Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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An isolated electrostatic system for waterbornes can have multiple automat- ic atomizers or it can have a single handgun. It cannot have both, nor can it have more than one handgun. National Fire Prevention Association (NFPA) reg- ulations dictate that the electrostatic voltage to any handgun must turn off when the trigger is released. Since all the atomizers in a waterborne system are connected electrically by their fluid hoses, the voltage remains "on" to an idle hand- gun as long as it is "on" to any atomizer in the system. This means that a hand- gun cannot be used with electrostatics if there are other atomizers in the system, and without electrostatics it is impossible to achieve the maximum TE. To summarize, completely isolated systems have the potential to allow the maximum TE for a given application because they allow the coating material to be directly charged with electrostatics. In practice, that potential is rarely achieved unless the application system is very small because it is difficult to keep the electrostatic charge isolated. Fully isolated systems can store too much electrical energy and become dan- gerous. To prevent operator injury, such systems need to be caged and equipped with interlocks to prevent access while the system is operating. Unfortunately, this means that even minor maintenance to the equipment is impossible while any part of the system is operating at high voltage because all the equipment is elec- trically connected by the fluid hoses. This is also why only one handgun can be permitted in a completely isolated system. Indirect Charging Indirect charging avoids many of the problems of completely isolated systems, but at a price. Indirect charging systems charge the coating material between the nozzle of the atomizer and the part, rather than at the atomizer. This is done by placing the high-voltage electrode in the air stream near the nozzle but not in direct contact.The coating material particles pick up a charge after they leave the atomizer. Because the high voltage never directly contacts the application equipment, there is no opportunity for the charge to drain away down paint hoses. On the oth- er hand, any charge inadvertently imparted to the application system drains away harmlessly to ground because the system is not isolated from ground. In fact these systems can, and should, be intentionally grounded to prevent storage of electrical energy. There is little capacitive storage of electrical energy in an indirect charged electrostatic system so any electrical hazard is greatly reduced. This means that safety caging and interlocks can be less intrusive, or eliminated completely. With handguns no longer connected electrically by their hoses, there is no need to limit the number of handguns in a particular application system. Several coating application equipment manufacturers offer atomizers spe- cially designed for indirect charging. These devices position the electrostatic electrode away from the coating material stream so that there is no direct elec- trical contact between the application equipment and components charged with high voltage. Some conventional atomizers can also be retrofit with indirect charging apparatus, making the conversion to waterbornes easy and relatively in- expensive. The downside of indirect charging is lower than optimum TE. Indirect charg- ing does improve TE over comparable nonelectrostatic systems. Unfortunately, tests prove that the TE with indirect charging is less than the TE that can be 144

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