Local ventilation performance

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. 

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. 

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

For supply inlets in rooms some performance measurements exist, such as air exchange and ventilation efficiencies (see Chapter 8). It is usually not possible to use these for local ventilation supply inlets, and for the moment there are no specific measurements to evaluate the influence of an inlet on contaminants. Some trials with comparison indices, which compare inhaled concentrations (or exposures) with and without a supply inlet, have been done. 

Jets used in local ventilation have the same forms and performance as jets in general ventilation, described in Sections 7.4 and 7.7. These sections describe usable equations for flow, velocity, temperature, and concentration distributions. The buoyancy plumes that can result at the end of a jet or from a warm source are described in Section 7..5. 

Different available measurement instruments and evaluation methods are described in Chapter 12. Some specific methods to evaluate local ventilation systems are described in this section. All local ventilation systems should be evaluated regularly. The evaluation procedures can be divided into detailed and simple, as well as direct and indirect, procedures. The detailed procedures need special instruments and competence, whereas it should be possible to use the simple procedures every day. Since the simple procedures do not measure directly the performance of the exhaust, it is usually necessary to calibrate a simple procedure by using a detailed procedure. ... 

The commissioning tests are typically performed before a local ventilation system is put into use the purpose is to confirm that the design conditions have been achieved. Hence, commissioning tests apply to methods used in type testing, but in a simple and modified way. In-use tests concentrate on the exposure effects of a local ventilation system. 

The initial performance test for all local ventilation systems is a smoke test, which provides easy airflow visualization between the source and the hood, it helps to identify, with little effort, the main features of airflow patterns. Such a test, recorded by a video camera, allows performance comparisons to be made before and after improvements. Real contaminant or tracer gas measurements are necessary in the case of more detailed testing. 

Hoods. A laboratory hood with 2.5 linear feet of hood space per person should be provided for every 2 workers if they spend most of their time working with chemicals (199) each hood should have a continuous monitoring device to allow convenient confirmation of adequate hood performance before use (200, 209). If this is not possible, work with substances of unknown toxicity should be avoided (13) or other t)q)es of local ventilation devices should be provided (199). See pp. 201-206 for a discussion of hood design, construction, and evaluation. 

Use of hood Use the hood for operations which might result in release of toxic chemical vapors or dust (198-9). As a rule of thumb, use a hood or other local ventilation device when working with any appreciably volatile substance with a TLV of less than 50 ppm (13). Confirm adequate hood performance before use keep hood closed at all times except when adjustments within the hood are being made (200) keep materials stored in hoods to a minimum and do not allow them to block vents or air flow (200). Leave the hood "on" when it is not in active use if toxic substances are stored in it or if it is uncertain whether adequate general laboratory ventilation will be maintained when it is "off (200). 

Spray cleaning of parts with solvent, using an airless gun similar to a paint sprayer, is also practiced. Spray cleaning is typically performed in a ventilated fume hood so as to protect the worker from solvent fumes. In addition to the fume hood, an emissions control system is often required. Many local air quality agencies prohibit the spray cleaning of parts with solvent or require an appropriate emission control system to be in place. 

B. Perform endotracheal intubation if personnel trained in the procedure are available. Intubation of the trachea provides the most reliable protection of the ainvay, preventing aspiration and obstmcfion and allowing for mechanically assisted ventilation. However, it is not a simple procedure and should be attempted only by those with training and experience. Complications include vomiting with pulmonary aspiration local trauma to the oropharynx, nasopharynx, and larynx inadvertent intubation of the esophagus or a main-stem bronchus and failure to intubate the patient after respiratory arrest has been... 

Selecting the correct combination of antioxidants is specific to the elastomer polymer type as well as the compound formulation and the end use application. It is important to note that many of these materials may be considered toxic or hazardous in nature. Chemical specific Material Safety Data Sheets (MSDS) should be consulted for safe handling practices. Particular attention should be given to the proper selection and use of personal protective equipment, including proper ventilation and/or the use of respiratory protection. The MSDS will also provide information on how to handle spills and proper disposal procedures. Disposal methods should not be overlooked since these chemicals are all regulated, and hence waste disposal must conform to EPA and local disposal regulations. Refer to Rubber Oxidation. (Source Handbook of Polymer Science and Technology Volume 2 - Performance Properties of Plastics and Elastomers, N. P. Cheremisinoff - editor, Marcel Dekker Inc., New York, 1989). 

Performance. Rate 4-12 room air changes/hour is normally adequate general ventilation if local exhaust systems such as hoods are used as the primary method of control (194). 

Fans, blowers, or ejectors provide air movement in local exhaust systems. Fans are most common. In most cases, fans are downstream of air cleaning devices. That prevents contaminants from affecting fan performance and service life. Blowers are usually upstream of ventilated space and often used in dilution ventilation. 

All electrical devices should be turned off except for the necessary ventilation pumps. Local gas tests should be performed before and during any welding or other hot work. Exposure to toxic substances should be recorded and the proper fresh air should be supplied to all underground areas. 

Local exhaust ventilation (LEV) equipment is intended to control mechanically the emission of contaminants such as dust and fumes that are given off during a manufacturing process or in a chemical laboratory. Normally this is done as close to the point of emission as possible using a stream of air to remove the airborne particulate matter and transport it to where it can be safely collected for ultimate disposal. The physical layout and setting of LEV equipment is critical for it to work effectively, and comparatively minor alterations can affect its performance. It is, therefore, important that LEV equipment should be properly designed, manufactured, installed, operated and maintained. Fume cupboards in chemical laboratories are included in this type of equipment. 

Air contamination control by dilution and local exhaust ventilation. Problems of design and efficiency fan, trunking and filtration systems location of exhaust outlets maintenance monitoring of performance. 

Application. Wherever dry grinding, dry polishing or buffing is performed, and employee exposure, without regard to the use of respirators, exceeds the permissible exposure limits prescribed in 1926.55 or other pertinent sections of this part, a local exhaust ventilation system shall be provided and used to maintain employee exposures within the prescribed limits. 

Welding, cutting, or heating in any enclosed spaces involving the metals specified in this subparagraph shall be performed with local exhaust ventilation in accordance with the requirements of paragraph (a) of this section, or employees shall be protected by air line respirators in accordance with the requirements of Subpart E of this part ... 

Fume cupboards, slot captor or receptor hood ventilation. These must be driven by a suitable and adequate fan, and will often need water wash, dust filtration or precipitators built in. Local air pollution control requirements may require capture of vapours, which cannot simply be discharged fiom an exhaust vent. The ventilation system pressures should be checked regularly to spot obstmctions or reductions in fan performance, and the pressure figures recorded. 

Performance of local exhaust ventilation systems checked for conformance... 

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