Trane RT-PRC010-EN manual Application Considerations, 2 100 Percent Exhaust System

Application Considerations

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 ofTheTrane 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.

Figure AC-1 — PlanView of Modulating 100 Percent Exhaust System

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