Trane PKG-PRC002-EN manual Selection Example Design Conditions, Unit Selection

Selection

Procedure Self-Contained

Selection Example

Design Conditions

Total gross capacity required = 420 MBh = 35.2Tons

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 toTable 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

Refer toTable 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 deltaT 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

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 toTable 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

60.4deg F leaving water temperature at nominal cfm.

Determine gross capacities at design cfm by applying the cfm correction factors from waterside economizer fromTable PD-1 on page 31. Use the following correction factors:

282.1MBh x 1.009 = 284.6 MBh

277.2MBh 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

239.8MBh and net sensible capacities of

239.9MBh.

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