Bryant manual SELECTION PROCEDURE WITH 548J*07 EXAMPLE

SELECTION PROCEDURE (WITH 548J*07 EXAMPLE)

(Selection software by Bryant saves time by performing many of the steps below.)

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)
TCLoad	65.0 MBH
SHCLoad	46.0 MBH
HCLoad	45.0 MBH
Outdoor-Air Winter Design Temp	0_F (-18_C)
Indoor-Air Winter Design Temp	70_F (21_C)
Vertical Supply Air	2100 CFM
External Static Pressure	0.66 in.wg
Electrical Characteristics	230-3-60

II.Make an initial guess at cooling tons.

Refrig. tons = TCLoad / 12 MBH per ton Refrig. tons = 65.0 / 12 = 5.42 tons

In this case, start by looking at the 548J*07.

III. Look up the rooftop’s TC and SHC.

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

548J*07 supplies: TCLoad = 69.0 MBH SHCLoad = 50.7 MBH.

The required heating capacity is 45,000 Btuh. There- fore, 20,700 Btuh (45,000 – 24,300) additional elec- tric heat is required.

Determine additional electric heat capacity in kW.

20,700 Btuh	= 6.1 kW of heat required.
3413 Btuh/kW

Enter the Electric Heating Capacities table for 548J*07 at 208/230, 3-phase. The 6.5-kW heater at 230V most closely satisfies the heating required.

6.5 kW x 3413 = 22,185 Btuh

Total unit heating capacity is 46,485 Btuh (22,185 + 24,300).

VI. Calculate RTU Latent Heat Capacity

LC = TC - SHC

LC = 69.0 MBH - 50.7 MBH = 18.3 MBH

VII. Compare RTU capacities to loads.

Compare the rooftop’s SHC and LC to the building’s Sensible and Latent Heat Loads.

See Notes 1 and 2.

VIII. Select factory options (FIOP)

Local code requires an economizer for any unit with TC larger than 65.0 MBH.

548J

IV. Calculate the building Latent Heat Load.

LCLoad = TCLoad - SHCLoad

LCLoad = 65.0 MBH - 46.0 MBH = 19.0 MBH

V.Select electric heat.

Enter the Instantaneous and Integrated Heating Rat- ings, Table 17 at 2100 cfm. At 70_F (21_C) return indoor air and 0°F (-18_C) air entering outdoor coil, the integrated heating capacity after interpolation, is 24,300 Btuh. (Select integrated heating capacity value since deductions for outdoor-coil frost and de- frosting have already been made. No correction is re- quired.)

IX. Calculate the total static pressure.
External static pressure	0.66 in. wg
Sum of FIOP/Accessory static	+0.14 in. wg
Total Static Pressure	0.80 in. wg

X.Look up the Indoor Fan RPM & BHP.

Table 34 shows, at 2100 CFM & ESP= 0.8, RPM = 1268 & BHP = 1.52

XI. Determine electrical requirements

Table 25 shows the MCA and MOCP of a 548J*07 (without convenience outlet) with 6.5 kW electric heater as:

MCA = 52.3 amps & MOCP = 60 amps

Min. Disconnect Size: FLA = 50 & LRA = 199.

Legend			NOTES:
BHP	—	Break horsepower	1. Selecting a unit with a SHC slightly lower than the SHCLoad is
FLA	—	Full load amps	often better than oversizing. Slightly lower SHC’s will help con-
HC	—	Heating Capacities	trol indoor humidity, and prevent temperature swings.
LC	—	Latent capacity	2. If the rooftop’s capacity meets the Sensible Heat Load, but not
			the Latent Heat Load.
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

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