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Controlling Ventilation Air Flows

Animation Series 1:
Controlling Ventilation Air Flows
  1. Desired Ventilation Flows
  2. Overcrowded Offices
  3. Air Handling Unit Contaminated
  4. Outdoor Air Damper Opening Reduced
  5. Exhaust Not Operating Properly
  6. Supply Air to Office Shut Off or Pinched Down
  7. Economizer Operating
  8. Contaminated Outdoor Air

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Desired Ventilation Flows

8. The exhaust in the laboratory is depressurizing 7. Air always flows from areas of high atmospheric pressure 6. Pressure relationships between rooms is designated at the bottom 5. The laboratory contaminants are exhausted directly to the outside 4. Building atmospheric pressure relative to the atmospheric pressure 3. Outdoor air is being drawn into the supply air stream to 'dilute indoor pollutants' 2. Air inside the offices is accumulating pollutants from occupants 1. Ventilation air is constantly being supplied to all occupied spaces

Mouse over the animated picture above to see messages which are referenced in more detail below.
Pressure relationships between rooms is designated at the bottom of each room (with a plus for positive and minus for negative pressure).

  1. [SUPPLY AIR] Ventilation air is constantly being supplied to all occupied spaces through the supply air stream. Notice that the supply air is made up of some clean outdoor air and some recirculated air.   Recirculated air often makes up the major portion of the supply air stream.

    [RETURN AIR] Air inside the offices is accumulating pollutants from occupants and their activities and from the building fabric or other indoor sources. Contaminated office air is leaving the offices through the return air vent. Some portion of this contaminated return air exits the building through the “relief air vent”. The remainder is being “recirculated” and “diluted by the outdoor air” entering the air handler from outside.
     
  2. [OUTDOOR AIR] Outdoor air is being drawn into the supply air stream to “dilute indoor pollutants”. The pollution from indoor sources to which occupants are exposed is directly proportional to the strength of the indoor sources, and inversely proportional to the amount outdoor air in the supply air stream. Since occupants and their activities are often the major source of pollution, ventilation standards for indoor air quality provide for a minimum quantity of outdoor air “per person”. This is designed to insure that as pollution sources from occupants increase, outdoor air increases proportionally to insure adequate dilution.
     
  3. [RELIEF VENT] Building atmospheric pressure relative to the atmospheric pressure outside is the force that is always equalizing the flow into the building with the flow out of the building. The relief vent provides a convenient way for flows to equalize.
     
  4. [EXHAUST VENTILATION] The laboratory contaminants are exhausted directly to the outside.   Since the laboratory is a significant potential source of toxic pollutants, none of the laboratory air is recirculated.   Ideally, an exhaust hood directly above and close to the laboratory chemicals should be used to trap the pollutants into the exhaust stream and prevent any pollutants from contaminating the breathing zone of the laboratory staff.
     
  5. [INDOOR RELATIVE PRESSURE]  Pressure relationships between rooms is designated at the bottom of each room. The pressure relationships indoors are being controlled mechanically through supply, return, and exhaust air flows. Since the office is being maintained at a higher pressure relative to the laboratory, air that may sometimes flow through wall cavities or doorways flows “FROM” the offices “TO” the laboratory but not vice versa.
     
  6. [BUILDING PRESSURIZATION–EXFILTRATION] Air always flows from areas of high atmospheric pressure to areas of low atmospheric pressure.   If more air is entering than is leaving the building, the atmospheric pressure inside increases relative to the outside, and thus increases the flow of air exiting the building until the incoming and outgoing flows are equal. The converse is also true.   Notice how, except for the laboratory, this building is slightly pressurized to insure that the direction of air flow is from the inside to the outside to prevent outdoor pollutants from infiltrating into the building. Where outdoor humidity is a problem, this might not be a good strategy.
     
  7. [LABORATORY DEPRESSURIZATION–INFILTRATION]  The exhaust in the laboratory is depressurizing the laboratory relative to the outside and relative to the office areas. That is why air is flowing into the laboratory.   Depressurization is insuring that pollutants from the laboratory do not migrate into the office environments.

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