Application considerations – Trane Fan User Manual
Page 17
17
Application
Considerations
The control static pressure can then be
expressed as a percent of peak static
pressure and plotted. The system
modulation curve is described by the
curve on the modulation overlay that
passes through the design point when
the axis is placed on the control static
pressure point.
The minimum inlet vane cfm can easily
be determined after the system
modulation has been established. It
will be one of two things, either a) the
point where the system modulation
curve intersects the surge line, or
b) 40 percent wocfm, whichever is
greater. Forty percent wocfm is the
minimum point a Q fan with inlet vanes
can modulate, due to inherent
instability that results when the vanes
close to a certain angle.
A plot of part load cfm versus brake
horsepower can also be made after the
system modulation curve is established.
At each intersection of the system
modulation curve with a cfm-sp curve
for a certain inlet vane opening, a
vertical line is traced to the appropriate
bhp-cfm curve. At each intersection of a
bhp-cfm curve, a horizontal line is
traced to the scale of percent brake
horsepower. This will lead to a percent
wocfm versus percent peak bhp plot.
The design rpm and bhp need to be
corrected to account for performance
losses due to inlet vanes being in the air
stream. A correction of one percent to
the rpm and three percent to the bhp is
made in order to get to design
conditions with the inlet vanes fully
open.
Part load fan power consumption with
inlet vanes can be determined by
entering Figure A-6 at the desired
percent wide open cfm. (Wide open
cfm is found on the fan curve by
following the fan rpm to the right until
it intersects the 0” static pressure axis.)
On Figure A-6, plot a system curve from
the control static pressure through the
point of operation defined as a
calculated percent wide open cfm. Read
vertically upward from the 25, 50 and 75
percent intersections to determine the
percent bhp at part load.
Intermediate operating points are found
by extrapolation. Inlet vanes increase
Q fan sound levels. See Table A-5.
Figure A-6 – Inlet Vane Performance