Local Exhaust Hood Research Articles

Numerical and experimental studies of airflows at exhaust hoods with inlet extensions

Local exhaust ventilation is used to capture contaminants and maintain a comfortable atmosphere in premises. Exhaust hoods with extension units (canopy hoods) are commonly used in open-type local exhaust devices. The goal of this study was to use computational and experimental methods to determine the effect of adding an extension to a local exhaust hood on the separated-flow streamlines, vortex zones, exhaust velocity distribution, and local drag coefficient. We used the discrete vortex method together with computational fluid dynamics to investigate air flows near round and slotted extended exhaust hoods. We then compared the computed airflow velocity field, vortex zone outlines at the inlet of a local exhaust hood, and local drag coefficient factor with data from a field experiment. Outlines of vortex zones resulting from flow separation at the inlet of the hood were considered for a hood length of 5 times gauge (the radius of a round hood or half-width of a slotted exhaust hood), hood tilt angles of 90°, 75°, 60°, 45°, and 30°, and extension lengths of 0; 0.5; 1; and 2 times gauge. We found that longer extensions caused the first vortex zone to enlarge, and the contaminant capture velocity of the local exhaust hood to decrease. We identified the behavior of the local drag coefficient at various hood tilt angles, extension lengths (as listed above), and hood lengths (including 2, 3, and 5 times gauge), and translated them into equations for computing the local drag coefficient.

Efficiency enhancing of the local capture hood due to air barriers

AbstractThis article is related to investigations of the capture hoods of the local exhaust ventilation. The purpose of the research: to increase the zone of action of local exhaust hoods and reduce the amount of air removed. It is equipped with two barriers for air: ring and cylindrical. The empirical dependences for air velocity determination near the suction zone are obtained. Graphs, chart and three-dimensional image visualizations of removed air jet velocity near capture hood with barriers for air are designed. The reduction of production energy consumption, material, and ventilation system maintenance costs due to the correction of the design of the capture hood are the main benefits of the new solution.

Reduced-scale experimental investigation on flow field characteristics of exhaust hood of double helix lifting transportation equipment in an industrial plant

In this study, the double screw lifting device of a rubber factory is taken as the case study object, and a scale model experiment was designed to determine the temperature field, velocity field and concentration field of pollutant escape of double spiral lifting transportation device are compared, and the capture efficiency is compared under the conditions of different dimensionless installation height and local exhaust hood air flow velocity Vexh. Reduced-scale experiments were conducted with a geometric scale of 1:4. Compared with no local exhaust hood, the concentration at the same measuring point decreases significantly under the action of local exhaust hood (dimensionless distances z/d = 3/4 and z/d = 2/4). Under the condition of maintaining the same local exhaust hood air flow velocity Vexh, the capture efficiency of local exhaust hood increased by 4% and 1% respectively on the dimensionless installation distance Z/D was 3/4 and 2/4 respectively. The experimental method of this case study and the air flow field experimental data obtained can provide a reference for the subsequent design and installation of exhaust hood.

Research on the relationship between the centerline velocity, aspect ratio and exhaust airflow rate for a slot and a rectangular capture hood in an local exhaust ventilation system.

When using a local exhaust hood to remove harmful substances from the production process, the exhaust airflow rate must be calculated according to the capturing velocity specified by the relevant regulations. The Numano and American Conference of Governmental Industrial Hygienists (ACGIH) equations are used in Japan and the US, respectively, for estimating the exhaust airflow rate of slot hoods. However, these equations differ from each other, and when using these equations to calculate the exhaust airflow rate of the capture hood, whether using Japan's equation or ACGIH, the hood type (slot or rectangular hood) should be distinguished at first. Therefore, this study performs experiments and a computational fluid dynamics (CFD) simulation to investigate the relationship between the centerline velocity and the aspect ratio for five types of capture hoods. The results showed good agreement between simulated and experimental centerline velocities when the distance from the hood face. A dimensionless velocity was introduced and a significant difference in the relationship between the centerline velocity and the distance from the hood face with different aspect ratios was found. A unified equation was obtained that can express the relationship between exhaust airflow rate and centerline velocity regardless of the aspect ratio of the hood face of the free-standing capture hood.

Developing a mathematical simulation method for three-dimensional separated airflow at inlet of local exhaust devices

For local exhaust ventilation systems to be capable of removing contaminants reliably, accurate data on the velocity field around local exhaust devices is needed. The boundaries of the separation airflow at its inlet must be determined as a prerequisite for shaping leading edges of the exhaust hood. This shaping technique makes it possible to reduce local resistance, improve acoustic properties, increase contaminant capture efficiency and reduce electricity costs associated with operating ventilation systems in buildings. The goal of our study is to develop a discrete mathematical model of three-dimensional stationary separated airflow at the inlet of a rectangular exhaust duct. A novel aspect in our study is the mathematical simulation technique developed by us for 3D separated airflow at inlet of an exhaust hood. The discrete model was constructed using quadrangular vortex clouds comprising straight and curved segments, straight and curved horseshoe vortices, and closed curved vortex polygonal lines. We traced out the flow separation surface at the inlet of a sharp-edged rectangular exhaust duct and determined the axial airflow velocity using a computational procedure and a computer program designed by us. We compared our findings with the specially conducted experiment and different researchers' reports as well as values computed using complex analysis techniques for slotted exhaust hoods and our past computational data using the discrete vortex method for a square exhaust opening in non-stationary and quasi-axisymmetric settings. We found the results to be adequate. Our method can be used to determine the boundaries of vortex zones and trace out the velocity field at inlet of local exhaust hoods in a fully three-dimensional setting.

Flow-field characteristics and ventilation performance of the high-temperature buoyant jet controlled by spray-local exhaust ventilation

High-temperature buoyant jets are common pollutants in hot-rolling plants. Owing to the constraints of the process equipment, local exhaust hoods cannot be set near the buoyant jet source, resulting in low ventilation efficiency and high energy consumption. To efficiently control the high-temperature buoyant jet, spray-local exhaust ventilation (SLEV) is used in this study. The aspects discussed the flow-field characteristics, ventilation performance and recommended optimization parameter order for initial spray droplet diameter, spray angle, and exhaust velocity. The results showed that when the initial droplet diameter was 50 μm ≤ D < 100 μm, the spray gained significant entrainment and cooling effect on the buoyant jet, and when it increased to 150 μm, the spray gained almost no effect. Besides, when the spray angle was 20°<α ≤ 80°, the control ability of the spray on buoyant jet was enhanced; when α ≤ 20°, it was difficult to control buoyant jet owing to the small coverage of the droplets. In addition, the spray helps the escaped buoyant jet to be captured by the exhaust hood. The capture efficiency of the SLEV was higher than that of the local exhaust ventilation (LEV) at the same exhaust velocity. Finally, the exhaust hood and spray parameters must be matched for efficient buoyant jet control since they are interactive. The interaction between the exhaust velocity and initial spray droplet diameter can exert a greater impact on the capture efficiency than the interaction between the exhaust velocity and spray angle. The research results provide guidance for the application of SLEV and reduce ventilation energy consumption.

Droplet control of a local exhaust hood enhanced by air curtains

Local exhaust hoods are widely used to control pollutants in industrial buildings. The hoods are often not installed in the optimum position due to the limitations of equipment layout and worker operation, which leads to a poor control effect. This study proposed a twin boundary curtain ventilation (TBCV) system using twin boundary curtains to assist an exhaust hood to address these limitations and improve ventilation efficiency. The airflow distribution and droplet control of the system were studied experimentally and numerically. It was found that the TBCV system had a low flow rate and good control effect on droplets while being flexible to install. The flow rate of the TBCV system was less than 1/5 of that of the traditional upper exhaust hood when the hood height was 3 times the tank width. The offset distance of the hood could reach twice the tank width, and the hood width could be reduced to half the tank width. Air curtain temperature close to ambient air temperature was conducive to forming the TBCV system. The escape rate of droplets below 100 μm was lower than 0.13% in the TBCV system. Only droplets larger than 50 μm in diameter escaped when the exhaust hood offset the source due to gravity. For the TBCV system, this study guided the installation position and size of the exhaust hood and clarified the removal efficiency for droplets with different diameters.

Optimization research based on local exhaust of oil mist particles in cutting machine tool

Oil particles generated from metalworking fluids(MWFs) in machining process can lead serious health problem to operator. Local exhaust hood is an effective engineering method to capture oil particles and other contamination which is wildly used in manufacture workshop. In this paper, exhaust hood capture efficiency with various height, air volume and particle size was gotten by Computational Fluid Dynamic(CFD) technology. Though further analysis of the CFD result, feature air velocity was introduced. Then an equation of feature velocity and capture efficiency was established by multi regression method. According to this equation one improvement solution was studied: Set to flexible enclosure for up exhaust hood. The solution raised particle capture efficiency on each size significant, the result is equivalent to low down up exhaust hood for 60cm.

Air-jet cover for heat treatment chamber

To increase the efficiency of local exhaust hoods for removing harmful substances emitted during technological processes, air is supplied by jets to create an air curtain (air-jet covers). The air curtain prevents the spread of harmful substances, does not interfere with the technological process, and leaves the equipment open for control and monitoring. The device of local supply and exhaust ventilation of the open doorway of the UKM Classic M 2005 chamber made by Mauting is considered. The chamber is designed for heat treatment of meat products in the food industry. Geometric models have been developed, including a heat treatment chamber, a room from which the chamber is loaded, a supply and exhaust hood above the door from the chamber to the room, and a supply air duct with air distribution devices. For comparison, an option of using an exhaust hood without supply air was also studied. The STAR-CCM+ software package was used as a calculation program. The results of computational experiments on modeling the operation of the local ventilation system of the heat treatment chamber are presented. The analysis of the calculation results and comparison of the efficiency of local ventilation devices are carried out. The studied design of the supply and exhaust hood ensures that heat and gases coming out of the heat treatment chamber are captured at a 17% lower supply and exhaust air flow rate compared to the exhaust hood.

Improving dust capture efficiency with local exhaust hoods in manicure shops

As a prerequisite for our goal of studying the entrainment of dust particles released by nail filing in nail salons we have identified their physical and mechanical properties that were previously unknown: density, particle-size distribution, dynamic shape coefficient and dust concentration in the manicurist's working zone. It was proven using a computational experiment a lateral exhaust would capture dust particles effectively. Using the discrete vortex method and numerical integration of the dust particle motion equation, a mathematical simulation was developed for the flow of dust-laden air around a flanged lateral hood. Relationships behind the dust particle capture range for an exhaust hood mounted on a horizontal table were identified in connection to the terminal velocity of dust particles, quantity and inclination angle of the initial escape velocity vector, velocity inside the duct, flange length, and duct inlet edge shaping along the identified vortex zone boundaries. Dust released by nail treatment can be trapped effectively with a tabletop-mounted lateral exhaust duct with a semicircular flange having a length of 2–4 times gauge (pipe radius). As a means of reducing drag, it is proposed to shape duct inlet edges along the identified boundaries of vortex zones occurring at duct inlet. For a local exhaust hood, such shaping will render the added benefit of reducing noise generated in vortex zones along with preventing contaminant release from these zones. Patterns in dust particle capture range identified by us can be used to determine the required performance of local exhaust ventilation.

高効率捕集性能を有する局所排気装置の設計手法に関する研究 (第2報)吸い込み口形状が横風条件下における排気性能に及ぼす影響

高効率捕集性能を有する局所排気装置の設計手法に関する研究 (第2報)吸い込み口形状が横風条件下における排気性能に及ぼす影響

Study on limit flow ratio method for a lateral exhaust hood above high-temperature buoyant jets

A local lateral exhaust hood can be used to capture the contaminated airflow in the industrial production process. For the design of the lateral exhaust hood, the flow ratio method is commonly adopted. However, there are three major concerns in the existing calculation formula of the limit flow ratio for the lateral exhaust hood: 1 The influence of the horizontal width of the hood is not considered. 2 The influence of the installation height of the hood is not accurately presented. 3 The influence of temperature difference between polluted air flow and environment is not considered. By analyzing the variation of the limit flow ratio of the hood with the above three different influencing factors, the existing calculation formula of the limit flow ratio for the lateral exhaust hood is corrected, and the limit flow ratio correction formula is obtained when the temperature difference Δt = 0∼1200 °C. Therefore, the improved formula of the limit flow ratio for a lateral exhaust hood could guide a more precise design of the exhaust system, which is significant to save energy, ensure the workers’ health, and step into sustainable development.

The investigation of the dust-air flow close to the vertical rotating cylinder local exhaust

The search for solution was made in the article to raise the efficiency of the open-type local exhaust and to decrease the dust losses by the closed-type local exhaust-hood into the aspiration system at the expense of the vertical rotating exhaust cylinder. The exhaust is regarded in the form of the slit-type gaps parallel to the z-axes with the drain from the end of the cylinder. The improvements are proposed for the rotating cylinder to increase the air flow entrainment efficiency. The results obtained can be useful when designing of the dust-collecting chambers-capable aspiration hoods.

Investigation of the Distribution of Velocities of the Air Flow Swirling by a Rotating Exhaust Cylinder

For the improvement of the efficiency of systems of local exhaust ventilation, it is necessary to localize the emission of pollutants with minimum losses of the electric power. The application of rotating exhaust cylinder in a shelter may lead both to a decrease in the dust losses into the aspiration system and to lowering the costs of transportation of dusty aerosols in air ducts. We study the velocities of distribution of the airflows near the rotating exhaust cylinder depending on the frequency of rotation and the discharge of exhaust air. The accumulated results can be used in designing closed local exhaust hoods (aspiration shelters equipped with a dust-collecting chamber).

A survey of separated airflow patterns at inlet of circular exhaust hoods

In this paper our objectives are to perform a numerical and experimental study of the velocity field in the area affected by a circular exhaust duct, to define the outline of flow separation area at its inlet, to come up with a reliable mathematical simulation of the separated flow at exhaust duct inlet, and to determine the effect of the duct inclination angle its serviced area.The stationary discrete vortex method has enabled development of an adequate and reliable technique for computation of separated flows at inlet of a local exhaust hood. We have studied airflow patterns around the exhaust duct using the discrete vortex method, CFD and natural experiment. Characteristic dimensions of the flow separation area at exhaust hood inlet have been determined with relation to the length and inclination angle of the hood. Shaping the exhaust duct to fit the resulting separation zone outline will enable savings of energy that would otherwise be wasted on overcoming local drag forces; furthermore, this will prevent escape of contaminants circulating in the separation area to the environment. Determining airflow velocity distribution patterns will facilitate the choice of the most efficient exhaust duct design to minimize the cost of contaminant removal.

換気効率指Net Escape Velocityを用いた不均一濃度場の形成構造解明に関する研究 第3報-各種の局所換気装置を対象とした空気質制御問題へのNEV適用

換気効率指Net Escape Velocityを用いた不均一濃度場の形成構造解明に関する研究 第3報-各種の局所換気装置を対象とした空気質制御問題へのNEV適用

Performance of constant exhaust ventilation for removal of transient high-temperature contaminated airflows and ventilation-performance comparison between two local exhaust hoods

Transient high-temperature contaminated airflows generated during the pouring of high-temperature materials are widespread in Chinese industrial plants. In this study, the performance of both the local exhaust and the general exhaust with a constant flow rate to remove these transient contaminants was experimentally investigated, and the ventilation performance was compared between two local exhaust hoods. This study focused on the dispersion and removal characteristics of contaminant concentrations under the conditions of high-temperature airflows. The results showed that the cumulative contaminant concentration could be divided into four time slots and that there was a time lag between the cumulative concentration of the local and general exhaust. Additionally, the fitting formula of time-varying concentration decay was discussed to highlight the advantages of the designed hood performance. Finally, the dynamic evaluation index (TCRRt) for evaluating exhaust hoods on removing transient contaminants was proposed, and the connections between the proposed index and the traditional capture efficiency were illustrated. The minimum TCRRt for the improved exhaust hood was invariably greater than that for the existing hood for each hood flow rate. This study demonstrates that increasing the containment volume of the exhaust hood can help to remove transient high-temperature contaminants in industrial plants.

Ventilation strategy for random pollutant releasing from rubber vulcanization process

This paper investigates the emission characteristic of a rubber vulcanization process in a workshop and tries to formulate a ventilation strategy for the rubber vulcanization production process with a lower exhaust rate. Measurements were performed to derive detailed source emission characteristic of rubber vulcanization process in a rubber workshop. The measurement results show that the primary source emission process was concentrated within 600 s of a vulcanization process, the corresponding cumulative emission percentage reached up to 95%. Based on random pollutant releasing from rubber vulcanization process, a single local exhaust hood was applied for a curing machine, the corresponding exhaust rate of 4000 m3/h was proven to be reliable for capturing rubber fume by the experimental and numerical methods. The corresponding cumulative capture efficiency of the single local exhaust hood was 92.1% at 600 s. A new ventilation system equipped with single local exhaust hood was further designed for the rubber vulcanization production process line, and the total exhaust rate of this system was only a quarter of the original one used in a large exhaust hood system. These findings have illustrated that the new ventilation system equipped with single local exhaust hood could largely reduce the exhaust rate in a rubber vulcanization workshop.

Reduced-scale experimental investigation on ventilation performance of a local exhaust hood in an industrial plant

Local ventilation systems are widely used in industrial production processes to capture heat release and/or gaseous/particulate contaminants. The primary objective of this study was to determine important empirical factors on local pollutant capture efficiency and characteristics of thermal stratification in the working areas of industrial plants. Investigated factors were confined airflow boundaries, flow rates of the exhaust hoods, source strengths, airflow obstacles and distances between sources and exhaust hoods. Reduced-scale experiments were conducted with a geometric scale of 1:15 corresponding to a portion of the blast furnace workshop of a steel plant. The dependency of capture efficiency on Archimedes numbers was established. The results showed that confined airflow boundaries, flow rates of the exhaust hoods and source strengths were important empirical factors on pollutant capture efficiency. Hood performance was also evaluated by thermal stratification heights in the plants. This study could help improve the capture efficiency of local ventilation systems used in industrial plants. Safe operation heights are recommended in the upper space of industrial plants based on the thermal stratification in the plants.

Influence of liquid and gas flow rates on sulfuric acid mist removal from air by packed bed tower

The possible emission of sulfuric acid mists from a laboratory scale, counter-current packed bed tower operated with a caustic scrubbing solution was studied. Acid mists were applied through a local exhaust hood. The emissions from the packed bed tower were monitored in three different categories of gas flow rate as well as three liquid flow rates, while other influencing parameters were kept almost constant. Air sampling and sulfuric acid measurement were carried out iso-kinetically using USEPA method 8. The acid mists were measured by the barium-thorin titration method. According to the results when the gas flow rate increased from 10 L/s to 30 L/s, the average removal efficiency increased significantly (p < 0.001) from 76.8 ± 1.8% to 85.7 ± 1.2%. Analysis of covariance method followed by Tukey post-hoc test of 92 tests did not show a significant change in removal efficiency between liquid flow rates of 1.5, 2.5 and 3.5 L/min (p = 0.811). On the other hand, with fixed pressure loss across the tower, by increasing the liquid/gas (L/G) mass ratio, the average removal efficiency decreased significantly (p = 0.001) from 89.9% at L/G of <2 to 83.1% at L/G of 2–3 and further to 80.2% at L/G of >3, respectively. L/G of 2–3 was recommended for designing purposes of a packed tower for sulfuric acid mists and vapors removal from contaminated air stream.