Bryant LEGACY 558J Product Data page 19

SELECTION PROCEDURE (WITH 558J*07 EXAMPLE)
I. Determine cooling and heating loads.
Given:
Mixed Air Drybulb
Mixed Air Wetbulb
Ambient Drybulb
THC
Load
SHC
Load
Supply Air
Heating Load
External Static Pressure
Electrical Characteristics
II. Make an initial guess at cooling tons.
Refrig. tons = THC
Refrig. tons = 69.0 / 12 = 5.75 tons
In this case, start by looking at the 558J*07.
III. Look up the rooftop's THC and SHC.
Table 11 shows that, at the application's supply air
CFM, mixed air and ambient temperatures, the
558J*07 supplies:
THC = 73.7 MBH
Load
SHC = 54.3 MBH.
Load
IV. Calculate the building Latent Heat Load.
LHC = THC - - SHC
Load Load
LHC = 692.0 MBH - - 51.0 MBH = 18.0 MBH
Load
LEGEND
THC — Total Heat Capacity
SHC — Sensible Heat Capacity
LHC
— Latent Heat Capacity
RPM
— Revolutions Per Minute
RTU — Rooftop Unit
BHP — Break Horsepower
MCA — Min. Circuit Ampacity
MOCP — Max. Over ---current Protection
FLA — Full Load Amps
RLA — Rated Load Amps
80_F (27_C)
67_F (19_C)
95_F (35_C)
69.0 MBH
51.0 MBH
2100 CFM
85.0 MBH
0.67 in.wg
230- - 3- - 60
/ 12 MBH per ton
Load
Load
V. Calculate RTU Latent Heat Capacity
LHC = THC - - SHC
LHC = 73.7 MBH - - 54.3 MBH = 19.4 MBH
VI. Compare RTU capacities to loads.
Compare the rooftop's SHC and LHC to the
building's Sensible and Latent Heat Loads.
VII. Select factory options (FIOP)
Local code requires an economizer for any unit with
THC less than 65.0 MBH.
VIII. Calculate the total static pressure.
External static pressure
Sum of FIOP/Accessory static
Total Static Pressure
IX. Look up the Indoor Fan RPM & BHP.
Table 26 shows, at 2100 CFM & ESP= 0.8,
RPM = 1268 & BHP = 1.52
X. Convert BHP (Step VIII) into Fan Motor Heat.
Fan Motor Heat = 2.546* BHP/Motor Eff.
Fan Motor Heat = 4.8 MBH
XI. Determine electrical requirements
Table 32 shows the MCA and MOCP of a 558J*07
(without convenience outlet) as:
MCA = 32.8 amps & MOCP = 50 amps
Min. Disconnect Size: FLA = 32 & LRA = 72.
NOTES:
1. Bryant's selection software saves time by performing
many of the steps above. Contact your Bryant sales rep-
resentative for assistance.
2. Selecting a unit with a SHC slightly lower than the
SHC is often better than oversizing. Slightly lower
Load
SHC's will help control indoor humidity, and prevent
temperature swings.
3. Indoor Fan Motor efficiency is available in Table 13---26.
Use the decimal form in the equation eg. 80% =.8.
19
2
0.67 in. wg
+0.13 in. wg
0.80 in. wg
3