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1. Define hood capture velocity.

2. Evaluate proximity of the pollutant source to the hood affects capture efficiency.

3. Calculate the required airflow rate to attain a desired hood capture velocity for a specific situation.

4. Describe some hood designs that improve capture effectiveness.

Hood Capture Velocity

Hoods are generally designed to operate under negative static pressure. The air is drawn into the hood due to static pressures that are lower inside the hood than those in the process equipment and the surrounding air. The fan, located downstream from the hood, creates the suction that draws the air into the hood. Since air from all directions moves toward the low-pressure hood, the hood must be as close as possible to the process equipment in order to capture the pollutant-laden air and not just the surrounding air. The capture velocity of a hood is defined as "the air velocity at any point in front of the hood or at the hood opening necessary to overcome opposing air currents and to capture the contaminated air at that point by pulling it into the hood" (American Conference of Governmental Industrial Hygienists, 1998). In other words, contaminated air located at some point in front of the hood must at least be moving at the capture velocity in order to be drawn into the hood.

At approximately one-hood-diameter away from the hood entrance, the gas velocities are often less than 10% of the velocity at the hood entrance. Figure 1 illustrates how quickly the gas velocity decreases as distance from the hood increases.

Figure 1 indicates that the hood has very little influence on gas flow except in the area very close to the hood entrance. In order to ensure good capture of the pollutant-laden gas streams, the hood must be close to the emission source.

In general, heavy particulate matter (e.g. from grinding and sandblasting operations) requires a much higher capture velocity than do volatile organic compounds (VOCs) from a tank. Hood capture velocities are often provided as a range for a particular type of operation. The following conditions should be assessed in determining which part of the capture velocity range should be used for a particular operation (i.e. low end vs. the upper). (American Conference of Governmental Industrial Hygienists, 1998)

• The surrounding air currents (e.g. minimal room air currents vs. more turbulent air currents)

• The level of toxicity of the pollutant to be captured

• The amount of pollutant (e.g. intermittent low production vs. high production, heavy use)

• The area of the hood opening, ft² (e.g. large hood with large air mass in motion vs. small hood with local control only)

Hood Capture Velocity Equation (Without Flange)

There are several different variations of the basic hood capture velocity equation. The correct one to use in a particular situation depends on the configuration of the hood and duct system. A variation of the basic equation is presented later in this lesson. These equations show which parameters are involved in hood capture of contaminants and highlight the importance of hood proximity to the pollutant source.

The following capture velocity equation is for a freely suspended hood without a flange.

Where:

It should be noted that the correlation between distance, gas flow rate, and capture velocity should be used for estimation purposes only because the vacuum from a hood does not create equal velocity lines or points. Equation 1 is also limited to the distance (X) being less than or equal to 1.5 hood diameters (American Conference of Governmental Industrial Hygienists, 1998).

Example Problem 1.
Calculating the Required Flow Rate as the Proximity of the Hood to the Pollutant Source Varies

The recommended capture velocity for a certain pollutant is 300 fpm entering a 16-inch diameter hood. What is the required volumetric flow rate for the following distances from the hood face (X)? Assume X is the farthest distance from the hood face to the released contaminant.

1. X = 12 in. (75% of hood diameter)
   
2. X = 24 in. (150% of hood diameter)
   

Solution:

Part i

1. Calculate the area of the hood opening.

   

2. Calculate the volumetric flow rate, Q, required to attain the recommended capture velocity of 300 fpm at a distance of 12 inches from the hood.

   

Solution:

Part ii

1. Calculate the volumetric flow rate, Q, required to attain the recommended capture velocity of 300 fpm at a distance of 24 inches from the hood.

   

The volumetric flow rate requirements increased approximately four times when the distance between the hood and the contaminant source doubled.

The capture velocity equations for a variety of hoods with different locations and arrangements can be obtained from the ACGIH, Industrial Ventilation - A Manual of Recommended Practice.

Hood Designs for Improved Performance

There are many ways to design hoods to improve capture effectiveness. When the pollutant-laden gas stream is hot, the hood is often positioned above the point of pollutant release to take advantage of the buoyancy of the low-density hot gas stream. This helps the gas rise into the area of hood capture velocity.

Side baffles or flanges can be used to help block the movement of clean air into the hood. In addition, side baffles and flanges help eliminate cross drafts, which can prevent the intended movement of the pollutant-laden gas into the hood. The possible beneficial effect of side baffles on the gas velocities near the hood entrance is shown in Figure 3. For hoods with side baffles or flanges, the required gas flow rate can be slightly lower than for hoods without these structures. Also, the hood capture velocity zones extend outward slightly.

Side baffles can be in the form of metal sheets, strips of fabric or plastic, or any other materials that block the movement of clean air into the low-pressure area of the hood. The recommended width of a flange for most situations should be equal to the square root of the hood area. (American Conference of Governmental Industrial Hygienists, 1998). Hood capture is greatly improved when the enclosure comprised of the hood and the side baffles can encompass the point of pollutant generation.

Some process equipment inherently creates an entirely enclosed area for pollutant-laden gas capture. For example, coal-fired boilers generate pollutants in an enclosed furnace area that is maintained at a slightly negative static pressure of -0.05 to -0.25 in. W.C. In this case, the boiler walls serve as the hood.

Another hood design that is used to improve capture effectiveness is called the push-pull hood. As shown in Figure 4, a high-velocity clean air stream is blown across the area of pollutant generation into the hood on the opposite side.

The high-velocity gas stream does not inherently disperse rapidly. Therefore, it flows toward the hood and is captured. The hood also effectively captures the pollutant-laden gas that is trapped in this strong cross draft. These types of hoods are sometimes used on open tanks and other sources where access from the top is necessary in order to operate the equipment. However, they may not be appropriate for tanks and other processes handling materials where the cross draft could significantly increase the quantities vaporized. Push-pull hoods can provide very high capture efficiencies where they are applicable.

Hood Capture Velocity Equation (Wide Flange)

The following hood capture velocity equation is used for a hood with a wide flange.

Where:

Example Problem 2.
Calculating the Required Flow Rate for a Flanged Hood

What is the required volumetric flow rate for a hood with a wide flange to attain the recommended capture velocity? Use the same data as presented in Example Problem 1 in this lesson. It is repeated below for your convenience.

Solution:

1. Calculate the area of the hood opening, A. In Example Problem 1 of this lesson, the area was calculated to be 201 inches².

2. Calculate the volumetric flow rate, Q, required to attain the recommended capture velocity of 300 fpm at a distance of 12 in. from the hood.

   

   The volumetric flow requirements decreased by approximately 25% when a flanged hood was used.

* data obtained from www.epa.gov