Applications Considerations
Exhaust/Return
Fan Options
When is it necessary to provide building exhaust? Whenever an outdoor air economizer is used, a building generally requires an exhaust system. The purpose of the exhaust system is to exhaust the proper amount of air to prevent over or
The goal is to exhaust approximately 10 percent less air than the amount of outside air going into the building. This maintains a slightly positive building pressure.
The reason for applying either a return, or exhaust fan is to control building pressure. The Trane 100 percent modulating exhaust system with Statitrac is an excellent choice for controlling building pressure in the majority of applications.
For more demanding applications, Trane's 100 percent modulating return fan system with Statitrac is an excellent choice for systems with high return static pressure losses, or duct returns. Both systems employ direct digital control technology to maintain building pressure. Either return or exhaust fan systems with Statitrac may be used on any air handler application that has an outdoor air economizer.
A building may have all or part of its exhaust system in the air handler unit. Often, a building provides exhaust external to the air handling equipment. This external exhaust must be considered when selecting the air handler exhaust system.
With an exhaust fan system, the supply fan motor and drives must be sized to overcome the total system static pressure, including return losses, and pull return air back to the unit during
However, a supply fan can typically overcome return duct losses more efficiently than a return air fan system.
Essentially, one large fan by itself is normally more efficient than two fans in series because of only one drive loss, not two as with return fan systems.
In a return fan system, the return fan is in series with the supply fan, and operates continuously whenever the supply fan is operating to maintain return air volume. The supply fan motor and drives are sized to deliver the design CFM based on internal and discharge static pressure losses only.
The return fan motor and drives are sized to pull the return CFM back to the unit based on return duct static. Therefore, with a return fan system, the supply fan ordinarily requires less horsepower than a system with an exhaust fan
IntelliPak™ II Rooftop Air
Handler Unit Offers Four
Types of Exhaust/Return Fan
Systems:
1
100 percent modulating exhaust with Statitrac™ direct space sensing building pressurization control (with or without exhaust variable frequency drives)
2
100 percent modulating exhaust without Statitrac
3
100 percent modulating plenum return airfoil fan with Statitrac direct space sensing building pressurization control with variable frequency drive
4
100 percent modulating plenum return airfoil fan without Statitrac
Drivers for applying either return or exhaust fan systems range from economy, to building pressure control, to code requirements, to generally accepted engineering practices
Application
Recommendations
100 Percent Modulating Exhaust with Statitrac Control, Constant Volume and VAV Units
For both CV and VAV air handlers, the 100 percent modulating exhaust discharge dampers (or VFD) are modulated in response to building pressure. A differential pressure control system, Statitrac, uses a differential pressure transducer to compare indoor building pressure to atmospheric pressure.
The FC exhaust fan is turned on when required to lower building static pressure to setpoint. The Statitrac control system then modulates the discharge dampers (or VFD) to control the building pressure to within the adjustable, specified deadband that is set at the Human Interface Panel.
Economizer and return air dampers are modulated independent of the exhaust dampers (or VFD) based on ventilation control and economizer cooling requests.
Advantages:
•The exhaust fan runs only when needed to lower building static pressure.
•Statitrac compensates for pressure variations within the building from remote exhaust fans and makeup air units.
•The exhaust fan discharges in a single direction resulting in more efficient fan operation compared to return fan systems.
•When discharge dampers are utilized to modulate the exhaust airflow, the exhaust fan may be running unloaded whenever the economizer dampers are less than 100 percent open.
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