Metal Finishing Guide Book

2012-2013

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tion to the surface of soiled parts. Although the vapor generally stays below the cool zone of an open-top degreaser there is always some solvent loss. Drafts in the area around the degreaser will cause solvent vapor to be pulled out. Parts loading causes losses as work to be cleaned disturbs the solvent/air interface. In addition cleaned parts may also carry solvent with them when removed from the degreaser. Up to 70% of the solvent in a traditional open-top degreaser can be lost through these factors over a year. Consequently, procedures are called for to minimize this loss, in order to provide a working environment in which vapor exposure is below the levels permitted by Occupational Safety and Health Administration (OSHA) regulations and an industrial environment, which meets requirements set by the EPA. These procedures are outlined below. In-line vapor degreasers include several types of conveyorized equipment ��� large, automatic units, which can handle a large volume of work and are enclosed to provide minimal solvent loss. These units include the monorail, crossrod, and vibratory degreasers. The monorail conveyorized degreaser uses a straight-line conveyor to carry parts into the degreaser, lower them into the vapor zone, raise them into a cooling zone, and finally out of the degreaser. This process is ideal when production rates are high and large parts to be cleaned can be suspended from hooks or hangers. The cross-rod conveyorized degreaser is generally used for processing small parts in baskets, trays, or even mesh cylinders. In this equipment the parts are placed in the degreaser and removed from it at the same opening, while the conveyor carries the work through immersion dips, vapor zones, and drying zones. The vibratory degreaser is a patented unit. In this process the work is dipped in solvent then rises on a vibrating spiral elevator trough through a counterflowing rinse of clean solvent distillate, a vapor zone, and finally a drying section. Although these units are enclosed there is still some solvent loss through the openings where work enters and leaves the equipment, and through the joints and seams of the equipment. CHARACTERISTICS OF CHLORINATED SOLVENTS For many years 1,1,1-trichloroethane (methyl chloroform or MCF) was the solvent of choice for vapor degreasing because of its status as a non-VOC; it was not classed as a VOC compound under federal regulations because it does not contribute to the generation of ground-level ozone and the consequent production of urban smog. This situation changed drastically at the end of December 1995, when MCF was phased out of production for all uses except chemical feedstock, under the provisions of the U.S. Clean Air Act and the international Montreal Protocol, due to its action as a depleter of stratospheric ozone. This led to a misunderstanding in many quarters that all chlorinated solvents were coming under a ban. In fact corporate edicts in some companies required their manufacturing divisions to cease using chlorinated solvents for degreasing as soon as practicable. This unfortunate misunderstanding has led a number of companies to experiment with a variety of alternate cleaning technologies. The fact is, chlorinated solvents are still very viable products and remain one of the best choices for surface cleaning and vapor degreasing. Trichloroethylene, perchloroethylene, and methylene chloride are not regulated for ozone-de104

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