How to Select the Right Heater for Your System

Q: Our shop is looking for recommendations for an electric heater setup for maximum uptime and optimum performance. What suggestions do you have?

A: When selecting equipment to operate on process lines, there is a wide variety of products to choose from. While a large number of these product combinations will be able to do the job, some selections will provide fewer maintenance and repair problems after installation. A liberal application of wisdom and foresight will help to identify the better product selections for specific processes.

First, choose compatible materials. Select heaters with surface materials that are compatible with the chemicals in the tank. Sometimes a particular material may be compatible with a chemical at certain concentrations and below certain temperatures, but this is not the best choice. The surface temperature of the heater will be much hotter than the bulk liquid temperature, and the internal operating temperature will be hundreds of degrees hotter than that. The increased energy coming from the heater can trigger a chemical reaction on the heater surface (corrosion). Instead, select a material that is compatible with this chemical at all concentrations and all temperatures. This will help prevent future problems.

Also, consider the effect of chemical drag-in from the prior bath when selecting heater material. A stainless steel covered heater may be suitable for a water rinse tank, but, if the previous bath is sulfuric acid, some pitting of the heater surface may occur.

Installing at least two immersion heaters in process baths will provide multiple advantages. First and foremost, it will allow production to continue if one of the heaters needs to be removed from service. The largest demand for heating the bath is the initial heat up to operating temperature. The total heating requirement was likely based upon this. Fortunately, once the bath reaches the operating temperature, it typically takes about 25 percent of that energy to maintain. Thus, while all of the heaters are needed to bring the tank up to temperature in time, only one is needed to maintain the temperature.

A second advantage will be a much lower cost for a replacement heater, if it’s needed in the future. Just like trying to find a place to live when relocating to a new city, some areas in the process tank are better than others for immersion heaters. Here are some examples of “bad neighborhoods” for electric heaters:

Directly in front of the exhaust intake. Positioning the heater heads directly in the path of the heated chemical fumes will shorten their service life. If this is the only location to install the heaters, it is wise to select a heater with a seamless heater head, such as one that is constructed of welded sheet metal.
Directly beneath dripping liquids. Parts moving between baths will be continuously exposed to dripping chemicals. If this is the only location to install them, it is wise to provide some sort of “umbrella” to deflect the dripping chemicals (preferably back into the bath) or again select a heater with a seamless heater head.
Underneath or sealed around tank covers. Tank covers concentrate the heated chemical fumes which may work their way into the heater heads. The best approach for installing heaters around or under tank covers is to select ones with seamless heads. Another choice would be to select bottom heaters and position the junction box above or away from the tank cover.
Avoid being buried in sludge. Bottom heaters are not a good selection for a tank where accumulations of sludge and precipitates settle to the tank bottom. This will be the same as if the heater was covered in buildup. In these baths, the better heater choice is a side-mounted or hanging style that will be above the sludge.

Scale or chemical buildup on the surface of the heater will inhibit heat transfer. This increases the internal operating temperature of the heater, which will shorten the effective service life of the heater and may trigger corrosion on the heater surface. Shops working with manganese phosphate baths or concentrated cleaning chemicals don’t need to be told this. Here are some points to consider when selecting heating equipment for these types of baths:

When heating baths have a tendency for buildup, choose a heater configuration that will simplify the cleaning process. Avoid heaters with complex surface contours and tight spacing, as well as conjoined heating tubes or bundles of multiple heating elements. A better approach would be a separate single-tube construction. This would make the heating tube very easy to separate for cleaning and allow easy access to clean all sides of the heater surface.
Plan ahead to install the heaters in a relatively easy location to access and remove for service. This will save time and trouble for the maintenance crew. Some customers select heaters with extra long lengths of wire and conduit. This allows them to remove heaters from their tank and clean the surfaces nearby without needing to disconnect the wiring from the junction box.
For the phosphate coating baths, consider specialty heater products that are specifically designed for operation in such a heavy buildup application. History has demonstrated that investing in these specialty heaters provides a short return on investment in the form of longer service life and durability.

Edward Dulzer is the technical support manager at Process Technology. Visit processtechnology.com.