College of Agricultural Sciences
Cooperative Extension
Agricultural and Biological Engineering
Selecting Tunnel Ventilation Fans
G 103
Eileen Fabian Wheeler, Associate Professor, Agricultural Engineering
F
broiler house with an average ceiling height of 8’10”
might be outfitted with ten 48-inch diameter tunnel
ventilation fans during hot weather. Most fans used
for tunnel ventilation are 48-inches in diameter with
a one-horsepower motor and will be the basis of the
following discussion. Even though the focus of this
fact sheet is on fan selection, an equal amount of
consideration should be given to other tunnel ventilation system features, such as inlet size, inlet position,
and controls.
an selection is critical to long-term successful
operation of a tunnel ventilation system. The
best and easiest way to select fans is by
considering only “rated” fans. Fans are rated when
they are run through a series of standardized performance tests by a certified laboratory, such as the Air
Movement and Control Association (AMCA) or the
BioEnvironmental and Structural Systems laboratory
(BESS). These standardized tests help in ventilation
system design and comparison shopping similar to
how the automobile standardized test for miles-pergallon fuel consumption allows you to fairly compare
features. Both AMCA and BESS publish books with
tables of agricultural fan data and provide certification to dozens of fans each year (see Additional
Resources). Fan manufacturers use these data for
product development and promotion.
A tunnel ventilation system is a hot-weather
strategy that employs a large number of exhaust fans
to move air through a building like air moves through
a wind tunnel. The large capacity fans are positioned
near one end of the building with inlets for air entry
on the opposite end. For example, a 48’ by 500’
Selection Criteria
Rated performance data consists of a table or curve
of airflow capacity (cubic feet per minute, cfm)
versus static pressure difference against which the
fan is operating (inches of water). An example of
rated fan data is shown in Figure 1 along with a
graph of these data, known as a fan curve. Notice
that as the resistance to air flow increases, as
measured by increased static pressure, air flow
delivered by a fan decreases. This is to be expected
with any fan but better fans will minimize this effect.
25000
Bare Fan
Figure 1. Rated fan
performance data showing
airflow capacity (cfm) versus
static pressure difference the
fan is operating against for a
bare fan and the same fan
outfitted with typical
accessories. The table of
data and curves represent an
example 48-inch diameter,
belt-drive, 1 Hp fan operating
at around 510 rpm.
Airflow (cfm)
20000
15000
10000
5000
Static
Pressure
Difference
(inches
water)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Fan with
Bare Guard &
Fan
Shutter
(cfm)
(cfm)
23963 19889
22703 18843
21503 17847
20011 16609
18328 15212
16215 13458
13883 11523
Fan with Guard & Interior Shutters
Normal Operating
Pressure Range
0
0.00
0.05
0.10
0.15
0.20
0.25
0.3
Static Pressure Difference (inches water)
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Better fans, being those that outperform their
peers, do so because of careful design, attention to
detail during manufacture, and use of high quality
components. Three important factors to consider
when selecting fans include the static pressure
difference against which the fan will be operating,
efficiency in delivering airflow for the electrical
power consumed, and the type of accessories
installed with the fan. A target capacity for a 48-inch
fan used in tunnel ventilation, which is outfitted with a
shutter and guard (and perhaps a discharge cone), is
20,000 cfm at 0.05-inches water static pressure
difference.
hence, more expensive to manufacturer. The payback takes about two to three years in reduced
electrical consumption. Higher efficiency motors are
readily available and are used on well-designed
agricultural fans.
Better fans also offer
There is too much
other design advantages in ventilation
emphasis on
performance. There
“cheap” fans.
has been too much
emphasis in the
agricultural industry on
“cheap” fans. This is a policy that is costing more in
operating expense and maintenance than if a better
model was selected.
Static Pressure. Tunnel fans operate against
pressure in their job of moving air through a building.
Most environment controllers monitor the static
pressure difference across the inlets, which is commonly 0.03 to 0.10 inches of water. Tunnel ventilation
systems should be operating in the lower end of this
range. For initial estimates, evaluate a tunnel fan’s
performance based on its airflow capacity at 0.05
inches static pressure. Be aware that some tunnel
fans operate under higher-pressure conditions. If the
tunnel fan will be discharging air into a headwind,
pulling air through evaporative cooling pads, or used
during mild
weather as part of
Energy-efficient
conventional
ventilation, select
fans payback in two
these fans for
to three years.
performance at
0.10 or 1/8-inch
(0.125-inch) static
pressure. Beware of manufacturers selling fans who
only furnish performance at zero static pressure
difference.
Accessories. Rated data are most useful when the
fan was tested under conditions that match your
application. Any increased static pressure caused by
operating conditions or accessories needs to be
accounted for in ventilation system design. For
example, since tunnel ventilation fans will have a
shutter and a guard, evaluate data where the fan was
rated with a shutter and guard in place.
BESS performance tests are commonly performed with the fan outfitted with a guard, shutter
and sometimes a discharge cone (AMCA data are
more often bare fan). Figure 1 provides fan data with
no accessories (bare fan) and the same fan with the
addition of a shutter and guard during the rating test.
Note that decreased airflow from fan accessories is
important. Design airflow will not be met if accessories are ignored.
If fan data with accessories are not provided, you
can add the static pressure resistance associated with
these accessories, as shown in Table 1, to your
estimate of the total static pressure against which the
fan will operate. Dirty shutters and fans discharging
into a headwind each provide resistance to air flow
and, hence, decrease fan performance.
Efficiency. For the same performance, choosing an
efficient fan can cut electricity costs in half. The
annual electricity cost of operating one of the most
energy-efficient 48-inch fans is about $300 compared
to $600 for an inefficient fan (at 10-cents per kWh).
This is based on 120 days of fan use for hot weather
ventilation. Summer fans are selected on their energy
efficiency since they have big motors. Fortunately,
bigger fans (i.e. 48-inch) are much more efficient
than smaller fans (i.e. 16-inch). Fan efficiency is
expressed in terms of cfm per Watt (W). For large
fans used in tunnel ventilation, choose fans with no
less than 20 cfm/W at 0.05-inches static pressure.
Efficient motors have more copper windings and are,
What kind of accessories will
a tunnel fan need?
Accessories are necessary for proper functioning of
the fan as part of a ventilation system, even though
they often reduce airflow and efficiency. Shutters
placed on the discharge side of fans are particularly
detrimental to air flow. Expect a 10 to 15 percent
airflow reduction using inlet-side shutters and a 15 to
25 percent reduction using discharge-side shutters.
Select shutters, whether power or gravity design, that
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Table 1. Typical Resistances to Air Movement. Total static pressure the fan must overcome is the sum of the
individual resistances. A discharge cone improves fan airflow by decreasing resistance.
Properly sized and managed inlet
Shutter
Exhausting against wind
(no wind shielding)
Fan guards, clean
Discharge cone
clean
dirty
5 mph
10 mph
15 mph
20 mph
wire mesh
round ring
improves airflow
Static Pressure
in. H20
0.04
0.02 – 0.10
0.05 – 0.20
0.02
0.05
0.10
0.20
0.05 – 0.15
0.01 – 0.02
-0.03 to - 0.08
Adapted from MWPS-32 Mechanical Ventilating Systems for Livestock Housing.
open to a full horizontal position as anything less than
horizontal will decrease airflow. Guards are necessary for the safety of people and animals around the
fan and to prevent objects from damaging the fan.
Properly designed guards will disrupt airflow and
efficiency by less than 5 percent. Round ring guards
with concentric circles of wire, like a barbeque grill,
disrupt airflow less than wire mesh guards with
rectangular or square openings. Fan performance will
be improved, through decreased air turbulence, with a
well-designed housing and discharge cone. A
discharge cone and well designed housing offer at
least a 15 % airflow improvement and will provide the
fan some protection from weather.
deliver as little as 14,900 cfm for the worst performer
up to 28,400 cfm for the best fan. Average for all the
tested fans was 21,090 cfm at 0.05-inches and 19,652
at 0.10-inches. These data underscore the necessity
to check rated fan data rather than relying on a “rule
of thumb” which states that a 48-inch fan provides
20,000 cfm. Only “above average” fans provide this.
A rule of thumb is acceptable for a first estimate, but
specific rated fan data should be used in final ventilation design specifications. Energy efficiency of the
tested fans ranged from 13.5 to 28.3 cfm/W, with an
average of 20.0 cfm/W at 0.05-inches (17.9 cfm/W
average at 0.10-inches).
Summary
Maintenance
Ventilation system design should use data from fans
tested in the condition and with the accessories, such
as guards and shutters, under which they will be
operated.
Dirty shutters can decrease airflow up to 40%. Dustladen shutters are a common problem for poultry
house fans. Remove accumulated dust from fan
shutters, blades, and guards every one to three
months. This is easier and hence, more likely to be
done when the shutters are on the inside of the fan.
While cleaning the shutters, check the belt on beltdriven fans as belt slippage reduces airflow and
increases belt wear. Plan on replacing tunnel fan
belts every Spring.
Choose 48-inch diameter tunnel ventilation fans that
have:
Rated capacity of at least 20,000 cfm at 0.05-inch
static pressure difference
Energy efficiency rating of at least 20 cfm/W at
0.05-inch
To achieve these performance features, the fan will
likely have:
Shutters on the intake side for at least 10% more
airflow than shutters on the exhaust side
Shutters that open to a fully horizontal position
Discharge cone and well designed housing for at
least a 15% airflow improvement
Regular maintenance and cleaning program
Comparing Fan Performance
Fan performance can vary widely among manufacturers and models. The days of buying fans by the
“inch” are over. Figure 2 shows average, worst, and
best cases of air flow performance for nearly threehundred 48-inch fans tested at the BESS laboratory.
At 0.05-inches static pressure, a 48-inch fan may
3
30000
Best fan
Figure 2. Fans vary
tremendously in their ability
to provide airflow. This
figure shows the average
airflow capacity of almost
three-hundred 48-inch fans
as rated at BESS
laboratory. The worst and
best fan airflows are also
shown.
25000
Average of all fans tested
Airflow (cfm)
20000
15000
10000
Worst fan
5000
Normal Operating
Pressure Range
0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Static Pressure Difference (inches water)
Additional Resources
To maintain performance:
Clean dust off shutters every other month
Replace fan belt every Spring
Agricultural Ventilation Fans Performance and
Efficiencies. 1999. S.E. Ford, L.L. Christianson, G.L.
Riskowski, T.L.Funk. BioEnvironmental and Structural
Systems (BESS) Laboratory, Department of Agricultural Engineering, University of Illinois, Urbana, IL. 115
pp. www.bess.uiuc.edu
As important as fan selection is, remember that
good ventilation depends on a complete system, and
the fan is just one component. Proper selection and
maintenance of fans will ensure adequate air exchange, while inlet design is tremendously important
for proper air distribution.
AMCA Directory of Agricultural Products with
Certified Ratings, Publication 262, Edition 12A. 2000.
AMCA (Air Movement and Control Association),
Arlington Heights, IL. 40 pp. www.amca.org
Acknowledgements
Thanks to Robert Graves, Professor of Agricultural
Engineering and Dennis Buffington, Professor of
Agricultural Engineering, for their suggestions in the
technical review of this document. Steve Ford,
manager of the BESS Laboratory at University of
Illinois, provided data and fan test experiences that
improved the usefulness of the information.
1st Edition 2/02
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