Attività dell'ISO TC 142 - Cleaning equipment for air and other gases

in "Il CTI Informa", marzo 201

Assessing the need for neutralizing KCl filter testing aerosol

American Association for Aerosol Research 28th Annual Conference, Minneapolis (MN), 26-30 October 2009, Abstract #81

Properties of nanoparticles affecting simulation of fibrous gas filter performance

Computational Fluid Dynamics (CFD) software now widely available allows detailed simulation of the flow of gases through fibrous filter media. When the pattern of gas velocity vectors in the interstices between fibers has been established, a simulated particle of any desired size can be “injected” into the entering gas stream, and its path under the influence of aerodynamic drag, Brownian motion, electrostatic forces and thermal gradients traced, until the particle either collides with a fiber, or passes through the entire filter medium. Successive simulated injection of many particles at random locations in the entering stream allows an average probability of capture to be calculated. If the particles can be assumed to adhere permanently to fibers after a collision, the average thus determined will represent the capture probability, or “efficiency” for that particle type and size of particle. Many particle properties must be available as parameters for the equations defining aerodynamic drag, electrical and thermal mobility, and Brownian velocity. Particle morphology, size, density, Knudsen Number, charge, and dielectric constant are examples of influential properties. These properties have been characterized and measured in many studies for micrometer-scale particles, but less so for nanoparticles. A particle which collides with a fiber does not necessarily remain permanently attached to that fiber. Under some conditions, a particle will bounce off the fiber, re-entering the gas stream with some velocity and direction dependent its shape, its velocity and angle of approach to the fiber surface, and the elastic properties of the particle-fiber combination. If the particle does not bounce, it may still be bound to the fiber surface weakly enough to be removed by the gas flow past the fiber, or by vibration. A particle retained on the fiber surface becomes a potential object for collisions by later arrivals of its own kind. Indeed, the formation of fiber-like chains of captured particles – dendrites –has been observed in filter media. Thus both particle-fiber and particle-particle bounce and adhesion properties are needed for a complete simulation of particle filtration. This is especially important in simulating the “loading” of filter media, which can enable prediction of the rise in pressure drop across a filter medium when some distribution of aerosol particles is fed to it over an extended period of time. Accurate values for all properties affecting particle dynamics are needed, not only for predicting particle capture in actual service, but also to validate models for media geometries and computational procedures used in CFD. We present a survey of existing literature on the properties influencing the effects listed above, with emphasis on nanoparticles where available. In some cases, effects which are significant for particles with micrometer dimensions are trivial for nanoparticles; in some cases, the reverse is true. We discuss the reliability of extrapolation of property values from micrometer-scale studies to nanoparticles, where nanoparticle data are not available

Looking for the minimum efficiency of fibrous air filters during their service life

Electret fibrous air filter media achieve high efficiencies while maintaining low air flow resistance by incorporating electrostatic charges on their fibers. However, captured ultrafine particles reduce this electrostatic enhancement. It is important to evaluate the behavior of such media when their electrostatic enhancement has been completely suppressed, leaving only “mechanical” effects such as sieving, impaction, interception and diffusion. Existing standards EN779:2002 and ASHRAE 52.2-2007 specify preconditioning procedures to eliminate or reduce electrostatic effects. ASHRAE 52.2 exposes the full air filter to KCl nanoparticle aerosols. ISO/TS 21220:2009 and EN 779 precondition filter media by a 2 minute soak in isopropyl alcohol (IPA), after which the filter media are dried before further efficiency measurement. These approaches present problems: • To control the size distribution of KCl nanoparticles requires additional expensive equipment and, even after completing this procedure, the electrostatic charge can be still partly active. • The structure of some media may be affected by immersion in liquid IPA. • Soaking full scale air filters is not practical and requires large amounts of IPA, which is then dispersed into the environment. The most recent efforts in getting the discharged efficiency of a filter have been made by ISO/TC142 “Cleaning equipment for air and other gases” Working Group 9, which is devoted at developing standards for gas turbine air intake applications. A new procedure proposed by Hayashi (Japan) eliminates electrostatic effects by exposing filters to IPA vapor for 8 hours or longer. This procedure has been successfully duplicated by Cai (China), at the Politecnico di Torino and in other laboratories. WG9 of ISO/TC142 organized inter-laboratory tests comparing IPA vapor treatment to liquid IPA immersion on samples of five media types. The data obtained indicate that IPA vapor treatment can replace the immersion approach successfully. The main result of these tests was the adoption of IPA vapor treatment on a flat sheet of medium in ISO/FDIS 29461-1 “Air intake filter systems for rotary machinery -- Test methods -- Part 1: Static filter elements”. The discharge of full scale filter elements is still under study. We summarize all these studies, and describe the new test protocols that could be adopted by future national and international test methods. The results presented here could also be useful to evaluate the need for further research in this area

Properties of nanoparticles affecting simulation of fibrous gas filter performance

Computational Fluid Dynamics (CFD) codes allow detailed simulation of the flow of gases through fibrous filter media. When the pattern of gas flow between fibers has been established, simulated particles of any desired size can be “injected” into the entering gas stream, and their paths under the influence of aerodynamic drag, Brownian motion and electrostatic forces tracked. Particles either collide with a fiber, or pass through the entire filter medium. They may bounce off the fiber surface, or adhere firmly to the surface or to particles previously captured. Simulated injection of many particles at random locations in the entering stream allows the average probability of capture to be calculated. Many particle properties must be available as parameters for the equations defining the forces on particles in the gas stream, at the moment of contact with a fiber, and after contact. Accurate values or all properties are needed, not only for predicting particle capture in actual service, but also to validate models for media geometries and computational procedures used in CFD. We present a survey of existing literature on the properties influencing nanoparticle dynamics and adhesion

Application of dust removal technologies for future lunar exploration

American Association for Aerosol Research 28th Annual Conference, Minneapolis (MN), 26-30 October 2009, Abstract #144

Qualidade do ar externo e a qualidade do ar interior com sistemas de ventilação

La qualità dell’aria esterna può essere tale da non consentire un’adeguata qualità dell’aria interna. In tal caso l’uso di dispositivi per la rimozione dei contaminanti è indispensabile per potere garantire un ambiente salubre. Viene discussa la procedura per effettuare una scelta ragionata degli elementi filtranti più adatti in relazione alle condizioni ambientali desiderate