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.