Baity Air and Engineering Laboratory at the
University of North Carolina at Chapel Hill

Current Projects

Performance of Metal Working Fluid Mist Collectors

In the first phase of this study, PhD student Pete Raynor developed a method to evaluate collector performance at mist removal. This protocol was then used in the Baity Air Lab to evaluate the efficiencies of eight commercial collectors under conditions that simulate plant practice. An Aerosizer was used to determine mist counts; this instrument sizes droplets larger than about 0.3 m in diameter at a rate of up to 50,000 droplets per second. Efficiency data against droplet diameter were generated for each stage of the collectors and for each collector with all stages installed. Pressure drop across each collection stage and the entire collector was also measured. Results show that although fiber filters can be effective when new, their collection efficiency decreases over time as the filters become loaded with oil mist. This increased loading on the filters also causes pressure drop across the filter to increase, resulting in increased fan operating costs. It is the balance between efficiency and pressure drop that determines the overall effectiveness of an industrial mist collector.

Because the length of the tests on the eight commercial collectors were limited to a maximum of
seventeen days each, the second phase of the study was to perform long-term tests on collector performance. A three-stage prototype collector was developed that defines the state-of-the-art; Maryanne Boundy and Steve Cooper conducted tests for one year on this collector to determine how performance changes over time and to develop the maintenance schedules necessary to keep performance high. With collaboration of investigators from NIOSH, these results are now being validated through additional, long-term tests on an identical collector at a Ford transmission plant in Sharonville, Ohio. Figure 1 shows the mist removal efficiency of a second-stage filter in this prototype collector plotted against droplet diameter for tests conducted over several months; notice the decrease in efficiency as the filter became loaded with oil mist. Although the fficiency of the second stage is low for small droplets, a third-stage filter downstream provides additional protection. The purpose of the second-stage filter is to reduce the mist load to the third stage, which has limited capacity and is expensive to replace. Figure 2 shows the pressure drop across the housing of the collector, each filter placed individually in the housing, and all filters placed in the housing for the Sharonville collector; again notice the increase in pressure drop over time as the filters become loaded with oil mist.

Figure 1. Efficiency against droplet diameter for second-stage filter of the prototype collector in
Sharonville plant.


Figure 2. Pressure drop across housing, each filter placed individually in the housing, and all filters placed in housing as a function of time.

The following abstract describes research done on collector performance at the Baity Air Lab.

Abstract: "Performance of Industrial Equipment to Collect Coolant Mist" David Leith, Peter C. Raynor, Maryanne G. Boundy, Steven J. Cooper. American Industrial Hygiene Association Journal. Dec 1996.

Because manufacturers offer many kinds of collectors to remove coolant mist from ventilation air, the best choice for a given application is seldom obvious. A protocol was developed to evaluate collector performance in the laboratory, under conditions that simulate plant practice. Seven collector manufacturers provided eight samples of their products, each designed to handle 1700 m3/hr (1000 cfm) of air flow. Each collector was tested using mists of mineral oil, soluble oil, and synthetic fluid; for a three-stage collector, tests with each metalworking fluid lasted for thirteen 24-hour days. For each collector stage as well as for each assembled collector, tests established the relationship between efficiency and droplet diameter for droplets from less than 0.3 (m to about 6 (m in diameter. Substantial differences in efficiency and pressure drop were found among the stages and assembled collectors. Some metal mesh filters worked well as first stages, whereas pocket filters and an electrostatic precipitator worked well as second stages. HEPA, 95% DOP, and candle filters provided excellent efficiency as final stages for droplets of all sizes. When collecting mineral oil mist, the efficiency of many first and second stages deteriorated substantially over the relatively short term of these tests. Most mineral oil that penetrated high efficiency collectors did so as vapor that had evaporated from incoming droplets or from collection elements laden with mist; penetration as liquid droplets was low by comparison.


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