Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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decorating and stenciling. Use of the dry-filter spray booths requires a regular schedule of filter re- placement. Codes require that filters be inspected after each period of use and that clogged filters be discarded and replaced immediately. Used filters must be re- moved 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 com- bination with high-holding-capacity dry filters, result in lower operating costs and higher production rates. There are two filtration principles that apply to dry fil- ters, 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 heav- ier overspray particulates are force- fully deposited at various depths in the filter. This process, called depth loading, is optimized with the slit and expanded kraft filter. Baffle fil- ters are available in metal panels, cor- rugated filters, pleated and expand- ed 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 limit- ed 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 re- duce 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 excel- lent holding capacity with fair effi- ciency. Fig. 14. Vehicular cross-draft spray booth. Fig. 15. Vehicular semidowndraft spray booth. Strainer Filters The second primary filtration prin- ciple 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. Non- woven cloth filters have excellent efficiency but poor holding capacity. Another dis- advantage is that they are face loaded. Fiberglass filters are a little less efficient, 213

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