Local ventilation

Use localized ventilation (flexible ventilation pick-up close enough to manway/addition port to effectively capture emissions)... 

Operator exposure Charge liquids and solids materials by means of a to fumes or inerts. closed system (e.g., hard piping, hopper and rotary airlock, screw feeder, double-dump valve system, etc.) Provide local ventilation Use proper personnel protective equipment (PPE) ACGIH 1986 CCPS G-22 CCPS G-23 CCPS G-29... 

With flammable gases, eliminate all ignition sources (refer to Chapter 6). Possibly provide additional high/low level ventilation background gas detectors to alarm, e.g. at 40% of the LEL. With toxic gases, possibly provide additional local ventilation monitors connected to alarms appropriate air-fed respirators. (The flammable/toxic gas detectors may be linked to automatic shutdown instrumentation.)... 

Building maintenance schedules Try to schedule maintenance activities that interfere with HVAC operation or produce odors and emissions (e.g., painting, roofing operations) so that they occur when the building is unoccupied. Inform occupants when such activities are scheduled and, if possible, use local ventilation to ensure that dust and odors are confined to the work area. 

Local ventilation systems are used for local controlled zones. These systems are based on local capture of contaminants. 

This chapter describes the aerodynamic principles, models, and equations that govern the flow and the contaminant presence and transport in a designated volume of a work room. The purpose of local ventilation is to control the transport of contaminants at or near the source of emission, thus minimizing the contaminants in the workplace air. 

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. 

Local ventilation systems can be divided into the following subsystems ... 

Posokhin, V. N. 1984. Design of Local Ventilation Systems for Vrocess Equipment with Heat and Gas Release (in Russian). Mashinostroyeniye, Moscow. 

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

Air curtains are local ventilation devices that are used in industrial buildings to reduce leakage of airflow through apertures in building enclosures and process equipment. Their operation is based on the damping effect of air jets that are supplied into the area of the open aperture. The advantages of air curtains include... 

The following equations separately outline calculating contaminant concentration inside a room with central and local recirculation. The assumptions for the room are that it has one main ventilation system with supply and exhaust air and that the contaminant concentration is the same in the whole volume (except very close to the contaminant source or in the ducts, etc.). The contaminant source is steady and continuous. The model for local ventilation assumes also one main ventilation system to which is added one local exhaust hood connected to a local ventilation system (see Chapter 10) from which all the air is recirculated. In the central system the number of inlets and outlets could vary. The flow rates are continuous and steady. 

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. 

The convection flows from the heat sources V and Z as well as contaminant flows from contaminant sources are flows loading the room. In the sources additional heat and pollutant flows may be generated, which are exhausted directly out by local ventilation and are not included in the balance calculation. 

Sections 9.2 to 9.8 describe processes and equipment mainly for general industrial ventilation but also for local ventilation. 

Local ventilation is used to diminish or, preferably, to prevent exposure to contaminants (including heat). This includes protection of persons, products, or animals from hazardous and/or nuisance contaminants. This task is not specific for local ventilation and different definitions exist. One is the following ... 

Local ventilation is often a very important part of the ventilation system, both in function and in construction, which makes a specific definition difficult. In addition to the above definitions it can be defined as ventilation of a separate volume inside a large room, as opposed to general ventilation, which is for the complete room or building. One problem with this definition is that local ventilation systems could function as localized ventilation in one surrounding and as general ventilation in another surrounding e.g., a ceiling inlet combined with floor exhaust is a local ventilation system in a... 

By using a local ventilation system of good design less air is needed to reach a specific contaminant level than is possible with general ventilation. It can also be said that the purpose of local ventilation is to achieve a more efficient (defined in some way) ventilation in a part of a room or in the wdiole room. Local ventilation also can be important from a process standpoint, e.g., removal of heat that might damage equipment. 

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. 

The demands and design of a local ventilation system (not only local ex hausts) should naturally start with the demanded target levels and the toxicity of the air contaminants (see Chapters 5 and 6). For best performance the exhaust should be close to the source and preferably enclose the source, there should be no disturbances of the flow, and at the same time it should have a low flow rate and be able to minimize the concentration of even quite dangerous air contaminants in the working zone. 

The parts of local ventilation systems, situated inside rooms, that influence the flow field are described here. This presumes that the inlet and outlet openings are properly connected to duct systems either directly or through flexible connections (tubes). These ducts and tubes and other parts of importance for the function of these systems are described in other chapters. 

All local ventilation systems can, in principle (and many are in practice), be manufactured for use in one or more of three different modes fixed, flexible, and mobile. 

It is difficult to determine the most common mode for local ventilation and naturally many systems fit into more than one mode. 

The necessary containment or transport capability of a local ventilation sy stem depends on the type of contaminant present and its health risks. There could be different demands for gases and particles, for contaminants that have immediate health risks and those that have long-term effects, for contaminants that affect the breathing system and those that affect the skin and eyes, for infectious contaminants, fot contaminants that follow the air streamlines closely and those that fall out on floor and work surfaces, etc. (See Chapter 5 for physiological and toxicological considerations.)... 

There are many possible ways to classify local ventilation systems. When local ventilation is used to describe exhaust hoods only, one classification is hoods that totally surround the contaminant source (enclosing hoods), hoods that partially surround the contaminant source (partially enclosing hoods), and hoods where the contaminant source is outside the hood (exterior hoods). A similar classification is used here for the exhaust hoods. Since local ventila tion, as described in this chapter, includes more than exhaust hoods, the following three main categories are used exhaust hoods, supply inlets, and combinations of exhaust hoods and supply inlets. (See Fig. 10.1.)... 

Many parameters influence the performance of a specific local ventilation system. The main parameters are... 

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