Hoods and enclosures

The classification of hoods into exterior hoods and enclosures could sometimes make it difficult to specify a hood. This classification is only an attempt to describe the hoods. Enclosures can be separated into partial and total enclosures partial enclosures have an opening to the surroundings big enough to use for work, and total enclosures do not. Both have the contaminant source inside a physical volume and for some of these hoods this volume is large enough for some workers to work inside. See Fig. 10.39. 

For exterior hoods and enclosures, the measurement of the breathing zone concentration provides a method of comparing the effects of changes in the supply and exhaust airflow rates,... 

Figure 28.10 Typical local extract hoods and enclosures...

Hood and enclosure design, (i) (A) It is the dual function of grinding and abrasive cutting-off wheel hoods to protect the operator from the hazards of bursting wheels as well as to provide a means for the removal of dust and dirt generated. All hoods shall be not less in structural strength than specified in the American National Standard Safety Code for the Use, Care, and Protection of Abrasive Wheels, B7.1-1970. 

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... 

Hoods and Enclosures The partial or complete enclosure around the wheel or disc through which air enters an exhaust system during operation. 

Hoods and enclosures shall be designed in accordance with OSHA standards. 

Exhaust hoods should enclose as effectively as practical the points where the contaminant is released. They should create air flow through the zone of contaminant release of such magnitude and direction as to carry the contaminated air into the exhaust system. Exhaust hoods and enclosures may also serve the important function of keeping materials in the process by preventing their dispersion. 

Total enclosure may be in the form of a room with grilles to facilitate air flow this functions as a hood and operates under a slight negative pressure with controls located externally. Entry is restricted and usually entails use of comprehensive personal protective equipment. Ancillary requirements may include air filters/scmbbers, atmospheric monitoring, decontamination procedures and a permit-to-work system (see page 417). 

Fixed systems are those where movement of the hood or other changes to the system, except perhaps opening and closing of lids and doors, is not possible. One example is the hood with a sliding door surrounding a drilling or a milling machine another is the laboratory fume hood and another is the canopy hood above or the enclosure around a paper machine. 

In practice there are many different combinations, such as two exhaust hoods close to each other or two or more air curtains placed around a horizontal (or v ertical) source or a hood that is partly an exterior hood and partly an enclosure. 

R. P. Harvey. In Proceedings of the Development and Use of Fume Cupboards, Fume Hoods and Ventilated Safety Enclosures in Laboratories . Symposium Organised by the Laboratory of the Government Chemist, London, March 1979, pp. 32-59. 

Total enclosure may be in the form of a room with grilles to facilitate air flow this functions as a hood and operates under a slight negative pressure with controls located... 

Charging and tapping emissions are controlled by a variety of evacuation systems and operating practices. Charging hoods, tapside enclosures, and full furnace enclosures are used in the industry to capture these emissions and send them to either the primary hood gas cleaner or a second gas cleaner.1516 Pollution prevention opportunities for the reduction of heavy metals at the BOF are limited as heavy metals are an inherent part of the iron ore material stream, so the higher the iron production, the greater will be the use of the ore. 

Visibly clean is not an acceptable cleaning criterion. A detailed, sequential cleaning procedure is advisable, for surfaces inside and outside laboratory chemical hoods and ventilated weighing enclosures. If settling of powder occurs on the floor, this indicates that material is escaping and that additional administrative and personal protective controls are warranted when using chemical fume hoods for subdivision applications. 

The degree of enclosure provided by the hood and the air capture velocity should be such that dust particles are prevented from entering the workplace. 

It is often possible to reduce air-input requirements- by removing the hazardous material at the point of discharge by loccd ventilation. This lowers the ta value in Eq. (8-5), which assumes possible disposal of hazardous material within the entire enclosed volume of the enclosure being ventilated. Hoods and exhaust ducts are placed over such equipment as open filter presses, pulverizers, open tanks, and over laboratory benches and equipment to catch the maximum amount of vapor or dust without interfering with normal operation and maintenance. Local air velocities in the region of pickup will depend on density of the hazardous material or its particle size if a dust or fume. Air velocities greater than 200 fpm are usually employed for industrial operations, while chemical laboratory fume hoods range from 70 to 125 fpm when fully opened. 

Ductless fume hoods are ventilated enclosures that have their own fan, which draws air out of the hood and through filters and ultimately recirculates it into the laboratory. The filters are designed to trap vapors generated in the hood and exhaust "clean" air back into the laboratory. These hoods usually employ activated carbon filta-s. The collection efficiency of the filters decreases o er time. Ductless fume hoods have extremely limited applications and should be used only where the hazard is very low, where the access to the hood and the chemicals used in the hood are carefully controlled, and under the supervision of a laboratory supervisor who is familiar with the serious limitations and potentially hazardous characteristics of these devices. If these limitations cannot be accommodated, then this type of device should not be used. 

By using extraction as close to the source of pollution as possible to minimise the escape of the pollutant into the atmosphere. The extraction devices can be either hoods, slots, enclosures or fume cupboards coupled to a system of ducts, fans and air cleaners. 

These clean rooms tend to be very expensive to build, so if your budget does not stretch to a full-blown clean room, it might be worth investing in special HEPA filter enclosures just for your instrument and sample preparation area. These are typically either mobile units that can be wheeled around the laboratory and placed around different equipment or hood-based enclosures that are placed over a particular instrument. Whatever system is used, their objective is to ensure that the area around the equipment is free of airborne contamination and the instrument background levels are as low as possible. 

Exhaust System A system consisting of branch pipes connected to hoods or enclosures, one or more header pipes, an exhaust fan, means for separating solid contaminants from the air flowing in the system, and a discharge stack outside. 

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