SELECTION PROCEDURE (WITH 581J*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) |
TCLoad | 73.6 MBH |
SHCLoad | 53.3 MBH |
Vertical supply air | 2100 CFM |
Heating load | 85.0 MBH |
External static pressure | 0.67 in. wg |
Electrical characteristics |
II.Make an initial guess at cooling tons.
Refrig. tons = TCLoad / 12 MBH per ton 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:
TC = 73.6 MBH2 SHC = 53.3 MBH2
IV. Calculate the building latent heat load.
LCLoad = TCLoad - SHCLoad
LCLoad = 72.0 MBH - 54.0 MBH = 18.0 MBH
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.3
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. |
RPM | — Revolutions per minute |
RTU | — Rooftop unit |
SHC | — Sensible heat capacity |
TC | — Total capacity |
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 | 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 = 712 & BHP = 1.17
X.Convert BHP (Step VIII) into fan motor heat.
Fan motor heat = 2.546* BHP/Motor Eff.4
Fan motor heat = 3.7 MBH
XI. Calculate RTU heating capacity. |
|
Building heating load | 85.0 MBH |
Fan motor heat | |
Required heating capacity | 81.3 MBH |
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 SHCLoad is often better than oversizing. Slightly lower 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.
581J
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