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Fantech on-line ventilation fans & noise control product catalogue
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Pressure Recovery
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Look for opportunities for pressure recovery at centrifugal exhaust fan outlets.
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To determine the system resistance the
discharge losses have to be added before
selecting the fan.
With a correctly designed discharge diffuser the
pressure recovery in the diffuser will reduce the
system total pressure.
Discharge losses are highest in this arrangement
and are equivalent to one velocity head.
Included angle of diffuser to be 15o or less.
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Discharge losses reduced by up to 75% in this
arrangement.
10:23:49 AM
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Spinning of air at inlet connection
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Ensure unintentional spinning of air at fan inlet does not take place.
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Unintentional spinning of air caused by a poorly
designed inlet box can reduce performance by
around 25%.
Splitters at the inlet help prevent spinning flow.
Turning vanes, where R/W<1.0 improves
uniformity of flow approaching the fan inlet.
Avoid small inlet boxes as they can cause
surging.
Inlet boxes should be amply sized.
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Eccentric inlet flow
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Do ensure inlet air to the fan is evenly distributed.
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Eccentric air distribution at fan inlet diminishes
fan performance by up to 45% because the main
flow is accelerated through an effectively smaller
duct area on one side while turbulence and high
intensity pressure fluctuations occur on the other.
Noise level is also increased
10:27:42 AM
Rated fan performance is only achieved when air
flows evenly into the fan impeller.
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Bends on fan discharge
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Bends near fan discharge should be in the direction of wheel rotation to prevent needless
pressure loss
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Air moves faster at the outside of the fan housing
and is not evened out before striking the abrupt
bend. A very high pressure loss results.
10:28:01 AM
Improved discharge arrangement with radiused
bends and splitters or different fan mounting
position will reduce system losses. Bends should
‘rotate’ in the same direction as the impeller.
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Inlet plenums
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Ensure eccentric flow is not caused by inlet plenum.
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With a single width fan the airflow takes a single
turn to enter it; eccentric flow is created.
Selection of a double inlet fan improves airflow.
Central positioning of the fan and plenum entry
grille will also assist.
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10:28:31 AM
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Motor position
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Position the motor of a belt-driven fan such that the belt leaving the motor pulley is
uppermost as this will increase belt arc of contact.
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Do
Belt leaving motor pulley is less taut than upper
length and the slight sag will reduce arc of
contact on the drive pulley.
Arrangement with the maximum arc of contact of
the drive pulley where energy is applied, reduces
belt squeal at start-up and improves efficiency.
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10:28:48 AM
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The correct fan for the application
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Don’t use a centrifugal blower if space is at a
premium and avoid unnecessary bends.
Do consider the space saving advantages of an
in-line fan. It can be centrifugal or axial, the
selection being governed by the fan duty and
noise level required.
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10:29:08 AM
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Inlet cones
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Use inlet cones for axial fan free air intakes to improve performance and
noise level.
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An abrupt entry will generate turbulence at the
impeller. Note blade tips will be starved of air
which reduces pressure development capability,
induce stall and increase the noise level
generated by the fan.
By fitting a correctly designed inlet cone the air
flow to the impeller will be uniform resulting in the
optimum performance being achieved and
minimum noise level.
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10:30:03 AM
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Flexible connection to fan
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Flexible connections must be taut or else turbulence at the fan inlet, noise level and
pressure loss are all increased.
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Slack or misaligned flexible connectors reduce
the effective duct area. This generates turbulence
and the blade tips are starved of air. Fan
performance is reduced and noise levels
increased.
If flexible connectors are fitted they should be
remote from the fan and ensure they are taut.
The air to the impeller tips is then not obstructed
allowing the fan to operate to its optimum and
minimises noise generation.
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Duct restrictions
at fans
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Ducts significantly smaller than the fan diameter create turbulence if transitions connect
directly to fan.
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An abrupt duct expansion at fan inlet causes air
separation from the duct, starves the impeller
tips, creates turbulence, reduces performance
and generates increased noise.
Duct expansions on the inlet to axial flow fans
should be avoided where at all possible.
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10:30:47 AM
If essential the transition should have an included
angle of not more than 15o.
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Turning vanes at sharp bends
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Fit turning vanes in elbows adjacent to axial fans
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Eccentric flow conditions at both inlet and outlet
will result in part of the impeller being starved of
air and the fan not operating satisfactorily.
Square-to-round transitions and turning vanes in
elbows assists uniform airflow; this is a
compromise only and by no means ideal.
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10:31:08 AM
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Isolating vibrating fans
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Vibration transmission through building structures is a frequent problem
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Do not bolt fan directly to the building structure as
noise and vibration can be transmitted directly in
to the building fabric.
Isolate fan and duct from the building structure
with fan on neoprene or spring isolators and
suitable flexible connectors.
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10:31:18 AM
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Guards
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Safety of personnel around rotating machinery is crucial.
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Don’t leave open fan inlets unguarded. This is
unsafe to personnel and machinery. In addition it
may be illegal and leave the building owner or
installer open to prosecution.
Ensure open fan inlets are fitted with suitable
guards. These should be designed to comply with
local regulations and not only protect against
injury but lessen the risk of a foreign object
entering the fan and causing damage.
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10:31:57 AM
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Obstructions at fan inlet
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Don’t obstruct fan inlet
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The impeller can be starved of air when the inlet
to the fan is obstructed. This effectively increases
the system resistance thereby reducing the air
flow being handled by the fan.
Allow a gap at least equal to 1 fan diameter
between the fan intake and nearby obstructions,
even then fan performance can be less than
rated.
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Always fit an inlet cone on open intakes.
10:32:17 AM
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Obstructions at fan outlet
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Don’t obstruct fan outlet.
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When the discharge is too close to a wall the
obstruction generates noise and increases
discharge losses.
Allow a gap of at least one fan diameter between
fan outlet and obstruction and fit a diffuser on the
discharge.
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10:32:51 AM
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Transitions
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Ensure symmetrical transitions from duct equipment to fan inlet.
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Poor airflow - little or no airflow through top of
coil.
Centralise the fan to ensure uniform airflow
through the coil
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10:33:54 AM
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Air flow dead zones
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Unducted air extract fans require careful location to obtain an even airflow across the room
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Part of room is starved of ventilation air
Because of cross-flow ventilation air circulation and
quality of ventilation is optimised.
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10:34:19 AM
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Roof ventilator short-circuiting of air
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Ensure the roof ventilator base has an airtight seal.
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If the unit does not have an airtight seal between
the base and the upstand, short-circuiting of
outside air can occur thereby reducing the
amount of air being exhausted from the building.
An airtight seal between the upstand and fan
base prevents short-circuiting of outside air
ensuring the fan is exhausting from the
designated space.
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10:34:58 AM
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Prevailing winds
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Mechanical roof ventilator units perform best if influence of prevailing wind is minimal.
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Unit above ridge is exposed to crosswinds.
Exhaust capacity may be reduced due to backpressure effects.
Roof unit relatively sheltered from prevailing
winds suffers less from back-pressure effects.
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10:35:11 AM
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Back-draft shutters
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Butterfly back-draft shutters on a roof unit should be installed so gravitational effect on
each leaf is the same.
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Don’t have shutters at 90o to roof fall. The lower
shutter must overcome a greater gravitational
effect for even air flow. Also the top damper may
go over top dead centre and not close.
Each leaf has the same gravitational effect when
shutters are parallel with roof fall.
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Mounting of Roof Units to ductwork
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Don’t connect to the ‘eye’ of the base as this
increases pressure losses on the intake.
Size inlet duct to fit roof unit overflashing.
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10:35:57 AM
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Transitions
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Abrupt transitions immediately adjacent to an
attenuator will cause attenuator pressure drop to
increase.
Ensure transitions close to attenuators are
gradual or, better still, remote.
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10:36:21 AM
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Transitions
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Abrupt transitions immediately adjacent to an
attenuator will cause attenuator pressure drop to
increase.
Ensure transitions close to attenuators are
gradual or, better still, remote.
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10:36:40 AM
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Volume control dampers
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Don't site volume control dampers or fittings too
close to attenuators as they can cause a dramatic
increase in attenuator pressure drop.
Allow for a setting duct between volume control
dampers and attenuators.
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10:37:20 AM
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Attenuators in
relation to bends
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Don't use attenuator splitters when sited close to
a bend.
Do ensure attenuator splitters are in the plane of
the bend as shown.
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Fit turning vanes only if R/W<1.0
10:37:44 AM
