Source-capturing systems

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). 

Local ventilation systems (see Fig. 2.2) are used for local controlled zones. These systems are air technological methods for local protection. Primarily, local protection should be made using process methods such as encapsulation or process modification (see Design Methodology, Chapter 3). Another use for local ventilation systems is source capturing. 

Working locations between the contaminant source and the capture openings dramatically reduce the efficiency of the capture system and should therefore be avoided. If the hood is enclosed on three vertical sides the sensitivity to cross-draft is low. 

Capture system performance on a nonbuoyant source is influenced by enclosure (hood) design and location of the exhaust point. 

The technology in the fume capture field Is not well developed, and performances of many capture systems are low and typically may be in the 30% to 60% range. There is a paucity of fundamental research and development in the fume capture field. In contrast, hundreds of million of dollars have iteen spent on research and development activities in the gas-cleaning area, which is mature and well developed. It is not uncommon to specify and to measure gas-cleaning equipment performances of over 99.9% colleaion efficiency. As shown in Eq. (13.75), the ovcTall fume control system performance is determined by the product of the capture efficiency and the gas-cleaning efficiency. This equation clearly shows the need to improve the efficiency of capture of the fume at the source in order to obtain significant improvements in the overall fume control system performance. 

Table 13.17 lists some of the important considerations for the different fume capture techniques. From the point of view of cost effectiveness, the usual preference is source collection or a low-level hood, provided an acceptable scheme can be developed within the process, operating, and layout constraints. The cost of fume control systems is almost a direct function of the gas volume being handled. Flence, the lower volume requirements for the source capture or low-level hood approach often results in significant capital and operating cost savings for the fume control system. 

The use of a source capture or a direct evacuation system is the most positive form of fume capture. A well-designed system can operate at high fume capture efficiencies. For many of these systems, the captured gas temperature for the processing operation is very high (1000-1500 °C), and gas cooling may be required... 

Despite limitations, specific ventilation capture systems provide effective control of emissions of toxic vapors or dusts if they are installed and used correctly. A separate, dedicated exhaust system is recommended. The capture system should not be attached to an existing hood duct unless fan capacity is increased and airflow to both hoods is properly balanced. One important consideration is the effect that such added local exhaust systems will have on the ventilation for the rest of the laboratory. Each additional capture hood will be a new exhaust port in the laboratory and will compete with the existing exhaust sources for supply air. 

Definition 2 is phrased in terms of knowledge-based systems rather than expert systems. No reference is made to expert human problem solvers. Definition 2 captures the sense that the representation and manipulation of knowledge is the source of such a system s power, whether or not that knowledge is dkecdy eHcited from a human expert. 

The sampling system consists of a condensate trap, flow-control system, and sample tank (Fig. 25-38). The analytical system consists of two major subsystems an oxidation system for the recovery and conditioning of the condensate-trap contents and an NMO analyzer. The NMO analyzer is a gas chromatograph with backflush capabihty for NMO analysis and is equipped with an oxidation catalyst, a reduction catalyst, and an FID. The system for the recovery and conditioning of the organics captured in the condensate trap consists of a heat source, an oxidation catalyst, a nondispersive infrared (NDIR) analyzer, and an intermediate collec tion vessel. 

It is also clear that it is difficult to relate cause and effect to any specific chemical since, with the exception of point source effluents, many waterways contain a multitude of chemicals, of which the active endocrine disruptor may not be that which has been measured in the water or tissue. For such reasons, many studies have used in vitro experiments in which isolated tissue, either from a control animal or one captured in a polluted water system, is exposed to a single pollutant in the laboratory. Such experiments have shown significant disruption to testicular activity by a wide range of xenobiotics, including cadmium, lindane, DDT, cythion, hexadrin and PCBs. ... 

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

Plane jets could be used to create a closed volume in which a contaminant source could be placed. In some ways, these systems are similar to Aaberg exhaust hoods (Section 10.4.4). The objective is to use plane jets instead of walls around an exhaust opening to create a vortex which enhances the capture efficiency of the exhaust. 

For an existing process plant, the designer has the opportunity to take measurements of the fume or plume flow rates in the field. There are two basic approaches which can be adopted. For the first approach, the fume source can be totally enclosed, and a temporary duct and fan system installed to capture the contaminant. For this approach, standard techniques can be used to measure gas flow rates, gas compositions, gas temperatures, and fume loadings. From the collected fume samples, the physical and chemical characteristics can be established using standard techniques. For most applications, this approach is not practical and not very cost effec tive. For the second approach, one of three field measurement techniques, described next, can be used to evaluate plume flow rates and source heat fl uxes. 

The use of canopy hoods or remote capture of fume is usually considered only after the rejection of source or local hood capture concepts. The common reasons for rejecting source or local hood capture are usually operating interference problems or layout constraints. In almost all cases, a canopy hood system represents an expensive fume collection approach from both capital and opetating cost considerations. Remote capture depends on buoyant ait curtents to carry the contaminated gas to a canopy hood. The rising fume on its way to the hood is often subjected to cross-drafts within the ptocess buildings or deflected away from the hood by objects such as cranes. For many of these canopy systems, the capture efficiency of fume may be as low as 30-50%. 

Local extract systems are designed specifically to remove fumes, dust, mists, heat, etc. at source from machinery and fume cupboards. The main design considerations are capture of the contaminant, which will normally involve special hoods or cabins, and extract at sufficient velocity... 

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