Bryant manual Selection Procedure with 558J*07 Example, Determine cooling and heating loads

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)
THCLoad	69.0 MBH
SHCLoad	51.0 MBH
Supply Air	2100 CFM
Heating 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	=	THCLoad / 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.

Table 11 shows that, at the application’s supply air CFM, mixed air and ambient temperatures, the

558J*07 supplies: THCLoad = 73.7 MBH SHCLoad = 54.3 MBH.

IV. Calculate the building Latent Heat Load.

LHCLoad = THCLoad - SHCLoad

LHCLoad = 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.

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.3 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.

558J

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

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 SHCLoad is often 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.

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