Steven E. Guffey

The effect of local exhaust ventilation controls on dust exposures during concrete cutting and grinding activities

This study assessed the effectiveness of commercially available local exhaust ventilation (LEV) systems for controlling respirable dust and crystalline silica exposures during concrete cutting and grinding activities. Work activities were performed by union-sponsored apprentices and included tuck-point grinding, surface grinding, paver block and brick cutting (masonry saw), and concrete block cutting (hand-held saw). In a randomized block design, implemented under controlled field conditions, three ventilation rates (0, 30, and 75 cfm) were tested for each tool. Each ventilation treatment was replicated three times in random order for a total of nine 15-min work sessions per study subject. With the exception of the hand-held saw, the use of LEV resulted in a significant (p < 0.05) reduction in respirable dust exposure. Mean exposure levels for the 75 cfm treatments were less than that of the 30 cfm treatments; however, differences between these two treatments were only significant for paver block cutting (p < 0.01). Although exposure reduction was significant (70-90% at the low ventilation rate and 80-95% reduction at the high ventilation rate), personal respirable dust [corrected] exposures remained very high: 1.4-2.8 x PEL (permissible exposure limit) at the low ventilation rate and 0.9-1.7 x PEL at the high ventilation rate. Exposure levels found under actual field conditions would likely be lower due to the intermittent nature of most job tasks. Despite incomplete control LEV has merit, as it would reduce the risk of workers developing disease, allow workers to use a lower level of respiratory protection, protect workers during short duration work episodes reduce exposure to nearby workers, and reduce clean-up associated dust exposures.

Effects of Face Velocity, Flanges, and Mannikin Position on the Effectiveness of a Benchtop Enclosing Hood in the Absence of Cross-Drafts

Although enclosing hoods are often deemed the most effective type for industrial processes, there is little published research concerning simple enclosing hoods similar to those used in industry. This study investigated the effects of face velocity, mannikin position, and flange angle on exposure to a mannikin standing at the face of an industrial benchtop enclosing hood. Sulfur hexafluoride (SF6) was released at low velocity inside a plain enclosing hood located on a tabletop. Plastic tubing transported sampled air from the breathing zone (BZ) and other sample sites to an infrared spectrophotometer. For some tests an anthropometrically scaled, 1.02 m (40 in.) mannikin stood in front of the hood opening with its arms straight; for others the mannikin was bent forward with its hands on each side of the source in a “working” position. Flange conditions were either none or a flange with a 30°, 45°, 60°, or 90° angle to the plane of the hood. Face velocities were varied from 0.30 to 0.86 m/sec (60 to 170 ft/m...

Friction Tables Determined from Colebrook's Equation for Standard Density Air Flow

Abstract To make it easier to compute friction losses in air flow for a wide range of roughness values, the author has developed tables useful for two different approaches: (1) velocity pressure coefficients and (2) exponential equations. The author has produced tables of velocity pressure coefficients (f table and Ftable) based on solution of the Colebrook equation for given velocities, absolute roughnesses, and duct diameters. The values were generated by ten iterative solutions of the Colebrook equation using Haalands approximation as a seed value. The velocity ranges were selected to minimize the required number of velocities while keeping deviations from values computed with the Colebrook equation to less than 4 percent at any point. The second approach involved generating data using solutions of the Colebrook equation over practical ranges of roughness, diameter, and duct velocities. Log10-transformed pressures were fitted to a linear model of log10-transformed velocities and diameters, producing r...

Airflow distribution in exhaust ventilation systems

The distribution of airflows converging at a junction can be estimated from the predicted pressures just upstream of the junction fitting. However, the total pressure approach requires iterative solutions at each junction, and the current implementation of the static pressure approach provides a poor fit to published empirical data. A model to predict pressures upstream of junctions is proposed and validated using experimental data.

Quantitative Troubleshooting of Industrial Exhaust Ventilation Systems

Abstract This article proposes two troubleshooting tools that may allow precise and accurate assessment of changes to ventilation systems of any type. Both are useful in discovering and quantifying most modifications that affect the distribution of airflows among the branches and static pressures throughout the system. The approaches are derived from energy balance considerations, using power loss coefficients (X) computed for any contiguous section of the system from the duct velocities and static pressures measured at that sections inlets and outlets. The value of X for a given portion of the system should be nearly constant with changes in airflow and with modifications to other portions of the system. A substantial change in the value of a coefficient determined from “before” and “after” measurements can be attributed only to a modification in that portion of the system. That responsiveness to local modifications and insensitivity to changes elsewhere in the system—including gross changes in fan perf...

Simplifying Pitot Traverses

Abstract This article details the advantages of traverses conducted with Pitot tubes and describes a device that will hold a Pitot tube in correct alignment to a round duct at predetermined insertion depths. With the device, one unaided individual can perform Pitot traverses without referring to traverse tables and do so with much better repeatability than is possible by hand. The device is suitable for use in the field and in the laboratory.

Hard Metal Exposures. Part 1: Observed Performance of Three Local Exhaust Ventilation Systems

Not every ventilation system performs as intended; much can be learned when they do not. The purpose of this study was to compare observed initial performance to expected levels for three saw-reconditioning shop ventilation systems and to characterize the changes in performance of the systems over a one-year period. These three local exhaust ventilation systems were intended to control worker exposures to cobalt, cadmium, and chromium during wet grinding, dry grinding, and welding/brazing activities. Prior to installation the authors provided some design guidance based on Industrial Ventilation, a Manual of Recommended Practice. However, the authors had limited influence on the actual installation and operation and no line authority for the systems. In apparent efforts to cut costs and to respond to other perceived needs, the installed systems deviated from the specifications used in pressure calculations in many important aspects, including adding branch ducts, use of flexible ducts, the choice of fans, and the construction of some hoods. After installation of the three systems, ventilation measurements were taken to determine if the systems met design specifications, and worker exposures were measured to determine effectiveness. The results of the latter will be published as a companion article. The deviations from design and maintenance failures may have adversely affected performance. From the beginning to the end of the study period the distribution of air flow never matched the design specifications for the systems. The observed air flows measured within the first month of installation did not match the predicated design air flows for any of the systems, probably because of the differences between the design and the installed system. Over the first year of operation, hood air flow variability was high due to inadequate cleaning of the sticky process materials which rapidly accumulated in the branch ducts. Poor distribution of air flows among branch ducts frequently produced individual hood air flows that were far below specified design levels even when the total air flow through that system was more than adequate. To experienced practitioners, it is not surprising that deviations from design recommendations and poor maintenance would be associated with poor system performance. Although commonplace, such experiences have not been documented in peer-reviewed publications to date. This publication is a first step in providing that documentation.

Experimental investigation of power loss coefficients and static pressure ratios in an industrial exhaust ventilation system.

A study tested whether measures of equivalent resistance (X values) and ratios of static pressure (SPratio) for given ducts of contaminant control exhaust ventilation systems were independent of substantial changes to airflow level and to changes to resistance of other ducts within the same full-scale five-branch system. In a factorial study design, four airflow levels were achieved by changing fan rotation rate while resistances to flow for specific branch ducts were changed independently by adjusting slidegate dampers to various settings. For each damper insertion depth (including fully open), the results demonstrated substantial invariance for branch X values (few greater than 5%), SPratio (few greater than 3%), and fraction of airflow to each duct (few greater than 2%). X-values for submains were much less stable, changing by 20% or more with changes to other parts of the system. For the same conditions, hood static pressures changed by as much as 96% (with standard deviation of 40%). The results suggest that before and after values of X and SPratios should be more reliable bases for indicating alterations than comparison of observed static pressures. The stability of airflow distributions with substantial changes in airflow suggests that one could adjust airflow distribution (e.g., with dampers) without considering whether the fan speed was set correctly, leaving fan adjustments for a final step.

MODELING EXISTING VENTILATION SYSTEMS USING MEASURED VALUES

Equations are derived to model installed exhaust ventilation systems using measured diameters, flows, and pressures. The system of equations does not require knowledge of system components or their loss coefficients except for those components that will be replaced. The approach also allows detection of physical changes in existing systems. Measurement errors limit the accuracy of predictions. The nonquadratic relationship between flow and friction loss limits the accuracy of predictions when there are extreme changes in duct velocities. The modeling procedure is demonstrated for an example problem in which “observed” values are created for “initial” and “final” conditions from velocity pressure loss coefficients.

Hard metal exposures. Part 2: Prospective exposure assessment.

Hard metal exposures may precipitate lung disease in exposed workers. This article reports on a project investigating the relationship between local exhaust hood air flow levels and workplace hard metal exposures. Airborne cobalt, chromium, and cadmium exposure concentrations, and ventilation system function were monitored for three consecutive days prior to installation of three new ventilation systems, and then were followed monthly for one year. Work activities included wet and dry grinding of saw blades, brazing, welding, and setup. Work task exposures were highly variable over the period of the study. Ventilation air flows failed to meet design goals due to low total air volume and poor distribution; however, worker exposures to metals were controlled in most cases. Hood design, worker acceptance, and use of the hoods were as important in controlling exposures as were exhaust hood air flow levels.