Solution heating and cooling are essential parts of many finishing processes. The temperature of solutions affects cleaning effectiveness, rates of electrodeposition, the ability to produce heavier anodic coatings on aluminum (hard anodizing) and many other finishing-related chemical processes. The most common equipment for controlling the temperatures of solutions include heat exchangers, immersion heaters and chillers.
Heat exchangers may be used to heat or cool a solution. They operate by recirculating a liquid (or steam) that is either hotter or cooler than the process solution. In the simplest installations the liquid circulates through pipes, coils or plate-type units immersed in the process solution. As the steam or liquid circulates through the immersed pipes, plates or coils, heat is exchanged and the process solution becomes warmer or cooler.
Larger installations use external heat exchangers. They occupy little space in the process tank, since they recirculate the process solutions through the external heat exchanger, using a pump. The tubes in the external heat exchanger circulating the process solution are surrounded by a shell or plate-and-frame enclosure containing liquid that is either cooler or warmer than the process solution. This fluid is also recirculated, using a pump. Heat is transferred from the water or other liquid in the shell to the process solution in the tubes if the objective is to heat the process solution. Heat is transferred from the process solution to the surrounding coolant if the objective is to lower process-solution temperature.
For either type of exchanger—external or internal—materials of construction are important. Plating and anodizing solutions may be very corrosive. Depending on the chemical composition of the process solution, various metals and plastics may be appropriate, ranging from mild steel to titanium, graphite, tantalum and fluoropolymers.
Sizing of heat exchangers requires calculations based upon the specific heat of the solution, the rate of heat transfer of the metal or plastic used in construction, evaporative losses and so on. To make these calculations, refer to the 2006 PRODUCTS FINISHING DIRECTORY, “Evaluation and Selection of Heat Exchangers”.
Electric Immersion Heaters
Immersion heaters offer a simple solution to heating process solutions. They use electric resistance heaters sheathed in various metals and plastic-coated metals. They are common in both large and small finishing systems and are available in a variety of sizes, shapes and configurations. These units can be mounted anywhere in the tank—on sides, front or bottom—to minimize work-flow problems.
As is the case with heat exchangers, sizing of heaters is important, requiring watt densities to be calculated based on initial heat-up speed, evaporative losses and work loads in the tank.
Materials of construction are vital to a long-term economical operation. They range from carbon steel to Incoloy and titanium. Quartz heaters are also used. Fluoropolymer-sheathed metal heaters have excellent resistance to most electroplating solutions, and like Teflon-coated fry pans, offer excellent release properties and easy cleaning.
It is vital in the interest of safety and fire protection to use temperature controllers that minimize the effects of short circuits, loss of solution from a tank—particularly a plastic tank—and incorporate redundant safety controls.
Detailed information on heater construction, sizing and installation may be found in the 2006 PRODUCTS FINISHING DIRECTORY “How to Choose an Electric Immersion Heater”.
Chillers absorb heat from process solutions such as anodizing or plating solutions, using heat exchangers circulating chilled water from a refrigerating condensing unit. For very large installations water from a cooling tower may be used. Water-cooled chillers require condensate water treatment to eliminate mineral buildup.