Thermal loading as a causal factor in exceeding the 0.1 PPM laboratory fume hood control level

Tracer gas testing per ANSI/ASHRAE 110-1995 Method of Testing Performance of Laboratory Fume Hoods was used to investigate the role of thermal loading in exceeding laboratory fume hood control levels. Three types of typical laboratory burners (blast, Meeker, and economy) were used to provide a thermal challenge. Heat outputs of between 0 and 61,610 Btu/hr were based on fuel heat capacity (for liquid propane gas) and fuel gas flow rates. Breathing zone concentrations were measured with a MIRAN 1B2 infrared gas analyzer. Also, for each test, the difference between the room and duct temperatures (delta temperature) was measured. Results indicated a linear relationship between heat loads and tracer gas breathing zone concentrations for both Btu/hr and delta temperature. Control levels of 0.1 ppm were exceeded at less than 12,000 Btu/hr. Also, control levels were exceeded at a lower heat load when the tracer gas generation rate was increased. These results indicate that thermal loads in laboratory fume hoods increase the risk of exceeding laboratory fume hood control levels. Some compensatory measures relative to hood configuration and flow rates are recommended for laboratory operations involving heat sources.     

Containment testing of laboratory hoods in the as-used condition

Airborne contaminants generated inside laboratory fume hoods during use leak into the breathing zone of the user. Concentration of the leakage is unknown and variable depending on laboratory design, work practices, arrangement of internal apparatus, face velocity, and sash height. Surrogate tracer gas tests have been developed using sulfur hexafluoride (SF6) and a manikin to estimate leakage. This study presents results of hood leakage tests using SF6 with a manikin and then a live operator performing a phenol:chloroform (P:C) extraction. Four hoods were tested in each of three institutions during normal work hours with the lab occupied. The purpose of the study was to determine leakage concentrations for the SF6-manikin with effects of sash height, hood contents as found and after being cleaned out, face velocity, and the actual P:C and SF6 exposure concentrations of the user during work. Results indicate P:C was not detectable in the breathing zone of the 12 operators (< 0.1 ppm) at their selected operating sash heights (7 to 15 inches). Simultaneous SF6 concentrations were also minimal (average 0.06 ppm). Average leakage was 0.02 percent for SF6 and less than 2 percent based on chloroform concentrations measured in the breathing zone of the operator and inside the hood. SF6 percent leakage was greater when sash height was above the breathing zone of the manikin (average 2.09 percent) and lower leakage (average 0.02 percent) when below the breathing zone (26 inches or less). Average face velocity did not appear to be a predictor of average hood leakage. Cleaning out the hoods did not reduce leakage in most tests. The data from this study shows that when providing training on proper work practices for lab hood use, lowering the sash should be stressed as being the major factor in reducing hood leakage.     

Effects of walk-by and sash movement on contaminant leakage of air curtain-isolated fume hood

The effects of the walk-by motion and sash movement on the containment leakage of an air curtain-isolated fume hood were evaluated and compared with the results of a corresponding conventional fume hood. The air curtain was generated by a narrow planar jet issued from the double-layered sash and a suction slot-flow arranged on the floor of the hood just behind the doorsill. The conventional fume hood used for comparison had the major dimensions identical to the air-curtain hood. SF tracer-gas concentrations were released and measured following the prEN 14175-3:2003 protocol to examine the contaminant leakage levels. Experimental results showed that operating the air-curtain hood at the suction velocity above about 6 m/s and jet velocity about 1 m/s could provide drastically high containment performance when compared with the corresponding conventional fume hood operated at the face velocity of 0.5 m/s. The total air flow required for the air-curtain hood operated at 6 m/s suction velocity and 1 m/s jet velocity was about 20% less than that exhausted by the conventional fume hood. If the suction velocity of the air-curtain hood was increased above 8 m/s, the containment leakage during dynamic motions could be reduced to ignorable level (about 10(3) ppm).     

Correlation between airflow patterns and performance of a laboratory fume hood

To understand the physical mechanisms of the contaminant dispersion and containment leakage during the ventilation process through a laboratory fume hood, the complicated three-dimensional flow patterns and the real-time tracer gas (SF6) leakage were studied via the laser-assisted flow visualization method and the standard/special gas sampling technique, respectively. Through flow visualization, the large-scale vortex structures and boundary layer separations were found around the side poles and doorsill of the hood. In the near-wake region of the manikin, large recirculation zones and wavy flow structures were also identified. When tracer gas concentration measurements were conducted point-by-point across the sash opening, the areas near the doorsill, the lower parts of the side poles, and the sides of the manikin showed significant contaminant leaks. These areas with high contaminant leaks exactly corresponded to where the flow recirculated or separated. However, when the ANSI/ASHRAE 110-1995 protocol was used to measure the concentration of SF6 at the breathing zone of the manikin, no appreciable leakage was detected. It is suggested that a method based on the aerodynamic features and multipoint leakage detections would reflect a more realistic evaluation of overall performance of laboratory fume hood than a single-point sampling method at the manikin's breathing zone.     

Required response time for variable air volume fume hood controllers

This paper describes results from tests made with the aim of investigating how quickly the exhaust air flow rate through fume hoods needs to be controlled in order to prevent contaminants from leaking out of the fume hood and putting the safety of the laboratory personnel at risk. The measurements were made on a laboratory fume hood in a chemical laboratory. There were no other fume hoods in the laboratory, and the measurements were made without interference from persons entering or leaving the laboratory or walking about in it. A tracer gas method was used with the concentration of dinitrogen oxide (N(2)O) being recorded by a Foxboro Miran 101 infra-red gas analyser. In parallel with the tracer gas measurements, the air velocity through the face opening was also measured, as was the control signal to the damper controlling the air flow rate. The measurements show an increased outward leakage of tracer gas from the fume hood if the air flow rate is not re-established within 1-2 s after the sash is opened. If the delay exceeds 3 s the safety function is temporarily defeated. The measurements were made under virtually ideal conditions. Under more typical conditions, the fume hood could be exposed to various other external perturbations, which means that the control system should re-establish the correct exhaust flow more quickly than indicated by the measurement results obtained under these almost ideal conditions.     

Effects of boundary-layer separation controllers on a desktop fume hood

A desktop fume hood installed with an innovative design of flow boundary-layer separation controllers on the leading edges of the side plates, work surface, and corners was developed and characterized for its flow and containment leakage characteristics. The geometric features of the developed desktop fume hood included a rearward offset suction slot, two side plates, two side-plate boundary-layer separation controllers on the leading edges of the side plates, a slanted surface on the leading edge of the work surface, and two small triangular plates on the upper left and right corners of the hood face. The flow characteristics were examined using the laser-assisted smoke flow visualization technique. The containment leakages were measured by the tracer gas (sulphur hexafluoride) detection method on the hood face plane with a mannequin installed in front of the hood. The results of flow visualization showed that the smoke dispersions induced by the boundary-layer separations on the leading edges of the side plates and work surface, as well as the three-dimensional complex flows on the upper-left and -right corners of the hood face, were effectively alleviated by the boundary-layer separation controllers. The results of the tracer gas detection method with a mannequin standing in front of the hood showed that the leakage levels were negligibly small (≤0.003 ppm) at low face velocities (≥0.19 m/s).     

Development and evaluation of an air-curtain fume cabinet with considerations of its aerodynamics

In order to avoid the inherent aerodynamic difficulties of the conventional fume hood, an innovative design--the 'air curtain-isolated fume hood' is developed. The new hood applies a specially designed air curtain (which is generated by a narrow planar jet and a suction slot flow at low velocities) across the sash plane. The hood constructed for the study is full size and transparent for flow visualization. The aerodynamic characteristics are diagnosed by using the laser-light-sheet-assisted smoke flow visualization method. Four characteristic air-curtain flow modes are identified in the domain of jet and suction velocities when the sash remains static. Some of these characteristic flow modes have much improved flow patterns when compared with those of the conventional fume hoods. From the viewpoint of the aerodynamics and mass transport, the results indicate that the air curtain properly setup across the sash opening allows almost no sensible exchange of momentum and mass between the flowfields of the cabinet and the outside environment. Two standard sulfur hexafluoride (SF6) tracer gas concentration measurement methods following the ANSI/ASHRAE 110-1995 standard and the prEN14175 protocol for static test are employed to examine the contaminant leakage levels. Results of the rigorous examinations of leakage show unusually satisfactory hood performance. The leakage of the tracer gas can approach almost null (<0.001 p.p.m.) if the jet and suction velocities are properly adjusted.     

Installation of a flow control device in an inclined air-curtain fume hood to control wake-induced exposure

An inclined plate for flow control was installed at the lower edge of the sash of an inclined air-curtain fume hood to reduce the effects of the wake around a worker standing in front of the fume hood. Flow inside the fume hood is controlled by the inclined air-curtain and deflection plates, thereby forming a quad-vortex flow structure. Controlling the face velocity of the fume hood resulted in convex, straight, concave, and attachment flow profiles in the inclined air-curtain. We used the flow visualization and conducted a tracer gas test with a mannequin to determine the performance of two sash geometries, namely, the half-cylinder and inclined plate designs. When the half-cylinder design was used, the tracer gas test registered a high leakage concentration at V_f ≦ 57.1 fpm or less. This concentration occurred at the top of the sash opening, which was close to the breathing zone of the mannequin placed in front of the fume hood. When the inclined plate design was used, the containment was good, with concentrations of 0.002–0.004 ppm, at V_f ≦ 63.0 fpm. Results indicate that an inclined plate effectively reduces the leakage concentration induced by recirculation flow structures that form in the wake of a worker standing in front of an inclined air-curtain fume hood.     

A review of published quantitative experimental studies on factors affecting laboratory fume hood performance

This study attempted to identify the important factors that affect the performance of a laboratory fume hood and the relationship between the factors and hood performance under various conditions by analyzing and generalizing the results from other studies that quantitatively investigated fume hood performance. A literature search identified 43 studies that were published from 1966 to 2006. For each of those studies, information on the type of test methods used, the factors investigated, and the findings were recorded and summarized. Among the 43 quantitative experimental studies, 21 comparable studies were selected, and then a meta-analysis of the comparable studies was conducted. The exposure concentration variable from the resulting 617 independent test conditions was dichotomized into acceptable or unacceptable using the control level of 0.1 ppm tracer gas. Regression analysis using Cox proportional hazards models provided hood failure ratios for potential exposure determinants. The variables that were found to be statistically significant were the presence of a mannequin/human subject, the distance between a source and breathing zone, and the height of sash opening. In summary, performance of laboratory fume hoods was affected mainly by the presence of a mannequin/human subject, distance between a source and breathing zone, and height of sash opening. Presence of a mannequin/human subject in front of the hood adversely affects hood performance. Worker exposures to air contaminants can be greatly reduced by increasing the distance between the contaminant source and breathing zone and by reducing the height of sash opening. Many other factors can also affect hood performance. Checking face velocity by itself is unlikely to be sufficient in evaluating hood performance properly. An evaluation of the performance of a laboratory fume hood should be performed with a human subject or a mannequin in front of the hood and should address the effects of the activities performed by a hood user.     

Influence of high heat load on flow and containment of an inclined air-curtain (IAC) fume hood

The inclined air-curtain (IAC) fume hood has been reported to have “almost null leakage”^[1 Huang, R.F., J.-K. Chen, and K.-C. Tang: Development and characterization of an inclined air-curtain (IAC) fume hood. Ann. Occup. Hyg. 59(5):655–667 (2015).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]^] at low suction flow rates when operated at regular temperatures. However, previous research has not investigated the performance or optimized operating parameters when a high heat load is used in the IAC fume hood. For the present work, the effects of a high heat load on the flow field and contaminant leakage characteristics of the IAC fume hood were examined. The heat load was supplied to an IAC hood according to the standard method of EN14175-7:2012. The laser-assisted smoke flow visualization technique was employed to identify the characteristic flow patterns. The standard tracer-gas concentration test method (EN14175-3:2003) was used to examine the leakage levels of the IAC fume hood under static conditions, sash movement, and simulated walk-by conditions. When the IAC fume hood was operated at a high heat load, the static test results showed negligibly small leakage levels at a face velocity greater than or equal to only 0.19 m/s (37.4 ft/min). The sash movement and simulated walk-by test results showed that to obtain negligibly small leakage levels at high heat load operation, the IAC fume hood required a face velocity greater than or equal to 0.32 m/s (63 ft/min). In addition, the IAC fume hood exhibited a superior hood containment performance with low energy consumption when compared with conventional fume hoods operated at a high heat load.     

Use of tracer gas technique for industrial exhaust hood efficiency evaluation--where to sample?

A tracer gas technique using sulfur hexafluoride (SF6) was developed for the evaluation of industrial exhaust hood efficiency. In addition to other parameters, accuracy of this method depends on proper location of the sampling probe. The sampling probe should be located in the duct at a minimum distance from the investigated hood where the SF6 is dispersed uniformly across the duct cross section. To determine the minimum sampling distance, the SF6 dispersion in the duct in fully developed turbulent flow was studied at four duct configurations frequently found in industry: straight duct, straight duct-side branch, straight duct-one elbow, and straight duct-two elbows combinations. Based on the established SF6 dispersion factor, the minimum sampling distances were determined as follows: for straight duct, at least 50 duct diameters; for straight duct-side branch combination, at least 25 duct diameters; for straight duct-one elbow combination, 7 duct diameters; and for straight duct-two elbow combination, 4 duct diameters. Sampling at (or beyond) these distances minimizes the error caused by the non-homogeneous dispersion of SF6 in the duct and contributes to the accuracy of the tracer gas technique.     

Flow Characteristics and Robustness of an Inclined Quad-vortex Range Hood

A novel design of range hood, which was termed the inclined quad-vortex (IQV) range hood, was examined for its flow and containment leakage characteristics under the influence of a plate sweeping across the hood face. A flow visualization technique was used to unveil the flow behavior. Three characteristic flow modes were observed: convex, straight, and concave modes. A tracer gas detection method using sulfur hexafluoride (SF₆) was employed to measure the containment leakage levels. The results were compared with the test data reported previously in the literature for a conventional range hood and an inclined air curtain (IAC) range hood. The leakage SF₆ concentration of the IQV range hood under the influence of the plate sweeping was 0.039 ppm at a suction flow rate of 9.4 m³/min. The leakage concentration of the conventional range hood was 0.768 ppm at a suction flow rate of 15.0 m³/min. For the IAC range hood, the leakage concentration was 0.326 ppm at a suction flow rate of 10.9 m³/min. The IQV range hood presented a significantly lower leakage level at a smaller suction flow rate than the conventional and IAC range hoods due to its aerodynamic design for flow behavior.     

Evaluation of Leakage From Fume Hoods Using Tracer Gas,Tracer Nanoparticles and Nanopowder Handling Test Methodologies

The most commonly reported control used to minimize workplace exposures to nanomaterials is the chemical fume hood. Studies have shown, however, that significant releases of nanoparticles can occur when materials are handled inside fume hoods. This study evaluated the performance of a new commercially available nano fume hood using three different test protocols. Tracer gas, tracer nanoparticle, and nanopowder handling protocols were used to evaluate the hood. A static test procedure using tracer gas (sulfur hexafluoride) and nanoparticles as well as an active test using an operator handling nanoalumina were conducted. A commercially available particle generator was used to produce sodium chloride tracer nanoparticles. Containment effectiveness was evaluated by sampling both in the breathing zone (BZ) of a mannequin and operator as well as across the hood opening. These containment tests were conducted across a range of hood face velocities (60, 80, and 100 ft/min) and with the room ventilation system turned off and on. For the tracer gas and tracer nanoparticle tests, leakage was much more prominent on the left side of the hood (closest to the room supply air diffuser) although some leakage was noted on the right side and in the BZ sample locations. During the tracer gas and tracer nanoparticle tests, leakage was primarily noted when the room air conditioner was on for both the low and medium hood exhaust airflows. When the room air conditioner was turned off, the static tracer gas tests showed good containment across most test conditions. The tracer gas and nanoparticle test results were well correlated showing hood leakage under the same conditions and at the same sample locations. The impact of a room air conditioner was demonstrated with containment being adversely impacted during the use of room air ventilation. The tracer nanoparticle approach is a simple method requiring minimal setup and instrumentation. However, the method requires the reduction in background concentrations to allow for increased sensitivity.     

Evaluation of the reverse flow around a worker's body produced by a local exhaust hood]

Recent studies have shown that a reverse flow often occurs in a unidirectional airflow in push-pull ventilation and may transport contaminants from the source into a worker's breathing zone. The same problem may arise in local exhaust ventilation when the contaminant source is located in the worker's wake region. In this study, organic solvent work with local exhaust ventilation was duplicated in a laboratory and the details of the reverse flow around the worker's body produced by the ventilation were experimentally investigated. In order to evaluate the influence of the reverse flow on the exposure of the worker, experiments with a mock-up mannequin (dummy worker) and a local ventilation system which was equipped with an exterior type hood and an enclosure type hood were conducted. The exposure level and the contaminant leakage from the hoods in several conditions were measured by means of a smoke test and tracer gas method. Ethanol vapor was used as a tracer gas. With the exterior type hood, the reverse flow visualized by the smoke was observed in front of the standing dummy worker but could not be observed when the dummy worker was seated. From the tracer gas measurements, it was proved that the exposure due to the reverse flow was not so serious at a capture velocity of > 0.4 m/s, but < 10 ppm contaminant leakage from the exterior hood had been recognized independently of the capture velocity. With the enclosure type hood, exposure due to the reverse flow could be controlled with a capture velocity of > 0.8 m/s. Although the contaminant leakage from the hood due to the reverse flow was not obvious with the enclosure type in any condition, caution should be exercised to prevent exposure when the worker is seated. Regardless of the hood type, the increase in the capture velocity was effective in decreasing exposure due to the reverse flow.     

Evaluation of industrial local exhaust hood efficiency by a tracer gas technique

Efficiency of industrial local exhaust ventilation is defined as the ratio of air contaminant quantity captured by the system per unit time to the total contaminant quantity produced by the process per unit time. To date, no direct method exists for this evaluation. This paper describes a tracer gas technique, using sulfur hexafluoride (SF6), which has been developed for the evaluation of local exhaust system efficiency. SF6 was discharged at a known rate into the industrial process generation area. Then, by comparing this quantity to that captured by the exhaust system, as measured in the exhaust duct, hood efficiency is determined. Major advantages of this technique are: The tracer gas technique is able to evaluate directly the hood efficiency. The tracer gas technique is not affected by cross-contamination from nearby industrial processes. The tracer gas technique can be conducted "on site" with minimal interruption of industrial process or interference with workers' duties. The tracer gas, using SF6 is non-toxic. Since SF6 is a gas, this technique may be limited to efficiency evaluation of hoods associated with gases, fumes, vapors, or fine particles.     

Flow and containment characteristics of an air-curtain fume hood operated at high temperatures

The flow and leakage characteristics of the air-curtain fume hood under high temperature operation (between 100°C and 250°C) were studied. Laser-assisted flow visualization technique was used to reveal the hot plume movements in the cabinet and the critical conditions for the hood-top leakage. The sulfur hexafluoride tracer-gas concentration test method was employed to examine the containment spillages from the sash opening and the hood top. It was found that the primary parameters dominating the behavior of the flow field and hood performance are the sash height and the suction velocity as an air-curtain hood is operated at high temperatures. At large sash height and low suction velocity, the air curtain broke down and accompanied with three-dimensional flow in the cabinet. Since the suction velocity was low and the sash opening was large, the makeup air drawn down from the hood top became insufficient to counter act the rising hot plume. Under this situation, containment leakage from the sash opening and the hood top was observed. At small sash opening and high suction velocity, the air curtain presented robust characteristics and the makeup air flow from the hood top was sufficiently large. Therefore the containment leakages from the sash opening and the hood top were not observed. According to the results of experiments, quantitative operation sash height and suction velocity corresponding to the operation temperatures were suggested.     

Fume hoods, open canopy type--their ability to capture pollutants in various environments.

Using field observations, modelling techniques and theoretical analysis, parameters describing the performance and collection efficiency of large industrial canopy fume hoods are established for, a) steady state collection of fume and b) collection of plumes with fluctuating flowrates. Hopper and pool type hoods are investigated. A baffle plate arrangement for placement within hoods is proposed. It prevents recirculation and spillage of fume. Temporary storage of fume surges within the hood is shown to be possible. At a cost of $6 per m3/hr ($10 per ft3/min) of installed fume control system capacity the arrangement promises to save millions of dollars on large new installations and to significantly improve the collection efficiency of many existing systems. A practical application of the results is proposed for the design of electric arc furnace canopy hoods.     

Manganese and welding fume exposure and control in construction

Overexposure to welding fume constituents, particularly manganese, is of concern in the construction industry due to the prevalence of welding and the scarcity of engineering controls. The control effectiveness of a commercially available portable local exhaust ventilation (LEV) unit was assessed. It consisted of a portable vacuum and a small bell-shaped hood connected by a flexible 2 inch (50.8 mm) diameter hose, in both experimental and field settings. The experimental testing was done in a semienclosed booth at a pipefitter training facility. Five paired trials of LEV control vs. no control, each approximately 1 hr in duration and conducted during two successive welds of 6 inch (152.4 mm) diameter carbon steel pipe were run in random order. Breathing zone samples were collected outside the welding hood during each trial. In the field scenario, full-shift breathing zone samples were collected from two pipefitters welding carbon steel pipe for a chiller installation on a commercial construction project. Eight days of full-shift sampling were conducted on both workers (n = 16), and the LEV was used by one of the two workers on an alternating basis for 7 of the days. All samples were collected with personal sample pumps calibrated at 2 L/min. Filter cassettes were analyzed for total particulate and manganese concentration by a certified laboratory. In the experimental setting, use of the portable LEV resulted in a 75% reduction in manganese exposure (mean 13 microg/m(3) vs. 51 microg/m(3); p < 0.05) and a 60% reduction in total particulate (mean 0.74 mg/m(3) vs. 1.83 mg/m(3); p < 0.05). In the field setting, LEV use resulted in a 53% reduction in manganese exposure (geometric mean 46 microg/m(3) vs. 97 microg/m(3); p < 0.05) but only a 10% reduction in total particulate (geometric mean 4.5 mg/m(3) vs. 5.0 mg/m(3); p > 0.05). These results demonstrate that LEV use can reduce manganese exposure associated with welding tasks in construction.     

Effects of packaging, equipment, and storage time on energy used for reheating beef stew.

Energy used to reheat 3 kg of a standard beef stew to 74 degrees C was measured to determine (a) the benefits of a retort pouch packaging processing system that keeps food microbially safe at room temperature compared with a system that packages food in a plastic bag that requires refrigerated storage; (b) the most economical form for reheating (in bulk in bags, in bulk out of bags, or in portions); (c) the most economical equipment for reheating (convection oven, infrared oven, microwave oven, compartment steamer, or steam-jacketed kettle); and (d) the influence of storage time (7, 28, or 85 days). Energy used for reheating the retort product was 18,883.7 British thermal units (BTU) compared with 31,035.6 BTU for the plastic bag product. Reheating in portions used 6,857 BTU; reheating in bulk out of bag used 23,419 BTU; and reheating in bulk in bag used 64,247 BTU. The order of least to greatest energy use for equipment was microwave oven, 324 BTU; infrared oven, 5,406 BTU; convection oven, 11,399 BTU; steam-jacketed kettle, 30,713 BTU; and steamer, 51,412 BTU. Storage time in the plastic bag significantly (P less than .05) affected initial product temperature and the energy required for reheating; this was not true for the retort product. Our findings indicate that microwave heating, heating in portions rather than in larger quantities, refrigerated storage of 7 days instead of 28 days, and use of retort pouch products achieve the least energy cost in reheating a product such as beef stew.     

Laboratory chemical hoods: a historical perspective

A historical review of laboratory fume hoods leads to a consideration of the current status of structural design, operating characteristics (with special reference to face velocity), safety (relative to standardized test results), energy conservation, and certification methods. Noteworthy are (1) the increasing complexity of instrumentation designed to assure full safety function plus airflow modulation to minimize energy consumption; (2) the extreme plasticity of accepted and recommended face velocity values; (3) the insensitivity of standardized hood test protocols to variations in face velocity; and (4) a serious lack of correlation between operator risk, face velocity, and standard hood test results. Safety considerations lead to the selection of laboratory fume hoods having the highest demonstrated containment capability. This is in spite of the fact that most hood operations have a low hazard rating. Energy savings could be realized if the face velocity of each hood could be modulated to match the risk associated with the work being conducted.