GENERAL VENTILATION

…examination of the use of “fresh air” to dilute contaminants to acceptable levels

Applications of General Ventilation



dilution of airborne contaminants to acceptable levels



control fire/explosion risks



minimize odors or other nuisances



maintain “comfort”

Natural Ventilation



always a factor via infiltration, etc.



design of natural ventilation via



pressure gradient



air density difference



equations/examples

Mechanical Ventilation



creation of air flow by “artificial” means



usually required because infiltration is low volume & not controllable



general ventilation vs local exhaust

General Ventilation May be OK

IF...



air contaminants are of low toxicity



concentrations are not high/hazardous



emission rate is fairly low & constant



contaminant sources are dispersed



other solutions are less practical



uncontaminated “fresh air” is available

…and usually is NOT Ok if...



contaminants are moderately toxic



concentrations are high/hazardous



emissions vary with time, e.g. batch operation.



air must pass through breathing zone



re-entrainment of contaminant is likely



control at source is more appropriate

DESIGN PRINCIPLES



determine amount of air required



locate exhaust near source



establish appropriate air flow pattern



replace exhausted air



avoid re-entry of exhausted air

HVAC Standards Affecting IAQ

    ASHRAE 62-1999: Ventilation for Acceptable Air Quality ASHRAE 55-1992: Thermal Environmental Conditions for Human Occupancy ASHRAE 52-1992: Methods of Testing Air Cleaning Devices Used in General Ventilation for Removing Particulate Matter Proposed OSHA IAQ Standard

OA Required for Specific Occupancies (ASHRAE 62-1999) Application

Commercial dry cleaners Office space Smoking lounge Conference room Laboratories

Estimated Max. Occupancy per 1,000 sq/ft

30

cfm/person OA

30 7 70 50 30 20 60 20 20

Causes of IAQ Problems

 Deficiencies in ventilation of the building  Indoor air contaminants  Outdoor sources  No identified cause

Equations

 Calculating Dilution Air Volumes  For health protection (where target concentration is PEL/TLV)  Pure  Mixtures • If solvent produce similar (additive) effects, the total dilution air required must be determined by calculating volume for each solvent and summing • • If solvents do not produce similar effects, the largest volume calculated for individual components should be used If any doubt about additive effects, assume they are additive  For fire and explosion protection (where target concentration is LEL)  Does not apply where workers are exposed to vapors. In these cases, the dilution rates above for health protection should always be used  Air density adjustment

K Factor

Some considerations that may be looked at in determining the K Factor:  The mixing and distribution of replacement air  The toxicity of the solvent  Duration of process  Location of workers to sources  Location and number of points of generation  Changes in ventilation systems

Role of IH?

 Understand HVAC system components  Determine operating parameters  Perform ventilation checks