Packaged Rooftop Air Conditioners - RT-DS-9 - Page 912

Table 13-1 — Economizer Outdoor Air Damper Leakage (Of Rated Airflow)

Note: Above data based on tests completed in accordance with AMCA Standard 575.

Application Considerations

Exhaust Air 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  under-pressurization of the building.

A building may have all or part of its exhaust system in the rooftop unit. Often, a building provides exhaust external to the air conditioning equipment. This external exhaust must be considered when selecting the rooftop exhaust system.

Voyager Commercial rooftop units offer two types of exhaust systems:

1

Power exhaust fan.

2

Barometric relief dampers.

Power Exhaust Fan

The exhaust fan option is a dual, nonmodulating exhaust fan with approximately half the air-moving capabilities of the supply fan system. The experience of The Trane Company is that a non-modulating exhaust fan selected for 40 to 50 percent of nominal supply cfm can be applied successfully.

The power exhaust fan generally should not be selected for more than 40 to 50 percent of design supply airflow. Since it is an on/off nonmodulating fan, 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 underpressurized 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 fan may be sized for more than 50 percent of design supply airflow. Consult Table 25-2 for specific exhaust fan capabilities with Voyager Commercial units.

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.

Altitude Corrections

The rooftop performance tables and curves of this catalog are based on standard air (.075 lbs/ft). If the rooftop airflow requirements are at other than standard conditions (sea level), an air density correction is needed to project accurate unit performance.

Figure 17-1 shows the air density ratio at various temperatures and elevations. Trane rooftops are designed to operate between 40 and 90 degrees Fahrenheit leaving air temperature.

The procedure to use when selecting a supply or exhaust fan on a rooftop for elevations and temperatures other than standard is as follows:

1

First, determine the air density ratio using Figure 17-1.

2

Divide the static pressure at the nonstandard condition by the air density ratio to obtain the corrected static pressure.

3

Use the actual cfm and the corrected static pressure to determine the fan rpm and bhp from the rooftop performance tables or curves.

4

The fan rpm is correct as selected.

5

Bhp must be multiplied by the air density ratio to obtain the actual operating bhp. In order to better illustrate this procedure, the following example is used:

Consider a 30-ton rooftop unit that is to deliver 11,000 actual cfm at 1.50 inches total static pressure (tsp), 55 F leaving air temperature, at an elevation of 5,000 ft.

1

From Figure 17-1, the air density ratio is 0.86.

2

Tsp=1.50 inches/0.86=1.74 inches tsp.

3

From the performance tables: a 30-ton rooftop will deliver 11,000 cfm at 1.74 inches tsp at 668 rpm and 6.93 bhp.

4

The rpm is correct as selected — 668 rpm.

5

Bhp = 6.93 x 0.86 = 5.96 .

Compressor MBh, SHR, and kw should be calculated at standard and then converted to actual using the correction factors in Table 17-2. Apply these factors to the capacities selected at standard cfm so as to correct for the reduced mass flow rate across the condenser.