Industrial Local Exhaust Systems
By Oleg Chetchel
Oct 1, 2009 - 9:53:50 AM
To design a Local Exhaust System we must know the:
* physical state of the chemical (Is it a dust, mist, fume, gas or vapor ?),
* chemical's toxicity and applicable exposure limits,
* physical properties of the chemical (Vapor pressure, boiling point, flash point),
* routes of worker exposure -- inhalation, ingestion, skin contact,
* how, where and when the chemical is used,
* how the worker does their job.
Local exhaust ventilation systems are designed to capture airborne chemicals at the source of generation and remove contaminants from the work area. When a local exhaust system does its job your workers are healthy and productive. It usually consists of:
- 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.
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, 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, 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, entries, and changes in size,
- type of air cleaner,
- type of hood,
- 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, 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.
For additional information please refer to http://www.olegsystems.com/fumeextractor/Index.html.
Oleg Chetchel
Industrial Process Equipment Designer
Oleg Systems Co.
http://www.olegsystems.com/dustcollector/Index.html
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