Surge, Stall, and Instabilities in Fans.pdf

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F
AN
E
NGINEERING
Information and Recommendations for the Engineer
®
FE-600
Surge, Stall, and Instabilities in Fans
Introduction
Users of fan systems desire a steady, continuous flow
of air. In this ideal situation, the pressure generated by
the fan is constant. A single instantaneous measurement
of the flow rate would be valid for extended periods of
time.
Figure 1 shows the flow for an ideal system. Figures
2 through 5 show a variety of conditions for time varia-
tion of flow.
Figure 1. Ideal Flow
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Figure 2. Typical Flow
FLOW
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0
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TIME
FLOW
Figure 3. Stall Condition
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FLOW
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Those involved with measurement of flow rates know
that ideal flow conditions are not common. Each point
of flow measurement is usually time averaged for ten
seconds or more to get an accurate reading. Variations
in flow and pressure readings of 10% over short time
periods are relatively common. However, fans which are
improperly selected or applied can produce variations far
greater than this. Conditions can become so severe that
the flow through the fan can oscillate between forward
and reverse (flow exiting the inlet) many times per min-
ute (see Figure 4).
The variations in flow and pressure not only make it
more difficult to measure the flow, they can create a
variety of problems:
1. Dramatic increases in noise.
2. Increases in vibration.
3. Structural fatigue damage to the fan due to continu-
ous loading and unloading of components.
4. Damage to the ductwork and other system compo-
nents.
5. A fan system that does not perform properly due to
unsteady flow and/or transmitted vibration.
An understanding of the causes of unsteady flow can
be helpful in avoiding these problems. Because some of
the causes are very complex, researchers have had
some interest. However, there has not been uniform
agreement in the conclusions as to what the exact
causes are. From their research we can learn the condi-
tions that tend to perform normally and avoid the condi-
tions that don’t.
Systems with unsteady flow can perform mysteri-
ously. Complicated terms to describe the phenomena
are often used and misused. The result is that there are
TIME
Figure 4. Surge Condition
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FLOW
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-300
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Figure 5. Bi-Stable Flow Condition
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FLOW
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TRIGGERING EVENT
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©2012 Twin City Fan Companies, Ltd.
few authoritative reference materials to use as diagnos-
tic guides. We will discuss some of the more common
terms and issues.
Stall
Elongated objects (such as fan blades) when passing
through the airstream will deflect the air. If we change
the orientation of the object relative to the flow direction,
we can increase or decrease the amount that the air is
deflected. If we progressively increase the attack angle
of the fan blade, it will increase the amount of deflec-
tion. It is this change of direction (and relative velocities)
that allows the fan to generate pressure. If the attack
angle becomes too severe, the air will no longer follow
the blade surface in a uniform manner. The amount of
deflection and the pressure being generated stops
increasing and normally will fall off. This is called the
stall point. Figure 6 shows a fan curve of a fan with a
significant dip in the stall region.
In a fan, the blades are normally rotating at constant
velocity. Therefore, to change the angle of attack, the
system to which the fan is attached must be changed.
Higher flow rates through the inlet increase the attack
angle, and lower flow rates decrease it. Therefore, if a
fan is operating in stall, it is because the CFM is too
low for it. On a given system, this is caused by select-
ing a fan which is too large (making the air velocities
too low in the fan).
Figure 6. Curve Showing a Dip in the Stall Region
4
STALL REGION
There is a time-varying nature for the flow of a fan
in stall. However, this is normally not the major cause
of concern. The increased noise being generated can be
a problem, but this too can often be dealt with. The
major concern for a fan operating in stall is the poten-
tial for mechanical damage. Those who have had a
“bumpy” airplane ride have a feel for how severe aero-
dynamic shock impacts can be.
A fan continuously operating in stall can sustain
structural metal fatigue. This is especially true for axial
flow fans having long slender blades, or blades fabri-
cated from sheet metal. Centrifugal fans are less prone
to damage. Centrifugal fans designed for relatively high
pressures but operating at very low pressures (less than
1" SP) have been known to operate continuously in stall
for many years without damage.
There is another downside to having a fan operate in
stall. It means the efficiency of the fan is less than
optimum. A smaller size fan costs less and has a lower
operating cost. It will also likely outlast a larger fan.
Rotating Stall
This is a special case of stall that normally only occurs
in backwardly inclined and airfoil centrifugal fans. Most
observers also report that inlet box dampers are
involved. Variable inlet vanes do a good job of prevent-
ing rotating stall because they provide a more stable
flow path for the air through the wheel. These fans are
encased in a scroll type housing that helps generate the
fan’s pressure. The pressure around the periphery of the
fan wheel varies relative to how near it is to the fan
outlet (where it is highest). These fans have several
blades, typically 9 to12.
We will call the passageway between each blade a
“cell.” The flow through each cell can vary since the
pressure around the periphery varies. Near the stall point
it becomes possible for most of the cells to have the
normal forward flow, while one or two cells have reverse
flow. The air that “squirts” backward through these cells
has nowhere to go so it moves to an adjacent cell,
deflecting the air which was already traveling through it.
This change of attack angle now forces this cell to stall,
it then also has reverse flow, passes on its bubble of
air, and on and on around the fan wheel.
Rotating stall typically occurs in fans which are
severely throttled (inlet box damper typically less than
30% open).
Most researchers have reported that the frequency of
travel of this rotating stall occurs at about two-thirds of
the fan rotational RPM(x). Some have observed two
traveling cells at once generating a four-thirds rotational
frequency. There are other reports of rotating stall rang-
ing from two-thirds and even higher harmonics (2/3x,
4/3x, 6/3x, 8/3x, ...). If these exciting frequencies coin-
cide with the natural frequencies of the wheel or hous-
ing, resonance occurs and damage can result. This
frequency will show up in both sound and vibration
measurements. Rotating stall is among the most destruc-
tive of instabilities in the fan.
STATIC PRESSURE (inWC)
3.5
3
2.5
2
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1
0.5
0
0
4
8
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CFM in (1,000s)
In some fans, the angle of attack is not uniform
across the full width of the blade. These are normally
not the most efficient fans, although the severity of the
stall is often less since only part of the blade is stalling
at any one flow rate. Some people say that radial
bladed centrifugal fans are always in stall since there is
a poor match between the directional velocity of the
blade and that of the approaching air. This is essen-
tially true. However, these types of fans can have severe
varying flows at very low flow rates since the internal
losses are dominated by stall and the pressure falls off
at this point.
A fan operating at or near the stall point usually will
have severe increases in noise. On some fans it will
sound almost as if the impeller is being impacted by a
solid object (hammering). Pure stall tends to have a
random frequency but there are special cases where a
pure frequency is generated. This will be discussed
later.
Surge
Several years ago there was a report of a grain drying
system that was pressurizing large grain bins. It utilized
a burner section near the inlet of a large centrifugal fan.
Periodically this system would “belch” fire back out the
inlet of the burner. This was likely a severe case of
system surge.
The sound a surging fan makes is commonly
described by observers as “whoosh” or “whoomp.” There
are several criteria which must be met to have surge:
Fan Engineering FE-600
2
STATIC PRESSURE (inWC)
1. There must be a relatively large volume of air which
is pressurized (such as the grain bins or a large ple-
num).
2. There must be a section of ductwork with relatively
high velocities.
3. The operating point of the system is to the left of
the peak pressure (at lower flow rates). In this region
the fan curve has a positive slope such that increas-
ing the flow also increases the fan static pressure.
In concept, a system in surge is like an oscillator.
The energy imparted to the air alternates between creat-
ing kinetic energy (high velocity in the duct) and poten-
tial energy (compressing the air in the plenum). The
positive slope on the fan curve allows large amplification
of this oscillation to occur. In extreme conditions, the air
can temporarily blow back through the inlet.
In a fixed system, the frequency of the surge is con-
stant. Usually the frequency is low enough that you can
count the number of cycles per minute since it is quite
audible. Most severe reports occur at a frequency below
300 cpm. One researcher reported that this effect seems
to disappear at frequencies above 450 cpm.
The frequency of surge can be be calculated for
simple systems:
Frequency (Hz) = 175 * Square Root [Duct-area /
(Plenum-volume * Duct-length)]
Note: Keep all dimensions in feet.
For those wishing further information, research the
term “helmholtz resonator.”
Figure 7. Bi-Stable Flow
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CFM
Parallel Flow Operation
It is relatively common for two or more fans to be oper-
ating in parallel. In the two fan system, each fan is
selected for half of the design flow rate. Fans that have
a large “dip” in the stall region can have another type
of problem. Vaneaxial and forward curve centrifugal fans
are two types of fans which can have large “dips.”
The problem with parallel flow systems can occur in
the starting sequence. If the fans are properly sized,
started simultaneously and brought up to speed at the
same rate, there is no problem. However, if one fan is
started first, the second fan is already exposed to back
pressure while it is coming up to speed. At full speed,
a condition can arise where one fan is operating at a
flow rate to the right of the peak static pressure point,
while the other fan is trapped on the left side of the
peak.
Skeptics will not think this is possible, so here is an
example. A fan was to be selected for 7000 CFM at 4"
SP. Due to severe space limitations, two 18" diameter
vaneaxials were selected to operate in parallel. Referring
to Figure 8 (B), we can see that each fan was to
deliver 3500 CFM.
Figure 8. Two Identical Fans Not Sharing the Load Equally
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Hunting
Some people use the term “hunting” to apply to any
time-varying flow. However, the appropriate use of this
term should apply to an under damped control circuit.
In variable volume systems, sensors are used to provide
information that controls dampers, vanes, speed controls,
or other means of setting the flow rate. If the control
system responds too quickly, it will overcorrect and have
to readjust the other direction. In the extreme condition,
a system may continuously “hunt” back and forth.
Stability and Bi-Stable Flow
Stability refers to the ability of a system if temporarily
displaced to return to its un-displaced position. A coin
balanced on edge is unstable. A coin at rest on a flat
surface is stable. Some fans are not stable for all flow
ranges. For example, walking by the inlet (don’t
try this!)
of a large centrifugal during an air test caused the flow
to reduce by over 15%. This fan continued to operate
at the lower flow rate until the test was restarted.
We can determine the stability of a fan by performing
two air tests. On one test, we start at full flow (free
delivery) and measure the flow and pressure as we
progressively add more resistance. In the second test,
we start at shut-off and progressively reduce the resis-
tance. We now have two flow vs. pressure fan curves.
If they do not overlay, we have a region of instability.
Figure 7 shows a sample fan curve with this property.
Since there are only two possible conditions on any
system, this is called bi-stable flow.
Although the noise changes between the two flow
conditions, neither is particularly objectionable. If the fan
is rated in the high flow condition and it “trips” to the
lower condition, the loss of flow can be a problem.
Bi-stable flow has been observed in backwardly
inclined centrifugal fans, usually at performances close
to free delivery and almost always at flow rates higher
than that where the best efficiency occurs.
3
STATIC PRESSURE (inWC)
7.5
6
4.5
D
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0
0
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3000
B
C
A
E
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6000
CFM
Fan Engineering FE-600
Those familiar with the system will recognize the
system curve as a parabola with the equation:
SP = CFM
2
÷ Constant
For this system we get:
4 = 7000
2
÷ Constant
or
Constant = 49,000,000 ÷ 4 = 12,250,000
We can determine what this system will do with one
fan running by plotting this parabola on our fan curve.
With fan #1 running we can see that we will get about
4750 CFM at 1.8" SP (A).
At this point, flow would be going backwards through
fan #2 unless we stop the leak. Commonly, heavy duty
backdraft dampers are used for this. When starting fan
#2, it will contribute no flow until it reaches a speed
such that the pressure (at shut-off) exceeds the back
pressure it sees from fan #1. As the speed continues
to increase, the flow will eventually reach point “D”.
Meanwhile, the pressure fan #1 sees continues to
increase until it reaches point “C”.
We then have the final condition:
Fan #1 is delivering 3800 CFM @ 3.50" SP
Fan #2 is delivering 2750 CFM @ 3.50" SP
From the system curve equation:
(2750 + 3800)
2
÷ 12,250,000 = 3.50"
• The system is happy because the flow (total of both
fans) and pressure readings are on the system
curve.
• Fan #1 is happy because point “C” is on the fan
curve.
• Fan #2 has mixed emotions. It is happy that point
“C” is on the fan curve, but unhappy about being
trapped in stall.
If these were your fans you would probably be
unhappy because:
1. The total CFM is only 6550, not 7000 as expected.
2. Fan #2 is probably noisy.
3. Fan #2 is prone to damage due to operation in
stall.
This example shows that it is possible to have two
identical fans not sharing the load equally. A more
severe condition can exist if non-identical fans are oper-
ating in parallel. Some years ago a complaint regarding
a system with two fans in parallel was received from a
customer. After installing a second larger fan in parallel
with a smaller fan that had been in operation, the com-
bined flow wasn’t what was expected. Measurements
revealed that the second fan by itself was generating
more pressure than the first fan was capable of at any
point on its fan curve. The original fan was completely
overpowered, and flow was blowing back out of its inlet.
The customer was advised to shut off the original fan
(saving the power) and block solid the duct branch to
Figure 10. Reduced Width Fan
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10.5
REDUCED WIDTH SELECTION
Figure 9. Normal Width Fan
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10.5
RPM=1282, BHP=13.14
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RPM=1342, BHP=12.56
32
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0
OPERATING POINT CLOSE TO STALL
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STATIC PRESSURE (inWC)
STATIC PRESSURE (inWC)
BRAKE HORSEPOWER
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CFM in (1,000s)
CFM in (1,000s)
Figure 11. Blade Angle=45°
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RPM=1388, BHP=15.64
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Figure 12. Blade Angle=35°
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8
RPM=1663, BHP=14.67
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16
STATIC PRESSURE (inWC)
STATIC PRESSURE (inWC)
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BRAKE HORSEPOWER
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CFM in (1,000s)
CFM in (1,000s)
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Fan Engineering FE-600
BRAKE HORSEPOWER
7
14
BRAKE HORSEPOWER
the original fan (plugging the leak). There were two les-
sons learned here:
1. Don’t mix two different fans (or operate two identical
fans at different speeds) for parallel operation.
2. If more flow is required on a constant system, boost
the pressure capability of the fan or add a second
fan in series.
8. For fans in parallel, there are four suggestions:
a. Make sure fans start simultaneously.
b. Select fans so that the operating static pressure is
lower than the lowest point of the dip in the curve.
(Below the pressure at point E on Figure 8.)
c. Select identical fans.
d. Operate fans at identical speeds.
Tips On How to Avoid Problems
1. Don’t select too large a fan. Some people think that
selecting a larger fan will give them an extra safety
margin should the system be miscalculated. This can
put you in the undesirable part of the fan curve
where stall and surge can occur. A fan too large for
an application is also uneconomical to operate (waste
of horsepower). You are better off selecting a higher
class of fan that can be sped up should the system
calculations be incorrect.
2. Centrifugal fans can be designed with “narrow width”
construction to help avoid stall. Comparing Figure 9
and Figure 10 shows how reducing the fan width not
only helps avoid stall, but also reduces power con-
sumption (BHP).
3. On adjustable pitch axial flow fans, reducing the
blade angle and increasing the speed can help avoid
stall. Comparing Figure 11 and Figure 12, the operat-
ing point moves away from stall and the power
requirement decreases by changing the blade angle
from 45° to 35°.
4. On variable volume systems, the use of inlet vanes
can allow lower turndowns and still avoid stall.
5. Severely undersized fans are not only inefficient; they
can make the fan operate in a condition where stabil-
ity can be a problem.
6. Avoid operating fans for long periods in a severely
throttled condition.
7. On fans that need to operate at low damper settings,
utilize one of the following:
a. Variable speed drive
b. Bypass duct from discharge to inlet
c. Variable inlet vanes (VIV)
Note: Rotating stall has not been observed with VIV*
* See attached references.
Quick Fixes for Eliminating Stall
1. Allow greater flow through the fan by creating a
“leak”. On closed systems, a small duct can often be
run from the outlet back to the inlet (with recircula-
tion).
2. On fans that have internal cones, reposition the fan
wheel so that air can leak from the periphery of the
fan wheel back into the inlet.
Note:
Both of these fixes will reduce the system effi-
ciency.
Conclusion
The words “stall” and “surge” often “put the fear” into
inexperienced people when applying fans. We all would
like to simply plug a fan into a system and have a
continuous steady flow. It would be nice if system cal-
culations were ultra precise making it easy to avoid bad
operating points. However, in the real world fans are
often applied in less than optimum conditions, and many
times in conditions where stall is likely. Even then,
severe problems are rare. When problems do occur,
there are methods to identify the problem type, and
once identified, solutions can be implemented.
Reference
A summary of experiences with Fan Induced Duct
Vibrations on Fossil Fueled Boilers; Presented at
American Power Conference; Chicago, IL; April 21-23,
1975.
5
Fan Engineering FE-600

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