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

Past Projects

Fate of Mist Droplets in a Fibrous Filter

The doctoral work of Pete Raynor focuses on understanding the fate of mist droplets that enter a filter. These droplets can evaporate from incoming air before they reach the filter or they can penetrate through the filter without collecting at all. Alternatively, they can collect and drain from the filter, collect then evaporate from the filter fibers, re-entrain into the air stream that leaves the back of the filter, or remain on the fibers. Design of an effective filter depends on understanding these fates and their relationships to filter design variables. Pete has developed methods to build filters with properties that can be controlled. In addition, he has designed the apparatus to test these filters and is monitoring their changes in efficiency, mass loading, and pressure drop over time. These results will be used to validate theory for droplet and filter interactions and behavior.

Considerable work on evaporation has already been done in the Baity Air Lab. For his master's thesis, Steve Cooper developed a GC/MS method to measure oil vapor concentrations upstream and downstream of mist collectors. He found that mists of mineral and hobbing oils can evaporate, despite their low vapor pressures, because the aggregate surface area of the droplets is much larger than that of the bulk fluid. Concentrations of oil vapor measured downstream of a collector controlling mineral oil and hobbing oil range from 2 to 5 mg/m3; these values are far higher than the mass concentration of mist droplets present in the same locations. Similar concentrations have been measured in both Baity Lab and at the Sharonville plant. Because vapor that penetrates through the collector can later condense to form droplets, mist collectors are less effective than expected at removing these fluids from plant air. Furthermore, the collector itself may be a source of oil vapors as air that passes through the collector strips away organic compounds from the collection elements, then recirculates within the plant.

The following abstracts describe research done at the Baity Air Lab on evaporation.

Abstract: Evaporation of Accumulated Multicomponent Liquids from Fibrous Filters, Peter
C. Raynor and David Leith, poster presented at AAAR '96, Orlando, FL, October 14 - 18,

Fibrous filters are employed to collect petroleum-based metalworking fluid mist generated by machining operations. Furthermore, industrial hygienists use fibrous filters to sample metalworking fluid mists to determine worker exposure. McAneny et al. (1995) and Cooper et al. (1996) have shown experimentally that oils accumulated within fibrous filters can evaporate into the air passing through the filters.  Zhang and McMurry (1987) developed a numerical model based on mass conservation principles to predict evaporation of accumulated liquid from a sampling filter when a saturated air stream enters the filter. Their model assumes that, as liquid collects beyond a threshold level, it spreads evenly over fiber surfaces. However, Liew and Conder (1985) showed that accumulated fluid collects at fiber intersections for filters that are sufficiently porous and not highly saturated with liquid.  Zhang and McMurry's model has been modified so that evaporation of retained liquid is modeled as the evaporation of individual spheres gathered at fiber intersections. Also, the updated model is capable of predicting evaporation for incoming aerosol streams that are not saturated with vapor.  The updated model's predictions exhibit good general agreement with experimental findings. When the incoming air stream is not saturated with vapor, significant quantities of organic liquid retained on filters can volatilize. Compounds evaporated from mist collection filters in factories may be able to recondense as a mist and harm worker health. Evaporation of liquid from sampling filters will lead to an underestimation of worker exposure to metalworking fluid mist.

Cooper, S. J., Raynor, P. C. and Leith, D. (1996). Appl. Occup. Environ. Hyg. 11:(1204-1211).
Liew, T. P. and Conder, J. R. (1985). J. Aerosol Sci. 16:497-509.
McAneny, J. J., Leith, D. and Boundy, M. G. (1995). Appl. Occup. Environ. Hyg. 10:783-787.
Zhang, X. Q. and McMurry, P. H. (1987). Atmos. Environ. 21: 1779-1789.

Abstract: "Evaporation of Polydisperse Multicomponent Oil Droplets" Peter Raynor,
Steven Cooper, David Leith. American Industrial Hygiene Association Journal. Dec 1996

During machining, polydisperse mist droplets are generated from multicomponent oils used as
metalworking fluids. As these droplets travel with an air stream, they are subject to evaporation. the
resulting vapor will pass through mist collectors and enter factory air where it may recondense on
particles or surfaces, be inhaled by workers, or be emitted to the atmosphere. Maxwell's equation can be used to predict the evaporation of multicomponent oil droplets at normal temperatures. Within each droplet size, relationships describing the change in mass with time for small droplets (diameter < 20 (m) are numerically integrated over time for the most prevalent compounds in the oil. Decreases or increases in mass for individual droplets are linked by their combined influence on vapor concentrations. Experiments conducted with mineral oil mist demonstrate that the model predicts evaporation accurately. The model indicates that, under some conditions, as much as 65% of the mass in a mineral oil mist can volatilize within 10 seconds. The amount of vaporization will depend on the initial concentration, size distribution, and composition of the mist. Substitution of an alternative oil as a machining fluid may decrease evaporation considerably.

Abstract: "Evaporation of Mineral Oil in a Mist Collector" Steven Cooper, Peter Raynor,
David Leith. Applied Occupational and Environmental Hygiene. Oct 1996 11(10) 1204-1211.

Many industrial processes generate mineral oil mist. Current control technologies focus only on
removing the mist droplets from the air. However, if these droplets evaporate, the vapor can pass
through a collector designed to remove droplets alone. The purpose of this study was to determine the extent of evaporation for mineral oil droplets introduced to a mist collection system. Vapor was
sampled using activated carbon downstream from the collector as high concentrations (45-55 mg/m3) of oil mist were injected upstream. After the system was loaded with mist, additional vapor samples were taken without mist injection to determine the amount of evaporation from mist-loaded collector components. The average vapor concentration measured for all samples taken when oil mist was introduced to the system was 4.02 mg/m3, or 8.8% of all oil introduced. Vapor concentrations caused by evaporation from collector components declined from 3-5 mg/m3 as clean air initially passed through freshly loaded components to 1-2 mg/m3 after air had passed through the components for about 12 hours. These findings suggest that even if a collector is 100% efficient for droplets of all diameters, some of the incoming mass will penetrate through the collector as vapor, and that evaporation of liquid oil retained on the collector itself can cause significant concentrations of vapor even if the collector has not been exposed to oil mist recently.

Abstract: "Evaporation of Metalworking Fluid Mist in Laboratory and Industrial Collectors" Steven Cooper and David Leith. submitted to American Industrial Hygiene Association Journal.

Many industrial processes generate metalworking fluid mists. Recent studies have shown that these
metalworking fluids will evaporate in mist form. If these droplets do evaporate, they will pass through a collector designed to capture droplets alone. The purpose of this paper was to measure the evaporation of three commercial metalworking fluids: a mineral oil, a hobbing oil, and a soluble oil. Evaporation of these fluids was determined for two laboratory collectors and one industrial field collector; the field collector was identical to one of the laboratory collectors and could be used to corroborate laboratory findings. Vapor was sampled using activated carbon upstream and downsteam from the collector as high concentrations of mist were generated upstream of the collector. After the system was loaded with mist, additional vapor samples were taken without mist injection to measure evaporation of droplets retained on collector components. For the laboratory collectors, evaporation measured downstream of the collectors during mist injection was greatest for mineral oil (4.90 mg/m3), then hobbing oil (2.90 mg/m3), and finally soluble oil (0.71 mg/m3). Field collector hobbing oil vapor concentrations during machining operations averaged 6.40 mg/m3 downstream of the collector; greater duct temperatures in the plant than in the laboratory resulted in increased vapor concentrations. Vapor concentrations measured as room air passed through mist-loaded collector components were greater downstream than upstream of the collector for both laboratory and field collectors. For instance, downstream vapor concentrations were 5-10 times greater than upstream vapor concentrations for the laboratory collectors. These findings suggest that even if a collector is 100% efficient for droplets of all diameters, some of the incoming mass will penetrate through the collector as vapor. This vapor may then re-condense once re-circulated back to the workroom to form droplets.

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