Pressure Losses from Fan Accessories.pdf

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F
AN
E
NGINEERING
Information and Recommendations for the Engineer
®
FE-2900
Pressure Losses from
Fan Accessories
For a fan in a system to perform as rated in a catalog,
it is necessary for the system to be constructed in such
a way that the airflow pathways into and out of the fan
are similar to the conditions present during the tests
performed to develop the fan’s ratings. Usually, this
means that the fan’s inlet and outlet are free from imme-
diate obstructions and there are no bends in the duct-
work close to the fan. Due to accessory requirements or
space limitations, this may not be possible. In such
cases, the effect of accessories and/or ducting conditions
must be taken into account during fan selection to get
the airflow desired. This newsletter will cover the effect
of accessories/appurtenances added to a fan system.
Performance losses are usually represented in units of
pressure. Performance loss values indicate how much a
fan’s static pressure needs to increase in order to
achieve the same flow at the system’s point of rating
with the addition of the appurtenance. Typically, the
magnitude of the performance loss is calculated as a
function of the velocity pressure at the appurtenance.
Velocity pressure is proportional to the air stream den-
sity and the square of the air stream velocity. Consider
a system with two locations A & B. If the velocity of
the air stream at point B is double the velocity at point
A, any appurtenance placed into the air stream at point
B will have four times the loss than if the appurtenance
is placed at point A. Appurtenances placed in the throat
of a fan’s inlet (where the velocities are usually much
greater than the ductwork) can have a considerable
impact on the fan’s performance.
Formulae for calculating performance losses may be
available from the fan/accessory manufacturer or the
loss can be estimated using one of many available ref-
erences (e.g.
AMCA Publication 201 — Fans and
Systems).
appurtenance. A screen with a fine mesh will have a
higher value for k than a screen with a loose mesh. The
value of k for a fan exhaust or supply hood is typically
much greater than k for a stack cap which has straight
through flow. The value for k for a filtered hood would
be even higher.
Evasé or Outlet Transitions
These appurtenances change the velocity of the air
stream by changing the cross sectional area of the
ductwork. When air stream velocity changes, velocity
pressure is converted to static pressure (or vice-versa).
The formula for the losses for changes in area is:
Loss = -1 x k x dPv
dPv is the change in the air stream velocity pressure and
k is the efficiency of the transition. The value for dPv is
positive when the air stream is slowed down (i.e. the
cross sectional area of the ductwork is increased) and
negative when the velocity is increased. When the loss
is negative (slowing down the air stream), the static pres-
sure generated by the fan with the transition will be
greater than the static pressure generated without the
transition. The efficiency of the transition is dependent
on its design. Long gradual transitions can have efficien-
cies around 85% while short abrupt transitions can have
efficiencies less than 25%. Since the flow at the outlet
is not uniform, regions of flow velocities well in excess
of the averages exist. Converting this extra velocity pres-
sure to static pressure can effectively raise the apparent
efficiency to 100%.
Variable Inlet Vanes
When 100% open, variable inlet vanes cause a pressure
loss proportional to the velocity pressure. When they are
not 100% open, the effect of variable inlet vanes on
performance is not so simple to predict. Inlet vanes
produce a pre-spin in the air stream at the fan’s inlet.
The spin produced is in the same direction as the fan’s
impeller rotation. This has the effect of lowering the fan’s
static pressure (and thus the airflow through the system)
as well as the power consumed by the fan. Typically,
the effect of variable inlet vanes on fan performance is
interpolated from a series of tests done at various vane
settings. Variable inlet vanes are generally more efficient
than dampers for regulating airflow because they
decrease the power consumed by the fan as well as the
flow.
Screens, Dampers, Discharge
Caps, and Hoods
These appurtenances place a system resistance on the
airflow. The function for the magnitude of the perfor-
mance loss is typically:
Loss = k x Pv = k x (flow rate / flow area)
2
x density –
converted to units of pressure.
Pv is the velocity pressure of the air stream at the
appurtenance and k is a constant value for the given
©2003 Twin City Fan Companies, Ltd.
Example of an Accessory Loss
In the graph below, the solid line represents the fan’s
catalog performance. The dotted line represents the fan’s
performance with some appurtenance added.
Reminders
1) Fan performance is usually published with disclaimers
indicating any appurtenances in the air stream present
when the ratings were developed. If the ratings were
developed with a stack cap present, do not correct the
performance for the stack cap. If the ratings were
developed with an evase’, correct the performance if
the evase’ is not used.
2) Losses must be calculated at the same density as
the point of rating.
3) Expanding transitions in the ductwork have a negative
loss (which is a gain). The corrected fan curve after
adding an expanding transition will be above the
uncorrected fan curve. If the selection program does
not allow for correction to an expanding area, the
static pressure entered into the program will be lower
than the desired static pressure. Once corrected, the
result for the addition of the transition (subtracting a
negative loss value) will pass through the desired
point of operation.
4) Failure to account for the effects of appurtenances in
the air stream could require significant changes to
achieve the desired airflow.
• Belt driven fans may be able to achieve the
desired performance by increasing the speed of
the fan. Before selecting/adjusting sheaves, it will
be necessary to check that the impeller, shaft,
bearings, and motor can handle the new speed
and power requirements.
• Direct drive fans may be able to achieve the
desired performance by using a higher pitch pro-
peller (axial fans) or an over width/diameter wheel
design (centrifugal fans). If the direct drive fan is
attached to an inverter, speeding up the fan may
be possible. Again, it will be necessary to check
that the impeller and motor can handle the
requirements.
• It may become necessary to select an entirely new
fan to meet the requirements.
The addition of the appurtenance lowered the airflow
produced by the fan by about 5% along the system
curve. If the effect of the appurtenance was ignored
during the fan’s selection process, it would be necessary
to increase the running speed of the fan, get a new
propeller/wheel for the fan, or get a new fan to achieve
100% of the desired flow.
Notice that the intersection of the system curve with the
corrected (dotted) fan curve occurs at about 91% of the
design static pressure. If the fan selection software used
to make the selection could not automatically correct for
the appurtenance, the selection could be made at the
required flow and at a static pressure 9% higher than
desired. When the resulting performance is corrected for
the appurtenance, the fan curve would pass through the
desired flow and pressure.
For example, if the desired fan performance is 10,000
CFM and 10.0 in. w.g., we would have calculated a loss
of 0.9 in. w.g. for the appurtenance. To select the correct
fan, we would enter 10,000 CFM and 10.9 in. w.g. into
the selection program. When the results of the selection
are corrected for the appurtenance, the fan’s corrected
performance curve will pass through 10,000 CFM at 10.0
in. w.g. The uncorrected fan curve will show 10.9 in. w.g.
at the design flow even though only 10.0 in. w.g. will be
measured in the installed system.
Conclusion
AMCA Publication 201 — Fans and Systems
is an excellent
reference for fan performance in a system. This publica-
tion covers such topics as fan ratings, fan laws, fan/
system interaction, and system effects (the accessories
covered in this newsletter are considered a system
effect). Formulae and charts are included for estimating
values for losses due to accessories. This publication
also covers losses due to ducting conditions, which are
not covered in this newsletter.
®
AERovENt | www.AERovENt.com
5959 trenton Lane N | minneapolis, mN 55442 | Phone: 763-551-7500 | Fax: 763-551-7501

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