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SELECTION PROCEDURE
I DETERMINE COOLING AND HEATING REQUIRE-
MENTS AT DESIGN CONDITIONS.
Given:
Required Gross Cooling
Capacity (TC) . . . . . . . . . . . . . . . . . . . . . . 168,000 Btuh
Gross Sensible Heat
Capacity (SHC) . . . . . . . . . . . . . . . . . . . . 121,000 Btuh
Required Heating Capacity . . . . . . . . . . . . . 110,000 Btuh
Condenser Entering Air Temperature . . . . . . . . . . . . 95 F
Indoor Air Temperature . . . . . . . . . . . .80 F edb, 67 F ewb
Evaporator Air Quantity . . . . . . . . . . . . . . . . . . . 6,000 cfm
External Static Pressure . . . . . . . . . . . . . . . . . 1.45 in. wg
Electrical Characteristics (V-Ph-Hz). . . . . . . . . . . 230-3-60
II SELECT UNIT BASED ON REQUIRED COOLING
CAPACITY.
Enter Cooling Capacities table for 551A180 at condenser
entering temperature of 95 F, evaporator air quantity of
6,000 cfm, and indoor air temperature of 67 F. The
551A180 unit will provide a total cooling capacity of
190,000 Btuh and a SHC of 136,000 Btuh. For evaporator
air temperatures other than 80 F, calculate SHC correction
using formula in notes under cooling capacity tables.
Unit meets design conditions for TC and SHC.
NOTE:
Unit ratings are gross capacities and do not
include the effect of indoor fan motor heat. To calculate net
capacities, see Step V.
III SELECT ELECTRIC HEAT.
Heating load required is 110,000 Btuh.
Enter the Electric Resistance Heater Data table, on
page 150, for the 551A180 unit at 230-3-60. The 56 kW
(52 kW at 230 v) heater most closely satisfies the heat
required.
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.
Tabulated fan performance includes filters and wet evapo-
rator coil. Refer to Accessory/FIOP Static Pressure table
for added pressure drops.
Enter the Accessory/FIOP Static Pressure — 551A155-
180 Units table at the selected unit size, cfm, and kW
heater size to determine heater pressure drop.
Calculate pressure drop:
Design External Static Pressure 1.45
56 kW Heater Static Pressure +0.12
1.57 in. wg (ESP)
Enter Fan Performance tables for 551A180 at 6,000 cfm
and 1.57 in. wg. The rpm is 1184 and the Bhp is
4.43 (interpolation required). The factory-installed 5 hp
motor and alternate high-static drive are sufficient for this
operation.
V DETERMINE NET COOLING CAPACITIES.
Cooling capacities are gross capacities and do not include
indoor (evaporator) fan motor (IFM) heat. To determine
input power to the motor, enter the Fan Performance
tables for 551A180 at 6,000 cfm and 1.57 in. wg. Input
watts to the motor are 3772 (interpolation required).
Determine net cooling capacity and net sensible cooling
capacity using the following formulas:
IFM Heat = Input Watts x 3.412 Btuh/Watt
= 3772 x 3.412
= 12,870 Btuh
Net Capacity = Gross Capacity – IFM Heat
= 190,000 – 12,870
= 177,130 Btuh
Net Sensible Cap. = Gross Sensible Cap. – IFM Heat
= 136,000 – 12,870
= 123,130 Btuh
The calculations show that a 551A180 unit with the 56 kW
heater is the correct selection for the given conditions.
VI SELECT THE UNIT THAT CORRESPONDS TO POWER
SOURCE AVAILABLE.
The electrical data table shows that the 230-3-60 unit is
available.
110,000 Btuh
= 32,239 Watts Required
3.412 Btu/W
= 32.2 kW required
0TFQ004-012
551A155-180