Metal Finishing Guide Book

2013

Issue link: https://metalfinishing.epubxp.com/i/218436

Contents of this Issue

Navigation

Page 207 of 843

ing material to flow from the grounded pumps or kitchen to charged atomizers, yet block voltage from leaking back from the atomizers to the pumps or kitchen. This means that the hardware in the pump house and distribution system can be virtually the same as for a conventional solvent-based system, or for an indirect charged waterborne system. Since the primary advantage of voltage block technology is that it limits the amount of hardware at high voltage, it is important to install these devices as close to the atomizer as possible. The connecting hoses between the voltage block and the atomizer are at high voltage, so keeping them short minimizes both the capacitance and the opportunity for accidental grounding. A voltage block for one atomizer is compact, requiring about as much space as a small electrical control box, so it can be mounted inside the spray or ventilation booth close to the atomizer. The mini-isolated systems created by voltage blocks do not have the problems found in large isolated systems because less hardware is charged with electrical energy. Capacitance is greatly reduced, making the system inherently safer. Safer systems mean easier access to the inside of the spray booths. Often a simple guard rail and warning sign can replace elaborate caging and interlocks. Voltage leakage problems are minimized, since only the atomizer and a short hose are charged, making it easy to keep the TEs up to a high level. By isolating atomizers from each other, mini-isolated systems have some unexpected advantages. First, the NFPA limitation concerning handguns no longer applies. Each handgun is independently isolated from every other handgun so the voltage to idled guns can be turned "off." In fact, spraying waterbornes with a handgun and voltage block can be easier than spraying the old solvent-based material with the handgun. Solvent-based material is charged at the gun barrel so a high-voltage cable to the gun is required. Since waterborne coating material conducts electricity, however, it can be directly charged at the voltage block and the cable to the gun can be eliminated. With the cable gone, the gun feels lighter and the hose bundle flexes more easily. Even automatic atomizers, such as rotaries or disks, require less maintenance if the coating material is charged at the voltage block rather than at the atomizer, as it was when spraying solvent-based material. This is because the high-voltage cables last longer when they don't get flexed over and over by the motion of the gun mover or robot. A second unexpected advantage of making each atomizer into a mini-isolated system is that every atomizer can operate at a different voltage, or at zero voltage, as desired. For example, in-plant experience might show that the rotary atomizers in a paint line run best at 60,000 V, but the handguns perform better at 45,000 V. With voltage blocks, the handguns and the rotary atomizers can run at different voltages, yet all can be supplied from a common paint distribution system. Finally, in an application system for waterbornes and using voltage blocks, any atomizer in the system can be shut down and repaired or changed out, even though the other atomizers are operating at high voltage. The ease of access to production application equipment is comparable between a voltage-blocked waterborne system and the solvent-based coating material system it replaces. CONCLUSION An often unstated goal when converting a coating application system to waterbornes is to disrupt "the way it's done now" as little as possible, particularly if the existing system has good equipment and is performing well. That goal is not out of reach because the existing process, and much of the existing equipment, 202

Articles in this issue

view archives of Metal Finishing Guide Book - 2013