Trane RT-PRC010-EN Manual page 18

18
Selection
Procedure
AIR DELIVERY PROCEDURE
Supply fan performance tables include
internal resistance of rooftop. For total
static pressure determination, system
external static must be added to
appropriate component static pressure
drop (evaporator coil, filters, optional
economizer, optional exhaust fan,
optional heating system, optional
cooling only extended casing, optional
roof curb).
Supply Fan Motor Sizing
The supply fan motor selected in the
cooling capacity determination was 15.3
bhp and 924 rpm. Thus, a 20 hp supply
fan motor is selected. Enter Table PD-45
to select the proper drive. For a 50 ton
rooftop with 20 hp motor, a drive
number 9 — 900 rpm is selected.
Exhaust Fan Motor Sizing
The exhaust fan is selected based on
total return system negative static
pressure and exhaust fan cfm. Return
system negative static include return
duct static and roof curb static pressure
drop.
Return duct static pressure = 0.65 inches
Trane roof curb (Table PD-43) = 0.12
inches
Total return system negative static
pressure = 0.77 inches
Exhaust fan cfm = 12,000 cfm
From Table PD-47 , the required bhp is
3.45 hp at 574 rpm. Thus, the exhaust fan
motor selected is 5 hp.
To select a drive, enter Table PD-49 for a 5
hp motor for a 50 ton unit. Drive
selection number 6 — 600 rpm.
Where altitudes are significantly above
sea level, use Tables PAF-2 and PAF-3 and
Figure PAF-1 for applicable correction
factors.
UNIT ELECTRICAL REQUIREMENTS
Selection procedures for electrical
requirements for wire sizing amps,
maximum fuse sizing, and dual element
fuses are given in the electrical service
section of this catalog.
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 PAF-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 PAF-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.
RT -PRC010-EN