Slot Hood Design

1. Kitchen Hood Design
2. Slot Hood Designs

What is covered in this document?

This document is part of a series of documents on industrial ventilation, and provides general information about hoods.

1. Hoods

Three plates were designed to change the geometry of a plain conical hood (slot area: 0.1334, 0.0963 and 0.0694 ft 2 They were tested at different airflow rates (243, 347, 467, 647, 897 cubic feet per minute). The volumetric exhaust flow rate through the system was one-half that which would have been required for a commonly used slot design. This reduction in exhaust volume was due to partial enclosure of the slot, which reduced the handling of uncontaminated air.

What is a hood?

A hood - correctly called a local exhaust hood - is the point where contaminated air is drawn into the ventilation system. The sizes and shapes of hoods are designed for specific tasks or situations. The air speed (velocity) at the hood opening and inside the hood must be enough to catch or capture and carry the air contaminants. To be most effective, the hood should surround 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.
• Capturing.

Enclosing Hood

Enclosing hoods, or 'fume' hoods, are hoods surrounding the process or point where the contaminants are generated. Examples of completely enclosed hoods (all sides enclosed) are glove 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 whenever possible.

Receiving Hood

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

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 welding or grinding bench table (figure 4) or a downdraft hood for hand grinding bench (figure 5).

What is meant by 'capture velocity'?

The ventilation system removes contaminants by 'pulling' the air (and the contaminant) into the exhaust hood and away from the worker or the source. Airflow toward the hood opening must be fast or high enough to 'catch and transport' the contaminant until it reaches the hood and ducts. 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 the hood. The ventilation system will require a higher airflow speed to overcome air disturbances. As much as possible, the other sources of air motion should be minimized or eliminated for the ventilation 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 grinding wheels, belt conveyor, etc.
• Material motion such as dumping or filling.
• Movements of the operator.
• Room air currents (which are usually considered 50 fpm (feet per minute), but may be much higher).
• Rapid air movement caused by spot cooling and heating equipment.

Where the contaminant is released with practically no other air currents in a room, the recommended capture velocity is generally around 0.5 m/s (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. It is easy to see how it will take very little air movement from other sources to affect how well a hood can capture contaminants. (See Figure 6).

In situations, such as grinding for example, where the contaminants are released in the air at high speed and where there is a rapid air circulation in the room, the necessary capture velocity may be 5 to 10 times higher.

What are the general rules for hood design?

Online poker hud software free version. Accommodation melbourne cbd near casinos. The shape of the hood, its size, location, and rate of airflow each play an important role in design considerations. Each type of hood has specific design requirements, but several general principles apply to all hoods:

• The hood should be placed as close as possible to the source of contamination, preferably enclosing it. The more completely enclosed the source is, the less air will be required for control. The required airflow rate varies with the square of the distance from the source as shown in 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 contaminant source and the hood.
• The natural movement of contaminants should be taken into consideration. For example, a hood should be placed above hot processes to trap rising gases and heat. A grinding wheel or woodworking machine should be equipped with a partial enclosure to trap the flying particles where they spin off.
• Flanges or baffles should be used around the hood opening to increase the capture effectiveness and reduce ventilation air requirements.

Kitchen Hood Design

How do I know which type of hood is adequate for the process?

The hood should be selected according to the characteristics of the process to ensure that the worker's exposure to airborne contaminants is minimal.

The following table contains a comparison of the three types of hoods.

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.

The primary goal of an industrial ventilation system hood is to capture and transfer environmental contaminants. A hood's size and shape is designed specific to its end application but is typically classified within the enclosing hood or exterior hood category.

Hood Types

Enclosing hoods:

An enclosing hood will completely or partially surround the point where contaminants are generated. An enclosing hood is typically preferred but may not be practical due to potential interference with employee workstations.

• A partial enclosing hood has two to three sides where an inward flow of air through the opening will contain the contaminant within the enclosure and prevent its escape. Examples include paint spray booths or grinder station.
• A completely enclosing hood has all sides and is preferred whenever possible. A laboratory hood is an example of this use.

Exterior hood:

Exterior hoods are placed next to the point where contaminants are generated without creating an enclosure. An exterior hood may be an opening on a welding table or slots on the side of a tank. The exterior hood should be located in the path of the emission if transferring larger particulates such as sand.

There are four main types of exterior hoods:

• Canopy: A one- or two-sided overhead hood that receives upward airflow from hot air or gas.
• Close-capture: Mounted directly over the source of a contaminant.
• Push-pull: A hood placed on the side of a push-pull ventilation system.
• Side-draft (also called lateral exhaust hood): This is not as efficient as other containment or down-draft hoods.

Hood Velocity Considerations

What is a hood?

A hood - correctly called a local exhaust hood - is the point where contaminated air is drawn into the ventilation system. The sizes and shapes of hoods are designed for specific tasks or situations. The air speed (velocity) at the hood opening and inside the hood must be enough to catch or capture and carry the air contaminants. To be most effective, the hood should surround 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.
• Capturing.

Enclosing Hood

Enclosing hoods, or 'fume' hoods, are hoods surrounding the process or point where the contaminants are generated. Examples of completely enclosed hoods (all sides enclosed) are glove 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 whenever possible.

Receiving Hood

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

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 welding or grinding bench table (figure 4) or a downdraft hood for hand grinding bench (figure 5).

What is meant by 'capture velocity'?

The ventilation system removes contaminants by 'pulling' the air (and the contaminant) into the exhaust hood and away from the worker or the source. Airflow toward the hood opening must be fast or high enough to 'catch and transport' the contaminant until it reaches the hood and ducts. 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 the hood. The ventilation system will require a higher airflow speed to overcome air disturbances. As much as possible, the other sources of air motion should be minimized or eliminated for the ventilation 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 grinding wheels, belt conveyor, etc.
• Material motion such as dumping or filling.
• Movements of the operator.
• Room air currents (which are usually considered 50 fpm (feet per minute), but may be much higher).
• Rapid air movement caused by spot cooling and heating equipment.

Where the contaminant is released with practically no other air currents in a room, the recommended capture velocity is generally around 0.5 m/s (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. It is easy to see how it will take very little air movement from other sources to affect how well a hood can capture contaminants. (See Figure 6).

In situations, such as grinding for example, where the contaminants are released in the air at high speed and where there is a rapid air circulation in the room, the necessary capture velocity may be 5 to 10 times higher.

What are the general rules for hood design?

Online poker hud software free version. Accommodation melbourne cbd near casinos. The shape of the hood, its size, location, and rate of airflow each play an important role in design considerations. Each type of hood has specific design requirements, but several general principles apply to all hoods:

• The hood should be placed as close as possible to the source of contamination, preferably enclosing it. The more completely enclosed the source is, the less air will be required for control. The required airflow rate varies with the square of the distance from the source as shown in 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 contaminant source and the hood.
• The natural movement of contaminants should be taken into consideration. For example, a hood should be placed above hot processes to trap rising gases and heat. A grinding wheel or woodworking machine should be equipped with a partial enclosure to trap the flying particles where they spin off.
• Flanges or baffles should be used around the hood opening to increase the capture effectiveness and reduce ventilation air requirements.

Kitchen Hood Design

How do I know which type of hood is adequate for the process?

The hood should be selected according to the characteristics of the process to ensure that the worker's exposure to airborne contaminants is minimal.

The following table contains a comparison of the three types of hoods.

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.

The primary goal of an industrial ventilation system hood is to capture and transfer environmental contaminants. A hood's size and shape is designed specific to its end application but is typically classified within the enclosing hood or exterior hood category.

Hood Types

Enclosing hoods:

An enclosing hood will completely or partially surround the point where contaminants are generated. An enclosing hood is typically preferred but may not be practical due to potential interference with employee workstations.

• A partial enclosing hood has two to three sides where an inward flow of air through the opening will contain the contaminant within the enclosure and prevent its escape. Examples include paint spray booths or grinder station.
• A completely enclosing hood has all sides and is preferred whenever possible. A laboratory hood is an example of this use.

Exterior hood:

Exterior hoods are placed next to the point where contaminants are generated without creating an enclosure. An exterior hood may be an opening on a welding table or slots on the side of a tank. The exterior hood should be located in the path of the emission if transferring larger particulates such as sand.

There are four main types of exterior hoods:

• Canopy: A one- or two-sided overhead hood that receives upward airflow from hot air or gas.
• Close-capture: Mounted directly over the source of a contaminant.
• Push-pull: A hood placed on the side of a push-pull ventilation system.
• Side-draft (also called lateral exhaust hood): This is not as efficient as other containment or down-draft hoods.

Hood Velocity Considerations

A specific velocity is required, depending on the type of contaminant being captured. To achieve the required velocity, carefully consider the hood's shape, size and location.

• Face velocity: Velocity right at the hood opening.
• Capture velocity: Velocity at the dust generation source to capture the contaminant and transfer it into the hood.

Ergonomic Considerations:

An industrial ventilation system hood is one of the most important components of an individual's workstation. A worker will be more likely to use the hood and the ventilation system properly if ergonomic elements are considered. Among these considerations are:

• Accessibility to parts within the hood
• Size, design and weight of objects handled
• Safety cables
• Overhead clearance
• Sharp edges
• Lighting
• Ease of cleaning

Slot Hood Designs

IVI's engineering and design team has years of experience designing and sizing industrial ventilation systems. IVI can assist in the design of a new system or the redesign of an existing system.