System Curves

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Next, the duct damper is moved to a slightly closed position.  The
additional friction through the damper tends to decrease the air flow
somewhat, causing the static pressure at the fan discharge to go up
In developing the fan curve data, for each different setting of
slightly.
the duct damper, the static pressure across the fan and the airflow are
measured.  When this data is plotted on a graph, we have a fan curve
(Figure 2-3).  The service technician may use the fan curve in order to
determine the airflow in an operating system.  By measuring the static
pressure at the fan inlet and the fan outlet and calculating the change in
the fan curve may be used to read the cfm.
We can create another fan curve for the same fan, but this time we
change the motor or the drive so that the fan operates at a slower velocity,
revolutions per minute (rpm).  When we plot these results on the same fan
curve as the original, we find that the new curve is similar in shape to the
original, but the flow in cfm and static pressure are lower.  Figure 2-4
shows a family of fan curves for one fan at many different rpm settings.
Table 2-1 shows a multirating table for a fan.  It presents the same
information as that contained in a fan curve, but some prefer the tabulated
form instead of the graph presentation.
2.4 SYSTEM CURVES.  For any duct system, the flow of air through the
ductwork will experience a pressure drop caused by the friction with the
duct walls.  Elbows, changes in duct size or direction, fittings, coils,
filters, dampers, and any other devices which the air must pass through will
The amount of friction depends upon the velocity of the air.  For a
given system, more air being pushed through the ductwork will result in a
larger pressure drop, and will require higher fan capacity to accomplish the
task.  Figure 2-5 shows the relationship between the air flow through one
specific duct system and the pressure drop that the air will experience.
This is called a system curve.  The pressure drop of the air due to friction
is proportional to the square of the velocity.
In practical terms, consider a duct system which is carrying some
1,200 cfm, and has a system pressure drop of 1.5 inches water column. If
you want to push twice as much air through the system, it will experience a
pressure drop of four times as much, or 6.0 (i.e. 4 X 1.5) inches water
column.
Example:  A duct is carrying 2,000 cfm.  The fan moving the
air through the system is overcoming a duct friction loss of
1.2 inches water column.  How much static pressure loss will
the fan need to overcome if you want to increase the airflow
to 2,300 cfm?
Solution:  The ratio of the new cfm to the old cfm is
(2,300/2,000) which equals 1.15. The ratio of the new duct
velocity to the old duct velocity will also be 1.15.  The
static pressure loss through the duct will increase with the
square of the velocity.  It will increase by a factor of
(1.15)  2 = 1.32.  The old pressure drop must be multiplied by a
factor of 1.32.  The new pressure drop is:
1.58 inches water column
(1.2) x (1.32) =
SP loss =
2-3