Trane Fan - Manual (Page 16)

16
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.
The Trane 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 — Inlet Vanes
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.
As VAV 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/Cfm
d
) 2 x (SP
d
-SP
c
) = SP
c
SP
d
= static pressure at design,
SP
c
= static pressure control setting,
Cfm
d
= cfm at design.
The VAV system modulation curve can
be drawn using a Trane 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.
Figure A-5 – VAV System Modulation Curve