11
RT-PRC007-EN
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
Application Recommendations
Power Exhaust Fan
The exhaust fan option is a dual, non-
modulating exhaust fan with
approximately half the air-moving
capabilities of the supply fan system. The
experience of Trane 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 non-modulating 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 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 fan may be sized for more than
50 percent of design supply airflow.
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 lb/ft) (.034 kg/cm). 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 PD-1 shows the air density ratio at
various temperatures and elevations.
Trane rooftops are designed to operate
between 40 and 90°F (4.4 and 32.2°C)
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 PD-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 29-ton (105 kW) rooftop unit
that is to deliver 9,160 actual cfm (4323 L/
s) at 1.50 inches total static pressure (tsp)
(38 mm, 373 Pa), 55°F (12.8°C) leaving air
temperature, at an elevation of 5,000 ft
(1524 m).
1
From Figure PD-1, the air density ratio is
0.86.
2
Tsp = 1.50 inches/0.86 = 1.74 inches tsp.
374/.86 = 434 Pa.
3
From the performance tables: a 29-ton
(105 kW) rooftop will deliver 9,160 cfm at
1.74 inches tsp 4323 L/s at 434 Pa) at 651
rpm and 5.51 bhp (4.11 kW).
4
The rpm is correct as selected –
651 rpm.
5
Bhp = 5.51 x 0.86 = 4.74 bhp actual.
kW = 4.11 x 0.86 = 3.5 kW
Compressor MBh, SHR, and kW should
be calculated at standard and then
converted to actual using the correction
factors in Table PD-2. Apply these factors
to the capacities selected at standard cfm
so as to correct for the reduced mass flow
rate across the condenser.
Heat selections other than gas heat will
not be affected by altitude. Nominal gas
capacity (output) should be multiplied by
the factors given in Table PD-3 before
calculating the heating supply air
temperature.
50 Hz
Application
Considerations
