By John Constance, Professional Engineer, P.E.
The Engineers Collaborative
Dust and vapor collection exhaust systems are designed for plant production operations to minimize employee exposure to nuisance and hazardous materials. The two most common industrial ventilation methods to control production generated dust and vapor are dilution ventilation and local exhaust ventilation. Dilution ventilation has the advantage of requiring very little knowledge of the practice of industrial hygiene engineering to apply it. It is deceptively easy to apply. Local exhaust ventilation requires a detailed knowledge of the production process, knowledge of capture hood design, avoiding interference with the process and the operator, and a detailed knowledge of duct design, exhaust fan and air pollution control equipment.
Collection of dust or vapor by dilution ventilation is accomplished by diluting the concentration of the contaminant with clean air but only after the concentrated contaminant passes through the operator's breathing zone. Dilution ventilation is accomplished by evenly exhausting relatively large amounts of air from the workspace. In the same category of dilution ventilation, we have displacement ventilation. Displacement ventilation with its usually downward laminar air flow has been adapted from clean room applications where this clean air is used to protect the product from people and other sources of contamination.
Dilution or displacement ventilation does not reduce the amount of dust or vapor released into the work space; over a period of time, dilution ventilation, evenly distributed, will remove most of the contaminant from the work space. There are situations where dilution or displacement ventilation can be effective:
- If the operator is far enough away from the dust or vapor generation source.
- If the generation source is producing contaminates in sufficiently low concentrations so that the operator will not have an exposure in excess of the contaminant's permissible exposure limit (PEL).
- If the velocity and rate of contaminant generation are relatively even and low.
Local Exhaust Ventilation
Local exhaust ventilation, using capture hoods at the point of contaminant generation, captures the dust or vapor at its source, minimizing the operator's exposure to breathing the contaminant in high concentrations. Local exhaust ventilation has the advantage of requiring less air volume than dilution and displacement ventilation for effective contaminant control at the least possible capital cost.
Both dilution ventilation and local exhaust ventilation require sufficient makeup air to operate effectively. Ideally, makeup air will be positively introduced into the work area and distributed in such a manner that the source of contamination is between the worker and the exhaust point(s). This is especially important with dilution ventilation. If the dilution ventilation exhaust is located close to the contamination source, all the makeup air will pass through the zone of contamination. The contaminant will then be diluted to safe levels as it is generated. However, in many plant environments, it is usually difficult to achieve ideal conditions. In some cases, dilution and displacement ventilation can be very effective when used together with local exhaust ventilation.
Dilution Ventilation and Local Exhaust Ventilation Working
In addition to capturing most of the contaminant before it disperses into the operator's breathing zone and the general work area, local exhaust ventilation provides excellent dilution ventilation in the area immediately occupied by the personnel involved with the production process.
Let's assume, for example, that a source of dust generation requires 550 CFM exhaust using a properly designed dust capture hood. The number of air changes per hour of dilution ventilation within an imaginary 3 foot radius sphere with the hood at its center is 292. If the source of dust was from a drum filing operation, the operatorís face most likely would be within the imaginary 3 foot radius sphere and experiencing 292 air changes per hour.
If you use displacement or dilution ventilation instead of local exhaust ventilation to achieve the same number of air changes (292), the amount of air required would be enormous. In a workspace 10 feet by 15 feet by 9 feet high, you would need about 6,500 CFM to achieve the same dilution effect that 550 CFM of local exhaust provides.
As always, material handling improvements to reduce or eliminate dust and vapor sources should be considered as a first line of defense to protect production personnel. This will minimize the cost of industrial ventilation exhaust and supply air systems.
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