Receptor hoods

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

A receptor hood reeeives a eontaminant driven into it by the souree of generation. The flowrate needs to ensure that the hood is emptied more rapidly than the proeess fills it and to overeome draughts. No operator should work between the hood and die souree of die eontaminants. 

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. 

Receptor hoods, also called canopy hoods, are designed to capture contaminants given off by heated processes. They take advantage of the thermal updraft caused by such processes by placing the hood in the path of the updraft, they receive the exhaust and capture the contaminants. 

Heated sources can cause strong updrafts that carry contaminants upward. Receptor hoods take advantage of this updraft, as shown in Fig. 10.31. The process shown to the left in Fig. 10.31 is at room temperature, while the process to the right in Fig. 10,31 is operating at elevated temperature. A canopy... 

The key variable in determining the applicability of a receptor hood to a particular source is the temperature of the heated source, and the resulting updraft. The temperature must be high enough to cause an appreciable updraft, or the hood will be ineffective. An estimate must be made of the total amount of buoyant airflow set in motion by the heated source the airflow through the hood must be greater than this buoyant airflow, in order to ensure complete contaminant capture. This principle is illustrated in Fig. 10.32, which shows the air spill that occurs when a hood s exhaust airflow is less than the thermal updraft airflow. 

Hemeon divides receptor hoods into low hoods, located within about 1 m of the heated source, and high hoods, located beyond this distance (Fig. 10.33). 

High Receptor Hoods The important variable that distinguishes receptor hoods from other exterior hoods is the upward airflow set in motion by the heated source. Let us first consider the more general (and difficult) case of a high hood. Assume for simplicity that the source and the hood are circular in cross-section. The basic geometry used in this case is shown in Fig. 10.36. 

The diameter of the receptor hood is also a critical design variable. The diameter of the heated plume, dp, can be determined geometrically if it assumed that the included angle of expansion is 18°. Alternatively, ACGIH° and Goodfellow give the following equation for the plume diameter ... 

Although the above approach for high receptor hoods is rigorous theoretically, it is difficult to use in practice. ACGIH- offers a simpler approach. First, the virtual source height is calculated using Eq. (10.61) where h = Rather than calculating the velocity of the hot air column... 

Low Receptor Hoods Low receptor hoods are much easier to design, since entrainment of air into the plume and the effects of turbulent cross-drafts are not significant problems. In this case, the diameter of the plume at the hood face, dp, is assumed to equal the diameter of the source, d,.. Accord-... 

The flow ratio method was first suggested for use in designing receptor hoods and then it was suggested for design of push-pull systems. The concept of the method is described as follows. 

One common combination is a jet and an exhaust hood. The jet can be circular or plane and situated around or in front of a (hot) contaminant source. The intention is to direct the contaminant into a basic opening or a receptor hood. Mostly these jets are directed upward into hoods, but may be directed sideways or downward. There is a difference to jets covering openings. When directed into a hood the jet is intended to help the natural flow into the hood and not to act as a shield, even though it sometimes also has this function. Figure 10.105 illustrates two principal ways that air jets could be used to direct contaminants into a hood (see also Section 10.4.5). 

Goodfellow, H. D., and P. Safe. Analysis of Remote Receptor Hoods Under the Influence cd Cross Drafts. ASHRAE Winter Meeting, Chicago, IL, Jan. 28-Feb. 1, 1989. 

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. 

Local exhaust uses captor hoods or receptor hoods . With captor hoods the contaminant source is outside the hood, e.g. a handpiece welding fume extractor, and with receptor hoods it is inside, e.g. spraypainting booth, laboratory fume hood. These take maty forms and a full review of design is not given here however, the principal points to note are given here. 

For small jobs, such as tissue work in medical laboratories, or shot blasting of small parts, afuUy enclosed ventilated glove box can be used. Figure 10.7 shows a receptor hood in use at the top of an adhesive mixer. Note also the use of a simple manual handling aid. 

A captor hood must generate sufficient capture velocity at the point of contaminant generation. A receptor hood must have sufficient face velocity (velocities vary from 0.5 m/s, e.g. laboratory fume hood, to lOm/s, e.g. disc grinder). 

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