Trane Fan - Manual (Page 13)

13
Application
Considerations
This section assists the system
designer in application and control of
Trane Q and Super Q II fans.
Satisfactory distribution of conditioned
air requires a properly chosen fan and
a well designed duct system.
Abbreviations
sp ...... static pressure (in. of water)
vp ...... velocity pressure (in. of water)
tp ....... total pressure (in. of water)
ov ...... outlet velocity (ft per minute)
rpm ... fan speed (revolutions per min.)
bhp ... brake horsepower
p ........ air density (lbs/ft
3 )
db ...... decibel (sound power or sound
pressure level)
cps .... cycles per second
cfm .... cubic feet of air per min. at any
density
scfm .. cubic feet per min. of standard air
clean, dry air with a density of
0.075 lbs/ft
3 at 70 F and a barometer
reading of 29.92- inches Hg)
The System
An air system may consist of a fan,
ductwork, air control dampers, cooling
coils, heating coils, filters, diffusers,
noise attenuation, turning vanes, etc.
The fan is the component in the system
which provides energy to the airstream
to overcome the resistance to flow of
the other components.
System Component Losses
Every system has a combined
resistance to flow which is usually
different from every other system and
is dependent upon the individual
components in the system. The
determination of the “pressure loss” or
“resistance to flow,” for the individual
components can be obtained from the
component manufacturers. The
determination of pressure losses for
ductwork and branch piping design is
well documented in standard
handbooks such as the ASHRAE
Handbook of Fundamentals.
System Curve
At a fixed volume flow rate (cfm)
through a given air system, a
corresponding pressure loss, or
resistance to this flow, will exist. If the
flow rate is changed, the resulting
pressure loss, or resistance to flow, will
also change. The relationship
governing this change for most
systems is:
PRESSUREc/PRESSURE = (CFMc/CFM) 2
The characteristic curve of a typical
“fixed system” plots as a parabola in
accordance with the above
relationship. Typical plots of the
resistance to flow versus volume flow
rate are shown with normalized duct
system curves, Figure A-1.
For a fixed system, an increase or
decrease in system resistance results
from an increase or decrease in the
volume flow rate along the given
system curve only.
Refer to Duct System A, Figure A-1.
Assume a system design point at
100 percent volume and 100 percent
resistance. If the volume flow rate is
increased to 120 percent of design
volume, the system resistance will
increase to 144 percent of the design
resistance in accordance with the
system equation. A further increase in
volume results in a corresponding
increase in system pressure. A
decrease in volume flow to 50 percent
results in a 75 percent reduction in
design resistance.
Figure A-1 — Normalized Duct System Curves