Application Considerations
Exhaust Fan Economizer
Figure A-3 depicts a typical Modular Climate Changer with an exhaust fan and
economizer (i.e., a standard unit with outdoor air and
exhaust). To balance the amount of air exhausted
from the building with the amount of air brought in, the exhaust fan modulates,
running at full capacity only when the economizer
brings in 100- percent outdoor air. When the economizer is at minimum and the exhaust
fan is idle, dampers on the mixing box close to prevent
outdoor air from being drawn into the air handler
through the exhaust module. The
exhaust-fan-and-economizer combination provides strict
space pressurization control, provided the supply fan is sized to handle total system static pressure. Its first cost and operating
cost are usually lower than the
return-fan-and-economizer alternative, too. (An exhaust
fan requires less capacity than a return fan, and runs less often.)
Application considerations:
Size the supply fan to
handle the static pressure requirements of the higher
of either 100-percent economizer
operation (i.e., OA ductwork, OA damper, filters, coils and other accessories
in the outdoor air stream, plus supply-duct static
pressure drop) or 100-percent return air
operation (i.e., RA ductwork, RA damper, filters, coils
and other accessories in the recirculated air stream, plus supply- duct
static pressure drop).
Size the exhaust fan to
handle 100- percent of the
return air (i.e., return duct, exhaust duct
and shutoff damper) when the unit is in full economizer
mode.
Control exhaust airflow to maintain the
outdoor/indoor static pressure differential within design limits.
Control the mixing-box dampers to prevent all of them from closing simultaneously;
otherwise, serious equipment damage could result.
Return Fan Economizer
Figure A-4 depicts a typical Modular Climate Changer with a return fan and economizer (i.e., a standard unit with outdoor
air). The return fan typically runs continuously to
balance the amount of air supplied to and removed
from the occupied space. While this approach makes precise space pressurization
control more difficult, it is better suited to
applications with high return static pressures than the
exhaust-fan alternative; if the supply fan is unable to handle system static pressure, the return fan is sized to overcome
the return duct’s external static pressure. Of
course, the return fan’s larger size and constant
operation also mean higher first and operating costs. Application
considerations:
Size the supply fan to
handle the static pressure requirements of 100-percent
economizer operation (i.e., OA ductwork, OA damper, filters, coils and other accessories in the outdoor air stream,
plus supply-duct static pressure).
Size the return fan to handle
the static pressure requirements of 100-percent
return air operation (i.e., return duct,
exhaust duct and exhaust damper).
Control the return fan to maintain the
outdoor/indoor static pressure differential within design limits.
Control the mixing-box dampers to prevent all of them from closing simultaneously;
otherwise, serious equipment damage could result.
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