Trane RT-PRC010-EN manual Selection Procedure, Air Delivery Procedure, Supply Fan Motor Sizing

Selection Procedure

AIR DELIVERY PROCEDURE

Supply fan performance tables include internal resistance of rooftop. For total static pressure determination, system external static must be added to appropriate component static pressure drop (evaporator coil, filters, optional economizer, optional exhaust fan, optional heating system, optional cooling only extended casing, optional roof curb).

Supply Fan Motor Sizing

The supply fan motor selected in the cooling capacity determination was 15.3 bhp and 924 rpm.Thus, a 20 hp supply fan motor is selected. EnterTable PD-45 to select the proper drive. For a 50 ton rooftop with 20 hp motor, a drive number 9 — 900 rpm is selected.

Exhaust Fan Motor Sizing

The exhaust fan is selected based on total return system negative static pressure and exhaust fan cfm. Return system negative static include return duct static and roof curb static pressure drop.

Return duct static pressure = 0.65 inches

Trane roof curb (Table PD-43) = 0.12 inches

Total return system negative static pressure = 0.77 inches

Exhaust fan cfm = 12,000 cfm

FromTable PD-47, the required bhp is

3.45hp at 574 rpm.Thus, the exhaust fan motor selected is 5 hp.

To select a drive, enterTable PD-49 for a 5 hp motor for a 50 ton unit. Drive selection number 6 — 600 rpm.

Where altitudes are significantly above sea level, useTables PAF-2 and PAF-3 and Figure PAF-1 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 rooftop performance tables and curves of this catalog are based on standard air (.075 lbs/ft). If the rooftop airflow requirements are at other than standard conditions (sea level), an air density correction is needed to project accurate unit performance.

Figure PAF-1 shows the air density ratio at various temperatures and elevations. Trane rooftops are designed to operate between 40 and 90 degrees Fahrenheit leaving air temperature.

The procedure to use when selecting a supply or exhaust fan on a rooftop for elevations and temperatures other than standard is as follows:

1

First, determine the air density ratio using Figure PAF-1.

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 rooftop 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.

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