Local exhaust ventilation systems

Certainly, some workplace operations involving highly toxic and/or valuable materials can be controlled more effectively by LVHV ventilation than by conventional local exhaust ventilation. These situations represent opportunities to improve worker protection, recover valuable materials, and to reduce replacement air requirements. Designers of local exhaust ventilation systems should be mindful of such opportunities and take advantage of LVHV control methods. 

Local exhaust ventilation systems LEV required at all dust- or vapor-generating operations and exhausted to the outside through HEPA filters. Full enclosure (containment) ventilation systems recommended... 

The idea of using airflows to collect a substance similar to explosive vapors and/or particles is not unique. From an engineering systems design perspective, collecting an explosive from a person can be directly compared to local exhaust ventilation systems used in industry for contamination control, e.g., welding hoods, open vapor tanks,... 

Ventilation is the standard method of controlling exposure to airborne vapors of epoxy resins and solvents. Ventilation involves controlling air flow to reduce exposure. Local exhaust ventilation systems capture the vapor at the source and either filter or remove it from the work area. The ventilation system needs to be designed so that vapors, aerosol, and dusts are pulled away from, and not into, the breathing zone of the workers. A constant supply of fresh, noncontaminated air should be available at all times. 

Examples of local exhaust ventilation system are drawdown exhaust tables, slot hoods, dust extraction systems, and portable vapor and dust collectors. Care must be taken that the ventilation systems are utilized and maintained as designed. The condition of the filters and air flow rate should be checked periodically as well as the condition of the duct work, motors, belts, etc. 

When designing a local exhaust ventilation system for a process that generates dust particles, it is important to consider the minimum air velocity. The minimum air velocity is the velocity required to prevent settling of dust particles in the air ducts. The minimum velocity is a function of dust particle size and particle density. Listed in the table below are the minimum air velocities recommended for the transport of various types of particulate contaminants. 

Local Exhaust Ventilation. Local exhaust ventilation is designed to intercept the flammable atmosphere at the source of release and directs it into a system where air is safely separated from the fuel. Correctly designed local exhaust ventilation systems could be very effective in limiting the spread of dust cloud atmospheres beyond the source of release. Local exhaust ventilation is generally less expensive to run than dilution ventilation because less air is used. 

A local exhaust ventilation system generally includes hood, ductwork, filter, and fan. 

One in series on improved local exhaust ventilation systems providing dramatic reductions in emissions. 

ANSI Z9.2, Fundamentals Governing the Design and Operation of Local Exhaust Ventilation Systems ANSI Z16, Accident Statistics (NSC)... 

The main purpose of local exhaust ventilation is to capture contaminants at their source before they contaminate a room or work station. An exhaust ventilation system captures contaminants and moves the contaminated air through ducts to another location. It may be necessary to remove the contaminants before dumping the air outdoors or before recirculating the air. Figure 25-1 illustrates the key components in a local exhaust ventilation system. Later sections of this chapter give more details about local exhaust ventilation systems. 

Very often a local exhaust ventilation system must include a capability to remove contaminants before air moves to the outdoors. Cleaning is essential for recirculated air. There are several types of air cleaning devices. Figure 25-5 illustrates properties of aerosols and related cleaning equipment. 

A long-standing publication of the American Conference of Governmental Industrial Hygienists is Industrial Ventilation (see note 2 in this chapter). It is a primary reference for the design of general and local exhaust ventilation systems. 

Engineering controls. Local exhaust ventilation systems, such as table systems, capture and remove contaminated air to the outdoors. A properly ventilated table is necessary (National Institute for Occupational Safety and Health 1999), and dust masks should be used if, for some reason, the exhaust system is not functioning. 

Understands the use and limitations of dilution and local exhaust ventilation systems. Understands the mechanism of hearing and the possible effects of high noise levels. Understands the more common terms used in noise measurement. 

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. 

A description of the local exhaust ventilation systems to be used and how they are to be tested ... 

Local exhaust ventilation systems placed on inspection and maintenance program... 

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

Are all local exhaust ventilation systems designed to provide sufficient airflow and volume for the application, and are ducts not plugged and belts not slipping ... 

Do you use general dilution or local exhaust ventilation systems to control dusts, vapors, gases,... 

Dust control systems usually include hoods and enclosures connected to a local exhaust ventilation system that discharges to cyclone collectors and scrubbers or a baghouse in series. The most common type of control equipment is the baghouse these are increasing in use as more stringent pollution control codes are adopted. These filters provide excellent collection efficiency with little or no visible emissions and the collected dry fines are sometimes usable in concrete mixes. However, if the exhaust gas temperature is at or near the dew point of the gas, condensation of moisture on the fabric filters is always a possibility and poses a serious fire hazard. The other type of the commonly used control equipment is the venturi wet... 

There are five components in a local exhaust ventilation system. These are the hood, the point at which the conta-... 

Just as pressure losses occur at the entrances to hoods, friction losses occur in the ductwork of local exhaust ventilation systems. This friction loss is described by the following equation ... 

Fans, exhausters and blowers are called air-moving devices. Fans are critical to the local exhaust ventilation system since they supply the energy to produce a continuous flow of air. resulting in the system s air movement. Whenever possible, the fan should be located downstream from the air cleaner so that it will not handle contaminated air and will pull, rather than push, air through the system. 

Under CCSHH, an employer is required to carry out a suitable and sufficient assessment of the risks to the health of their employees created by work involving hazardous substances. The assessment must be kept under review and any necessary changes made following the review. In addition, control measures to reduce the risks posed by the substances must also be subject to maintenance, e.g. local exhaust ventilation systems (LEV) for the removal of flammable mixtures from the atmosphere. 

Are local exhaust ventilation systems examined, tested and maintained on a regular basis ... 

In the case of substances identified as hazardous to health, the Control of Substances Hazardous to Health Regulations 1988 (COSHH) require that exposure to them must be either prevented or controlled. Prevention of exposure can be achieved by either a prohibition on the use of the substance, total enclosure of the source or by substituting a less hazardous substance. Control, on the other hand, may entail an engineering solution, such as the installation and operation of local exhaust ventilation systems, or by changing the process to afford better operator protection. In relatively low risk situations the provision and use of specific items of personal protective equipment may afford the appropriate level of operator protection. In all cases, prevention and control strategies should be supported by the provision of information, instruction and training for staff and adequate supervision. 

Any employees engaged in dry grinding, dry polishing, or buffing exposed beyond the limits specified in Sec. 1910.1000 of the OSHA Standards shall be provided with a local exhaust ventilation system to maintain employee exposure within prescribed limits. 

Substitution Where feasible, less toxic solvents should be substituted for highly toxic solvents. As discussed earlier, aqueous systems are the least toxic and are nonflammable. Water with safe additives may be effective however, if water has insufficient solvent power, safety solvents, such as aliphatic hydrocarbons and fluorinated hydrocarbons should be considered before the more toxic solvents, such as toluene, ethylene dichloride, and trichloroethylene. When the more toxic solvents must be used, control measures such as local exhaust ventilation systems should be employed to remove volatile vapors and mists at the source. A review system should be in place to identify highly toxic solvents such as benzene, carbon tetrachloride, and gasoline and to ensure that appropriate substitutions are considered and/or controls are in place to address potential hazards. 

Mobile local exhaust ventilation system used for the removal of rock dust during mechanical preparation. 

The Control of Exposure. This should be achieved primarily by engineering means and extraction, i.e. containment (see Figures 3, 4 and 5 use of three types of local exhaust ventilation systems). The use of protective clothing and personal protective equipment (see Figures 2 and 3 use of gloves, eye and ear protection, respirators and laboratory coats) is very much a secondary option, which, however, often becomes the preferred course of action in the workplace. 

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