Exterior exhaust hoods

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.)... 

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. 

Gases, vapors, and fumes usually do not exhibit significant inertial effects. In addition, some fine dusts, 5 to 10 micrometers or less in diameter, will not exhibit significant inertial effects. These contaminants will be transported with the surrounding air motion such as thermal air current, motion of machinery, movement of operators, and/or other room air currents. In such cases, the exterior hood needs to generate an airflow pattern and capture velocity sufficient to control the motion of the contaminants. However, as the airflow pattern created around a suction opening is not effective over a large distance, it is very difficult to control contaminants emitted from a source located at a di,stance from the exhaust outlet. In such a case, a low-momentum airflow is supplied across the contaminant source and toward the exhaust hood. The... 

An exterior hood is often a natural choice for an exhaust. It is usually easy to install, less expensive, and does not need any large changes in the outlay of the room or the process. Often it is possible to connect this type of hood to an existing exhaust duct system and when the flow rate is relatively small, the ex isting supply air system may be maintained without changes. 

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. 

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. 

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. 

Rim exhausts are a specific application of slot hoods, which in turn are a type of exterior or capture hood. Rim exhausts are slot hoods placed along the rim or edge of an area source such as an open surface tank or vessel opening. Open surface tanks are widely used in indu.strial settings for cleaning, stripping. 

If the contaminated airflow rate that is to be exhausted, or the internal pressure, varies too much it could be advantageous to use an exhaust connection with a small distance between tube and duct, acting as an opening for additional air when contaminant flow rare is low. This could be in the form of a large exterior hood covering the outlet from the process and leaving only a very small opening gap for external air (thimble). See Fig. 10.40. 

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. 

The low-momentum air, which is supplied from a relatively wide supply inlet, functions to transport contaminants to near the exterior hood. In addition, it functions to change the direction of contaminants toward the exterior hood when the direction of contaminated air is initially different from the exhaust direction. The momentum or velocity of the supply air to reach to the exterior hood will be sufficient when the motion of contaminants can be neglected. However, when the contaminated air exhibits significant motion or flows in a direction different from the exterior hood, the supply air velocity should have a sufficient momentum to control the contaminant flow. 

Since the low-momentum supply system should enhance the efficiency of an exterior hood by supplying low-momentum airflow to a source, the system can be applied to practically any sources where an exterior hood can be used. In particular, it is effective to apply the system when an exterior hood cannot be placed close to a source or the exhaust direction is different from the initial contaminant release direction. 

The final example is shown in Fig. 10.86. Several workers are breaking gates off of castings on the conveyor by hand. Much dust is generated by this operation and the dust rises due to buoyancy. To remove the dust, an exterior hood was placed beside the conveyor and a supply inlet was placed above the workers. The supply airflow is blown toward the breathing zone of the workers and the dust source. In this case, as the workers and the dust source are located within the supply airflow, the airflow functions to supply the workers with clean air and to transport the dust toward the exhaust inlet. The velocity of supply air is relatively low, 1.1 m s , and the exhaust velocity at the hood face is 2.75 m s . The dimensions of the system are indicated in the figure, and the depth of the device is 6.0 m (compare with Sections 10.3.3 and 10.4.6). 

The inlet opening that supplies the low-momentum airflow should be sufficiently wide to cover the contaminant source and should face toward the inlet of exterior hood. The airflow functions to transport contaminants emitted within the flow to the exterior hood. The exhaust airflow created around the suction opening must exhaust all of the contaminants transported by the supplied airflow. From this point of view, the low-momentum... 

In the low-momentum supply system, the contaminants are emitted within the low -momentum airflow blown from the supply inlet and they are transported to near the exhaust opening. If the contaminants diffuse into the whole of the supply airflow, the exterior hood must exhaust the whole of the airflow. To diminish the exhaust flow rate, some methods to prevent the contaminants from diffusing into the whole of the airflow are required. One possible method is to supply the air as slowly as possible but with enough velocity to reach the exhaust outlet and to control the surrounding air motion. Another method is to blow supply air with uniform... 

An established design method for this type of system is not available. The practical design of the low-momentum supply with exterior hood system described in the previous part of this section used the flow ratio method. How-evec, the actual exhaust flow rate was adjusted visually to the appropriate value in order to exhaust only the contaminants transported by the supply airflow. 

Applying the flow ratio method to the low-momentum supply system, the required exhaust flow rate is often in excess of practical values. This is because the value of is given as the value at which all the supplied airflow should be exhausted by the exterior hood. In the low-momentum supply system, contaminant sources should usually be between the supply inlet and the exterior hood. The supply airflow is contaminated at the position of the sources and it flows to the exterior hood. Therefore, all of the airflow is not always contaminated. Unfortunately, a design method considering such cases (the diffusion of contaminants within the airflow) has not been established yet, and the appropriate exhaust flow rate has to be adjusted after the system is installed. 

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,... 

An exterior hood continuously draws contaminants into an exhaust from some distance away. 

PHOTO 20.26 Exterior of Level A suit. The exterior tents covering the one-way exhaust valves are visualized just above the waist on the responder to the left, and on the top rear of the hood of the responder on the right in this photograph. (Courtesy of the Center for Domestic Preparedness, Department of Homeland Security, Anniston, Alabama. This image is the work of an employee of the center for Domestic Preparedness, taken or made during the course of official duties. As a work of the U.S. Federal Government the image is in the public domain.)... 

A local exhaust system is used to collect air contaminants at the source, as contrasted with general ventilation, which allows the contaminant to spread throughout the workroom, later to be diluted by exhausting quantities of air from the room. Local exhaust may be achieved using an enclosure, a receiving hood, or an exterior hood. Exhaust Ventilation... 

New alloys have been introduced for specific automobile applications including type 434 stainless steel for exterior trim, type 409 and 430 stainless steels for exhaust systems, and 6XXX and 7XXX aluminum aUoys for auto body and structural parts. Use of aluminum for body panels such as hoods and deck lids has prompted the development of new processing techniques for this material. 

Figure 12.6 Fume hood exhaust blowers. Locate the exhaust fan as far from the hood as possible. Here they are located in the mechanical penthouse, only a few meters from the duct outlet to the building exterior.

---