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7PKG-PRC010-EN
Selection
Procedure
Self-Contained
Selection Procedure
The selection of a deluxe self-contained
unit can be accomplished in three easy
steps.
Step 1 - Load
Determine the load requirements for
heating and cooling (include outside air)
using Trane’s load estimate program or
any standard method.
Step 2 - Unit Type
Self-contained air conditioners are
available in either water-cooled or
remote air-cooled models to match
individual needs.
Step 3 - Select the Unit
The conditions under which the unit must
operate and the design load will give the
final selection.
Selection Example
Design conditions for a water-cooled unit:
Entering air temperature = 80/67°F
Total gross capacity required = 64,400
Btu/h
Entering water temperature = 85°F
Leaving water temperature = 95°F
Airflow = 2,400 cfm at 0.5-inch duct static
pressure
Unit Selection
Tentatively select a 5-ton unit: Model
SCWB-B50. Refer to Table PD-10 on
page 21 to obtain gross total and sensible
unit capacities, gpm, and leaving water
temperature at nominal conditions:
Total capacity = 66 MBh
Sensible capacity = 45 MBh
Water flow rate = 20 gpm
Leaving water temperature = 93.1°F
Since the design cfm is greater than the
nominal cfm, the capacities and con-
denser water delta T must be adjusted to
reflect the higher cfm.
Design cfm = 2,400
Nominal cfm = 2,000 + 20% of nominal
cfm
Refer to Table PD-6 on page19 to obtain
the capacity correction factors for +20%
of nominal cfm:
Cooling capacity multiplier = 1.03
Sensible capacity multiplier = 1.05
Multiply the capacities by the correction
factors:
66 MBh x 1.03 = 68.0 MBh
45 MBh x 1.05 = 47.3 MBh
The SCWB-B50 meets the total and
sensible design requirements.
Subtract the entering water temperature
from the leaving water temperature to
determine condenser water delta T of
8.1°F. Multiply the delta T by the cooling
capacity correction factor of 1.03 to
obtain the new delta T of 8.3°F and add
this to the entering water temperature to
obtain the actual leaving water tempera-
ture of 93.3°F. The leaving condenser
water temperature is within the design
delta T of 10°F.
Refer the fan data Table PD-1 on page 14
to determine approximate brake horse-
power and fan rpm:
Fan brake horsepower = 1.09 bhp
Fan rpm = 934 rpm
Determine net capacities by subtracting
fan motor heat from gross capacities:
2.8 X 1.09 bhp = 3.1 MBh
Net total capacity = 67.8 MBh - 3.1 MBh
= 64.7 MBh
Net sensible capacity = 48.7MBh - 3.1
MBh = 45.6 MBh
Refer to the Trane psychrometric chart to
determine leaving air temperatures.