Friedrich Löffler

Realistic modelling of the behaviour of fibrous filters through consideration of filter structure

Abstract A new model is proposed to describe the effects of structural inhomogeneities in fibrous filters on pressure drop and particle collection. The filter is subdivided into a series of elements with different structural characteristics, defined by local directionally-dependent permeabilities, and the resulting three-dimensional flow field is calculated. Using published relationships, local particle collection is calculated. Spatially nonhomogeneous fibre distribution is shown to play a dominant role. The difference in fibre collection efficiency between a real and a homogeneous filter depends on the dominant mechanism transporting particles to the fibres. In the interception-inertia range, the predictions are verified quantitatively by measurements on industrial filters with characterised structure.

Dust Generation from Handling Powders in Industry

This article explores the hypothesis that the amount of dust generated by an industrial process that handles granular material depends on (a) the size distribution of the granules and (b) the ratio of impaction to cohesion. Here, impaction characterizes particle separation forces caused by energy input from the process, and cohesion characterizes interparticle binding forces within the material itself. A mathematical model based on this hypothesis is presented and evaluated using 441 data sets from an industrial-scale apparatus designed to measure dust generation. Dust generation was characterized in experiments using titanium dioxide, limestone, glass beads, and lactose with moisture contents ranging from 0% to 6% and size distributions of d 25 µm in aerodynamic diameter, dropped from heights of 0.25, 0.7, and 1.25 m at material flows of 0.1, 0.3, and 0.6 kg/sec. Results are presented in equations for impaction, cohesion, and dust generation as a function of particle size. The equa...

Preparation of dust cakes for microscopic examination

Abstract In ‘barrier’ or ‘surface’ filters, a dust cake gradually builds up on the upstream side, with relatively little penetration of dust into the filter medium itself. The performance of such a filter — its rise in pressure drop during the filtration cycle, for example — is strongly influenced by the structure of the growing dust cake. This paper describes a method by which a loose dust layer may be ‘prepared’ for subsequent microscopic examination, which may yield quantitative data on its structure. The first part deals with the ‘fixation’ of the dust cakes, which is achieved in two separate steps. The second part describes the further treatment of the hardened cakes up to the point at which photographs can be taken.

Influence of high temperatures on particle adhesion

Abstract In many technological fields high-temperature processes are becoming increasingly important. In this context, the cohesive and adhesive behavior of particles at high temperatures is particularly problematic. For reliable operation of industrial high-temperature processes, it is essential to understand the fundamental adhesive forces and the mechanisms which can enhance them. The temperature, the chemical composition of the particles and the gas atmosphere surrounding them have a considerable effect on particle adhesion. The various adhesive force mechanisms cannot be considered in isolation when studying real systems. One important application is the regeneration of filters at high temperatures. Reliable design and operation of such filters requires knowledge of the local values of adhesive and cohesive forces which must be overcome during the filter cleaning cycle to remove the dust deposit.

Electrostatic agglomeration and centrifugal separation of diesel soot particles

Abstract For the collection of diesel soot particles, a two-stage collection system is employed consisting of a cyclone in combination with a preceding electrostatic agglomerator. By virtue of its design, the agglomerator is, in principle, a tubular electrostatic precipitator. The agglomeration mechanism is based on the fact that the electrically conductive soot particles accumulate at the collecting electrode of the agglomerator as flake-like structures. These are re-entrained when they exceed a certain size. The process of agglomeration can therefore be divided into three steps, i.e. particle transport, formation of agglomerates and emission. In order to assess the efficiency of the agglomerator, a measuring methodology has been developed which allows the determination of the size distribution of the soot particles over the range 17 nm to 40 μm. Investigations carried out in connection with this work show that the proportion of particles bound in agglomerates depends essentially upon the operating level of the engine. Depending on the various conditions employed, more than 90% of the particle mass may be bound in agglomerates whose sizes exceed 3 μm. With increasing engine load, the proportion of agglomerates decreases to ca. 40%. Varying the gas volume flow at a constant engine operating level demonstrates that such a dependency on load is not related to residence time, i.e. low efficiency does not arise from insufficient particle transport, but appears to be associated with one of the subsequent stages of the agglomeration process. Details of these processes have not been investigated so far. The total collection efficiency achieved via this system varies between 43% and 87% according to the various operating states of the engine. This significant difference can be explained in terms of the dependence of agglomeration and agglomerate collection upon engine load and speed.

Influence of temperature on particle separation in granular bed filters

Abstract The present study deals with the influence of temperature on the separation of fine dust particles in granular bed filters. Collection within the diffusion regime for particles smaller than 0.5 μm becomes more effective with increasing temperature. However, for larger particles subject to the influence of inertial forces, a drop in separation efficiency is to be expected due to increasing drag forces. With the aid of experimentally determined grade efficiencies at temperatures up to 800 °C, the influence of temperature and other important filtration parameters such as the filter mediums grain size or the filter face velocity, are quantitatively derived and discussed. Investigations concerning the long-term filter behavior have shown that the formation of a filter cake significantly improves separation efficiency. This not only depends upon the characteristics of the dust in question, but also is a function of the grain size of the filter medium and the filtration velocity. Under cake filtration conditions, high collection efficiencies may also be attained at temperatures up to 800 °C.

Fabric cleaning in pulse-jet filters

Abstract In pulse-jet filters, bag cleaning is accomplished by a compressed air pulse being injected into the clean side of a filter bag. This causes an abrupt increase in pressure within it, leading to a flow reversal as well as to acceleration of the filter cloth. Investigations with a 2.5 m long filter bag have shown that it is not the forces arising from the fabric acceleration, but those arising from the reverse air flow which play the major role in bag cleaning, and that the dust-cake release is related to the peak pressure built up inside the bag. In order to achieve complete dust dislodgement, a critical overpressure is necessary at every location along the length of a bag for a certain period of time. This overpressure depends on the dust-cake-fabric interaction. Experiments with flat-geometry fabric samples have shown that the critical overpressure can be found from a laboratory-scale experiment.

Deep-Bed Electret Filters: The Determination of Single Fiber Charge and Collection Efficiency

Deep-bed electret filters are used to separate small solid or liquid particles from gas flows. Compared with conventional deep-bed filters of similiar pressure loss, they operate with substantially higher collection efficiencies. During the course of filtration, however, the electrical charges of the deposited particles neutralize the electret fibers, temporarily decreasing the collection efficiency. In this paper, experimentally determined fractional separation functions of different types of electret filters are presented for the particle size range from 10 nm to 10 , um at a filter face velocity of 10 cm/s. The initial efficiencies are compared to that of a conventional (glass fiber) filter and also to those of the electret filters in discharged state. The results of long-term filtration experiments show that a complex time-dependent behavior exists for different filter materials. Furthermore, two experimental methods are presented and discussed which determine the charge characteristics of single electret fibers.

Zur Berechnung der Partikelabscheidung in Zyklonen

Zusammenfassung In der vorliegenden Arbeit ist ausgehend von den Ergebnissen neuerer experimenteller und theoretischer Untersuchungen zur Gasstromung sowie Bewegung und Abscheidung der Partikeln im Zyklon ein neues, anwendungsorientiertes Modell zur Berechnung des Abscheideverhaltens von Zyklonen entwickelt worden. Die Partikelbewegung wird als cin Vorgang aufgefasst, bei dem einer determinierten Bewegung eine zufallige infolge der Stromungsturbulenz uberlagert ist. Die Berechnung der Partikelstrome im Abscheideraum, der in 4 charakteristische Abscheidezonen eingeteilt ist, erfolgt auf der Grundlage eines vereinfachten Stromungsmodells. Das neue Auslegungsmodell erfasst bei vergleichsweise geringem Rechenaufwand alle wichtigen Einflussgrossen. Ein Vergleich der nach diesem Modell berechneten Trenngrade mit gemessenen Trenngradkurven verdeutlicht die Nutzlichkeit der Auslegungsmethode.

Reflections on similarity laws concerning particle transport in electrical precipitators

Abstract Measurements in three geometrically similar wire-plate precipitators show that in the cases of constant current density at the collecting plates or constant electrical field strength, larger geometries give better grade efficiencies. If a dimensionless voltage (found by dimensional analysis) is held constant, the grade efficiencies are no longer dependent on the absolute precipitator size. Introducing an ‘electrical drift parameter’, a generalized representation of measured and (according to Deutsch) calculated grade efficiencies can be found which turns out to be a scale-up invariant in the case of constant dimensionless voltage. Overall, the results show that with decreasing particle size the measured efficiencies become increasing better than the calculated ones. For particles smaller than 1 μm, there is little difference between the Deutschian and Laminar models. When common models are applied with space averaged values for the electrical field strength (with consideration of the ionic space charge) instead of the simple field strength value, then a qualitatively correct scale-up behaviour emerges with respect to the grade efficiency trends. Two parameters are decisive: the electrical drift parameter, which includes the operational state of the precipitator, and the relative length of the collecting zone.