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What is covered in this document?What is a hood?What are the common types of hood?What is meant by "capture velocity"?What are general rules for hood design?How do I know if a hood is working as designed?

What is covered in this document?

This document is part of a series of documents on industrial ventilation:

1. Introduction2. Units and Measures3. Ducts4. Fans5. Hoods6. Air Cleaning Devices7. Installation and Maintenance (general)8. Troubleshooting9. Glossary of Common Terms

What is a hood?

A hood - correctly called a local exhaust hood - is the point where contaminated air is drawninto the ventilation system. The sizes and shapes of hoods are designed for specific tasks orsituations. The air speed (velocity) at the hood opening and inside the hood must be enough tocatch or capture and carry the air contaminants. To be most effective, the hood shouldsurround or enclose the source of contaminant or be placed as close to the source as possible.

What are the common types of hood?

The three common classes of hoods are:

enclosing,receiving, andcapturing.

Enclosing Hood

Enclosing hoods, or "fume" hoods, are hoods surrounding the process or point where thecontaminants are generated. Examples of completely enclosed hoods (all sides enclosed) areglove boxes and grinder hoods. Examples of partially enclosed (two or three sides enclosed)hoods are laboratory hoods or paint spray booths. The enclosing hood is preferred wheneverpossible.

Figure 1Partially Enclosed Hood

Receiving Hood

These hoods are designed to "receive" or catch the emissions from a source that has someinitial velocity or movement. For example, a type of receiving hood called a canopy hoodreceives hot rising air and gases as shown in Figure 2. An example is a canopy hood locatedover a melting furnace.

Figure 2Receiving Hood

Capturing Hood

These hoods are located next to an emission source without surrounding (enclosing) it.

Examples are a rectangular hood along the edge of a tank (as shown in Figure 3) or a hood on a

Examples are a rectangular hood along the edge of a tank (as shown in Figure 3) or a hood on a

welding or grinding bench table (figure 4) or a downdraft hood for hand grinding bench (figure5).

Figure 3Capturing Hood

Figure 4Capturing hood for welding or grinding bench

Figure 5Downdraft Hood for hand grinding

What is meant by "capture velocity"?

The ventilation system removes contaminants by "pulling" the air (and the contaminant) into theexhaust hood and away from the worker or the source. Airflow toward the hood opening mustbe fast or high enough to "catch and transport" the contaminant until it reaches the hood andducts. The required air speed is called the "capture velocity".

Any air motion outside of the hood and surrounding area may affect how the air flows into thehood. The ventilation system will require a higher airflow speed to overcome air disturbances. Asmuch as possible, the other sources of air motion should be minimized or eliminated for theventilation system to work effectively.

Common sources of external air movement include:

thermal air currents, especially from hot processes or heat-generating operations,motion of machinery such as by a grinding wheel, belt conveyor, etc,material motion such as dumping or filling,movements of the operator,room air currents (which are usually taken at 50 fpm (feet per minute) minimum and may bemuch higher), andrapid air movement caused by spot cooling and heating equipment.

Most of the capture velocities are around 100 feet per minute (fpm). How fast is 100 fpm?Blowing lightly on your hand so that you can just barely feel air movement is about 100 fpm. Itis easy to see how it will take very little air movement from other sources to affect how well ahood can capture contaminants. (See Figure 6).

Figure 6Competitors to Capture Velocity

What are general rules for hood design?

The shape of the hood and its size, location, and rate of airflow each play an important role indesign considerations. Each type of hood also has specific design requirements, but severalgeneral principles apply to all hoods:

The hood should be placed as close as possible to the source of contamination, preferablyenclosing it. The more completely enclosed the source is, the less air will be required forcontrol. The required volume varies with the square of the distance from the source as shownin Figure 7.The air should travel from source of the contaminant and into the hood with enough velocity(speed) to adequately capture the contaminant.The hood should be located in a way that the operator is never between the contaminantsource and the hood.The natural movement of contaminants should be taken into consideration. For example, ahood should be placed above hot processes to trap rising gases and heat. A grinding wheel orwoodworking machine should be equipped with a partial enclosure to trap the flying particleswhere they spin off.The flanges or baffles should be used around the hood opening to increase the capture

Date Modified: 2008-01-10

effectiveness and reduce ventilation air requirements.

Figure 7If a hood moved from two inches away from a source to four inches away (twice thedistance), the required airflow will be four times greater to provide the same degree ofcapture.

How do I know if a hood is working as designed?

The ASHRAE 110 (test) is the recognized method for evaluating the performance of fume hoods.A qualified person should do the testing.

Document last updated on January 10, 2008

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