Metal Finishing Guide Book


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tal area for increased capacity in comparison to the bench-top booth. It is designed with a table height shelf. Both the bench-top and bench booths are perfect for spraying small objects and decorating and stenciling. Use of the dry-filter spray booths requires a regular schedule of filter replacement. Codes require that filters be inspected after each period of use and that clogged filters be discarded and replaced immediately. Used filters must be removed to a safe, well-detached location or placed in a water-filled metal drum and disposed of at the close of the day's operation. A draft gauge is typical standard equipment with dry-filter spray booths. The gauge is designed to indicate when paint filters have become sufficiently loaded and replacement is required. Keep in mind that high-transfer-efficiency spray systems, when used in combination with high-holding-capacity dry filters, result in lower operating costs and higher production rates. There are two filtration principles that apply to dry filters, baffle and strainer, each having advantages and disadvantages. Baffle Filters The baffle principle creates a high turbulence in the air flow as the air moves through the filter. The heavier overspray particulates are forcefully deposited at various depths in the filter. This process, called depth loading, is optimized Fig. 14. Vehicular cross-draft spray booth. with the slit and expanded kraft filter. Baffle filters are available in metal panels, corrugated filters, pleated and expanded kraft, and Styrofoam pads. Metal panels have excellent holding capacity, but their ability to trap a high percentage of solids from the spray booth is limited and the exhaust air is poor. Also their efficiency is low. The metal panels are most efficient when intermittent production exists, or when used as a precollector to reduce the replacement frequency of more efficient filters. Corrugated filters also have excellent holding capacity and poor efficiency/performance. Pleated kraft filters have excellent holding capacity with fair efficiency. Generally, pleated kraft is used in light production situations and with slow-drying coatings. Expanded kraft filters exhibit good efficiency but only fair holding capacity. And lastly, Styrofoam pads have excellent holding capacity with fair efficiency. Fig. 15. Vehicular semidowndraft spray booth. Strainer Filters The second primary filtration principle is the strainer filter. This filter simply screens overspray from the air stream. Particles finer than the screen work through the screen, where as larger particles become trapped. Strainer filters come in two types. Nonwoven cloth filters have excellent efficiency but poor holding capacity. Another disadvantage is that they are face 782

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