| Selection Procedure (contd.)
Economizer Capacity
1 After
determining that the unit will meet the required
capacity with mechanical cooling, then the waterside economizer capacity can be
determined by referring to the appropriate two-row or
four-row waterside economizer capacity tables listed for each unit size.
2 Determine
entering air temperature dry bulb and wet bulb and
condenser water flow (gpm) and entering water temperature to the waterside economizer
coil.
3 Refer to the
appropriate waterside economizer table to find gross
total capacity and gross sensible capacity and the leaving water temperature.
4 Apply the cfm
correction factor for the waterside economizer from the
appropriate table to determine the gross total and sensible capacities at desired
cfm.
5 Multiply the
condenser water delta T by the total capacity
correction factor to determine the new delta T.
6 Calculate
supply fan motor heat by using the following equation:
Fan motor heat (MBh) = 2.8 x fan motor brake horsepower
7 Determine net
total capacity and net sensible capacity by subtracting
fan motor heat from gross total capacity and gross sensible capacity.
8 Refer to Trane
psychometric chart to determine leaving air
temperatures.
Selection Example
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
Leaving water temperature — 95
Airflow — 14840 cfm at 2.5-inch duct static pressure
Unit to include:
Inlet guide vanes
Waterside economizer
Medium velocity throwaway filters
Unit Selection
Tentatively select a 35 ton unit — Model SWUD 35.
Refer to Table 51-1 to obtain gross total and sensible unit capacities, gpm, and leaving water
temperature at the design conditions:
Total MBh –419
Sensible MBh — 309
Gpm — 105
LWT — 94.7
Since the design cfm is greater than the nominal cfm, the capacities and condenser water delta T must be adjusted to
reflect the higher cfm:
| design cfm = |
14840 = |
+6% from nom.cfm |
| nominal cfm |
14000 |
|
Refer to Table 51-2
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 X 1.009 = 423 MBh
309 X 1.027 = 317 MBh
The SWUD 35 meets the total and sensible design requirements.
Subtract the entering water temperature from the leaving water temperature
to determine condenser water delta T of 9.7 degrees.
Multiply the delta T by the cooling capacity correction factor of 1.009 to obtain new delta T of 9.78 degrees and this to the
entering water temperature to obtain the actual leaving water temperature. 94.78 within
the design delta T of 10 degrees.
Determine static air pressure drops through the accessories at the design cfm from Chart 50-2: 4-Row
Waterside Economizer — .37 in.
Medium Velocity Filters — .28 in., add this to the 2.50 inch duct static pressure for a total external static pressure
of 3.15 inches. Refer to the fan curve with inlet guide
vanes on page 49 to determine approximate brake horsepower and fan rpm: 16 bhp
and 1020 rpm.
Determine net capacities by subtracting fan motor heat from gross
capacities:
2.8 X 16 bhp = 44.8 MBh
Net total capacity = 423 – 44.8 = 378.2 MBh
Net sensible capacity = 317 - 44.8 = 272.8 MBh
Determine waterside economizer capacity by referring to Table 50-2. Use
entering air of 80/67 and entering water temperature of
55 degrees at 105 gpm. The table provides a gross total and sensible capacity of 271 MBh and 60 degree leaving
water temperature at nominal cfm.
Determine gross capacities at design cfm by applying the cfm correction factors from waterside economizer from Table
51-2. Since the total and sensible capacity are the
same in this example, use only the cooling correction factor: 271 x 1.024 = 277.5
total and sensible MBh.
Apply the cooling correction factor to the water delta T to determine new delta T of 5.12 degrees.
Determine net capacities by subtracting fan motor heat for net total and
sensible capacities of 232.7 MBh. |
