25
SELECTION PROCEDURE (WITH 548J*07 EXAMPLE)
(Selection software by Bryant saves time by performing many of the steps below.)
I. Determine cooling and heating loads.
Given:
Mixed Air Drybulb 80_F(27_C)
Mixed Air We tbulb 67_F(19_C)
Ambient Drybulb 95_F(35_C)
TC
Load
65.0 MBH
SHC
Load
46.0 MBH
HC
Load
45.0 MBH
Outdoor-Air Winter Design Temp 0_F(--18_C)
Indoor-Air Winter Design Temp 70_F(21_C)
Vertical Supply Air 2100 CFM
External Static Pressure 0.66 in.wg
Electrical Characteristics 230--3--60
II. Make an initial guess at cooling tons.
Refrig. tons = TC
Load
/ 12 MBH per ton
Refrig. tons = 65.0 / 12 = 5.42 tons
In this case, start by looking at the 548J*07.
III. Look up the rooftop’s TC and SHC.
Ta ble 11 shows that, at the applica tion’s supply air
CFM, mixed air and ambient temperatures, the
548J*07 supplies:
TC
Load
= 69.0 MBH
SHC
Load
= 50.7 MBH.
IV. Calculate the building Latent Heat Load.
LC
Load
=TC
Load
-- S H C
Load
LC
Load
= 65.0 MBH -- 46.0 MBH = 19.0 MBH
V. Select electric heat.
Enter the Instantaneous and Integrated Heating Rat-
ings, Table 17 at 2100 cfm. At 70_ F(21_C) return
indoor air and 0°F(--18_C) air entering outdoor coil,
the integrated heating capacity after interpolation, is
24,300 Btuh. (Select integrated heating capacity
value since deductions for outdoor-coil frost and de-
frosting have already been made. No correction is re-
quired.)
The required heating capacity is 45,000 Btuh. There -
fore, 20,700 Btuh (45,000 – 24,300) additional elec-
tric heat is required.
Determine additional electric heat capacity in kW.
20,700 Btuh
=6.1kWofheatrequired.
3413 Btuh/kW
Enter the Electric Heating Capacities table for
548J*07 at 208/230, 3--phase. The 6.5-kW heater at
230V most closely sati sfies the heating required.
6.5 kW x 3413 = 22,185 Btuh
Total unit heating capacity is 46,485 Btuh (22,185 +
24,300).
VI. Calculate RTU Latent Heat Capacity
LC = TC -- SHC
LC = 69.0 MBH -- 50.7 MBH = 18.3 MBH
VII. Compare RTU capacities to loads.
Compare the rooftop’s SHC and LC to the building’s
Sensible and Latent Heat Loads.
See Notes 1 and 2.
VIII.Select factory options (FIOP)
Local code requires an economizer for any unit with
TC larger than 65.0 MBH.
IX. Calculate the total static pressure.
External static pressure 0.66 in. wg
Sum of FIOP/Accessory static +0.14 in.
wg
Total Static Pressure 0.80 in. wg
X. Look up the Indoor Fan RPM & BHP.
Ta ble 34 shows, at 2100 CFM & ESP= 0.8,
RPM = 1268 & BHP = 1.52
XI. Determine electrical requirements
Ta ble 25 shows the MCA and MOCP of a 548J*07
(without convenience outle t) with 6.5 kW ele ctric
heater as:
MCA = 52.3 amps & MOCP = 60 amps
Min. Disconnect Size: FLA = 50 & LRA = 199.
Legend
BHP — Break horsepower
FLA — Full load amps
HC — Heating Capacities
LC — Latent capacity
LRA — Lock rotor amp
MBH — (1,000) BTUH
MCA — Min. circuit ampacity
MOCP — Max. over---current protection
RPM — Revolutions per minute
RTU — Rooftop unit
SHC — Sensible heat capacity
TC— Total capacity
NOTES:
1. Selecting a unit with a SHC slightly lower than the SHC
Load
is
often better than oversizing. Slightly lower SHC’s will help co n-
trol indoor humidity, and prevent temperature swings.
2. If the rooftop’s capacity meets the Sensible Heat Load, but not
the Latent Heat Load.
548J
