Trane PKG-PRC002-EN Manual page 13

Selection Example
Design Conditions
Total gross capacity required =
420 MBh = 35.2 Tons
Total sensible capacity required =
315 MBh
Entering air temperature = 80/67
Entering water temperature = 85
Water flowrate = 105 gpm
Airflow = 14840 cfm at 2.5-inch
duct static pressure
Unit includes:
Inlet guide vanes
Waterside economizer
Medium velocity throwaway filters
Unit Selection
Tentatively select a 35 ton unit
Model SCWF 35.
Refer to Table PD-19 on page 43 to obtain
gross total and sensible unit capacities,
and gpm at the design conditions:
Total capacity = 419.0 MBh
Sensible capacity = 309.0 MBh
Leaving water temperature = 94.7 F
Since the design cfm is greater than the
nominal cfm, adjust the capacities and
condenser water delta T to reflect the
higher cfm:
design cfm 14840 = +6% of nom. cfm
nominal 14000
cfm
PKG-PRC002-EN
Selection
Procedure
Refer to Table PD-1 on page 31 to obtain
the capacity correction factors for +6% of
nominal cfm:
Cooling capacity multiplier = 1.009
Sensible capacity multiplier = 1.027
Multiply the capacities by the correction
factors:
419 MBh x 1.009 = 422.8 MBh
309 MBh x 1.027 = 317.3 MBh
The SCWF 35 meets the total and
sensible design requirements.
Multiply the delta T of 9.7 F , by the cooling
capacity correction factor of 1.009 to
obtain new delta T of 9.79 and add this to
the entering water temperature to obtain
the actual leaving water temperature of
94.79 F .
Determine static air pressure drops
through the accessories at the design cfm
from Chart PD-3 on page 22:
4-row waterside economizer = 0.37 in.
Medium velocity filters = 0.28 in.
add this to the 2.5 inch duct static
pressure for a total external static
pressure of 3.15 inches.
Refer to the fan curve with inlet guide
vanes, Chart PD-38 on page 43, to
determine approximate brake
horsepower and fan rpm:
Fan brake horsepower = 16 bhp
Fan rpm = 1020 rpm
Self-Contained
Determine net capacities by subtracting
fan motor heat from gross capacities:
2.8 x 16 bhp = 44.8 MBh
Net total capacity = 422.8 MBh -
44.8 MBh = 378.0 MBh
Net sensible capacity = 317.3 MBh -
44.8 MBh = 272.5 MBh
Determine waterside economizer
capacity by referring to Table PD-17 on
page 42. Use entering air of 80/67 and
entering water temperature of 55 deg F
at 105 gpm. The table provides a gross
total capacity of 282.1 MBh and gross
sensible capacity of 277.2 MBh and
60.4 deg F leaving water temperature at
nominal cfm.
Determine gross capacities at design cfm
by applying the cfm correction factors
from waterside economizer from Table
PD-1 on page 31. Use the following
correction factors:
282.1 MBh x 1.009 = 284.6 MBh
277.2 MBh x 1.027 = 284.7 MBh
Apply the cooling correction factor to
water delta T to determine new delta T of
5.45 F .
Determine net capacities by subtracting
fan motor heat for net total capacity of
239.8 MBh and net sensible capacities of
239.9 MBh.
13