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SELECTION PROCEDURE (WITH 580J*07A EXAMPLE)
1
I. Determine cooling and heating loads.
Given:
Mixed air dry bulb 80_F(27_C)
Mixed air wet bulb 67_F(19_C)
Ambient dry bulb 95_F(35_C)
TC
Load
72.0 MBH
SHC
Load
54.0 MBH
Vertical 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 = TC
Load
/ 12 MBH per ton
Refrig. tons = 72.0 / 12 = 6.0 tons
In this case, start by looking at the 580J**07.
III. Look up the rooftop’s TC and SHC.
Table 15 shows that, at the a pplication’s supply air
CFM, mixed air and ambient temperatures, the
580J*07A supplies:
TC = 73.7 MBH
2
SHC = 54.4 MBH
2
IV. Calculate the building latent heat load.
LC
Load
=TC
Load
-- S H C
Load
LC
Load
= 72.0 MBH -- 54.0 MBH = 18.0 MBH
V. Calculate RTU latent heat capacity .
LC = TC -- SHC
LC = 73.7 MBH -- 54.4 MBH = 19.3 MBH
VI. Compare RTU capacities to loads.
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Compare the rooftop’s SHC and LC to the
building’s sensible and latent heat loads.
VII. Select factory options (FIOP)
Local code requires an economize r for any unit with
TC greater 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.
Table 36 shows, at 2100 CFM & ESP= 0.8,
RPM = 1358 & BHP = 1.52
X. Convert B HP ( Step VIII) into fan motor heat.
Fan motor heat = 2.546* BHP/Motor Eff.
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Fan motor heat = 4.9 MBH
XI. Calculate RTU heating capacity.
Building heating load 85.0 MBH
Fan motor heat --4.9
MBH
Required heating capacity 80.1 MBH
XII. Select a gas heater.
Table 4 shows the heating capacities of the
580JE07A = 93.0 MBH. Sel ect the 580JE07A
XIII.Determine electrical requirements.
Table 56 shows the MCA and MOCP of a 580J*07A
(without convenience outlet) as:
MCA = 30.5 amps & MOCP = 45.0 amps
Min. disconnect size: FL A = 30 & LRA = 157.
LEGEND
BHP — Brake horsepower
FLA — Full load amps
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. Selection software by Bryant saves time by perfor ming
many of the steps above. Contact your Bryant sales
representative for assistance.
2. Unit rati ngs are gross capacities and do not include the
effect of e vaporator fan m otor heat. S ee Step X. for de-
termining a mount of evaporator fan m otor heat to sub-
tract from total and sensible capacities to obtain net
coo ling and net sensible capacities.
3. Selecting a u nit with a SHC slightly lower than the
SHC
Load
is often better than oversizing. Slightly lower
SHC’s will help control indoor humidity, and prevent
temperature swings.
4. Indoor fan m otor efficiency i s available in Table 45. Use
the decimal form in the equation, eg. 80% = .8.
580J