Contaminated-air System

8 Air Finn Dntn. The system is based on the removal of a minimum of SO cfm of gases from each hoodv with a maximum of 10 hoods in use at any time. The system is sized for SOO cfm up to and including the scrubber. This is adequate tO provide for a considerable increase over the present number of IS hoods, since it is expected that normally only about 15% of the total off-gas openings would be in use at any one time. The capacity of the system after the scrubber is 1000. cfm this is to provide for the future disposal of additional small, volumes of radioactive. gases. The pressure drop through the system is approximately 25 in. of water. 

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Introduction. The Blower and Fan House contains the blowers and fans for the reactor-cooling- and contaminated-air systems along with their associated equipment. The equipment, function, and some phases of operation have already been discussed in Sections 8 1 and 8-2. The present section describes the general layout of the building equipment. 

The duct of the contaminated air system enters the building below grade at. the south end of the east wall. This duct leads to th.e exhaust fans., and ah exhaust duct leads from the fans, beneath the building floor to connect with the main exit duct near the east wall of the building. 

Activity of Stuck Guscb. The activity in the stack gases due to air from the contaminated-air system will generally be negligible compared to that due to the.air from the reactor. Thus the design of the stack is based principally on.the radioactivity of the reactor-cooling air. 

Leyse, C.F., Pressure Drop Through the Contaminated Air System, ANL-EKF-37,... 

An opening leading to the contaminated-air system is provided in each hood. These off-gas openings are for use with dry-box work and experiments which may give off radioactive or acidic vapors. Each off-gas opening provides an exhaust flow of about 50 cfm.. ... 

The most overlooked hazard and contaminant is water (99). Water reacts with isocyanates at room temperature to yield both ureas and large quantities of carbon dioxide. The presence of water or moisture can produce a sufficient amount of CO2 to overpressurize and mpture containers. As Httle as 30 mL of water can result in 40 L of carbon dioxide which could result in pressures of up to 300 kPa (40 psi). For these reasons, the use of dry nitrogen atmospheres is recommended during handling. If a plant air system must be used, purification equipment, such as oil traps and drying beds, should be installed between the source and the isocyanate vessel. 

The air inside a factory building can be polluted by release of contaminants from industrial processes to the air of the workroom. This is a major cause of occupational disease. Prevention and control of such contamination are part of the practice of industrial hygiene. To prevent exposure of workers to such contamination, industrial hygienists use industrial ventilation systems that remove the contaminated air from the workroom and discharge it, either with or without treatment to remove the contaminants, to the ambient air outside the factory building. 

Step 4 Define the System Boundaries. This depends on the nature of the unit process and individual unit operations. For example, some processes involve only mass flowthrough. An example is filtration. This unit operation involves only the physical separation of materials (e.g., particulates from air). Hence, we view the filtration equipment as a simple box on the process flow sheet, with one flow input (contaminated air) and two flow outputs (clean air and captured dust). This is an example of a system where no chemical reaction is involved. In contrast, if a chemical reaction is involved, then we must take into consideration the kinetics of the reaction, the stoichiometry of the reaction, and the by-products produced. An example is the combustion of coal in a boiler. On a process flow sheet, coal, water, and energy are the inputs to the box (the furnace), and the outputs are steam, ash, NOj, SOj, and CO2. 

In recapping, DAF is the process of removing suspended solids, oils and other contaminants via the use of bubble flotation. Air is dissolved into the water, then mixed with the wastestream and released from solution while in intimate contact with the contaminants. Air bubbles form, saturated with air, mix with the wastewater influent and are injected into the DAF separation chamber. The dissolved air then comes out of solution, producing literally millions of microscopic bubbles. These bubbles attach themselves to the particulate matter and float then to the surface where they are mechanically skimmed and removed from the tank. Most systems are versatile enough to remove not only finely divided suspended solids, but fats, oils and grease (FOG). Typical wastes handled include various suspended... 

One of the unidirectional flow system modifications is air supply through diffusers located above the occupietl zone. The supply air temperature is lower than the desired room air temperature in the occupied zone, and air velocity is lower compared to a mixing-type air supply, bur higher than for a thennal displacement ventilation. Polluted air of the occupied zone is suppressed by an tiverlying air cushion that displaces the contaminated air toward floor-level exhausts (Fig. ". 12). 

File exhaust air system must be protected from contamination by a filter of at least F5 (EU5) quality. 

More sophisticated applications of a pressurized system combined with a suitable extractor are found in hospital operating theaters. The positive pres-sure produced by sterile air entering the room ensures that all leakages are outward. This outward leakage ensures that contaminated air at a lower pressure in the surrounding rooms will not enter the space. 

Proper design and construction of a local ventilation system must account for hood flow rate, contaminant generation process and rate, and the generated flow rate of contaminated air. Thus, knowledge about airflow mechanics, process performance, and the contaminant source is essential. The descriptions of different sources are included in Chapter 7 and here only short descripiions are included as necessary to identify different processes and source types. 

Combined systems extraction with additional supply air) It is advantageous to increase the range of the capture system by an additional supply system. The supply air blows the contaminated air toward the capture openings (see Section 10.4.6). 

Additional calculations are necessary if significant heat loads inside the booth cause thermal stratification. A capture system in the ceiling would be advantageous in this case. A check of the pressure in the booth is necessary to avoid spilling of contaminated air near the top of face opening due to the thermal pressure. The height-dependent inflow or spilling velocity due to pressure differences can be calculated as... 

Most plants are highly dependent upon their compressed air supply and it should be assured that the air is in at least reasonable condition at all times, even if the drying system is out of use for maintenance or repair. It is possible that the line condensation would be so bad that some air applications would be handicapped or even shut down if there were no protection. A vital part of the entire endeavor to separate water in the conventional compressed air system is also the trapping of dirt, pipe scale and other contaminates. This is still necessary with a dried air system. As a minimum, all branch lines should be taken off the top of the main and all feeder lines off the top of branch lines. 

Spray-type collectors In this system water is sprayed or cascaded onto the contaminated air directly or through packed towers, and the fumes or dust are washed away by absorption. These collectors are used extensively on the treatment of fumes of all types and have low pressure drops and hence low power requirements compared to induced spray. A development of this collector is the venturi scrubber, which injects high-pressure water into a venturi through which the fume-laden air is passing. The intimate contact of the two ensures absorption and removal from the air stream. These collectors are used in fume removal and have efficiencies of more than 99 per cent on sub-micron particles. 

U.S. EPA has shown that 90% of process water can be recycled to the front end of the system for slurry preparation, and the rest must be treated on site or transported to an off-site facility.80 During the aerobic process, some contaminated air may be formed and emitted from the reactor. Depending on the air characteristics, a compatible air pollution control device may be used, such as activated carbon. Slurry biodegradation has been shown to be successful in treating soils contaminated with soluble organics, PAHs, and petroleum waste. The process has been most effective with contaminant concentrations ranging from 2500 mg/kg to 250,000 mg/kg. 

Ventilation systems are composed of fans and ducts. The fans produce a small pressure drop (less than 0.1 psi) that moves the air. The best system is a negative pressure system, with the fans located at the exhaust end of the system, pulling air out. This ensures that leaks in the system draw air in from the workplace rather than expel contaminated air from the ducts into the workplace. This is shown in Figure 3-5. 

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