Pressure recovery Look for opportunities for pressure recovery at centrifugal e

Pressure recovery Look for opportunities for pressure recovery at centrifugal exhaust fan outlets. 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 15º or less. Discharge losses reduced by up to 75% in this arrangement. Look for opportunities for static pressure recovery at axial exhaust fan outlets. 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 15º or less. Discharge losses reduced substantially in this arrangement. Spinning of air at inlet connection Ensure spinning of air at fan inlet does not take place Don't A poorly designed inlet box can generate spinning of the air which can reduce performance by around 25%. Also, 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. Eccentric inlet flow Do ensure inlet air to the fan is evenly distributed. 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 Bends on fan discharge Bends near fan discharge should be in the direction of wheel rotation to prevent needless pressure loss. 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. Inlet plenums Ensure eccentric flow is not caused by inlet plenum. 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. Motor Position 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. 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. Most important aspect is that the motor is accessible and to ensure belts are correctly tensioned and aligned. The correct fan for the application 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, mixed flow or axial, the selection being governed by the fan duty and noise level required. Inlet Cones Use inlet cones for axial fan free air intakes to improve performance and noise level. Don't An abrupt entry will generate turbulence at the impeller. Note blade tips will be starved of air which reduces pressure development capability, induces 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 performance being optimised and minimum noise level generated. Flexible connection to fan Flexible connections must be taut or else turbulence at the fan inlet, noise level and pressure loss are all increased. 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. Duct restrictions at fans Ducts significantly smaller than the fan diameter create turbulence if transitions connect directly to fan. 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 that 15º Turning vanes at sharp bends Fit turning vanes in elbows adjacent to axial fans 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. Isolating vibrating fans Vibration transmission through building structures is a frequent problem. Don't Do not bolt fan directly to the building structure as noise and vibration can be transmitted directly to the building fabric. Do Isolate fan and duct from the building structure with fan on neoprene or spring isolators and suitable flexible connectors. Guard Safety of personnel around rotaing machinery is crucial. 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 disigned 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. Obstructions at fan inlets Don't obstruct fan inlets. 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. This applies to all fan types. Do Allow a gap at least equal to one 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 axial fan inlets. Obstruction at fan outlet Don't obstruct fan outlet. Don't Do Allow a gap of at least one fan diameter between fan outlet and obstruction and fit a diffuser on the discharge. Transitions Ensure symmetrical transitions from duct equipment to fan inlet. 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 Abrupt transitions immediately adjacent to an attenuator will cause the attenuator pressure drop to increase. Do Ensure transitions close to attenuators are gradual or, better still, remote. Air flow dead zones Unducted air extract fans require careful location to obtain an even airflow across the room. Don't Part of room is starved of ventilation air. Do Because of cross-flow ventilation air circulation and quality of ventilation is optimised. Roof ventilator short-circuiting of air Ensure the roof ventilator base has an airtight seal. 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. Prevailing winds Mechanical roof ventilator units sperform best if influence of prevailing wind is minimal. Don't Unit above ridge is exposed to crosswinds. Exhaust capacity may be reduced ue to back-pressure effects. Do Roof unit relatively sheltered from prevailing winds suffers less from back-pressure effects Back-draft shutters Butterfly back-draft shutters on a roof unit should be installed so gravitational effect on each leaf is the same. 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. Mounting of Roof Units to ductwork 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. Volume controle dampers 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 settling duct between volume control dampers and attenuators. Attenuators in relation to bends Don't uploads/Management/ fan-installation-guide.pdf

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H © FANTECH 2016 ATTENUATOR SELECTION PROCEDURE Low F Freq Bias N Noise dB Over 0 0

• Détails
• Publié le Dec 18, 2021
• Catégorie Management
• Langue French
• Taille du fichier 2.4371MB