Basic Air-Handling Tasks

Once the airflow requirements are determined, the air handling units can be configured based on the tasks that they must perform. These tasks are categorized into three basic functions: mix and recirculate air to the occupied space, bring in outdoor air and treat it, and expel air from the building.

Mix and Recirculate Air to the Occupied Space

Generically termed standard air handling units (Figure A-2), these air handlers collect air from the occupied space, mix it with outdoor air, treat it and discharge it into the supply air stream.

Bring In Outdoor Air and Treat It

An air handler dedicated to treating outdoor air is referred to as an outdoor air unit, but may also be called a makeup air unit in instances where outdoor air is brought in to replace air exhausted locally from lavatories, kitchens, etc. See Figure A-2. Outdoor air units either discharge air directly into the occupied space, or serve as a "preconditioner" for other air handling units downstream. For example, a "preconditioning" outdoor air unit might pretreat outdoor air before it reaches other standard air handlers, effectively removing the outdoor air load from the standard air handlers. An outdoor air unit with recovery, generically termed an "energy recovery unit" (Figure A-2) not only draws in outdoor air, but also exhausts air from the building and contains a device that transfers energy between the two air streams. Such a design can be used to precondition (dehumidify) large percentages of humid outdoor air with low sensible heat ratios (SHR’s), and allows packaged units, standard air handling units and terminal units to satisfy space loads with proper humidity control.

Applications with clean outdoor air and low to medium humidity levels may be effectively served by a standard air handling unit.

Expel Air from the Building An exhaust unit is an air handler that does nothing more than collect air from the occupied space and discharge it outside the building. Remember: to balance building pressure, any outdoor airflow entering the building must approximate the exhaust airflow leaving the building.

Maintain Proper Building Pressurization

An important aspect of establishing outdoor air requirements is equalizing outdoor air and exhaust air volumes to maintain proper building pressurization. "Building pressurization" describes an air-handling strategy that regulates pressure differences across the building envelope and between zones or rooms by adjusting the amount of air supplied and removed. The goal of this strategy is to:

• Assure proper distribution of conditioned and ventilation air throughout the occupied space.
  
• Avoid discomfort due to temperature stratification and drafts.
  
• Prevent infiltration of unconditioned air.
  
• Confine odors and contaminants to specific areas within the building.

Building envelope pressurization is typically achieved by incorporating either an exhaust fan and economizer or a return fan and economizer in the air handler’s design. A brief summary of their respective operating characteristics and application considerations follows. Careful analysis is required to determine which approach best suits each application’s unique requirements. To better understand the differences between exhaust-fan and return-fan systems, consult your local Trane sales engineer or refer to Building Pressurization Control (Trane publication number AM- CON-17).