Other Local Exhaust Systems

RooBng plants (asphalt saturators) Felt or paper saturators spray section, asphalt tank, wet looper Crushed rock or other minerals handling Asphalt vapors and particulates (liquid) Particulates (dust) Exhaust system with high inlet velocity at hoods (3658 m/s [>200 ft/min]) with either scrubbers, baghouses, or two-stage low-voltage electrostatic precipitators Local exhaust system, cyclone or multiple cyclones... 

Flow rate changes are sometimes used in the design of local exhaust systems. An example of this is the use of dampers, blastgates, or valves that are interlocked with the machinery of interest. When the tool is on, the damper is opened and air is exhausted from that hood. When the tool is off, the damper is closed and more exhaust air is available to other parts of the system. These kinds of systems diminish the cost for running the system but increase the cost for installation as well as periodic preventive maintenance. The supply system... 

LVHV nozzles require very careful design. They can be effective in capturing and removing dust from operations that are otherwise difficult to control, e.g., hand-held tools and some fixed-machine grinding and other operations. Installation costs for LVHV systems can be considerably higher than for conventional local exhaust systems. However, operating costs can be lower be-... 

Roofing plants Crushed rock or other minerals Particulates (dust) Local exhaust system, cyclone or... 

Frequently exhaust vents for local exhaust systems end at building roof locations. Also on roofs there are inlets for air conditioning and recirculation systems. When inlet and exit vents are close to each other and when wind conditions are just right, exhausted contaminants may travel directly to inlets and return to the building interior. There should be adequate separation of exhaust vents both vertically and horizontally from any type of air inlet to ensure that contaminants do not reenter the building. 

Capture Efficiency One can accomplish this most easily by enclosing the source as much as possible. Enclosures increase the efficiency of capture, reduce operating costs, and require less air flow to capture contaminants. If there is no way to enclose a source of contamination, the entry (hood) to the local exhaust system should be as close to the source as possible. The farther a hood is from the source of contaminants, the lower the efficiency. Air volume required to accomplish capture increases with distance between a contaminant source and the face of a hood. The shape of a hood can also affect the likelihood of capture. The profile of air movement at the entry extends farther in front of a hood for certain types of hoods compared to others. 

Provision and use of appropriate health surveillance, e.g. for signs of dermatitis, asthma, effects of specific solvent exposures. Full use of any spray booth, enclosure, exhaust ventilation or dilution systems, and automatic handling equipment. (The efficiency of all local exhaust ventilation and other control systems should be maintained, and checked by testing.) Where appropriate, atmospheric monitoring of airborne pollution levels. 

The redesigned airflow system, consisting of a broader air curtain (the broadness needed also for other reasons) and a local exhaust below the oven, ensures class A air quality in the oven when the door is opened for (see Fig. 11.3). 

Local exhaust ventilation serves to remove a contaminant near its source of emission into the atmosphere of a workplace. A system normally comprises a hood, ducting which conveys exhausted air and contaminants, a fan, equipment for contaminants collection/ removal and a stack for dispersion of decontaminated air. Hoods normally comprise an enclosure, a booth, a captor hood or a receptor hood. Those relying on other than complete enclosure should be as close a practicable to the source of pollution to achieve maximum efficiency. 

PERSONAL PROTECTION wear NIOSH/MSHA-approved respirator, chemical-resistant gloves and other protective clothing use only in chemical fume hood avoid prolonged or repeated exposure wash thoroughly after each exposure use dust-proof safety goggles a system of local exhaust ventilation is recommended to control emissions at the source and to prevent dispersion into general work area use positive pressure self-contained breathing apparatus maintain eyewash fountains and quick-drench facilities in work area. 

This section is only a very brief introduction to the design and function of a chemical hood that should allow you to use these devices safely in common lab operations. The issue of overall lab ventilation is much larger and involves a consideration of lab design, the quality of all of the air in a lab, and other kinds of local containment and exhaust systems besides the standard chemical hood. Section 7.2.3 will present a more thorough analysis of these issues. 

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. 

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... 

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