Efficiency capture

Approximate estimates of solar energy capture efficiencies probably too high. 

The complementai y is the cumulative capture efficiency Z (= 1 — Y), which is defined as the feed particles of a given size and smaller which are captured in the cake, which in most dewateiing applications is the product stream. 

Centrifugation - Centrifugation has been demonstrated to be capable of thickening a variety of wastewater sludges. Centrifuges are a compact, simple, flexible, self-contained unit, and the capital cost is relatively low. They have the disadvantages of high maintenance and power costs and often a poor, solids-capture efficiency if chemicals are not used. 

Capture zones are zones in which source emissions will be captured by a source-capturing system, and where the capture efficiency is determined and shall be maintained over the working period. From the pollutant concentration point of view, the capture zone is uncontrolled (e.g., workers shall not enter a capture zone without additional protection). 

The pollutant-capturing efficiency of local ventilation systems depends on hood design, the hood s positioning near the source of contamination, and the... 

T hese factors can significantly reduce the capturing efficiency of local exhausts and should be accounted for by the correction coefficient on room air movement, K, > I, in Eqs. (7.205) and (7.206). For example, Eq. (7.206) is replaced with... 

Local ventilation in industry usually differs from the description above in that it is connected to a local exhaust hood (Chapter 10), which has a capture efficiency less than 100%. The capture efficiency is defined as the amount of contaminants captured by the exhaust hood per time divided by the amount of contaminants generated per each time (see Section 10.5). Figure 8.3 outlines a model for a recirculation system with a specific exhaust hood. Here, the whole system could be situated inside the workroom as one unit or made up of separate units connected with tubes, with some parts outside the workroom. For the calculation model it makes no difference as long as the exhaust hood and the return air supply are inside the room. 

This latter equation can also be used for systems without a local exhaust hood by setting the capture efficiency to zero. It could also be used to show the result of recirculation from, e.g., a laboratory fume hood with immediate recirculation. In such a hood all contaminants are generated within the hood and usually also all generated contaminants are captured, so the capture efficiency is 1. The equation demonstrates that if the... 

FIGURE 8.3 Model of a local recirculating system with a local exhaust hood, used for calculating the connection between contaminant concentrations, airflow rates, contamirtartt source strength, q , air cleaner efficiency, n and hood capture efficiency, a. is the concentration in the supply (outside) air c (equal to c h) is the concentration in the room Is the concentration in the returned air is the supply flow rate to the room equal to the exhaust flow rate, the recirculated flow rate (through the cleaner) is T is the time constant for the room and V is the room volume. 

Many different measures of local ventilation performance exist. These measures can be divided into three main categories capture velocities, capture efficiencies, and containment efficiencies. Table 10.1 shows the connections between hood types and different efficiency measurements. Section 10.5 describes procedures for measuring each of these performance measures. 

Capture efficiency is the fraction of contaminant generated outside an exhaust that is captured by the exhaust (see Fig. 10.6). 

The advantage of using capture efficiency is that it is possible to calculate how much of the contaminant is released into a workspace (if the source rate is known) and thus to judge if the exhaust will reduce workplace exposures to acceptable levels. Its disadvantage is that it is rather difficult to measure and, moreover, it is usually impossible to calculate source generation rate. 

FIGURE 10.6 Definition of capture efficiency, a, M = contaminant source rate, m = contaminant transport (directly) into the exhaust hood, a — m/M. 

A laboratory study of surface-treatment tanks by Braconnier et al." showed the effects of cross-drafts and obstructions to airflow on capture efficiency. They found that, without obstructions, capture efficiency decreased with increasing cnrss-draft velocity but the importance of this effect depended on freeboard height. In their study, cross-draft direction was always perpendicular to the hood face and directed opposite to the hood suction flow. Follow cro.ss-draft velocities (less than 0.2 m s ), efficiency remained close to 1.0 for the three freeboard heights studied. With higher cross-draft velocities, efficiency decreased as freeboard height decreased. For example, when the crossdraft velocity was 0.55 m s , efficiency decreased from 0.90 to 0.86 to 0.67 as freeboard height decreased from 0.3 m to 0.15 m to 0.1 m, respectively. 

A similar effect was observed for changes in hood flow rate. With a fixed cross-draft velocity, capture efficiency decreased with decreasing hood flow rate. This effect was much more important when freeboard height was small. Their results showed that when hood flow rate was 1.5 m s m, efficiency remained close to 1.0 as long as the cross-draft velocin. was less than 0.45 in s. The most severe conditions tested were a hood flow rate equal to 0.33 m s" nr- and crossdraft velocity equal to 1.15 m s. Under these conditions, capture efficiency was equal to 0.83 for freeboard hei t equal to 0.3 m, but decreasing to 0.4 when freeboard height was decreased to 0.1 m. 

The effects of obstacles in the flow field were also studied. A bar with dimensions of 0.04 m high x 0.003 m wide x 1 m long was used. Capture efficiency was... 

Cross-draft velocity was normalized by dividing the measured cross-draft ve-locit by the capture velocity calculated at the tatik centerline. Capture velocity at the tank centerline was calculated using Silverman s - centerline velocity (Eq. (JO.l)) for unflanged slot hoods. There was considerable scatter in the data, show ing chat cross-draft velocity alone is not responsible for low capture efficiency. 

FIGURE 10.9 Hood capture efficiency versus normalized cross-draft velocity, ... 

Detailed Evaluation Detailed evaluation is performed by measuring the capture efficiency, either by using the actual contaminant or by using a tracer gas. (In principle, it is possible to use particles as tracers, but gases are usually used as tracers.) The most reliable evaluation is to use the process-generated contaminant, since there are always problems with a tracer, due to the difficulties of feeding the tracer to the source in the same way and in a similar amount as the generated contaminant. ... 

Capture efficiency can also be measured by first estimating workspace emission rates and local exhaust emissions. The local exhaust emission rate equals the duct concentration (mass/volume) multiplied by the duct flow rate (volume/time). The workspace emission rates can be calculated using appropriate mass balance models and measured ventilation rates and workspace concentrations. Capture efficiency is the ratio of duct emission rate to total emission rate (duct plus workspace). ... 

Capture efficiency could be measured in the same way with a tracer gas. The difficulties explained above could make this measurement le.ss reliable even if it usually is easier to measure the concentration of a tracer gas than the concentration of a specified dust or gas mixture. 

In theory it should be possible to calculate the capture efficiency without measurements. Some attempts have used computational fluid dynamics (CFD) models, but difficulty modeling air movement and source characteristics have shown that it will be a long time before it will be possible to calculate the capture efficiency in advance. ... 

This type of dependence of capture efficiency on the exhaust flow rate and cross-draft velocity has also been seen by Fletcher and Johnson who determined the capture efficiency of a flanged square exhaust hood in a cross flow. 

Another design method uses capture efficiency. There are fewer models for capture efficiency available and none that have been validated over a wide range of conditions. Conroy and Ellenbecker - developed a semi-empirical capture efficiency for flanged slot hoods and point and area sources of contaminant. The point source model uses potential flow theory to describe the flow field in front of a flanged elliptical opening and an empirical factor to describe the turbulent diffusion of contaminant around streamlines. 

FIGURE 10.22 Predicted versus measured capture efficiency of vapor degreasers under operating conditions. 

Capture efficiency measurements may be used to evaluate the function of a canopy hood (see Section 10.5). Capture velocity is not a feasible evaluation tool, since a canopy hood does not generate an air velocity close to the source. It is also possible to use exposure measurements for workers outside the plume area. Since most hot processes generate visible contaminants, visual inspection of the flow, especially around hood edges, might provide a qualitative evaluation. Many contaminants could however be invisible when diluted and smoke generators (Section 10.5) may be necessary to find leakages (temporary or permanent) around the hood edges. 

An increase of the capture airflow rate normally results in increased capture efficiency, but the relationship between these quantities is not linear. A case-by-case evaluation is necessary to establish this relationship. In every case an increase of the airflow rate causes an increase of the operating costs. Analogously, a decrease in airflow rate leads to a decrease in capture efficiency and in some cases, a total breakdown of the capture effect (e.g., capture dev ices working with the vortex principle require minimum airflow rates). 

Evaluation can be performed by measuring capture efficiency using real contaminants and applying the real process or by substituting with tracer materials. A simpler, but qualitative, method of evaluation is the visualization of the airflow. If the relationship between capture efficiency and airflow rate is known, a measurement of the airflow rate can be used for frequent evaluation. See Section 10.5. 

Booths are partially enclosed workplaces with one or more open facefs) for access by workers. These openings at one or more sides of the enclosure function not only to capture air contaminants directly through their short-distance capture capability but also to cause an airflow in a certain direction (normally away from the worker/work process and into the enclosure). The capture efficiency could be increased by using an existing main flow direction (e.g., thermal flows caused by heat sources) to support the capture process. 

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