Bryant 581A - Manual (Page 11)

SELECTION PROCEDURE (with 581A240 270 example)
I DETERMINE COOLING AND HEATING REQUIRE-
MENTS AT DESIGN CONDITIONS:
Given:
Total Cooling Capacity Required . . . . . . . . 205,000 Btuh
Sensible Heat Capacity . . . . . . . . . . . . . . . 162,000 Btuh
Required Heating Capacity . . . . . . . . . . . . . 190,000 Btuh
Condenser Entering Air Temperature . . . . . . . . . . . . 95 F
Evaporator Entering Air Temperature . . . . . . . . 80 F Edb/
67 F Ewb
Evaporator Air Quantity . . . . . . . . . . . . . . . . . . . 8000 cfm
External Static Pressure . . . . . . . . . . . . . . . . . 1.30 in. wg
Power Supply (V-Ph-Hz) . . . . . . . . . . . . . . . . . . 460-3-60
edb — Entering dry bulb
ewb — Entering wet bulb
II DETERMINE UNIT SIZE:
Select unit based on cooling requirements.
Enter Gross Cooling Capacities tables on pages 12-14 at
condenser entering temperature of 95 F, evaporator air en-
tering at 8000 cfm and 80 F db and 67 F wb. The 581A240
270 unit will provide a gross cooling capacity of
254,000 Btuh and a sensible heat capacity of 193,800
Btuh. For evaporator air temperatures other than 80 F, cal-
culate sensible heat capacity correction, using the formula
found in Note 3 following the Gross Cooling Capacities
tables.
NOTE: Unit ratings are gross capacities and do not include
the effect of evaporator-fan motor heat. To calculate net
capacities, see Step V.
III SELECT HEATING CAPACITY OF UNIT TO PROVIDE
DESIGN CONDITION REQUIREMENTS:
In the Heating Capacities table on page 4, note that unit
581A240 270 will provide output capacity of 216,000 Btuh,
which is adequate for the given application.
IV DETERMINE FAN SPEED AND POWER REQUIRE-
MENTS AT DESIGN CONDITIONS:
Before entering the Air Delivery tables on pages 15-17,
calculate the total static pressure required based on unit
components. From the given and the Pressure Drop table
on page 18, find:
External static pressure 1.30 in. wg
Economizer 0.10 in. wg
Total Static Pressure 1.40 in. wg
Enter the Air Delivery table for standard 581A240 270
460-v unit at 8000 cfm and 1.40 in. wg external static pres-
sure. Find that the rpm is 1409 and the bhp is 6.84. The
factory-installed standard motor will suffice.
V DETERMINE NET COOLING CAPACITY:
Cooling capacities are gross capacities and do not include
indoor (evaporator) fan motor (IFM) heat. Use the watts in-
put power to the motor from the Air Delivery table.
IFM Watts = 6297
Determine net capacity using the following formula:
Net capacity = gross capacity – IFM heat
Btuh
= 254,000 Btuh – 6297 Watts (3.412 )
Watt
= 254,000 Btuh – 21,485 Btuh
Net capacity = 232,515 Btuh
Net sensible capacity = 193,800 – 21,485 Btuh
Net sensible capacity = 172,315 Btuh
The 581A240 270 is the correct selection for the given
conditions.
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