Selection Procedure
Exhaust Fan Motor Sizing
The exhaust/return fan is selected based on total return system negative static pressure and exhaust fan CFM. Return system negative static includes return duct static, and any other job site applicable static pressure drop.
Return duct static pressure = 0.30 inches.
Total return system negative static pressure = 0.30 inches.
Exhaust fan CFM = 36000 CFM
From Table 39, p. 77 the required BHP is 21.44 BHP at 400 RPM. Thus, the exhaust fan motor selected is 25 HP.
To select a drive, enter Table 37, p. 75 for a 25 HP motor and air handler "C". Drive selection number 4 - 400 RPM.
Return Fan Motor Sizing
The same static pressure and CFM considerations must be taken for return fan size, horsepower, and drive selection as are required for exhaust fan sizing. However, since the return fan runs continuously the sensible heat generated by the return fan motor must be included in the entering evaporator coil mixed air temperature equation.
In this selection, if the return motor BHP is equal to the exhaust motor BHP, 21.44 BHP = 58.1 MBH x 1000÷ (1.085 x 36000 Return CFM) = 1.5°F
an air density correction is needed to project accurate unit performance.
Figure 18, p. 37 shows the air density ratio at various temperatures and elevations.
The procedure to use when selecting a supply or exhaust/return fan at elevations and
temperatures other than standard is as follows:
1.First, determine the air density ratio using Figure 18, p. 37.
2.Divide the static pressure at the nonstandard condition by the air density ratio to obtain the corrected static pressure.
3.Use the actual CFM and the corrected static pressure to determine the fan RPM and BHP from the performance tables or curves.
4.The fan RPM is correct as selected.
5.BHP must be multiplied by the air density ratio to obtain the actual operating BHP.
In order to better illustrate this procedure, the following example is used:
Consider an air handler"C" that is to deliver 32000 actual CFM at 3-inches total static pressure (tsp), 55°F leaving air temperature, at an elevation of 5000 ft.
1.From Figure 18, p. 37, the air density ratio is 0.86.
2.Tsp =
3.From fan performance Table 17, p. 48 air handler"C" (without inlet vanes) will deliver 32000 CFM at 3.49 inches TSP at 997 RPM and 30.27 BHP.
4.The RPM is correct as selected - 997 RPM.
5.BHP = 30.27 x 0.86 = 26.3 BHP actual.
Cooling coil MBH should be calculated at standard and then converted to actual using the correction factors in Table 6, p. 37, Table 7, p. 37, Table 8, p. 37. Apply these factors to the capacities selected at standard CFM so as to correct for the reduced mass flow rate across the condenser.
Heat selections other than gas heat will not be affected by altitude. Nominal gas capacity (output) should be multiplied by the factors given in Table 8, p. 37 before calculating the heating supply air temperature.
added to the return air temperature. Where altitudes are significantly above sea level, use Table 6, p. 37, Table 7, p. 37 and Table 8, p. 37 for applicable correction factors.
Unit Electrical Requirements
Selection procedures for electrical requirements for wire sizing amps, maximum fuse sizing, and dual element fuses are given in the electrical service section of this catalog.
Altitude Corrections
The air handler performance tables and curves of this catalog are based on standard air (.075 lbs/ft). If the airflow requirements are at other than standard conditions (sea level),
30
FAN MOTOR HEAT
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| 250 |
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| Std Motor |
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MBH | 200 |
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Heat | 150 |
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Fan Motor | 100 |
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50 |
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| 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |
Motor Brake Horse Power | PM1206 |