IH Ventilation 04/05/2026

Local Exhaust Ventilation (LEV)

  • Definition:

    • Local exhaust ventilation (LEV) is a system designed to capture contaminated air from as close to the source as possible to prevent workers from inhaling harmful pollutants.

  • Key Components of LEV:

    • Hood: Central part where contaminated air gets captured.
    • Ductwork: Transports the captured air to an air cleaner or scrubber.
    • Air Cleaner/Scrubber: Removes contaminants from the air before it moves to a fan.
    • Fan: Creates necessary airflow to draw air from the hood through the system to either indoors or outdoors.

  • Applications of LEV:

    • Common uses include processes such as:
    • Dipping tanks (to contain vapors)
    • Media blasting

Differences Between LEV and Other Ventilation Systems

  • LEV vs. Dilution Ventilation:

    • LEV removes contamination directly at the source while dilution ventilation allows contaminants to accumulate and works to dilute them with fresh air over time.

  • Dilution Ventilation Process:

    • Introduces fresh air into a contaminated environment to mix and lower the concentration of pollutants.
    • Goals:
    1. Reduce the contamination to a target level.
    2. Maintain contamination below permissible levels while allowing processes (e.g., evaporation from dip tanks) to continue.

  • General Ventilation:

    • Primarily for thermal comfort (temperature control) rather than contamination reduction.
    • Not considered an effective industrial hygiene control.
    • Examples include HVAC systems and natural ventilation.

Control Methods in Ventilation

  • Types of Control:
    • Source Control: LEV is considered a source control because it directly targets contamination at its origin.
    • Path Control: Although LEV can act this way, it is primarily focused on source control.
    • Receiver Control: Less clear-cut; LEV might indirectly affect this when used in enclosures.

Makeup and Exhaust Air

  • Makeup Air (Supply Air): Air coming into the space to replace contaminated air.
  • Exhaust Air (Return Air): Air being expelled out of the space.
  • Ratio Insight:
    • An effective room will maintain a balance of at least 1.2 times the amount of makeup air to exhaust to ensure slightly positive pressure inside.

Pressure in Spaces

  • Positive Pressure Room: More supply air than exhaust, preventing outside contaminants from entering.
  • Negative Pressure Room: More exhaust than supply air, keeping contaminants contained within the room.
    • Common applications include:
    • Pharmaceutical labs
    • Isolation in hospitals for airborne diseases.

Ventilation Rates and Flow Dynamics

  • Flow rates are crucial in determining room pressures (positive or negative) based on the volume of makeup and exhaust air.
  • Example calculation: A room with six supply ducts (150 CFM each) and one exhaust (900 CFM) would create positive pressure if the supply exceeds the exhaust.

Local Exhaust Efficiency

  • Local exhaust ventilation efficiency depends on:
    • Proximity to contamination sources (the closer, the better).
    • Capture velocities, determined by the type of contaminant properties and their dispersal energy.

Summary of Key Formulas and Methods

  • Air Changes per Hour (ACH):
    • ACH=60imesQextVolumeoftheRoomACH = \frac{60 imes Q}{ ext{Volume of the Room}}
  • Capture Velocity Formula:
    • Q=VximesAfQ = V_x imes A_f where:
    • $V_x$ = desired capture velocity
    • $A_f$ = area of the hood face.
  • Dilution Time Calculation:
    • extDeltat=−extVolumeofRoomQimesextlnCg2Cg1ext{Delta } t = \frac{- ext{Volume of Room}}{Q} imes ext{ln}\frac{C_{g2}}{C_{g1}}

Types of Air Cleaning Devices

  • Dry Particulate Filters:
    1. Baghouse Filters: Captures dry particulates (not for hot/wet particles).
    2. Electrostatic Precipitators: Uses electrical charge to capture dry particulates.
    3. Cyclones: Utilizes centrifugal force for dry particulates.
  • Wet Scrubbers:
    • Effective for capturing both wet and dry particles using liquid to clean air streams.

General Ventilation Standards and Practices

  • Set by ASHRAE:

    • Minimum fresh air should be 5 CFM per person and 0.06 CFM per square foot for office spaces.

  • Thresholds for CO2:

    • Should not exceed the outdoor levels by more than 700 PPM, aiming for indoor concentrations to stay below 1000 PPM.

  • Humidity Control:

    • Relative humidity should be maintained at or below 65% to prevent discomfort and microbial growth.

Final Remarks

  • Understanding local exhaust ventilation is critical to creating safe work environments and understanding air quality control measures. Adjustments in flow rates, control mechanisms, and air quality thresholds play essential roles in workplace safety.