Don't

To determine the system resistance the discharge losses have to be added before selecting the fan.

Discharge losses are highest in this arrangement and are equivalent to one velocity head.

Do

With a correctly designed discharge diffuser the pressure recovery in the diffuser will reduce the system total pressure.

Included angle of diffuser to be 15° or less.

Discharge losses reduced by up to 75% in this arrangement.

Don't

To determine the system resistance the discharge losses have to be added before selecting the fan.

Discharge losses are highest in this arrangement and are equivalent to one velocity head.

Do

With a correctly designed discharge diffuser the pressure recovery in the diffuser will reduce the system total pressure.

Included angle of diffuser to be 15° or less.

Discharge losses reduced by up to 75% in this arrangement

Don't

Unintentional spinning of air caused by a poorly designed inlet box can reduce performance by around 25%.

Avoid small inlet boxes as they can cause surging.

Do

Splitters at the inlet help prevent spinning flow. Turning vanes, where R/W<1.0 improves uniformity of flow approaching the fan inlet.

Inlet boxes should be amply sized.

Don't

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

Do

Rated fan performance is only achieved when air flows evenly into the fan impeller.

Don't

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.

Do

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.

Don't

With a single width fan the airflow takes a single turn to enter it; eccentric flow is created.

Do

Selection of a double inlet fan improves airflow. Central positioning of the fan and plenum entry grille will also assist.

Don't

Belt leaving motor pulley is less taut than upper length and the slight sag will reduce arc of contact on the drive pulley.

Do

Arrangement with the maximum arc of contact of the drive pulley where energy is applied, reduces belt squeal at start-up and improves efficiency.

Don't

Don’t use a centrifugal blower if space is at a premium and avoid unnecessary bends.

Do

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.

Don't

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.

Do

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.

Don't

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.

Do

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.

Don't

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.

Do

Duct expansions on the inlet to axial flow fans should be avoided where at all possible.

If essential the transition should have an included angle of not more than 15°.

Don't

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.

Do

Square-to-round transitions and turning vanes in elbows assists uniform airflow; this is a compromise only and by no means ideal.

Don't

Do not bolt fan directly to the building structure as noise and vibration can be transmitted directly in to the building fabric.

Do

Isolate fan and duct from the building structure with fan on neoprene or spring isolators and suitable flexible connectors.

Don't

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.

Do

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.

Don't

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.

Do

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.

Always fit an inlet cone on open intakes.

Don't

When the discharge is too close to a wall the obstruction generates noise and increases discharge losses.

Do

Allow a gap of at least one fan diameter between fan outlet and obstruction and fit a diffuser on the discharge.

Don't

Poor airflow - little or no airflow through top of coil.

Do

Centralise the fan to ensure uniform airflow through the coil

Don't

Part of room is starved of ventilation air

Do

Because of cross-flow ventilation air circulation and quality of ventilation is optimised.

Don't

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.

Do

An airtight seal between the upstand and fan base prevents short-circuiting of outside air ensuring the fan is exhausting from the designated space.

Don't

Unit above ridge is exposed to crosswinds. Exhaust capacity may be reduced due to back-pressure effects.

Do

Roof unit relatively sheltered from prevailing winds suffers less from back-pressure effects.

Don't

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.

Do

Each leaf has the same gravitational effect when shutters are parallel with roof fall.

Don't

Don’t connect to the ‘eye’ of the base as this increases pressure losses on the intake.

Do

Size inlet duct to fit roof unit overflashing.

Don't

Abrupt transitions immediately adjacent to an attenuator will cause attenuator pressure drop to increase. .

Do

Ensure transitions close to attenuators are gradual or, better still, remote.

Don't

Don't site volume control dampers or fittings too close to attenuators as they can cause a dramatic increase in attenuator pressure drop.

Do

Allow for a setting duct between volume control dampers and attenuators.

Don't

Don't use attenuator splitters when sited close to a bend.

Do

Do ensure attenuator splitters are in the plane of the bend as shown. Fit turning vanes only if R/W<1.0

Don't

Don't site a rectangular attenuator close to an axial fan intake or discharge. Air is accelerated through an attenuator and fan discharge air is uneven and swirling.

Do

Allow adequate distance between attenuator and fans

Don't

Don't site pod attenuators immediately upstream of fan as the attenuator pod will block air flow to fan impeller. However, if the pod is the same size or smaller than the impeller hub bolting directly to the fan is acceptable.

Do

Allow 1 diameter of straight duct between pod attenuator and fan inlet to ensure impeller obtains even flow.

Don't

If attenuator is installed as shown, noise from the fan can bypass the attenuator and enter the conditioned space. This is known as flanking transmission and can seriously denigrate the attenuator performance.

Do

Installing the attenuator through or against the wall minimised the chance of flanking transmission via the duct system.

Don't

Don’t place a silencer immediately in front of a louvre.

Do

Do ensure that the splitter orientation is at 90° to the louvre orientation and place a spacer between the silencer and discharge louvre.

Don't

Don’t place a silencer immediately downstream of a louvre.

Do

Do place a spacer between the louvre and the silencer and ensure that the splitter orientation is at 90° to the louvre orientation.

Don't

Don’t select a silencer with high attenuation and low percentage open area without checking the airflow generated noise level.

Do

Do check silencer airflow generated noise levels when these conditions occur. If airflow generated noise is too high, try using a longer silencer with a larger open area to get the same attenuation at a lesser airflow generated noise, or acoustically treat the duct after the silencer.

As a general rule of thumb, a pressure drop of 30 Pa should limit the airflow generated noise of a silencer to an acceptable amount in normal office areas.

Don't

Don’t bridge the isolated wall to the non-isolated wall with the silencer fixings.

Do

Do fix the silencer to one wall only, and place a flexible or resilient seal on the isolated wall.

Don't

Don’t: When trying to achieve very high attenuation or very low noise levels (eg. NR20) do not use an inadequate seal between the silencer and wall opening.

Do

Do: Place a concrete sleeve around the silencer to increase the acoustic seal between rooms. For less stringent requirements, pack around the silencer with a resilient material to affect a complete seal between the attenuator and the opening.

Don't

Don’t place a rectangular silencer hard against the discharge of a centrifugal fan. Note the splitter orientation.

Do

Do place a spacer between the fan and the silencer, and rotate the splitter orientation by 90°.

Don't

Do not place fans in ceiling spaces directly above noise sensitive areas.

Do

Better: Do place fans in ceiling spaces away from noise sensitive areas.

Best: If fan cannot be relocated, wrap fan and surrounding ductwork with a noise barrier material (When wrapping fans pay particular attention to ensuring there are no holes at the joins). Allow sufficient overlap in the wrap to ensure adequate coverage. Remember that flexible connections will be the weakest

Don't

Don't overload spring mounts.

Do

Ensure that spring mounts are sized correctly for the job. Be careful to include the weight of ductwork (and any other piece of equipment) if they are going to be supported by the vibration isolators. Also ensure spring mounts incorporate neoprene or rubber pads to eliminate noise which can travel through a steel spring.

Don't

Don't allow short circuiting of vibration mounts.

Do

Do ensure vibration mounts are free from short circuiting by making sure that the hanger rods are central and not in contact with the hanger cage. Ensure no debris, or other connection is creating another path through which the vibration could transmit.

Don't

Do not hard mount roof mounted fans to upstands.

Do

Do isolate roof mounted fans from the duct work, and upstand by using a foam tape between the top of the upstand and the fan. Where the fan is fixed to the upstand with bolts or screws use rubber / neoprene grommets.

Don't

When generated noise could be a problem, and a quieter fan cannot be selected.

Caution: No acoustic treatment.

Do

Better: Replace windband of fan with silencer.

Best: Change to an inline fan running parallel to the roof, and place a rectangular silencer on the outdoor side. Direct the duct opening away from noise sensitive areas.

Don't

When the fan could exceed the internal background noise, and a quieter fan is not available for selection.

Caution: No Acoustic treatment.

Do

Place an acoustic baffle below the fan inlet / discharge.

Don't

Don't allow high velocity air to pass through grilles.

Do

Do ensure velocities through supply and exhaust air grilles are low by increasing the grille size or number of grilles.

Size ductwork for constant static pressure to each grille, thus eliminating or minimising the need for balancing damper adjustment (which could can generate excessive noise).

Don't

Don't use sharp bends or takeoffs.

Do

Do use sweeping bends and takeoffs. Use long chord turning vanes where possible. Keep velocities low to reduce airflow generated noise levels.