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Ventilation

General or Dilution Ventilation
Ventilation is one of the most important engineering control techniques for controlling the quality of air in an environment. The application or process requiring ventilation will determine whether to use general (dilution) ventilation or local exhaust ventilation. General ventilation is a term used to describe the supply and exhaust of large amounts of air to a room or entire building for the purpose of controlling room temperature and preventing the build-up of low concentrations of toxic or non-toxic contaminants or odors from minor and decentralized sources. Ventilation can be used to control one or more variables in a work environment including:

Temperature control.
Removing a hazardous contaminant.
Diluting the concentration of a contaminant to an acceptable level.
Supplying make-up air.
Note general or dilution ventilation should not be used as a control for chemical emissions released from equipment or specific processes. Local exhaust ventilation, as described below, should be used for this purpose.

Exhaust
Exhaust ventilation is used in major applications for one or more of the following reasons:

Preventing fire or explosions due to chemical build-up
Controlling the release of chemical emissions to concentrations which will not produce adverse health effects. .
Controlling the heat and humidity.

Exhaust Ventilation and Applications
Exhaust ventilation is used to minimize the risk to health or life due to the potential for employee exposures to chemical emissions. Exhaust ventilation applications include:

As a primary control when chemical emissions from a tool or operation cannot be controlled. This might occur when the flow of chemicals to a tool malfunctions resulting in worker exposure or chemicals are generated / released during normal operations.
As a secondary control when a process , tool or interlock malfunctions resulting in chemical exposure to the employee.
As a supplemental control during a maintenance task.

These types of applications are referred to as local exhaust ventilation (LEV).

It is critical to use good ventilation design to create efficient equipment exhaust which will result in energy savings and cost savings.



Ventilation Design Principles
The efficient flow of air through a duct is dependent upon minimizing the static pressure losses caused by the various components of the exhaust ventilation system. Each of the parameters listed below can negatively affect static pressure losses thereby making the exhaust ventilation system ineffective or inefficient to control chemical exposures. Some of the exhaust ventilation design considerations include:

Friction Losses from Material used in the ventilation system.
Type of duct opening used to collect the contaminant (hood design).
The geometry of the duct work, i.e. Straight runs verses numerous bends.
Presence of turbulence
Proper fan selection
Presence of internal fittings can contribute to the air turbulence

.

Other important design consideration to remember include:

Minimize volume of exhausted enclosure
Minimize distance between the contaminant source to the hood
Perform computer modeling to observe the way the system operates
Consider heat recapture from electrical equipment (energy conservation)

Key Elements for Specification
There are numerous elements to consider and specify when designing a local exhaust ventilation system and these are identified below. The lack of specification of any of these elements can result in an inadequate and inefficient local exhaust ventilation system which will not meet the needs of the system and possibly cause unnecessary chemical exposures to workers.

Point of Measurement
Duct Size
Air Density
Temperature
Static Pressure
Physical Condition of Equipment
Peak, Normal, Special Requirements
Environmental Conditions
Stability and Tolerance Requirements
Priority of Exhaust
Instruments and Practices to Take Measurements
Materials of Construction
Constituents of Exhaust

Measurement/Validation
Exhaust ventilation systems must be properly evaluated in order to determine if the ventilation system will properly control chemical emissions which might otherwise expose workers. A number of ventilation measurements must be collected to properly evaluate how well the ventilation system is operating and these measurements include:

Volumetric flow rate of air in the duct
Capture Velocity (where the airborne contaminants are generated outside an enclosure)
Face velocity (where applicable)
Hood entry loss factor due to the hood design (Ce)
Hood static pressure
Duct diameter at the point of connection to the facilities

Collection of this data allows us to thoroughly evaluate the performance of the exhaust ventilation system and identify the optimal exhaust levels. Furthermore, we can confirm whether the process and safety requirements of the tool are being met to prevent chemical exposure. In some instances, these measurements may simply not be enough to properly evaluate the exhaust ventilation system and other tests may need to be performed which include:

Velocity Profiling ­ This method will confirm the expected airflows, the direction of flow, and the effect of distance on the collection of a contaminant.

Vapor Visualization- This method uses aerosols (using water, liquid nitrogen or dry ice) so that exhaust flow patterns can be observed.

Tracer Gas Testing ­ This testing provides a method to test the integrity of hoods and gas boxes by simulating gas emission (sulfur hexafluoride) and measuring the effectiveness of the controls.

Chemical Air or Wipe Monitoring ­ This method measures the concentration of a chemical contaminant in the air or the quantity of chemical on the equipment by wiping the equipment with a special filte