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