Introduction
Hood design is one of most important areas to consider when designing Local Exhaust Ventilation
(LEV). It influences the effectiveness of the system in terms of contaminant control and the amount
of air captured, which in turn influence ducting, air mover and air cleaner size.
The aim of this paper is to provide information that will help the reader to identify the correct type
of hood for the particular application.
Problems with poor hood design
There are many problems that can be associated with poor hood design:-
- Inadequate capture of the contaminant, thereby leaving the operator exposed to the hazard.
- Excessive air volumes being captured, resulting in larger air movers, air cleaners and
- generally a more expensive system when compared with a correctly designed system.
- Restriction on the required user activity.
- Too much product being removed from the process and potentially wasted.
Advantages of good hood design
With good hood design the contaminant is correctly and effectively controlled and the captured air
volume is restricted to the minimum. This leads to smaller ducting, smaller air filter and smaller air
mover thereby providing the most efficient design and giving the following benefits when compared
with poor design :-
- Lower initial purchase price
- Generally lower installation costs
- Lower running costs
- A solution the user is less likely to interfere with
- Compliance with legislation – CoSHH, EPA, ATEX etc
- An LEV that is meeting its specification
These are important considerations and commercially should not be ignored.
How to select the correct hood type
Good hood design starts with a thorough understanding of the contaminant to be captured, the
process that is generating the contaminant, type of source (large, small moving) and how the user
interacts during the process that generates the contaminant.
All of these factors will determine:-
- The type of hood used
- The position of the hood
- Any requirement for enclosing the source
- The size of the hood
- If the hood is fixed or moving
Contaminant to be captured
The purpose of this guidance is to cover hood design types and therefore will not spend a great deal
of time on the type of contaminant other than state that gases, vapours, dust, fibres, fumes and
mists are all forms of contaminants that we are often faced with when designing hoods. Each one
has its own properties but once in air the cloud will move with the air surrounding it – i.e. into the
hood and through to the air cleaner.
These contaminant properties need to be taken into account when designing the hood.
Processes
The process carried out influences the way the contaminant is generated and there are many (for
example blasting) that can create mist, vapour or dust. Crushing would create dust as the
contaminant.
Understanding the process will also give greater understanding of the creation of the source and
also determines the possible control mechanism and hence the type of hood.
The source
This is where the process generates the contaminant and provides information that is used to
determine what method of capture is required. For example how large the contaminant cloud is,
how it is presented into the air and what direction it takes.
Types of hoods
There are three basic types of hood:-
Enclosing (fully or partially enclosing)
Receiving
Capturing
Enclosing Hood
Also known as a containment hood this type is often considered the most effective type. The source
to be controlled will be located internal to the hood and is so designed to prevent escape of the
contaminant due to the side wall constraints and the in draft of air from any openings.
Also known as a containment hood this type is often considered the most effective type. The source
to be controlled will be located internal to the hood and is so designed to prevent escape of the
contaminant due to the side wall constraints and the in draft of air from any openings.
Receiving Hood
Less effective than the enclosing hood is the receiving hood which has the contaminant source
external to the hood and relies upon the inertia and direction of the source to guide the
contaminant into the suitably designed and positioned hood.
Typical processes that would require a receiving hood would be ones that use high speed cutting,
grinding or blasting.
This type of hood can either be fixed or moveable. If moveable, it is good practice to provide the
user with instruction on how the hood should be positioned to remain effective.
The hood should be adequately sized and have a high enough velocity of air to ensure that the
contaminant is captured.
Capturing hoods
Again less efficient than the enclosing hood, this is the most commonly used hood type in Local
Exhaust Ventilation (LEV) where the process and source are external to the hood. This type of hood
relies on the air flow into the hood to capture and control the contaminant.
Whilst the position of the receiving hood is critical for performance, this is even more so for the
capturing hood. Ideally the hood should be fixed in the correct position relative to the source where
the contaminant is pulled away from the operator. However, often the hoods are moveable to
match the process taking place and, in these cases, it is important to provide the user with a clear
instruction on how the hood should be used in order to be effective.
Capture hoods are often used where the enclosing or receiving hood types are not suitable and
where the contaminant has no strong movement and direction.
One way to demonstrate how important the position of the hood is to the effectiveness of the
control is to look at a velocity contour map – see figure 6 below.
The particular velocity contour map is for a circular plain opening and it can be seen clearly that the
velocity falls off rapidly with increasing distance from the hood opening – velocity decreases
inversely with the square of the distance from the hood. At a distance of just one diameter from the
opening, the velocity has fallen to below 7.5% of the velocity at the opening.
Since there is no particular movement of the source into the hood, movement must be generated
from the hood itself.
Capture relies upon the velocity of the air at the location of the source, where the actual velocity
required is dependent on the how the source is presented and any influencing factors such as other
air movements such as draughts etc. The position of the hood is therefore critical to maintain the
velocity at the point of capture.
The following example shows the effect on distance of the hood from the source on the air volume
required.
An articulated arm (see figure 7) has an open face area equivalent to a 250 mm diameter opexning (0.05 m2) and needs a capture velocity at the point of source of 0.5 m/s. The hood will be placed 250
mm away from the source to allow the process to take place.
The required air volume is obtained from the following:-
Q = V (10 X 2 + A)
where
Q = air volume.
V = capture velocity at distance X from the hood.
X = distance of the hood from the source.
A = open face area of the hood.
Q = 0.5 (10 * 0.252 + 0.05) = 0.34 m3/s 1215 m3/hr
If however the distance from the hood to the source was increased to 300 mm the required air
volume would increase to:-
Q = 0.5 (10 * 0.32 + 0.05) = 0.48 m3/s 1710 m3/hr
40% increase in air volume required for 20% increase in distance.
If this is not understood at the design or user stage, the effect is to design a system suitable for acapture velocity at source of 0.5 m/s requiring a total air volume of 1215 m3/hr BUT be faced with a system that delivers inferior results, which may not control the contaminant when the hood is placed further away as in the example above.
LEVs are designed to handle a given air volume and the result of placing the hood at a further 50 mm from the source results in the capture velocity reducing to just 0.36 m/s
Finally.
The above describes the three basic types of hoods that are used in LEV but there are variations on
these types for example slotted, canopy and fish tail to mention some of the common ones. Each
hood has been created to handle specific processes and type of source of contaminant.
Some hoods are used in a push-pull arrangement where a jet of air is pushed across the contaminant
source into a receiving hood. When used on open tanks for example, it works due to the pushed jet
of air maintaining its velocity over large distances, pushing the contaminant into the capture zone of
the receiving hood.
Any designer of a hood should ask themselves:
“What type of hood requires the LEAST air volume to control the
contaminant when considering the process and source?”
and should bear in mind:
- It is better to enclose
- It takes four times the amount of air to pull twice the distance
- You can push air forty times farther than you can pull it
- Consider disturbing air movements that may affect capture hoods
- If extracting large amounts of air consider the impact on the local area – do you need to
- introduce lost air?
