Exterior exhaust hoods


This phenomenon is more common with large flow rates and large openings, such as use of a laboratory fume hood or a unidirectional horizontal flow field, than with small flow rates and small openings, such as exhaust hoods for welding, soldering, and grinding. Designing the system to place the worker beside the airflow path is generally recommended for all exterior hoods. When using partial enclosures with large openings to the surroundings, a person may also influence the flow field, e.g., by changing the flow into the hood. It is possible to counteract such wakes by using vertically directed supply air around the worker (see Sections 10.3.3 and 10.4.6).  [c.815]

The first of these conditions is the most important factor when deciding to use an exterior hood. Exterior hoods are allowed when the demands on the exhaust, and the hazard of the contaminants, are moderate.  [c.819]

The supply air for exterior hoods should be brought into the room without disturbing the exhaust flow. The supply openings are usually placed far away from the exhausts and it is necessary to check that the supply air velocity near the exhaust is much lower than the velocity due to the exhaust. If the supply air velocities are less than one-tenth of the exhaust air velocities at distances less than three exhaust opening diameters from the exhaust, then the influence of the supply on the exhaust should be negligible. This is also true for cross-drafts caused by other activities. For cases with higher supply air velocities or higher cross-draft velocities, supply air velocity or cross-draft velocity must be included in the design of the exhaust, the supply air or cross-draft should be redirected, or the type of exhaust should be changed to one with less sensitivity to supply air or cross-draft velocities.  [c.820]

All exterior hoods should be evaluated regularly. The evaluation procedures can be divided into detailed and simple procedures. Detailed procedures need special instruments and competence, whereas simple procedures may be performed daily. Since simple procedures do not directly measure the performance of the exhaust, it is usually necessary to calibrate them using detailed procedures.  [c.825]

As mentioned above, a basic exhaust can be of nearly any shape with the most common being round, rectangular, or slot openings, with or without flanges. Openings in walls could be said to have the largest flanges. There also are exhausts consisting of multiple holes or perforated plates in a wall or ceiling or floor or table. The latter ones (holes in a floor or table) ol ten have special designs and are treated separately in a later section. The exhaust opening can be tapered to have both a large opening area and a smooth velocity increase inside the opening to the connecting duct, resulting in lower pressure los.s. As mentioned above, exterior hoods can be connected directly to a process or to equipment. These have some similarity to industrial vacuum cleaners.  [c.831]

Exterior hoods intended to capture contaminants should be placed as close to contaminant sources as possible. In actual practice, however, the hoods can not always be placed close to the source due to circumstances such as working conditions. In such cases, to enhance the exhaust efficiency of exterior hoods, it is useful to use a low-momentum air supply directed toward the exhaust outlet. The supply airflow, which functions to transport contaminants emitted from sources located at a distance from the exhaust outlet,. should be relatively low with a uniform velocity but high enough so that it is not disturbed by the. surrounding air motions. The advantages of using low-momentum supply with exterior hoods are that (1) a lower supply airflow rate to the workspace is possible, (2) a lower exterior hood exhaust flow rate is possible, and (3) it is possible to supply clean air to the breathing zone of the worker.  [c.966]

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,  [c.1015]

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.)  [c.812]

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.  [c.812]

Contaminants captured by an exterior hood can cause exposure if allowed to enter the hood after passing through the operator s breathing zone. The capture efficiency describes the percentage of the generated contaminant that is captured directly by the hood. Occupational hygiene efficiency describes the effect of the use of the hood on the operator s exposure to the contaminant, The occupational hygiene efficiency is performed with the exterior hood working and not working. For example, many wood-processing machines produce large volumes of wood chips and dust and cannot operate without the exhaust hoods in use. With very toxic contaminants, it is essential to work with the hood exhaust operating at all times.  [c.1014]

The exterior hoods described here are divided into basic openings, rim exhausts, low-volume high-velocity (LVHV) hoods, receptor hoods (canopy hoods), and downdraft ventilation tables. Many varieties of these types of hoods exist. Some of these have been described and investigated more thoroughly than others because they are used more often or they are of more general use and applicability than the more specialized hoods.  [c.819]

The exhaust could be placed above, below, or beside the source dependiujj on generation direction. If possible, the main exhaust direction (center axis of opening area) should be the same as the main contaminant generation direction. Small changes in location, both sideways and in distance, could result in large changes in the efficiency of exterior hoods. If the exhaust is designed to have the highest efficiency in one specific placing, small moves could decrease the efficiency substantially.  [c.821]

Since exterior hoods are unshielded, nearly all disturbances can dramatically change the performance of the hood. Changes in direction of contaminant generation could result in contaminant spread outside the exhaust a variable generation rate could result in intermittent contaminant spread directly to the surroundings when the source volume rate becomes larger than the exhaust flow rate tools moving around or rotating could either disturb the flow field or redirect the contaminants or both cross-drafts from moving persons or vehicles, or leakage through door openings or w all cracks could temporarily or permanently disturb the exhaust flow field and result in spreading of the contaminant in the room.- A moving source demands a moving exhaust or a very large opening to ensure that the hood is as close as possible to the source. An exhaust can be moved virtually, i.e., the openings are not moved, but instead the exhaust flow rate is connected to different openings or parts of one opening (at different locations) depending on where the moving source is at that moment. I his is a way of minimizing the exhau,st flow rate without diminishing the efficiency. I his type of hood puts high demands on the control system for the exhaust. The misplacing of a baffle in the exhaust opening (to diminish the necessary flow rate) could also result in contaminants not being captured.  [c.822]

Partial enclosures are a compromise between containment and access. Most people misunderstand the function of partial enclosures. It is not possible to completely separate the interior from the surroundings with partial enclosures. Complete separation is only possible with total enclosures. The function of a partial enclosure is as dependent on the flow rate, the flow field, the working procedures, the contaminant generation process, etc. as is the function of exterior hoods. The advantage with a partial enclosure is that the physical walls diminish the possibilities for the contaminants to escape from the hood to the surroundings. Thus these hoods could be used when relatively high demands are put on the contaminant concentration outside the hood. Some of the most commonly used enclosures, such as fume cupboards and booths, are described. Many variations of these exist, e.g., enclosure of the complete process, and some of these are described here.  [c.878]


Industrial ventilation design guidebook (2001) -- [ c.0 ]