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13RT-PRC007-EN
Selection
Procedure
The supply air fan motor selected in the
previous cooling capacity determination
example was 7.27 bhp with 652 rpm.
Thus, the supply fan motor selected is 7.5
hp.
To select the drive, enter Table PD-15 for
a 30-ton unit. Select the appropriate
drive for the applicable rpm range. Drive
selection letter C with a range of 650
rpm, is required for 652 rpm. Where
altitude is significantly above sea level,
use Table PD-2 and PD-3, and Figure PD-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
selection of this catalog.
Unit Designation
After determining specific unit
characteristics utilizing the selection
procedure and additional job
information, the complete unit model
number can be developed. Use the
model number nomenclature on page
14.
Heating capacity selection:
1
Winter outdoor design conditions—5 F.
2
Total return air temperature — 72 F.
3
Winter outside air minimum ventilation
load and cfm — 1,200 cfm and 87.2 MBh.
4
Peak heating load 225 MBh.
Utilizing unit selection in the cooling
capacity procedure.
Mixed air temperature = RADB + %O.A.
(OADB - RADB) = 72
+ (0.10) (0-72) = 64.8 F.
Supply air fan motor heat temperature
rise = 20,600 BTU ÷ (1.085 x 12,000) cfm
= 1.6 F.
Mixed air temperature entering heat
module = 64.8 + 1.6 = 66.4 F.
Total winter heating load = peak heating
+ ventilation load - total fan motor heat =
225 + 87.2 - 20.6 = 291.6 MBh.
Electric Heating System
Unit operating on 480/60/3 power supply.
From Table PD-9, kw may be selected for
a nominal 30-ton unit operating on 480-
volt power. The high heat module — 90
KW or 307 MBh will satisfy the winter
heating load of 291.6 MBh.
Table PD-9 also shows an air
temperature rise of 23.6 F for 12,000 cfm
through the 90 kw heat module.
Unit supply temperature at design
heating conditions = mixed air
temperature + air temperature rise = 66.4
+ 23.6 = 90 F.
Natural Gas Heating System
Assume natural gas supply — 1000 Btu/
ft
3
. From Table PD-11, select the high heat
module (486 MBh output) to satisfy 291.6
at unit cfm.
Table PD-11 also shows air temperature
rise of 37.3 F for 12,000 cfm through
heating module.
Unit supply temperature design heating
conditions = mixed air temperature + air
temperature rise = 66.4 + 37.3 = 103.7 F.
Air Delivery Procedure
Supply air fan bhp and rpm selection.
Unit supply air fan performance shown
in Table PD-12 includes pressure drops
for dampers and casing losses. Static
pressure drops of accessory
components such as heating systems,
and filters if used, must be added to
external unit static pressure for total
static pressure determination.
0 5 10 15 20 25 30 35 40
0
10
20
30
40
50
60
70
80
90
100
110
120
STANDARD MOTOR
HIGH EFFICIENCY MOTOR
FAN MOTOR HEAT - MBH
MOTOR BRAKE HORSE POWER
Figure SP-1 — Fan Motor Heat