A Sonirvey of Wnnnd Speeds Inn Imioor Workplaces
( The applicability of the inhalable convention for sampling aerosols relies on its being a valid model for typical sampling environments. The current convention is based on measurements carried out in external wind speeds between 1 and 4 ms~'. However these measurements show a degree of wind speed dependence, and it is uncertain at present how valid the convention is for describing human aspiration efficiency outside these wind speed limits. Following concerns that wind speeds in many indoor workplaces may be significantly below this range, measurements have been made in 55 work areas covering a wide range of workplaces. Measurements have concentrated on 'background' wind speeds where Hie influence of specific air movement sources is minimised. The pooled wind speed measurements show a highly skewed distribution with an arithmetic mean of approximately 0.3 m*s '. Approximately 85% of all individual measurements were below this mean value. No obvious correlation was found between wind speed distribution parameters and industry type, room size or ventilation type. A limited number of comparisons were made between static anemometers and devices mounted on workers. It was found that modal wind speeds experience by workers were typically 0.05 m-s~' higher than those measured using a static anemometer. These measurements agreed well with previously published data for similar work- places as well as housesJCrown copyright © 1998 Published by Elsevier Science Ltd on behalf of
Scale model studies of the transport of airborne pollutants on coalfaces.
Studies have been carried out to investigate airflows in coalmine models, with special regard to the transport of airborne pollutants, and to examine how they relate to what happens at full-scale in an actual underground mine. If such models can be shown to provide data representative of actual mine ventilation engineering, then they can provide cost-effective alternatives to full-scale investigations. The work set out in the first instance to identify the properties of: (a) the bulk airflow and associated transport of airborne pollutants along a longwall coalface; and (b) the transport of material out of regions that were partially enclosed or poorly ventilated (e.g. in the cutting zone, in headings). For the former, an appropriate quantity is the dispersivity of the coalface airflow, for the latter the mean retention time. Both quantities may be rendered non-dimensional with respect to dimensions characteristic of the system and to velocities of the airflow. Their behaviour in relation to a third dimensionless quantity, the flow Reynolds' number is also important. Experiments were performed, using smoke or dust tracers, to investigate how these properties are interrelated and how they scale between small-scale and full-scale systems. They were carried out in a 1/10-scale laboratory model, in a full-scale surface model, and underground in an actual coalmine. The basis of most of the experiments was the 'tracer decay' method, in which the transport properties of the aerodynamic system under investigation were determined from observations of the changes in tracer concentration with time immediately following the removal of the tracer source. During these experiments, the feasibility of using small-scale models to investigate ventilation problems was clearly indicated and preliminary scaling relationships which may be used as an initial basis for predicting the transport and local build-up of pollutants in mines were developed. It is expected that applications of the ideas and methodology described will be relevant to other industries.
Characterization of ceramic composite membrane filters for hot gas cleaning
Abstract Novel composite ceramic membrane filters suitable for hot gas cleaning operations have been prepared from fly ash and titania particles on stainless steel woven mesh substrates. Both membranes had a structure like a solid packed bed with a thickness of less than 100 μm. In the domain of laminar flow, significant differences in the pressure drop between experimental results and those predicted from the Kozeny-Carman relationship were found, particularly for flow through the titania membrane. Membrane filters with extremely small pores such as the titania membrane are greatly affected by slip up to 80% in permeability at elevated temperatures. Efficiencies of filtration of typical coal fired power station exhaust fly ashes were found to be greater than 909% for both titania and ash membranes, whilst there was greater variation in the pressure drops in the ash membrane than in the titania at typical gas flows.