Trane Fan manual Application Considerations, Sp = Cfm/Cfmd2 x SPd-SPc= SPc

Application Considerations

Q and Super Q II Fan Modulation — AC Inverter Capacity Control

Q fans and Super Q II fans can be modulated with AC frequency drives. TheTrane Company recommends Magnetek low noise inverter drives and Century high efficiency motors for optimum modulation performance.

Operating the Q or Super Q II fan on AC frequency drives requires the Q fan to be strengthened and balanced in the factory.This option “beefs up” the mechanical bracing of the Q fan inlet bearing assembly and calls for a precision factory balance. Precision balancing covers 10 operating points on the system curve from 10 percent load to full load.

Minimum cfm with AC inverters — Above 1.5” static pressure, the minimum cfm is the surge (do not select) line. Below 1.5” static pressure, it is 1000 cfm.

Q Fan Modulation — InletVanes Inlet vanes are a widely used form of fan modulation. As inlet vanes close, they impart a spin on the incoming air in the direction of the fan wheel rotation.This reduces airflow, static pressure and brake horsepower. However, inlet vanes do increase sound levels. If a job is acoustically sensitive, AC inverters are recommended for modulation. As shown in Figure A-5, a separate cfm static pressure curve (cfm-sp) is generated per each inlet vane position. Likewise, the figure shows brake horsepower curves that apply for various inlet vane positions.

Inlet vanes are controlled by placing a static pressure sensor in the downstream ductwork, typically about two-thirds of the way down the longest trunk duct. This sensor is set at a static pressure that will ensure sufficient pressure is available to move air from that point through the remaining duct work. The sensor will respond to duct pressure changes and signal the inlet vane operator to open or close the vanes to maintain the control setting at the sensor location.

AsVAV terminal units begin to close in response to a decreasing cooling load, static pressure in the ductwork increases. This causes the fan operating point to temporarily move upward to the left on a constant rpm curve as shown in Figure A-5 (point A to point B). The static pressure sensor will detect an increase in duct pressure and signal the inlet vane operator to begin to close the vanes. The inlet vanes will close until the static pressure sensor is again satisfied, moving the operation point to C (Figure A-5). As the cooling load continues to decrease, the modulation curve will be formed (point C to D, and point D to E) on Figure A-5. This curve passes through the design point and through the static pressure sensor control point. The static pressure of any point on this curve can be calculated using the formula:

Sp = (Cfm/Cfmd)2 x (SPd-SPc) = SPc

SPd = static pressure at design,

SPc = static pressure control setting, Cfmd = cfm at design.

Figure A-5 – VAV System Modulation Curve

The VAV system modulation curve can be drawn using aTrane system modulation overlay.The axis of the overlay is placed on a static pressure control setting. The curve that intersects the design points is the system modulation curve.

Because the axes of the inlet vane performance graph are in terms of percent wide open cfm (wocfm) and percent peak static pressure, the first step in establishing the system modulation curve is to find the proper design points. By plotting the design point on the performance curve for the fan in question, one can easily determine the percent wocfm. Knowing this, plot a point on the cfm-sp curve (Figure A-6) for inlet vanes wide open, at the design point

of wocfm. By tracing to the left, one can determine the percent of peak static pressure. By knowing the design cfm, static pressure and the percent of wide open cfm and percent peak static that these values represent, one can calculate wocfm and peak static pressure.

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