VAV DESIGN FOR IMPROVED INDOOR AIR QUALITY

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VAV DESIGNFORIMPROVEDINDOOR AIR QUALITY
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• 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

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q CFM (1.08) (tr - ts)
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HEAT-COOL-OFFDUAL STREAMREHEAT
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DUAL STREAM qs CFM (1.08) (tr
- ts) CONSTANT CFM, VARIABLE DTEXAMPLES

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 airInadequate ambient air
circulationDumpingReduced 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

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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)

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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

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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.)

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