The second type of hoods are capturing hoods. In capturing hoods, the source is external to the hood, and the airflow must generate sufficient
velocity at the source to "reach out" and capture the contaminant.
In general, many issues can reduce performance of capturing hoods, such as cross drafts and user placement.
The required capture velocity to attain good effectiveness with a capturing hood depends on the contaminant.
Dusts require higher velocities than fumes and gases. The actual capture velocity at the source depends on how far away the hood is.
For an open round duct, we can model capture efficiency as a sphere with radius x, where x is the distance from the duct.
The capture area can then be expressed as pi times x squared.
Capture velocity is equal to the airflow divided by the capture area, which is dependent on distance squared.
Thus, the capture velocity is related to 1 over distance squared, or the inverse square law.
The bottom line is that capture velocities are high near the duct and rapidly diminish with distance away from the opening.
This is depicted in the image where smoke is being used to visualize capture velocity entering an open duct.
Capture velocity is low away from the duct, where the smoke follows curly paths,
and then increases close to the duct opening. The smoke begins to follow straight paths.
This welder is using an exterior hood to capture fumes and gases emitted by welding.
The hood must be placed in the proper position so that the air velocity at the contaminant
source entrains the fumes and gases, pulling them into the ventilation system.
Capture is most effective when the hood is positioned close to and angled to face the source.
If the hood is not angled toward the fume source, capture will be poor regardless of
proximity because air velocity is insufficient to entrain the contaminant.
Likewise, if the hood is positioned too far away from the fume source, capture will be poor again
because of insufficient air velocity where it is needed – at the source.
The slotted hood is a special type of capturing hood. Often, slotted hoods are used to ventilate tanks.
In this tank, hexavalent chromium is being used to plate chrome onto the wheels of a car, producing highly toxic vapors.
A slotted hood can be used to capture these vapors.
The slot ensures that air velocity is sufficient over the entire surface of the tank for good capture.
The third type of hoods are receiving hoods.
Receiving hoods use the momentum of the process to improve capture efficiency over other types of hoods.
Here, we will consider a grinding wheel that throws particle contaminants with high inertia into the air.
High efficiencies at low airflows can be achieved with a receiving hood aligned with the
typical trajectory of particles thrown from the grinding wheel.
In contrast, a capturing hood not aligned with the inertia of the particles is a poor choice
because the high velocities required to achieve high capture efficiency, if high capture efficiency is possible at all.
A canopy hood is a type of receiving hood that receives vapors or fumes rising due to the thermal energy from a hot source.
Here, a hot sodium bath is being used to blacken a steel part.
The resulting vapors, rising because the bath is hot, are often captured with a canopy hood easily at a reasonable low fan speed.
However, when large parts are dipped, the exhaust volume may be insufficient to capture vapors.
Although the fan may be operated at a higher speed to achieve adequate capture,
this configuration requires more power and may not be economical.
Canopy hoods are not recommended for cold processes or those that emit vapors that are more dense than air.
When a steel gear is dipped in room- temperature hydrochloric acid in descaling,
the heavier-than-air vapors do not rise to meet the canopy hood.
If possible at all, much higher fan speeds are need to capture these vapors - perhaps 10 times or more than the hot process.
The hood used to capture dust from an angle grinder is a good example of a receiving hood.
In this case, a worker is using an angle grinder to remove mortar between bricks as part of a process known as tuck pointing.
Without capture, the grinder throws debris and large quantities of dust into the air,
often resulting in very high exposures to dust and more importantly crystalline silica from the sand in the mortar.
A hood consisting of a shroud with a heavy-duty vacuum can be used to dramatically reduce dust exposures.
The shroud is specially designed with the vacuum positioned to receive the dust as it is
thrust into the air by the inertial energy of the grinding wheel.
A special type of ventilation, called push-pull ventilation, can be used to ventilate tanks.
We have already seen how a slotted capture hood can be used effectively to reach out and pull vapors into an exhaust ventilation system.
These systems are quite common and very effective in many situations.
However, the distance that a pull-only system can reach out is quite limited. Thus, vapor capture is often not sufficient for wide tanks.
We can, however, dramatically increase the width of effective capture by pushing clean air over the tank.
The clean air pushes the vapor to an area of effective capture, where it can be pulled into the ventilation system.
Thus, even for wide tanks, push-pull ventilation can effectively prevent vapors from entering the workplace.