12
PKGP-PRC003-EN
Cooling Capacity
Step 1
Calculate the building’s total and sensible
cooling loads at design conditions. Use
the Trane calculation methods or any
other standard accepted method.
Factors used in unit selection:
A
Total Cooling Load: 59 MBh
B
Sensible Cooling Load: 40 MBh
C
Airflow: 2000 cfm
D
Electrical Characteristics: 460/60/3
E
Summer Design Conditions: Entering
Evaporator Coil: 80 DB, 67 WB Outdoor
Ambient: 95
F
External Static Pressure: 0.36 in. wg
G
Downflow Configuration
H
High Efficiency
I
Economizer
Step 2
As a starting point, a rough determination
must be made of the size of the unit. The
final selection will be made after
examining the performance at the given
conditions. Divide the total cooling load by
nominal BTUH per ton (12 MBh per ton);
then round up to the nearest unit size.
59 MBh / 12 MBh = approx. 5 tons
Step 3
Table PD-3 shows that a WSC060A4 has
a gross cooling capacity of 63.1 MBh and
47.5 MBh sensible capacity at 2000 cfm
and 95 DB outdoor ambient with 80 DB,
67 WB air entering the evaporator.
To Find Capacity at Intermediate
Conditions Not in the Table
When the design conditions are between
two numbers that are in the capacity
table, interpolation is required to
approximate the capacity. Note:
Extrapolation outside of the table
conditions is not recommended.
Step 4
In order to select the correct unit which
meets the building’s requirements, the
fan motor heat must be deducted from
the gross cooling capacity. The amount of
heat that the fan motor generates is
dependent on the effort by the motor -
cfm and static pressure. To determine the
total unit static pressure:
External Static (duct system)
0.36 wg
Standard Filter 1 in.
0.09 wg
from Table PD-23
Economizer
0.18 wg
(100% Outside Air)
from Table
PD-23
Electric Heater Size 6 kW
0.056 wg
from Table PD-23
(reference “Heating Capacity” section on
this page for determination of heater
size)
Total Static Pressure
0.69 wg
Note: The Evaporator Fan Performance
Table PD-12 has deducted the pressure
drop for a 1 in. filter already in the unit
(see note below Table PD-12). Therefore,
the actual total static pressure is 0.69 -
0.09 (from Table PD-23) = 0.60 wg.
With 2000 cfm and 0.60 wg.
Table PD-46 shows .90 bhp for this unit.
Note below the table gives a formula to
calculate Fan Motor Heat,
2.829 x bhp + .4024 = MBH.
2.829 x .90 + .4024 = 2.95 MBH.
Now subtract the fan motor heat from
the gross cooling capacity of the unit:
Net Total Cooling Capacity
= 63.1 MBH - 2.95 = 60.15 MBH.
Net Sensible Cooling Capacity
= 47.5 MBH - 2.95 = 44.55 MBH.
Step 5
Compare results to original load
requirements.
If the performance will not meet the
building’s required total or sensible
cooling load, try a selection at the next
higher size unit.
Heating Capacity
Step 1
Calculate the building heating load using
the Trane calculation form or other
standard accepted method.
Step 2
Size the equipment using Table PD-31 to
match the heating loads at design
conditions.
A
Total heating load: 50 MBH
B
Outdoor Ambient (Winter) 17 DB
C
Indoor Return Temperature: 70 DB
D
Airflow: 2000cfm
Use the integrated portion of Table PD-31
for the WSC060A4 to determine
capacity at winter design conditions. The
mechanical heating portion of the heat
pump will provide 32.7 MBh.
Step 3
Because 32.7 is less than the building’s
required heating capacity at winter
design conditions, a supplementary
heater must be selected.
50 MBh-32.7 MBh = 17.3 MBh
The auxiliary eletric heat capacities are
listed in Table PD-38. From the table, a 6
kW heater will deliver 20.48 MBH at 480
volts. In order to determine capacity at
460 volts, the heater voltage correction
factor from Table PD-39 must be used.
Therefore, 20.48 MBH x .918 (voltage
correction factor) = 18.80 MBH. A 6kW
heater should be selected.
Air Delivery Selection
External static pressure drop through
the air distribution system has been
calculated to be 0.60 inches of water.
Enter Table PD-12 for a WSC060A4 at
2000 cfm and 0.60 static pressure. The
belt drive motor will give the desired
airflow at a rated bhp of 0.90 and 998
rpm.
Selection
Procedures
