Application Considerations
Figure 12-1 — Plan View of Modulating 100 Percent
Exhaust System
2 .
100 Percent Exhaust System
Competitive rooftops use a return air fan system for
controlling the amount of exhaust air during economizer operation. The return fan is in
series with the supply fan and must operate whenever the supply fan is operating. During
economizer operation, the economizer outdoor air dampers control the position of the
return and exhaust air dampers, to exhaust the proper amount of air. The
disadvantage of a return air fan is that it runs continuously, versus an exhaust fan
system which runs only when needed to lower or maintain building static pressure. Also,
the return fan must discharge air in two directions, through the return air dampers and/or
exhaust air dampers, resulting in less efficient operation compared to an exhaust fan.
The IntelliPak® Rooftop unit offers modulating 100 percent
exhaust system. This fan system has performance capabilities equal to the supply fan. The
FC exhaust fans are started by the economizer’s outdoor air damper position and the
exhaust dampers track the economizer outdoor air damper position. The amount of air
exhausted by this fan is controlled by modulating discharge dampers at the fan outlet. The
discharge damper position is controlled by a signal that varies with the position of the
economizer dampers. When the exhaust fans start, the modulating discharge dampers are
fully closed, and exhaust airflow is 15 to 20 percent of total exhaust capabilities.
3.
50 Percent Exhaust System
The 50 percent exhaust system is a single FC exhaust fan
with half the air-moving capabilities of the supply fan system. The experience of The
Trane Company is that a non-modulating exhaust system selected for 40 to 50 percent of
nominal supply CFM can be applied successfully.
The 50 percent exhaust system generally should not be
selected for more than 40 to 50 percent of design supply airflow. Since it is an on/off
nonmodulating system, it does not vary exhaust CFM with the amount of outside air entering
the building. Therefore, if selected for more than 40 to 50 percent of supply airflow, the
building may become under-pressurized when economizer operation is allowing lesser amounts
of outdoor air into the building. If, however, building pressure is not of a critical
nature, the non-modulating exhaust system may be sized for more than 50 percent of design
supply airflow.
4.
Barometric Relief Dampers
Barometric relief dampers consist of gravity dampers which
open with increased building pressure. As the building pressure increases, the pressure in
the unit return section also increases, opening the dampers and relieving air. Barometric
relief may be used to provide relief for single story buildings with no return ductwork
and exhaust requirements less than 25 percent. |
