Although the basic principles of fibrous filters have been well understood for capture of micron and submicron sized particles, questions arise when they are applied to nanoscale particles. In the first part of this review, the classical theory of fibrous filters is described with focus on the principles that are applicable to nanoparticle collection. The areas of recent developments reviewed include thermal rebound of nanoparticles and the effects of particle shape, aggregate morphology, flow regime, humidity, fiber size, and particle loading. One of the outstanding questions in nanoparticle collection is the particle size at which the effect of thermal rebound on collection efficiency can be observed. Theoretical calculations indicate that the effect probably can be observed only for particles smaller than 1 nm, but experimental confirmation is difficult at present because of lack of instruments for classifying and counting subnanoscale particles. Two promising devices based on filtration principles hav...

http://calliope.dem.uniud.it/CLASS/IMP-CHIM/iecr-filter-review.pdf

Removal of Nanoparticles from Gas Streams by Fibrous Filters: A Review

The filtration efficiency for nanoparticles down to 1 nm in size through glass fibrous filters was measured using an improved PSM-CNC system. In addition, the effects of relative humidity and particle charge were investigated for various nanoparticle diameters. The results show that the filtration efficiencies were independent of humidity and affected by particle charge in the case of particles below 100 nm in size. For particles smaller than 2 nm, the particle penetrations increased with decreasing particle size. These results suggest that the thermal rebound phenomena would be operative for nanoparticles with diameters below 2 nm, even though it would depend on the states of both the particles and the filter media. These results are particularly important for experimental investigations of the behavior of nanoparticles on a filter.

Filtration efficiency of a fibrous filter for nanoparticles

Problems of atmospheric and space electricity: Proceedings of the third international conference, Montreux, May 1963. (edited by S. C. Coroniti.) Elsevier, Amsterdam, 1965, 616 pp., £10

High resolution size analysis of nanoparticles and ions

Penetration of nanometer-sized aerosol particles and ions through a single wire screen and a laminar flow tube has been measured using a tandem differential mobility analyzer system to accurately determine the size of the particles. The results obtained using a condensation nucleus counter as a particle detector showed that penetration of particles down to 2 nm in particle diameter follows the predictions of the theories of Cheng and Yeh and Gormley and Kennedy. This result suggests that thermal rebound of particles on wire or tube surfaces does not occur. The measured penetrations for negative ions smaller than 2 nm also agreed well with the theoretical predictions. In this case, however, concentration was measured with an electrometer, and there exists the possibility of electron transfer between the ions and the surfaces without the (neutralized) ions being attached to them.

Penetration of nanometer-sized aerosol particles through wire screen and laminar flow tube

Generation of nanometer size particles and their removal from air

Removal of airborne particles by a tubular particle-removal device using UV/photoelectron method

Experimental evaluation of ion-induced nucleation in nanometeraerosol formation by α-ray radiolysis of SO2/H2O/N2 mixtures

The flow pattern of aerosol and sheath air in a cylindrical annulus of a Differential Mobility Analyzer (DMA) is critical to determine the classification performance of the DMA. In the present work, a new technique to check aerosol flow inside the DMA is proposed and the influences of aerosol flow distortion and broadening on the classification performance are investigated by means of the Tandem DMA method. As a result, it is found that (1) the annular flow inside the studied DMA is a fully developed laminar flow with a negligible mixing between aerosol and sheath air when the aerosol flow rate is less than 3 l/min and the sheath air flow rate is 17.5 l/min, (2) a larger width of aerosol entrance slit causes uneven distribution of aerosol flow in the circumferential direction, and (3) both the uneven distribution and broadening of the aerosol flow lead to a broader band of classified particle electrical mobility, as well as a shift of peak electrical mobility to a smaller range. These results suggest that aerosol flow inside a short-type DMA is more sensitive to the flow rates and its structure compared to the conventional TSI type DMA.

Sang-Joon Cho

Papers

Generation of nanometer size particles and their removal from air

Removal of airborne particles by a tubular particle-removal device using UV/photoelectron method

Experimental evaluation of ion-induced nucleation in nanometeraerosol formation by α-ray radiolysis of SO2/H2O/N2 mixtures

Technique for aerosol flow check in differential mobility analyzer and its influence on classification performance