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139
SELECTION PROCEDURE (with 558F240 Example)
I DETERMINE COOLING AND HEATING REQUIRE-
MENTS AT DESIGN CONDITIONS.
Given:
Required Cooling Capacity . . . . . . . . . . . . . 230,000 Btuh
Sensible Heat Capacity . . . . . . . . . . . . . . . . 170,000 Btuh
Required Heating Capacity . . . . . . . . . . . . . . . . . . . 75 kW
Condenser Entering Air Temp . . . . . . . . . . 95 F (Summer)
Evaporator Entering Air Temp . . . . . . . . . . . . . . .80 F edb,
67 F ewb
Evaporator Air Quantity . . . . . . . . . . . . . . . . . . . 8,000 cfm
External Static Pressure . . . . . . . . . . . . . . . . . . 0.4 in. wg
Electrical Characteristics (V-Ph-Hz). . . . . . . . . . . 460-3-60
Vertical discharge unit with optional economizer required.
edb — Entering dry-bulb
ewb — Entering wet-bulb
II SELECT UNIT BASED ON REQUIRED COOLING
CAPACITY.
Enter Cooling Capacities table for 558F240 (page 140 at
condenser entering temperature 95 F, evaporator air enter-
ing at 8,000 cfm and 67 F wb. The 558F240 unit will pro-
vide a total cooling capacity of 249,000 Btuh and a
sensible heating capacity of 188,000 Btuh. For air entering
evaporator at temperatures other than 80 F edb, calculate
sensible heat capacity correction as required using the for-
mula in the notes following the Cooling Capacities tables.
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 Electric Resistance Heater table (page 130) select
the 75 kW output heater.
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 Accessory/FIOP Static Pressure
table on page 145 find:
External static pressure 0.40 in. wg
Electric heater static pressure 0.22 in. wg
Total static pressure 0.62 in. wg
Enter the Fan Performance table 558F240 (page 144) at
8,000 cfm and 0.62 in. wg external static pressure. By
interpolation, find that the rpm is 1061 and the watts are
4552.
V DETERMINE NET COOLING CAPACITY.
Cooling capacities are gross capacities and do not include
indoor (evaporator) fan motor (IFM) heat. Use the watts
input power to the motor calculated in Section IV above.
IFM Watts = 4552
Determine net cooling capacity using the following
formula:
Net capacity = Gross capacity – IFM heat
= 249,000 Btuh – 4552 Watts
= 249,000 Btuh – 15,531 Btuh
= 233,469 Btuh
Net sensible capacity = 188,000 Btuh – 15,531 Btuh
= 172,469 Btuh
The calculations show that a 558F240 unit with the
standard motor and standard low-medium static drive is
the correct selection for the given conditions.
(3.412
Btuh
)
Watts
0TFQ004-012
558F180-300