Trane RT-DS-10 - Manual (Page 13)

13
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, non-
modulating exhaust fan with
approximately half the air-moving
capabilities of the supply fan system.
The experience of The Trane Company
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 lbs/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 17-1 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 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 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 17-1, 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 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.
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 17-3 before calculating the
heating supply air temperature.