Bryant COMMERCIAL SINGLE PACKAGE ROOFTOP GAS HEAT/ELECTRIC COOLING 581B - Manual (Page 17)

17
SELECTION PROCEDURE (With 581B048 Example)
I DETERMINE COOLING AND HEATING REQUIRE-
MENTS AT DESIGN CONDITIONS.
Given:
Required Cooling Capacity (TC) . . . . . . . . . . 44,600 Btuh
Sensible Heat Capacity (SHC). . . . . . . . . . . . 32,000 Btuh
Required Heating Capacity . . . . . . . . . . . . . . 85,000 Btuh
Condenser Entering-Air Temperature . . . . . . . . . . . . 95 F
Evaporator Entering-Air Temperature . . . . . . . . .82 F edb,
67 F ewb
Evaporator Air Quantity . . . . . . . . . . . . . . . . . . . .1600 cfm
External Static Pressure (ESP) . . . . . . . . . . . . 0.75 in. wg
Electrical Characteristics (V-Ph-Hz). . . . . . . . . . . 230-1-60
II SELECT UNIT BASED ON REQUIRED COOLING
CAPACITY.
Enter the Cooling Capacities table (page 18) at condenser
entering temperature of 95 F, evaporator-air entering at
1600 cfm and 80 F db and 67 F wb. The 581B--048115
unit will provide cooling capacity of 48,900 Btuh and a
sensible heat capacity of 34,700 Btuh. For evaporator-air
temperature other than 80 F edb, calculate sensible heat
capacity correction, as required, using the formula found in
Note 3 following the Cooling Capacities tables.
For this example:
Correction factor = 1.10 x (1 – .21) x (82 – 80) = 1.738
Multiply the correction factor of 1.738 by 1600 cfm (a total
of 2781). From the Gross Cooling Capacities tables find
that the sensible heat capacity at 80 F is 34.7 MBtuh
(equivalent to 34,700 Btuh). Add 34,700 and 2781 to get
the corrected sensible heat capacity of 37,481.
NOTE: Unit ratings are gross capacities and do not
include the effect of evaporator-fan motor heat. To calcu-
late net capacities, see Step V.
III SELECT HEATING CAPACITY OF UNIT TO PROVIDE
DESIGN CONDITION REQUIREMENTS.
In the Heating Capacities and Efficiencies table (page 5)
note that unit 581B--048115 will provide output capacity of
92,000 Btuh, which is adequate for the given application.
IV DETERMINE FAN SPEED AND POWER REQUIRE-
MENTS AT DESIGN CONDITIONS.
Before entering the Fan Performance tables, calculate the
total static pressure required based on unit components.
From the given and the Pressure Drop tables (page 40),
find:
External static pressure .75 in. wg
Durablade economizer .05 in. wg
Total static pressure = .80 in. wg
Enter the Fan Performance table for 581B--048115 vertical
discharge unit on page 27. At 1600 cfm, the standard
motor will deliver 1.1 in. wg static pressure and 1.17 Bhp.
This will adequately handle the job requirements.
Using evaporator-fan motor efficiency table and formula on
page 40, determine watts.
V DETERMINE NET COOLING CAPACITY.
Cooling capacities are gross and do not include indoor-fan
motor (IFM) heat. Determine net capacity using the follow-
ing formula:
Watts =
1.17 (746)
.75
Watts = 1164
Net capacity = gross capacity – IFM heat
= 48,900 Btuh – 1164 Watts
(3.413
Btuh
)
Watts
= 48,900 Btuh – 3972 Btuh
Net capacity = 44,928 Btuh
Net sensible capacity = 37,481 Btuh – 3972 Btuh
= 33,509 Btuh