Transcript ENERGY CONSERVATION IN ENGINEERING DESIGN

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. 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
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
q = CFM (1.08) (tr - ts)
HEAT-COOL-OFF
DUAL STREAM
REHEAT
DUAL STREAM
qs = CFM (1.08) (tr - ts)
CONSTANT CFM, VARIABLE DT
EXAMPLES:
DOUBLE DUCT
MULTIZONE
TERMINAL REHEAT
qs = CFM (1.08) (tr - ts)
CONSTANT CFM, VARIABLE DT
VARIABLE AIR VOLUME (VAV)
qs = CFM (1.08) (tr - ts)
VARIABLE CFM, CONSTANT DT
Poor mixing of supply air
Inadequate ambient air circulation
Dumping
Reduced ventilation rates as load
decreases
• 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
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
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
• Cost
• Load dynamics
• Occupancy patterns
• Ratio of Outdoor Air to
Recirculated Air
1. In warm humid climates the age-old
problem of poor part-load humidity
control in the spaces was solved, and
2. The major sources of indoor air quality
problems are eliminated. These are
a) Uncontrolled microbial growth
b) Poor air circulation and mixing
c) Inadequate ventilation air quantities
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)
4. USE THE SIMPLEST BUT HIGHEST
QUALITY VAV TERMINAL POSSIBLE
5. USE FAN POWERED TERMINALS
SPARINGLY
6. 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
7. BELOW THE MINIMUM PERFORMANCE
QUANTITY, IF THE LOAD CONTINUES TO
DROP, REHEAT IS NECESSARY
8. RESET THE COLD DECK IF THIS CAN BE
DONE WITH OUT ADDING TOO MUCH
COMPLEXITY
9. DESIGN THE AIR HANDLING SYSTEM FOR
THE LOWEST TOTAL PRESSSURE THAT
IS ECONOMICALLY FEASIBLE (TRY TO
HOLD THE UPPER LIMIT AT 4” w.c.)
10. DON’T DEPEND ON DIRECT DIGITAL
CONTROL TO SOLVE PROBLEMS THAT
THE DESIGNER DOESN’T UNDERSTAND