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145
SELECTION PROCEDURE (WITH 542J180 EXAMPLE)
I DETERMINE HEATING AND COOLING REQUIRE-
MENTS AT DESIGN CONDITIONS:
Given:
Required Cooling Capacity (TC) . . . . . . . . . 164,000 Btuh
Sensible Heat Capacity (SHC). . . . . . . . . . . 120,000 Btuh
Required Heating Capacity . . . . . . . . . . . . . 170,000 Btuh
Outdoor-Air Temperature (Cooling) . . . . . . . . . . . . . . 95 F
Indoor Entering-Air Temperature
(Cooling). . . . . . . . . . . . . . . . . . . . . .80 F Edb/67 F Ewb
Outdoor Coil Entering-Air Temperature
(Heating) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0° F
Indoor Winter Design Temperature . . . . . . . . . . . . . . 70 F
Indoor-Air Quantity. . . . . . . . . . . . . . . . . . . . . . . 6000 Cfm
External Static Pressure (ESP) . . . . . . . . . . . . 1.30 in. wg
Electrical Characteristics (V-Ph-Hz). . . . . . . . . . . 230-3-60
Economizer is specified.
Edb — Entering dry bulb
Ewb — Entering wet bulb
II SELECT UNIT BASED ON REQUIRED COOLING
CAPACITY.
Enter Cooling Capacity table on page 146 at outdoor-
entering temperature of 95 F, indoor air entering at
6000 cfm and 67 F ewb. The 542J180 will provide a total
capacity (TC) of 180,800 Btuh, a sensible heat capacity
(SHC) of 136,400 Btuh and a total unit kW of 15.88.
For indoor-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
table on page 146.
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.
Enter the Integrated Heating Capacities table on page 147
at 6000 cfm (interpolation required). At 70 F return indoor
air and 0° F air entering the outdoor coil, the integrated
heating capacity is 58,500 Btuh with a unit input kW of
11.71.
The required heating capacity is 170,000 Btuh; therefore,
111,500 Btuh (170,000 – 58,500) additional electric heat is
necessary.
Determine additional electric heat capacity in kW.
Enter the Electric Resistance Heater Data table at
230-3-60. The 34-kW heater satisfies the electric heater
requirements.
IV DETERMINE FAN SPEED AND MOTOR HORSE-
POWER REQUIREMENTS AT DESIGN CONDITIONS.
Before entering Fan Performance table, calculate the total
static pressure required based on unit components. From
the given and the Accessory/FIOP Static Pressure table
on page 151, find:
Enter the Fan Performance table on page 149. At
6000 cfm and 1.46 in. wg external static pressure, the fan
speed is 1214 and the bhp is 3.95 (interpolation required).
The standard motor and alternate high static drive and
suitable.
V DETERMINE NET CAPACITIES.
Capacities are gross and do not include the effect of
indoor-fan motor (IFM) heat.
Determine net cooling capacity as follows:
Determine net heating capacity as follows:
Net capacity =
Gross capacity + IFM heat + Electric heat capacity
= 56,800 + (IFM kW x 3412) + (Elec. kW x 3412)
= 56,800 + 11,827 + (34 x 3412)
= 56,800 + 11,827 + 116,008
= 184,635 Btuh
111,500 Btuh
= 32.7 kW
3412 Btuh/kW
External static pressure 1.30 in. wg
Economizer static pressure 0.07 in. wg
Electric heat static pressure 0.09 in. wg
Total static pressure 1.46 in. wg
Net capacity= Gross capacity – IFM heat
2545 Btuh
= 180,800 – 3.95 bhp x
hp
.85
= 180,800 – 11,827
= 168,973 Btuh
Net sensible capacity = 136,400 – 11,827
= 124,573 Btuh
542J