| Application Considerations
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). |
