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RT-PRC007-EN10
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.
Application Recommendations
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 PD-16 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 PD-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 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 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 PD-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 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.