Fates of Airborne Droplets Entering a Mist Filtration SystemEPA Grant Number: U914812
Title: Fates of Airborne Droplets Entering a Mist Filtration System
Investigators: Raynor, Peter C.
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Broadway, Virginia
Project Period: January 1, 1995 through January 1, 1996
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1995) Recipients Lists
Research Category: Academic Fellowships , Engineering and Environmental Chemistry , Fellowship - Engineering
The main objective of this research project is to use Maxwell's equation for evaporation of an isolated drop to model the evaporation of polydisperse multicomponent oil droplets at normal temperatures.
During machining, polydisperse mist droplets are generated from multicomponent oils used as metalworking fluids. As these droplets travel with an airstream, 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. Within each droplet size, relationships describing the change in mass with time for small droplets (diameter < 20 mm) 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 percent 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.
Industry uses metalworking fluids during the machining of metal parts to cool, clean, and lubricate tools and workpieces. Petroleum-based mineral oils, mixtures of liquid organic compounds that boil over a broad range of temperatures, are commonly utilized as metalworking fluids, either directly as straight oils or as emulsions of oil in water called "soluble oils." As the fluids are poured or sprayed liberally over the cutting zones, polydisperse mist droplets may be generated by the rotational motion of flooded workpieces and tools, the spray application of the oil, and condensation of hot vapor from the cutting zone. Because most airborne droplets generated by machining operations have diameters smaller than 10 mm, they may travel large distances through factory buildings to deposit on distant surfaces or enter worker's breathing zones.
More than one million workers are exposed to droplets generated by machining operations. Oil mist droplets that are inhaled or contact skin can cause a variety of health problems. Worker exposure to cutting fluids can result in dermatitis and respiratory problems. Epidemiologic studies indicate that long-term exposure to metalworking fluids can lead to increased incidence of several types of cancer. The International Agency for Research on Cancer has concluded that there is "sufficient evidence" that mineral oils used in the workplace are carcinogenic.
To reduce amounts of metalworking fluid mist in factory air, hoods or enclosures are installed to capture droplets near machining operations. Captured aerosol is fed through ducts to mist collectors, which efficiently remove droplets from the airstream before the cleaned air is recirculated.(15) As oil droplets travel to collectors, they may experience partial or complete evaporation, depending on the composition of the oil. Semivolatile organic compounds (SOCs) that evaporate from the droplets will penetrate through the collectors and return to the room air. These organic vapors may adversely affect worker health or serve as a source of fugitive.