Adequate levels of ventilation reduce the likelihood of sick building syndrome (a condition where indoor air quality leads to acute health problems and comfort issues for building occupants). The outdoor air used for ventilation must be conditioned, and this requires energy. Consider the air handling system for the commercial building illustrated in the Figure, consisting of ducting, two dampers labeled A and B, a vapor-compression dehumidifier, and a heater. The system supplies 40 m ³ /s of conditioned air at 20⁰C and a relative humidity of 45% to maintain the interior space at 25⁰C and a relative humidity of 50%. The recirculated air has the same conditions as the air in the interior space. A minimum of 8 m ³ /s of outdoor air is required to provide adequate ventilation. Dampers A and B can be set to provide alternative operating modes to maintain required ventilation rates. On a given summer day when the outside air dry-bulb temperature and relative humidity are 32⁰C and 80%, respectively, which of the following three operating modes is best from the standpoint of minimizing the total heat transfer from the conditioned air to the cooling coil and to the conditioned air from the heating coil?

1. Dampers A and B closed.
2. Damper A open and damper B closed with outside air contributing one-quarter of the total supply air.
3. Dampers A and B open. One-quarter of the conditioned air comes from outside air and one-third of the recirculated air bypasses the dehumidifier via open damper B, the rest flows through damper A.

Outside air -Damper A Cooling coil - Condensate Damper B Heating coil www. Recirculated air Supply air, Interior air-conditioned space at 25 C, 0-50%