Southern Metal Finishing

Re: Prevent Solution Overheating when using Immersion Heaters

This article was published in the June 2005 issue of Southern Metal Finishing. If you would like register to receive our free newsletter and review our online archives please visit www.southernmetalfinishing.com

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Ray Lokar and Dennis Rezabek  of  Process Technology

Electric immersion heaters represent 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 prevent overheating of your elements and possible damage to your products.

All heaters operate on the principle of heat exchange via temperature differential. In the case of electric heaters, the operating temperature of the resistance wire is about 7000 C (12920 F). 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 (7000 C) if heat is not readily removed from the heater surface, as may occur with insulating materials. Some typical insulators are air, sludge, solids build-up on the heater surface (incrustation), and fluids having low thermal conductivity.

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 50 mm (2 in.) of solution remains above the hot zone of the element(s) at all times. Failure to fully submerge an element hot zone can cause damage to the heater, the tank and its contents, as well as pose a substantial fire risk. Liquid level controls and thermal protection devices should always be employed to prevent the possibility of flammable materials igniting.

Sludges are hard to avoid in many process tanks, but selecting a heater style, configuration, or height to avoid sludge contact while periodically removing sediment will prevent this common form of overheating.
Solids build-up, or fouling of the heater surface, is influenced by the heater surface material and the solubility of certain ions present in the solution. In every solution heating application there is a film boundary between the heater surface and the solution being heated. This results in a heater surface temperature from 110 C to over 560 C (200 F to 1000 F) above solution temperature. Although agitation across the heater surface will reduce these temperatures, it is usually impractical to provide in-tank agitation sufficient to cover the entire heater surface area and prevent "hot spots" which will occur at these stagnation zones. Reduction in heater watt density, often called de-rating, will reduce element surface temperatures. De-rating is usually achieved either by increasing the surface area of the element or lowering the internal wire temperature for a given wattage heater. Published data gives recommended watt density ranges of 6 W/cm2 (38 W/in2) for alkaline baths, 2.5 W/cm2 (16 W/in2) for dilute acids, and 1 W/cm2 (6.5 W/in2) for phosphatizing baths. Obviously, your individual experience may provide a more effective starting point than these recommended.

Similar to the result of solids build-up, fluids with low thermal conductivity will cause an increase in heater surface temperature as well as localized overheating of solutions in contact with the element. Care should be exercised to keep temperature sensitive components from these areas or provide additional mixing to decrease these 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 often neglected or not given proper consideration. If you expect a foot of sludge in the bottom of your tank, don t 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 by the placement and removal of parts, i.e., the side(s) parallel to the parts  "direction of travel."

When selecting a heater, ask yourself these questions:

   •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 and can it be easily removed 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 get you started on the road to a 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 varying from simple on/off thermostats to self-tuning PID digital controls. Each have their respective cost and /or operating advantages. In this case we will focus on the sensing device itself, as its relative placement in your process can have more influence on your final product than the accuracy of the temperature control does.

The temperature controller s sensor location will play a major role in how your parts will clean, etch or plate in your process. This is, of course, provided that the other components of the system have been properly matched. Since the controller can only respond to the temperature changes it obtains through feedback from the sensor, the location of the sensor will greatly influence the ability of the controller to regulate the temperature about the desired set point. 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.

Temperature Control Sensor below the Heat Source
Since convective heat rises in most applications, placing the sensor below the heat source can result in the control measuring a consistently colder section of the process tank, causing overheating of the balance of the tank contents. This condition can cause excessive evaporation, as well as damage to the tank, tank contents and heater elements. Care should be taken to assure that the temperature sensor is secured in a location above the bottom of any heat source.

Temperature Control Sensor above Heat Source
Conversely, locating a sensor above the heated zone may result in its being removed from fluid contact and cause an equally dangerous operating condition, as the sensor will read a much cooler air temperature and keep the heaters on. This condition can also cause excessive evaporation, damage to the tank, tank contents and heater elements. Care should be taken to assure that the temperature sensor is secured in a location that will never be exposed to operation out of the fluid to be heated. Liquid level controls can help assure the minimum fluid depth in the tank to prevent this occurrence.

Temperature Control Sensor near the Heat Source
In most applications relying on convective heat transfer and long product cycle times, placing the sensor close to 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 (minimal thermal lag); therefore, the heater will cycle frequently, reducing the potential for overshoot and undershoot at the workload. With the sensor placed at or near the heat source, it can quickly sense temperature changes of the element, thus maintaining tighter control.

Temperature Control Sensor near the Workload
When the system experiences frequent workload changes, placing the sensor closer to the workload will enable the sensor to measure the load temperature change faster, and allow the controller to take the appropriate regulation more quickly. However, in this type of arrangement, 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) temperature at the workload. Solution agitation or mixing can minimize these differences.

Sensor halfway between Heat Source and Workload
When the heat demand fluctuates, placing the sensor halfway between the heat source and workload will divide the heat transfer lag times equally, producing reduced overshoot and/or undershoot. This sensor location is typically the most practical in the majority of thermal systems. Solution agitation or mixing can again minimize these differences.