Experimental study on centerline velocities of a rectangular capture hood under realistic conditions.

Capture hoods are an important component of a local ventilation system designed to reduce exposures to airborne contaminants. The velocity at any point along the centerline of the hood (Vx) is currently estimated using one of many predictive equations developed since the 1930s. It is unproven that those predictive equations for Vx are accurate, despite the prodigious number of studies concerning them. Among other issues, almost all experimental verifications were conducted for conditions that were either unrealistically ideal without competing air currents (e.g., zero cross draft) or were not described. This study measured values of Vx along the midline using Particle Image Velocimetry (PIV) at distances of 1-14 inches in front of a rectangular capture hood. The experiments were conducted in a large wind tunnel (9' × 12' × 40', H × W × L) using a heated, breathing, anthropomorphically sized manikin. Three 0 degree draft velocities (Vdraft = 4, 14, and 50 ft/min) were tested, all directed toward the hood face and the back of the manikin (if present). For each value of Vdraft, the velocity fields were measured in a factorial design with and without the manikin, and with and without a worktable underneath the hood. An ideal condition was represented by a freestanding hood at the 4 fpm draft. Nonideal conditions included the presence of a worktable or manikin, and the combination of table and manikin. Each condition was tested at the three levels of Vdraft. The experimental results found significant effects (p < 0.001) for Vdraft, the presence of the manikin, the presence of the worktable, and all combinations of those factors. The effects of the independent variables were most pronounced at distances greater than 10 in (25.4 cm) from the hood face. It is concluded that none of the previously published models accurately predicted Vx under the realistic conditions tested in this study. A satisfactory model will have to include terms for Vdraft and the presence of a worktable and a worker.

Quantitative evaluation of the performance of an industrial benchtop enclosing hood.

Plain benchtop enclosing hoods are assumed to be highly effective in protecting workers from airborne contaminants, but there is little research published to support or rebut that assumption. The purpose of this research was to investigate the performance of a 36 in. wide, 30 in. high, and 40 in. deep benchtop enclosing hood. The study consisted of two parts: (1) investigating the effects of hood face velocity (five levels: 111, 140, 170, 200, and 229 ft/min) and wind tunnel cross-draft velocity (five levels: 14, 26, 36, 46, and 57 ft/min) on a plain benchtop enclosing hood, and (2) studying the effects of specific interventions (no-intervention, collar flange, bottom flange, cowling, and sash) added onto the same enclosing hood. A tracer gas method was used to study the hood's performance inside a 9 ft high, 12 ft wide, and 40 ft long wind tunnel. Freon-134a concentrations were measured at the mouth and nose of an anthropometrically scaled, heated, breathing manikin holding a source between its hands while standing at the enclosing hood's face. Roughly 3 L/min of pure Freon-134a mixed with 9 L/min of helium was released from the source during all tests. Results showed that hood face velocity, wind tunnel cross-draft velocity, and interventions had statistically significant effects (p < 0.05) on the concentrations measured at the manikin's breathing zone. Lower exposures were associated with higher face velocities and higher cross-draft velocities. The highest exposures occurred when the face velocity was at the lowest test value (111 ft/min), and the cross-draft velocity was at its lowest test value (14 ft/min). For the effects of interventions to the hood face, the results showed that flanges and the cowling failed to consistently reduce exposures and often exacerbated them. However, the customized sash reduced exposures to less than the detection limit of 0.1 ppm, so a similar sash should be considered when feasible. The hood face velocity should be at least 150 ft/min if a sash is not used.

Flow dynamics and contaminant transport in industrial-type enclosing exhaust hoods.

The present study concerns the flow dynamics and associated contaminant transport in the near wake of a worker using an industrial-type benchtop enclosing hood. The primary focus is on evaluating the effects on the dynamics of the wake flow and the exposure level of various extraneous factors, such as the strength and direction of cross-drafts and the worker's body heat and shape. Three-dimensional Unsteady Reynolds-Averaged Navier-Stokes simulations were carried out for a model of a simple mannequin and a model of an anthropometric mannequin. Estimated flow patterns and concentrations near the simple mannequin were compared with the observations from concurrent smoke visualization experiments and with the experimental concentration measurements, respectively. Results for both visualizations indicated that the flow in front of the worker is dominated by dynamic vortical structures and that body heat may have negative effects on the exposure level, especially at low flow rates. Using simple rounded shapes to simulate the human form was a fair approximation from the viewpoint of flow structures and exposure trends, which agreed well with the experimental measurements and observations. However, the quantitative values of the predicted concentrations in the breathing zone were sensitive to the mesh resolution.

Investigation into pedestrian exposure to near-vehicle exhaust emissions.

BACKGROUND: Inhalation of diesel particulate matter (DPM) is known to have a negative impact on human health. Consequently, there are regulations and standards that limit the maximum concentrations to which persons may be exposed and the maximum concentrations allowed in the ambient air. However, these standards consider steady exposure over large spatial and time scales. Due to the nature of many vehicle exhaust systems, pedestrians in close proximity to a vehicle's tailpipe may experience events where diesel particulate matter concentrations are high enough to cause acute health effects for brief periods of time. METHODS: In order to quantify these exposure events, instruments which measure specific exhaust constituent concentrations were placed near a roadway and connected to the mouth of a mannequin used as a pedestrian surrogate. By measuring concentrations at the mannequin's mouth during drive-by events with a late model diesel truck, a representative estimate of the exhaust constituent concentrations to which a pedestrian may be exposed was obtained. Typical breathing rates were then multiplied by the measured concentrations to determine the mass of pollutant inhaled. RESULTS: The average concentration of diesel particulate matter measured over the duration of a single drive-by test often exceeded the low concentrations used in human clinical studies which are known to cause acute health effects. It was also observed that higher concentrations of diesel particulate matter were measured at the height of a stroller than were measured at the mouth of a mannequin. CONCLUSION: Diesel particulate matter concentrations during drive-by incidents easily reach or exceed the low concentrations that can cause acute health effects for brief periods of time. For the case of a particularly well-tuned late-model year vehicle, the mass of particulate matter inhaled during a drive-by incident is small compared to the mass inhaled daily at ambient conditions. On a per breath basis, however, the mass of particulate matter inhaled is large compared to the mass inhaled at ambient conditions. Finally, it was determined that children, infants, or people breathing at heights similar to that of a passing vehicle's tailpipe may be exposed to higher concentrations of particulate matter than those breathing at higher locations, such as adults standing up.

Experimental validation of a target method for balancing exhaust ventilation duct systems with dampers.

This study tested the "Target Method" for adjusting ventilation systems. The Target Method is based on target hood static pressures (SPh(target)) computed in a manner designed to take into account the estimated effects of dampers on the fan, the order of damper adjustments, and the ratio of the prebalancing branch airflows to their goals. It is aimed at achieving a desired relative distribution of airflows even if the fan output is far from ideal. The method assumes the fan output will be adjusted after the dampers are adjusted. The method is expected to produce lower fan pressure requirements than some commonly used methods. The method was tested on a working seven-branch, full-sized exhaust ventilation system in the West Virginia University Exposure Assessment Laboratory. Two radically different target distributions were tested with two replications apiece. Both target distributions of airflows were substantially different from the initial distribution, providing a high degree of challenge to the methodology. For each distribution, SPh(target) values were computed for the first round of adjustments. Each damper was adjusted until the observed value of the hood static pressure was nearly equal to that damper's computed SPh(target) value for that distribution. Each of the other branch dampers was adjusted similarly in turn. After the first round of adjustments, the median ratio of SPh to SPh(target) provided the targets for the partial second round of adjustments. Twenty-point Pitot traverses were used to determine the airflow in each branch duct both before and after employing the adjustment method, providing the basis to determine the success in reaching each of the two desired distributions. The percentage of excess airflow (assuming ideal adjustment of the fan speed) was below 2.2% for all experimental trials. An unpublished study by Vivek Balasubramanian showed that excess airflow was 4.8% to 8.5% in the same experimental system after two full rounds of adjustment using the customary Target Method. Under poor measurement conditions, the greater uncertainty of pressure measurements would likely produce somewhat higher excess airflows.

A numerical study of worker exposure to a gaseous contaminant: variations on body shape and scalar transport model.

Three-dimensional computational fluid dynamics simulations are used to investigate the distribution and level of contaminant concentrations in the true breathing zone (at the nose and mouth) when toxic airborne contaminants are released within an arm's length in front of the worker who has his back to the airflow. The effects of different body shapes on fluid flow and concentration patterns around the body in a wind tunnel were evaluated and clarified that a sharp body or a block may not be a good surrogate for the human form in consideration of occupational and environmental health studies. The comparison of the concentration field calculated with the Eulerian and Lagrangian methods revealed that the Eulerian method has a more diffusive nature than the Lagrangian method. The concentrations at different locations were also compared to determine the optimum sampling location. It was found that the concentration at the breathing zone may be significantly different from the one at the chest area. The influence of the heat flux from the body was studied at two different Reynolds numbers. Predictions indicate that the heat flux may have a significant impact on exposure especially when the convection induced by buoyancy dominates the flow.

Applying open-path FTIR with computed tomography to evaluate personal exposures. Part 1: simulation studies.

This paper presents the theoretical background and the numerical evaluation results obtained using computed tomography coupled with open-path Fourier transform infrared (CT-FTIR) measurements to estimate personal exposures. In this simulation study, we first tested the one-dimensional scenario with a five-beam segment geometry. A series of Gaussian plumes and the corresponding path-integrated concentrations (PICs) were simulated. The personal exposures were estimated as the average of the point estimates calculated from the workers' locations and the concentration profiles reconstructed from the Smooth Basis Function Minimization algorithm. It was found that the running-average PIC updating strategy has similar performance as the spline PIC updating strategy. However, the latter strategy gives delayed estimates of the workers' exposures since it requires additional measurements before and after the time period of interest. In the two-dimensional scenario, we simulated a series of single-mode bivariate Gaussian plumes with a nine-beam radial geometry. The average of the estimated exposures from the CT-FTIR approach was close to the average of the true exposures. The concordance correlation factors between the true and estimated exposures were reasonably good (between 0.50 and 0.58). This study demonstrated that the CT-FTIR approach is feasible for industrial hygiene monitoring.

Field evaluation of methods for determining the obstructed section of branches of industrial ventilation systems.

This study proposes and evaluates the effectiveness of three methods in determining the location of obstructions and alterations in the branches of industrial ventilation systems. Three branch-screening methods (reference ratio, static pressure ratio, and power loss coefficient methods) were adapted to determine if obstructions lay in four distinct areas of a branch. The areas were the hood, middle, and end sections, with the middle and end sections combined to form the fourth area. These methods were evaluated for their ability to detect obstructions in each specific section and also for their ability to detect obstructions in sections after the branch was surveyed for obstructions using a screening method. The results of this study indicate that each method is very useful in determining the presence and location of an obstruction in every section of the branch except the hood section. Each method performed poorly when determining obstructions in the hood section. However, because of the ability of the methods to detect obstructions in the other three sections, the process of elimination can be used with confidence to determine obstructions in the hood.

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.