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:

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.