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#1 10-08-2018 11:55:29

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Date d'inscription: 08-02-2018
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Air will only be

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.

To design a Local Exhaust System we must know the:

* physical state of the chemical (Is it a dust Rush Ben Gedeon Jersey , 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.

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

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