Selection Procedure; Determine Cooling And Heating Loads; Make An Initial Guess At Cooling Tons; Select Factory Options - Bryant PREFERRED 581J Product Data

SELECTION PROCEDURE (WITH 581J*07A EXAMPLE)
I.

Determine cooling and heating loads.

Given:
Mixed air dry bulb
Mixed air wet bulb
Ambient dry bulb
TC
Load
SHC
Load
Vertical supply air
Heating load
External static pressure
Electrical characteristics
II.

Make an initial guess at cooling tons.

Refrig. tons = TC
Load
Refrig. tons = 72.0 / 12 = 6.0 tons
In this case, start by looking at the 581J**07.
III. Look up the rooftop's TC and SHC.
Table 15 shows that, at the application's supply air
CFM, mixed air and ambient temperatures, the
581J*A07 supplies:
2
TC = 73.6 MBH
2
SHC = 53.3 MBH
IV. Calculate the building latent heat load.
LC = TC - - SHC
Load Load
LC = 72.0 MBH - - 54.0 MBH = 18.0 MBH
Load
V. Calculate RTU latent heat capacity.
LC = TC - - SHC
LC = 73.6 MBH - - 53.3 MBH = 20.3 MBH
VI. Compare RTU capacities to loads.
Compare the rooftop's SHC and LC to the
building's sensible and latent heat loads.
LEGEND
BHP — Break 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
80_F (27_C)
67_F (19_C)
95_F (35_C)
73.6 MBH
53.3 MBH
2100 CFM
85.0 MBH
0.67 in. wg
230- - 3- - 60
/ 12 MBH per ton
Load
3
VII. Select factory options (FIOP)
Local code requires an economizer for any unit with
TC greater 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 36 shows, at 2100 CFM & ESP= 0.8,
RPM = 712 & BHP = 1.17
X. Convert BHP (Step VIII) into fan motor heat.
Fan motor heat = 2.546* BHP/Motor Eff.
Fan motor heat = 3.7 MBH
XI. Calculate RTU heating capacity.
Building heating load
Fan motor heat
Required heating capacity
XII. Select a gas heater.
Table 4 shows the heating capacities of the
581JEA07 = 103.0 MBH. Select the 581JEA07
XIII. Determine electrical requirements.
Table 45 shows the MCA and Breaker Size of a
581J*A07 (without convenience outlet) as:
MCA = 32.0 amps & MOCP = 50.0 amps
Min. disconnect size: FLA = 31.0 & LRA = 148.
NOTES:
1. Selection software by Bryant saves time by performing
many of the steps above. Contact your Bryant sales
representative for assistance.
2. Unit ratings are gross capacities and do not include the
effect of evaporator fan motor heat. See Step XI. for de-
termining amount of evaporator fan motor heat to sub-
tract from total and sensible capacities to obtain net
cooling and net sensible capacities.
3. 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.
4. Indoor fan motor efficiency is available in Table 41. Use
the decimal form in the equation, eg. 80% = .8.
25
1
0.67 in. wg
+0.13 in. wg
0.80 in. wg
4
85.0 MBH
- - 3.7 MBH
81.3 MBH