of parts, i.e., the position parallel to the parts' direction of travel (figure 1).
- Will the configuration interfere with the parts being processed?
- Is the watt density appropriate for the solution being heated?
- Will the heater guard protect the element from contact with parts? Can it be removed easily to permit periodic inspection and cleaning?
- Can the heater be securely anchored to prevent movement that may cause damage to the element or parts being processed?
- Is the sheath material selected compatible with the solution to be heated?
- Are there any flammable materials near the proposed heater location?
Careful consideration of these basic principles will start you on the road to successful operation.
The other critical component of any aqueous processing application is the selection of an appropriate temperature control system. Controls are available in many designs from simple on/off thermostats to self-tuning PID digital controls. Each has cost and operating advantages. Focus on the sensing device itself, as its relative placement in the process can have more influence on your final product than the temperature control's accuracy.
The temperature controller's sensor location will play a major role in how your parts will clean, etch or plate in your process (assuming, of course, that the other system components have been properly matched). Because the controller can respond only to the temperature changes it receives through feedback from the sensor, the sensor's location greatly influences the controller's ability to regulate the temperature at the desired setpoint. To minimize the chance of any sensor movement, all temperature-sensing devices should be secured in a protected area of the tank or placed in a protective thermal well.
Where to Put ItDepending on your process, you may want to locate the sensor near the heater, near the load or equidistant between them.
Temperature Control Sensor Near the Heat Source. In most applications relying on convective heat transfer and long product cycle times, placing the sensor near the heat source will keep the heat fairly constant throughout the process tank. In this type of system, the distance between the heat source and the sensor is small, minimizing thermal lag. The heater will cycle frequently, reducing the potential for overshoot and undershoot at the work load. With the sensor placed at or near the heat source, it can quickly sense element temperature changes, thus maintaining tighter control.
Temperature Control Sensor Near the Work Load. When the system experiences frequent work load changes, placing the sensor closer to the work load will enable it to measure load temperature changes faster. This allows the controller to take the appropriate regulation more quickly. One drawback with this type of arrangement, though, is that the distance between the heat source and the sensor may be large, causing excessive thermal lag or delay across the tank. Therefore, the heater cycles will be longer, causing a wider swing between the maximum (overshoot) and minimum (undershoot) temperatures at the work load. Solution agitation or mixing can minimize these differences.
Sensor Between Heat Source and Work Load. When the heat demand fluctuates, placing the sensor halfway between the heat source and work load will divide the heat transfer lag times equally (figure 2). This sensor location typically is the most practical.
Just as there are positions that will provide accurate temperature regulation in your tank, there also are locations that will not provide a true picture of your process. Here are a few places to avoid.
Temperature Control Sensor Below the Heat Source. Because convective heat rises, placing the sensor below the heat source can result in the control measuring a consistently colder section of the process tank. This causes the balance of the tank to overheat and can cause excessive evaporation as well as damage to the tank, tank contents and heater elements. Be sure the temperature sensor is located above the bottom of any heat source.
Temperature Control Sensor above Heat Zone. Conversely, locating a sensor too far above the heated zone may result in its being removed from fluid contact and cause an equally dangerous operating condition: The sensor will read a much cooler air temperature and keep the heaters on. This condition also can cause excessive evaporation, damage to the tank, tank contents and heater elements. Care should be taken to ensure that the temperature sensor is secured in a location that will never be exposed to operation out of the fluid to be heated.