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VAV DESIGN FOR IMPROVED INDOOR AIR QUALITY

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VAV DESIGN FOR IMPROVED INDOOR AIR QUALITY Air conditioning is the control of the humidity of air by either increasing or decreasing its moisture content. – PowerPoint PPT presentation

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Title: VAV DESIGN FOR IMPROVED INDOOR AIR QUALITY


1
VAV DESIGN FOR IMPROVED INDOOR AIR QUALITY
2
  • Air conditioning is the control of the humidity
    of air by either increasing or decreasing its
    moisture content. Added to the control of
    humidity is the control of temperature by either
    heating or cooling the air, the purification of
    the air by washing or filtering the air, and the
    control of air motion and ventilation.
  • Willis H. Carrier

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  1. COMPLETE AND PERFECT MIXING OCCURS BETWEEN ALL OF
    THE ROOM AIR AND THE SUPPLY AIR
  2. THE RETURN AIR IS LEAVING THE ROOM AT THE
    TEMPERATURE OF THE ROOM
  3. THE INTRODUCTION OF THE SUPPLY AIR GENERATES AN
    ACCEPTABLE AMBIENT AIR VELOCITY IN THE ROOM
  4. EITHER THE FLOW RATE (CFM) OR THE DT OR SOME
    COMBINATION OF BOTH CAN BE VARIED TO SPAN THE
    ENTIRE RANGE OF CAPACITY REQUIREMENTS
  5. ADEQUATE VENTILATION AIR MUST BE CIRCULATED
    THROUGH THE SPACE AT ALL TIMES TO REMOVE THE
    CONTAMINENTS

5
q CFM (1.08) (tr - ts)
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HEAT-COOL-OFF DUAL STREAM REHEAT
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DUAL STREAM qs CFM (1.08) (tr
- ts) CONSTANT CFM, VARIABLE DT EXAMPLES

DOUBLE DUCT MULTIZONE
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TERMINAL REHEAT qs CFM (1.08) (tr
- ts) CONSTANT CFM, VARIABLE DT
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VARIABLE AIR VOLUME (VAV) qs CFM (1.08)
(tr - ts) VARIABLE CFM, CONSTANT DT
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Poor mixing of supply air Inadequate ambient air
circulation Dumping Reduced ventilation rates
as load decreases
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  • The dehumidification of the outdoor ventilation
    air and the space can be separated from the space
    temperature control
  • The ventilation supply need not be integrated
    with the space temperature control
  • The Economizer System is an option

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RULES
  1. The building envelope must be well defined and
    must be secure from any opportunity for mass air
    flow at conditions different from the indoors
    and must prevent migration of water vapor due to
    vapor pressure difference
  2. The envelope itself must prevent vapor migration
    by use of continuous vapor barriers properly
    located
  3. The point at which outdoor air is introduced into
    the space must be well defined
  4. No air should be introduced into the space at
    moisture conditions different from those desired
    in the space

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  1. The conditioned makeup or ventilation air can be
    supplied to and distributed to the space in a
    separate duct and distribution system, or
  2. The makeup air can be introduced into the return
    side of the recirculating units
  3. There could be a single makeup air unit for a
    building with multiple recirculating units, or
  4. There could be a makeup air unit for each
    recirculating unit

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  • Cost
  • Load dynamics
  • Occupancy patterns
  • Ratio of Outdoor Air to Recirculated Air

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  • In warm humid climates the age-old problem of
    poor part-load humidity control in the spaces was
    solved, and
  • The major sources of indoor air quality problems
    are eliminated. These are
  • Uncontrolled microbial growth
  • Poor air circulation and mixing
  • Inadequate ventilation air quantities

38
DESIGN GUIDELINES
  1. UNDERSTAND THE LOAD AND DESIGN THE SYSTEM TO
    SERVE THE ANTICIPATED LOAD PLATEAUS
  2. CONTROL THE BUILDING AT ALL TIMES
  3. PROVIDE A SEPARATE MINIMUM VENTILATION AIR
    CONDITIONING SYSTEM OR UNIT (PARTICULARLY WITH
    DIRECT EXPANSION)

39
  1. USE THE SIMPLEST BUT HIGHEST QUALITY VAV TERMINAL
    POSSIBLE
  2. USE FAN POWERED TERMINALS SPARINGLY
  3. DESIGN AN AIR DISTRIBUTION SYSTEM THAT WILL
    PROVIDE EFFECTIVE MIXING WITHOUT DUMPING, DOWN TO
    THE MINIMUM COOLING LOAD THE SYSTEM WILL EVER SEE
    ON AN OCCUPIED CYCLE, OR THE MINIMUM DICTATED BY
    THE EFFECTIVE PERFORMANCE

40
  1. BELOW THE MINIMUM PERFORMANCE QUANTITY, IF THE
    LOAD CONTINUES TO DROP, REHEAT IS NECESSARY
  2. RESET THE COLD DECK IF THIS CAN BE DONE WITH OUT
    ADDING TOO MUCH COMPLEXITY
  3. DESIGN THE AIR HANDLING SYSTEM FOR THE LOWEST
    TOTAL PRESSSURE THAT IS ECONOMICALLY FEASIBLE
    (TRY TO HOLD THE UPPER LIMIT AT 4 w.c.)

41
  1. DONT DEPEND ON DIRECT DIGITAL CONTROL TO SOLVE
    PROBLEMS THAT THE DESIGNER DOESNT UNDERSTAND
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