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19
SELECTION PROCEDURE (WITH 558J*07 EXAMPLE)
I. Determine cooling and heating loads.
Given:
Mixed Air Drybulb 80_F(27_C)
Mixed Air Wetbulb 67_F(19_C)
Ambient Drybulb 95_F(35_C)
THC
Load
69.0 MBH
SHC
Load
51.0 MBH
Supply Air 2100 CFM
Heati ng Load 85.0 MBH
External Static Pressure 0.67 in.wg
Electrical Characteristics 230-- 3 --60
II. Make an initial guess at cooling tons.
Refrig. tons = THC
Load
/ 12 MBH per ton
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.
Ta ble 11 shows that, at the application’s supply air
CFM, mixed air and ambient temperatures, the
558J*07 supplies:
THC
Load
= 73.7 MBH
SHC
Load
= 54.3 MBH.
IV. Calculate the building Latent Heat Load.
LHC
Load
=THC
Load
-- S H C
Load
LHC
Load
= 692.0 MBH -- 51.0 MBH = 18.0 MBH
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.
2
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 0.67 in. wg
Sum of FIOP/Accessory static +0.13 in.
wg
Total Static Pressure 0.80 in. wg
IX. Look up the Indoor Fan RPM & BHP.
Ta ble 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.
3
Fan Motor Heat = 4.8 MBH
XI. Determine electrical requirements
Ta ble 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.
LEGEND
THC — Total Heat Capacity
SHC — Sensible Heat Capacity
LHC — Latent Heat Capacity
RPM — Revolutio ns Per Minute
RTU — Rooftop Unit
BHP — Break H o rsepower
MCA — Min. Circuit Ampacity
MOCP — Max. Over---current Protection
FLA —FullLoadAmps
RLA — Rated Load Amps
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
Load
is o ften better than oversizing. Slightly lower
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.
558J