Mist Filters Research Articles

"Worst case" aerosol testing parameters: I. Sodium chloride and dioctyl phthalate aerosol filter efficiency as a function of particle size and flow rate.

The efficiency of filter media is dependent on the characteristics of the challenge aerosol and the filter's construction. Challenge aerosol parameters, such as particle size, density, shape, electrical charge, and flow rate, are influential in determining the filter's efficiency. In this regard, a so-called "worst case" set of conditions has been proposed for testing respirator filter efficiency in order to ensure wearer protection. Data collected on various types of filters (dust and mist; dust, fume, and mist; paint, lacquer, and enamel mist; and high efficiency) challenged with a worst case-type sodium chloride (NaCl) and dioctyl phthalate (DOP) aerosol are presented. The particle size of maximum penetration varies as a function of filter type and was less than 0.25-micron count mean diameter (CMD) in all cases. The count efficiency for high efficiency filters was greater than 99.97% at worst case testing conditions, but the worst case count efficiencies for dust and mist; dust, fume and mist; and paint, lacquer and enamel mist filters were not nearly as efficient as existing test methods indicate. Also, as the test flow rate is increased, the count efficiency decreases. Thus, respirator filters were found to conform to the prediction of single-fiber filtration theory.

Fine mist filtration by wet filters—III: Sintered disc filters

Sintered glass discs have been studied as collectors for dense mists of approx. 1 μm particles. The liquid saturation (holdup) attained when the rate of drainage equals the rate of collection agrees approximately with a literature correlation for thin beds of loose packing materials. Correlations are presented for the filter resistance caused by collected liquid. Plots of collection efficiency against flow velocity show a pronounced minimum. The behaviour of these plots under various operating conditions can be interpreted in terms of diffusion, impaction and direct interception as collection mechanisms. At superficial velocities over 50 cm s −1, sintered discs are capable of high efficiencies comparable with fibrous filters, but the pressure drop is typically 1.5–1.8 times greater. Both sintered discs and fibrous filters operate more efficiently wet than dry.

Fine mist filtration by wet filters—II: Efficiency of fibrous filters

Stainless steel fibrous filters of 4–22 μm wire diameter have been studied as collectors for dense mists of approx. 1 μm particles. The liquid collected in the filter in the steady state when equilibrium has been reached with the aerosol stream is found to increase the filter collection efficiency, sometimes very greatly, under most conditions studied. Only in the diffusional collection regime at very low fibre volume fractions (packing densities) or large fibre diameters does collected liquid diminish the efficiency. These findings are explained in terms of the effect of the liquid distribution on the collection mechanisms. In particular, collected liquid usually increases the efficiency of all significant single fibre collection mechanisms by increasing the effective packing density of the fibres, i.e. by adding to the distortion of a single fibre's flow field caused by neighbouring fibres. An existing model for dry fibrous filters has been modified to take account of the liquid distribution and provides good quantitative prediction of the relative collection efficiency of wet and dry filters having packing densities over 0.04.

Effect of Facial-seal Leaks on Protection Provided by Half-mask Respirators

Abstract Applications are presented of a computer model that gives the performance (overall protection factor including filter penetration and facial-seal leakage) of half-mask respirators for protection against aerosols. Input variables for the model are type of filter used, work rate of the wearer, QNFT-measured fit factor, and size distribution of the exposure aerosol. Based on the model, measurement errors due to loss of particles in leaks during qualitative fit testing (QNFT) measurements with current equipment are estimated to be less than 5 percent, but the change from high efficiency test filters to regular dust, fume, and mist (DFM) filters in use can reduce protection significantly. Overall protection factors are predicted for 33 industrial exposures based on assumed QNFT fit factors of 20 and 50. In these examples, DFM respirators with a QNFT fit factor of 50 have predicted protection factors from 21 to 2900, depending on aerosol size distribution. Overall protection factors for disposable and DFM filter respirators having a given facial-seal leak are presented graphically for a wide range of aerosol size distributions and two work rates. Also given is a contour chart showing the QNFT fit factor required to achieve an overall protection factor of 20 or 50 for a wide range of aerosol size distributions. For an unknown aerosol size distribution and average respirator performance (half-mask with DFM filters), a QNFT fit factor of 15 or more is required to ensure an overall protection factor of at least 10.

Performance of Dust Respirators with Facial Seal Leaks: II. Predictive Model

A performance model for half-mask and single-use respirators is presented. It represents a possible alternative to field measurements of respirator performance. Experimental data on filter and leak performance given in Part I were used to develop a model that allows one to predict 1) the overall respirator penetration as a function of particle size for any work rate and 2) overall total mass penetration for any work rate and exposure aerosol-size distribution for a known respirator filter and facial seal leak condition. A simplified method based on general regression equations is presented that allows one to estimate these quantities based on QNFT (quantitative fit testing) measurements and a knowledge of the exposure aerosol-size distribution. Example calculations are given for a situation in which QNFT gives a fit factor of 50 for a half-mask with dust, fume and mist filter cartridges, but predicted protection factors for various use conditions range from 20 to 81 depending on exposure particle-size distribution and work rate of the wearer.

High efficiency oill mist filtration with ESP

It has been tested a new electrode configuration on a two stage ESP, to filter oil mist. After optimizing, a efficiency over 99% has been obtained for concentrations up to 300 mg/m 3.

Fine mist filtration by wet filters—I. Liquid saturation and flow resistance of fibrous filters

Abstract Mist filters are often operated wet, i.e. with the substantial liquid saturation (hold-up) attained when the rate of drainage equals the rate of collection. In this condition, their performance is considerably modified by the liquid distributed over the fibres; the type of distribution depends on the packing density of the fibres. A capillary model originally devised for porous media is discussed and improved and an analogous model is then developed for wet fibrous filters. It predicts that the liquid saturation and filter resistance are functions of the capillary number and two other dimensionless groups, a and l / d f where a is the filter packing density, l the filter thickness, and d f the fibre diameter. The experimental data are fitted satisfactorily by correlations of this form for packing densities over about 0.04. The pressure drop of a wet filter is 2–40 times higher than that of a dry filter.

Increase of pressure drop in a model filter during mist filtration

The experimental increase of pressure drop across a grid of parallel fibers during mist filtration depends upon accumulated liquid on the fibers. Experimental data were compared in calculations obtained by three theoretical models. It was shown that when the equilibrium contact angle was low, the pressure drop across a grid with drops on the fibers can be calculated using a model which treats the fibers as polydisperse.

Balance of Payments and Government Policies

Articles cover the effects of urban meteorology on air pollution distribution; effects of air pollutants on plant life, metals and coatings; and pollution control and equipment, including mist eliminators, filtration and incineration.