- hoods for capturing the contaminant
- ducts for transporting the contaminant
- air cleaner for removing contaminants from the air stream
- fan to create airflow in the system
- stack to discharge the air outside the workplace.
To design a Local Exhaust System we must know the:
* physical state of the chemical (Is it a dust Giorgio Tavecchio Jersey , mist, fume, gas or vapor ?),
* chemical's toxicity and applicable exposure limits Connor Cook Jersey ,
* physical properties of the chemical (Vapor pressure, boiling point, flash point),
* routes of worker exposure -- inhalation Shilique Calhoun Jersey , ingestion, skin contact,
* how, where and when the chemical is used Johnny Townsend Jersey ,
* how the worker does their job.
HOOD DESIGN
A well-designed hood is the most important component of an effective LEVS. The hood must be positioned so that it does not
pull contaminated air through the worker's breathing zone. It should be easy to
use and not interfere with the job that the worker is trying to do. It should be
positioned as close to the point of contaminant generation as possible. The
further it is from the point where the chemical is released into the air, the
more airflow is required to capture the contaminant.
AIR VOLUME AND CAPTURE VELOCITY
The air volume (cubic feet per minute) that must be exhausted by LEVS is determined by the type of hood, the distance of the hood
from the source of the contaminant and the velocity needed to capture the
contaminant (Capture Velocity). Capture velocity for a hood is determined by the
properties of the chemical and how it is being used. Examples of capture
velocities are shown in Table 1.
MAKE UP AIR
Air will only be exhausted to the extent that air enters the workplace. If you don't provide make
up air in the amount at least equal to the amount of air being exhausted, your
LEVS will not work properly and the workplace will be very drafty Nick Nelson Jersey , doors will be difficult to open, and furnaces, heaters or other combustion equipment may back draft.
TRANSPORT VELOCITY AND DUCT SIZE
Once a contaminant is captured by the hood it moves into the duct system. The velocity in the duct must be sufficient to transport the contaminant
through the LEVS. The velocity in the duct necessary to carry the contaminant
through the system is referred to as the transport velocity. The heavier the
contaminant the higher the velocity needed for transport. Some examples of
transport velocities for different contaminants are shown in Table 2. Once you
know the airflow volume and transport velocity needed for a LEVS, the duct size
can be calculated using the formula shown in Table 3.
STREAMLINE AIR FLOW
Local exhaust systems should always use round ducts Arden Key Jersey , because airflow is more uniform and streamlined, which makes the system more efficient and provides better transport for contaminants.
The duct runs should be as straight as possible; curves should be smooth and
gradual; and an elbow should have a radius of 2 to 2.5 times the duct diameter.
Branch entries into the main duct should be at an angle of 45?; there should be
no 90? entries. All changes in size should be smooth and
gradual.
STACKS
A stack should discharge contaminated air vertically upward and away from the building. Stacks should be located as far
from air intake units as possible to prevent reintroduction of contaminated air
into the building. The top of the stack should be 1.3 to 2 times the building
height above the ground. Avoid exhausting air out of the sides of buildings. The
pressure of prevailing winds blowing into the exhaust can severely affect the
performance of the LEVS.
FAN SELECTION
The fan you select for your LEVS should be based on the needs of the system. It should not only deliver the
volume of air (in cubic feet per minute) necessary to capture the contaminant
but be able to do so against the resistance to airflow in the system. The
resistance to airflow is measured in inches of water and is usually referred to
as static pressure losses. Static pressure losses in LOCAL EXHAUST SYSTEM are
determined by the:
- size of the duct,
- roughness of the duct material,
- number and type of elbows Brandon Parker Jersey , entries, and changes in size,
- type of air cleaner,
- type of hood Mario Edwards Jr Jersey ,
- volume of air flowing in the system,
- stack design.
It should be clear from this list that a fan cannot possibly be selected successfully until the system has been
designed.
SYSTEM INSTALLATION
Insist that you get a system installed as designed, with round ducts and smooth streamlined airflow. Since a
rough duct increases static pressure losses and requires a larger fan, you
should keep the use of a flexible duct Karl Joseph Jersey , which is very rough, to a minimum. Use a flexible duct only where you need flexibility and use as little of it as possible.
Fans will operate more efficiently if they are installed with a length of straight
duct entering and leaving the fan. A rule of thumb is to provide a straight run
of duct at least six duct diameters long on the entrance side of the fan and at
least two duct diameters long on the exit side. After installing the system,
measure to ensure that the LEVS delivers the airflow volume and velocity that is
needed to do the job.
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