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.     

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.     

A simple and inexpensive method for determining the effective ventilation rate in a negatively pressurized room using airborne particles as a tracer

The ventilation rate within a negatively pressurized room is usually determined by measuring the exhaust air flow rate. This method does not account for air mixing factors and gives limited information on ventilation efficiency within the room. Effective ventilation rates have been determined using tracer gases such as sulfur hexafluoride (SF6). The objective of this study was to determine whether artificially generated airborne particles could be used as a tracer to directly measure ventilation efficiency. We monitored the decay of artificially generated particles within negatively pressurized rooms. Separate trials were conducted at air exhaust rates ranging from about 6 to 20 room air changes per hour. Particles were generated to a minimum of 20 times the ambient concentration using a simple ventilation smoke bottle and measured with handheld light-scattering airborne particle counters. Data were obtained for aerodynamic particle size ranges of: 0.5 micron (microM) and larger, and 1.0 microM and larger. The time rate of decay of particles was plotted after subtracting the background concentrations. Results were compared with simultaneously conducted tracer gas decay analyses (ASTM method E741-95) using SF6. Particle concentrations followed an exponential decay (R2 = 0.98-0.99+) and mirrored the decay curve of the tracer gas. The air change rates predicted by the particle count procedure differed from the tracer gas results by a mean of 4.0 percent (range 0%-12%). The particle count procedure was substantially simpler and less expensive than the SF6 tracer gas method. Additional studies are needed to further refine this procedure and to explore its range of applicability.     

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.     

Comparison of simulated respirator fit factors using aerosol and vapor challenges

Although not well established, mask leakage measured using submicron aerosol challenges is generally accepted as being representative of vapor challenges. The purpose of this study was to compare simulated respirator fit factors (FFs) measured using vapor challenges to those measured using an aerosol challenge. A full-facepiece respirator was mounted on a headform inside a small enclosure and modified with controlled leaks (laser-drilled orifices) to produce FFs ranging from about 300 to 30,000. A breathing machine was used to simulate breathing conditions of 1.0 L tidal volume and 25 breaths/min. A monodisperse aerosol consisting of 0.72 micron polystyrene latex spheres (PSL) was used for the reference test aerosol, and FFs were measured using a laser aerosol spectrometer. An inert gas, sulfur hexafluoride (SF6), and an organic vapor, isoamyl acetate (IAA), were used as the vapor challenges. The in-mask concentration of SF6 was measured using a gas chromatograph (GC). A GC was also used to quantify in-mask IAA concentration samples actively collected with sorbent tubes. FF measurements made with the PSL aerosol challenge were conducted in sequence with the SF6 and IAA challenges, without disturbing the mask, to yield matched data pairs for regression analysis. FFs measured using the PSL reference aerosol were found to correlate well with those measured with the SF6 (r2 = 0.99) and IAA (r2 = 0.98) vapor challenges. FFs measured using IAA tended to be higher at values below 10,000. The best agreement was observed with the inert gas, SF6. The results of this study suggest that submicron aerosols are suitable as quantitative fit test challenges for assessing the performance of respirators against inert vapors.     

Impact of air distribution on efficiency of dust capture from metal grinding--bench test method

According to the Machinery Directive 2006/42/EC, one of the essential requirements relating to occupational safety and health hazards is to prevent dust pollution emitted by machinery during the implementation processes. Research on evaluation of emissions from machinery, according to the method of test bench using tracer gases, are currently being conducted in CIOP-PIB. This article presents some aspects of dust emission and efficiency of local exhaust ventilation (LEV) during metal grinding. Studies were performed with 10 sources of dust emissions during grinding. To evaluate the pollutants emission in the process of grinding metal products sulfur hexafluoride (SF(6)) was selected as a tracer gas. The results show that wherever dust is emitted, the LEV should be supported by the general ventilation. Ensure good interaction between all elements of modifying the air flow and the spread of pollutants in the surroundings of the LEV is essential to effective protection of human working zone against pollutants. We used five variants of ventilation: ventilation turned off, the LEV, one-way general ventilation, mixed general ventilation and displacement general ventilation. An increase in the efficiency of dust capture depending on the source of emission by 2.5-14% was observed. This confirms that characteristics of flow resulting from the operation of ventilation is important in the spread of pollutants in the room.     

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.     

A numerical method of reconstructing the pollutant concentration field in a ventilated room

Pollutant source emission flow rates in the workplace are typically unknown in occupational hygiene. Similarly, a restricted number of concentration measurements can provide only spatial limited information on the pollutant distribution in the room. This paper presents a numerical method to evaluate the intensities of pollutant sources and to reconstruct the associated concentration field at every point of a ventilated enclosure containing one or several pollutant sources of unknown emission rate. This reconstructed concentration field is obtained both from the geometric and ventilation characteristics of the enclosure and from a limited number of fixed-station concentration measurements. The method is currently applicable to steady situations. The predictions obtained are then compared with concentration measurements in a laboratory closed cabin under controlled ventilation. Pollutant sources generated tracer gas emissions at known flow rates. Comparisons were performed successively for three different physical configurations.     

Use of a directional spray system design to control respirable dust and face gas concentrations around a continuous mining machine

A laboratory study assessed the impacts of water spray pressure, face ventilation quantity, and line brattice setback distance on respirable dust and SF6 tracer gas concentrations around a continuous mining machine using a sprayfan or directional spray system. Dust levels were measured at locations representing the mining machine operator and the standard and off-standard shuttle car operators, and in the return airway. The results showed that changes in all three independent variables significantly affected log-transformed dust levels at the three operator sampling locations. Changes in setback distance impacted return airway dust levels. Laboratory testing also identified numerous variable interactions affecting dust levels. Tracer gas levels were measured on the left and right sides of the cutting drum and in the return. Untransformed gas levels around the cutting drum were significantly affected by changes in water pressure, face ventilation quantity, and setback distance. Only a few interactions were identified that significantly affected these concentrations. Gas levels in the return airway were grouped by face ventilation quantity. Return gas levels measured at the low curtain quantity were generally unaffected by changes in water pressure or curtain setback distance. At the high curtain quantity, return airway gas levels were affected by curtain setback distance. A field study was conducted to assess the impact of these parameters in an actual mining operation. These data showed that respirable dust levels may have been impacted by a change in water pressure and, to a lesser extent, by an increase in curtain setback distance. A series of tracer gas pulse tests were also conducted during this study. The results showed that effectiveness of the face ventilation was impacted by changes in curtain flow quantity and setback distance. Laboratory testing supported similar conclusions.     

Mixing in a square and a rectangular duct regarding selection of locations for extractive sampling of gaseous contaminants

Tests were conducted to characterize the uniformity of velocity and tracer gas profiles in a square and a rectangular duct with respect to defining the suitability of locations for single point sampling of gaseous contaminants. Several configurations, such as a straight duct with unidirectional flow at the entrance section and straight ducts preceded by mixing elements (a 90 degrees mitered bend and double 90 degrees bends in S- and U-type configurations) were tested. Results are compared with those from circular ducts. For a straight duct of square cross section, which is not preceded by a mixing element, the coefficients of variation (COV) of tracer gas concentration at 19 duct diameters downstream of the gas release location is 143% (center release of tracer gas). COVs of velocity and tracer gas concentration in a straight square duct 9.5 duct diameters downstream of a 90 degrees mitered bend are 6% and 24.3% (top inside release), respectively, which does not meet the ANSI N13.1 limit of 20% for the tracer gas COV. In case of the rectangular duct with a 3:1 (width to height) aspect ratio, COVs of velocity and tracer gas concentration at 9 duct diameters downstream of a 90 degrees mitered bend are 29% and 62% (bottom inside release), respectively. A mixing element in a square duct comprised of two 90 degrees mitered bends in a U-configuration produces results similar to those obtained with a single 90 degrees bend. However, COVs of velocity and tracer gas concentration in a square duct 6 duct diameters downstream of an S-type double bend are 10.6% and 8.3% (top inside release), respectively, which comply with the ANSI tracer gas and velocity criteria for single point representative sampling. When mixing elements were employed in square ducts, the COV results were comparable with those of other researchers for circular ducts.     

The effect of thermal loading on laboratory fume hood performance

A modified version of the ANSI/ASHRAE 110-1995 Method of Testing Performance of Laboratory Fume Hoods was used to evaluate the relationship between thermal loading in a laboratory fume hood and subsequent tracer gas leakage. Three types of laboratory burners were used, alone and in combination, to thermally challenge the hood. Heat output from burners was measured in BTU/hr, which was based on the fuel heat capacity and flow rate. Hood leakage was measured between 2824 and 69,342 BTU/hr. Sulfur hexafluoride (SF6) was released at 23.5 LPM for each level of thermal loading. Duct temperature was also measured during the heating process. Results indicate a linear relationship for both BTU/hr vs. hood leakage and duct temperature vs. hood leakage. Under these test conditions, each increase of 10,000 BTU/hr resulted in an additional 4 ppm SF6 in the manikin's breathing zone (r2 = 0.68). An additional 3.1 ppm SF6 was measured for every 25 degrees F increase in duct temperature (r2 = 0.60). Both BTU/hr and duct temperature models showed p < 0.001. For these tests, BTU/hr was a better predictor of hood leakage than duct temperature. The results of this study indicate that heat output may compromise fume hood performance. This finding is consistent with those of previous studies.     

用示踪气体法评价和优选吸气罩

目的 在粉尘发生源处设置适用的局部吸气罩,可以有效地控制粉尘向周围扩散,是预防尘肺病发生的有效措施。为了提供适宜吸气罩的设计,使用了示踪气体法评价吸气罩的效率,对吸气罩的设计进行优选,以提高捕集效率和降低能耗。方法 建立示踪气体评价吸气罩效率的实验风道和方法,采用人工煤气作为示踪气体。结果 实验了长方形和无延伸挡板及有特殊延伸挡板的条缝形吸气罩在不同罩口风速下对示踪气体的捕集效率。导出了捕集效率和距离的关系方程式。实验结果表明:(1)吸气罩与污染源的距离和罩口风速对捕集效率有明显的影响。当罩口风速一定时。吸气罩越靠近污染源,捕集效率越高;而在同一距离上,罩口风速越大,捕集效率就越高。(2)有延伸挡板的条缝形吸气罩的捕集效率高于无延伸挡板条缝形吸气罩,前者采用较低的罩口风速(抽风量)可以得到相应的高捕集效率。结论 使用示踪气体对吸气罩进行优选的结果表明,通过改进吸气罩的形式可以降低所需风量并达到要求的效率,从而为减少通风设施的费用提供了一种重要途径。     

Environmental tobacco smoke leakage from smoking rooms

Twenty-seven laboratory experiments were conducted in a simulated smoking room to quantify rates of environmental tobacco smoke (ETS) leakage to a nonsmoking area as a function of the physical and operational characteristics of the smoking room. Data are presented for the various types of leakage flows, the effect of these leaks on smoking room performance and nonsmoker exposure, and the relative importance of each leakage mechanism. The results indicate that the first priority for an effective smoking room is to maintain it depressurized with respect to adjoining nonsmoking areas. The amount of ETS pumped out by the smoking room door when it is opened and closed can be reduced significantly by substituting a sliding door for the standard swing-type door. An "open doorway" configuration used twice the ventilation flow of those with smoking room doors, but yielded less reduction in nonsmoker exposure. Measured results correlated well with results modeled with mass-balance equations (R(2) = 0.82-0.99). Most of these results are based on sulfur hexafluoride (SF(6)) tracer gas leakage. Because five measured ETS tracers showed good correlation with SF(6), these conclusions should apply to ETS leakage as well. Field tests of a designated smoking room in an office building qualitatively agreed with model predictions.     

Development of a push-pull ventilation system to control solder fume.

Hand soldering using rosin core solder wire is common in the electronics industry and several studies have implicated the aerosol produced when rosin flux is heated in causing respiratory sensitisation. Control of solder fume is generally achieved using local exhaust hoods, simple blowers with a filter or low-volume high-velocity (LVHV) ventilation systems. None of these provide an ideal control system and so a push-pull ventilation design was developed as an alternative. Laboratory tests of the system's capture efficiency were carried out using nitrous oxide tracer gas. Capture efficiency was generally greater than 90% with the push airflow operating. However, without the push airflow, capture efficiency decreased sharply with increasing distance from the exhaust hood (between 38 and 58% at 420 mm from the front of the exhaust hood with the same exhaust airflow used by the push-pull system). The push-pull system was found to be relatively insensitive to obstructions placed in the path of the air flow or the influence of cross draughts.The system was tested in five electronics factories and the effectiveness was compared to their existing ventilation systems. Where only a small amount of soldering was carried out both the in-house and push-pull systems seemed to provide adequate control of inhalation exposure to rosin-based solder flux fume measured as total resin acids. However, the push-pull system provided more consistent control than the existing ventilation systems when larger quantities of solder were used. In these situations the mean personal exposure level was reduced to below the UK Maximum Exposure Limit (MEL) of 0.05 mg/m(3) 8-h time weighted average in most instances. The corresponding mean personal exposure level with the in-house systems in operation was about three to four times the long-term MEL. Interpretation of these data is complicated because of high background contribution to exposure from poorly controlled soldering operations elsewhere in the factories. However, this study suggests that the in-house systems were relatively inefficient.     

Effect of cross current due to worker's arm movement on local exhaust ventilation hood

The effect of cross drafts caused by a worker's arm movements on the capture efficiency of a local exhaust ventilation hood was examined in a laboratory. The performance of the local exhaust hoods (rectangular type and slot type) and the transportation of gaseous contaminants from an emission source to the breathing zone were studied by means of the tracer gas method. Acetone vapor was used as a tracer gas. The worker's arm movement was simulated by a dummy worker and a moving forearm model. The results suggest that a worker's arm movements disturb the exhaustion efficiency and may lead to exposure or leakage from a hood according to exhaust velocity.     

“华龙一号”核电厂烟囱气态流出物取样系统验证的试验与分析

目的 通过试验验证以确保中国自主研发的第三代压水堆核电站"华龙一号"堆型烟囱气态流出物取样系统（采用单嘴取样头设计）充分满足取样代表性要求。方法 本文基于美国国家标准ANSI/HPS N13.1-1999要求,建造了"华龙一号"反应堆烟囱1：5的比例模型,在比例模型上完成了3个不同标高取样截面的平均气旋角、气体流速分布、示踪气体分布、示踪气溶胶分布验证试验,并在福清核电站1、2号机组的烟囱上开展了气旋角、气体流速、示踪气体浓度分布的验证试验。结果 在"华龙一号"比例模型烟囱三个预选取样截面（Q1、Q2、Q3）中心2/3区域内,在两种设计通风工况下,气体流速分布变异系数（COV）≤ 1.1%,所有测点最大气旋角为11.38°,示踪气体分布浓度分布COV ≤ 4.4%,示踪气溶胶浓度分布COV ≤ 4.7%;在实际烟囱预选取样截面中心2/3面积内气体流速分布COV ≤ 8.4%,所有测点气旋角平均绝对值为11.3°（且最大值<20°）,并且由DVN碘排风、DVN正常排风系统注入示踪气体时,测量截面上示踪气体浓度分布COV分别为2.2%、1.3%。结论 "华龙一号"模型烟囱和实际烟囱的所有测试指标,全部符合ANSI/HPS N13.1-1999标准对于取样截面上污染物混合均匀性的要求,即可以采用单点取样方式来设计"华龙一号"烟囱气态流出物取样系统。     

The design of local exhaust ventilation hoods for grinding wheels

The simplest method of controlling airborne contaminants produced during grinding is the free-standing local exhaust ventilation hood. However design data are sparse and not related to individual applications, for example neither extract rate nor hood size is related to the wheel size. Hoods for use with grinding wheels are usually designed as captor hoods but it is shown in this study that such hoods act as receptors and should be designed on that basis. Flow visualization, using both smoke pellets and a heated oil-impregnated wire, showed the air flow patterns which are induced by a rapidly rotating wheel. Air speeds around the wheels were measured using omnidirectional probes whilst hood capture efficiencies were measured using tracer gas techniques. These observations and measurements identified several key parameters which should lead to a more effective and efficient design of control systems for use with surface grinders.     

Measuring the emission rate of an aerosol source placed in a ventilated room using a tracer gas: influence of particle wall deposition

A method to measure the emission rate of an airborne pollutant source using a tracer gas was tested in the case of an aerosol source. The influence of particle deposition on the walls of a test room of 72 m3 was studied. The deposition rate of an aerosol of MgCl2 was determined by means of two methods: one based on measuring the aerosol concentration decay inside the ventilated room, the other based on calculation of the material mass balance. The concentration decay was monitored by optical counting and the aerosol mass concentration determined by means of sampling on a filter and analysis of the mass deposited by atomic absorption spectrometry. Four series of measurements were carried out. The curve giving the deposition rate according to the particle aerodynamic diameter (d(ae)) was established and shows deposition rates higher than those predicted using the model of Corner. The decay method gives the best results. The study carried out has shown that the phenomenon of deposition has little effect on the measurement of the aerosol source emission rate using a tracer gas for particles of aerodynamic diameter < 5 microm (underestimation < 25%). For particles of a greater diameter, wall deposition is an extremely limiting factor for the method, the influence of which can, however, be limited by using a test booth of small volume and keeping the sampling duration as short as possible.     

Development of evaluation procedures for local exhaust ventilation for United States postal service mail-processing equipment

Researchers from the National Institute for Occupational Safety and Health (NIOSH) have conducted several evaluations of local exhaust ventilation (LEV) systems for the United States Postal Service (USPS) since autumn 2001 when (a) terrorist(s) employed the mail system for acts of bioterrorism. As a part of the USPS 2002 Emergency Preparedness Plan, the development and installation of LEV onto USPS mail-processing equipment can reduce future exposures to operators from potentially hazardous contaminants, such as anthrax, which might be emitted during the processing of mail. This article describes how NIOSH field testing led to the development of recommended testing procedures for evaluations of LEV capture efficiency for mail-processing equipment, including tracer gas measurements, smoke release observations, air velocity measurements, and decay-rate testing under access hoods.