Axial Flow Fans Good Design Guide

Transcription

1 Axial Flow Fans Good Design Guide 45 Heales Road, Lara VIC, Australia 3212 Phone Facsimile INTRODUCTION: Air Radiators have been manufacturing heat transfer and air movement solutions for more than 25 years. The Airscrew range of Axial Flow Fans was introduced to compliment the existing Air Radiators range, and provide efficient, soundly engineered fans that are robust, reliable, compactly designed and versatile, at an economical cost. The Airscrew range covers short and long casings, from 500 to 2000mm in diameter, with an extensive array of accessories to meet most exacting requirements. FEATURES: High Efficiency: Airscrew Axial Flow Fans are specifically designed to handle large volume flow rates at low to medium pressures with high efficiency. Reliable and Robust: Airscrew Axial Flow Fans are simple, robust and reliable, requiring very little maintenance. Compact: Airscrew fans are compact, and their in-line configuration minimises the duct cost. Their light weight is often an advantage. Proven Performance: Airscrew Fans have been manufactured and proven over many years, and a wide variety of applications. Constant Velocity Profile: Airscrew Fans have a large flow area with an approximate constant velocity profile. This makes them ideal for use in all types of heat exchange equipment. Fully Adjustable Pitch: Pitch angles can be adjusted to match system requirements and available motor power without having to dismantle the impeller. The Airscrew range utilises a standard range of largely Australian Made components, to ensure economical high quality products with minimal delivery times. BASIC COMPONENTS: Impellers: Airscrew impellers are fitted with manually adjustable pitch tapered airfoil section blades. The superior airfoil sections give high efficiency and low sound power. Advanced engineering plastics and aluminium are used for high strength, low weight and small starting torques, creep resistance and wide operating temperature ranges. Casings Long and Short: The case design ensures a robust fan suitable for most applications. The motor supports carry the loads directly into the feet, which are built in as standard for long casings, for maximum rigidity. All casings are paint finished as standard unless otherwise specified. Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 1 of 15

2 Motors: Special care has been taken to allow the use of almost any motor, provided it is foot mounted. This can minimise delivery times. Additionally, it allows one brand of motor to be used for different equipment on a specific project. The dimension tables show only the most commonly used totally enclosed fan cooled D- frames in accordance with AS1359, IEC34 and IEC72. Other frames, including B56 fractional horsepower, multi-speed and flame proof, can also be supplied. Fans may be operated in any attitude, subject only to the motor limitations. Typically this requires that medium to large motors have to be specifically ordered for shaft up or down operation. Large impellers on applications requiring only very low power may need oversized motors to ensure adequate starting torque and bearing life. Non Standard Fans Our Speciality Fan units can be assembled to meet higher pressure requirements. Details available on application. For smaller diameter fans, refer to the separate brochure. Specialised non-standard axial fan configurations are available for both general ventilation and industrial use, eg. Belt driven fans and truly reversible fans. SELECTING THE RIGHT FAN. The selection of the type of fan to suit your requirements depends on the following factors; Volume flow rate and pressure required Acceptance Noise Level Space Available Preferred Configuration Cost of the installed fan unit Once you have decided that an axial fan meets your parameters, the QUICK SELECTION GUIDE (p5) should be used to select the approximate fan size and speed. In most cases, alternatives are available. The final choice will depend on the following factors: The Cheapest Fan will usually be the one in the smallest casing possible using the highest speed impellers. The Quietest Fan will usually be the one with the lowest tip speed. The Most Efficient Fan will frequently be the one having the largest diameter. The specific performance curves for the fan selected will give more detailed information on noise level, efficiency and motor power. PERFORMANCE DATA The basic units used for the performance data in this catalogue are Systems International (SI) units. Fans have been tested as a fully ducted fan, in accordance with Test method No. 1 of BS.848: Part 1: Individual fans may vary in performance from that given on the curves due to slight variations in manufacture, but will be within the class B Tolerances of BS.848: Part 1: 1963 when used in the fully ducted arrangement. Variations in the performance due to differences of the installation from the fully ducted configuration are explained in the section on PERFORMANCE TESTING. Fan Static pressure has been retained for the presentation of fan pressure due to its wide acceptance, although in fact Fan Total Pressure is more appropriate to fans used with ducting on the outlet side, because the outlet kinetic energy is not lost. See Fan Engineering Data, for the relationship. Note: Performance curves show Total Efficiency, not Static Efficiency. Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 2 of 15

3 FAN DESIGNATION Fans can be supplied as either Type A pusher units, or Type B tractor units, see Fan Designation diagram below. When installed as a type B there is a slight improvement in duty and a reduction in noise level compared with a type A unit. However, all variations in duty come within the permitted tolerances laid down with BS.848. Ducted fans are designed and tested with ducting connected to the inlet and outlet. If a type B unit is to be installed at the inlet end of the run, unless an inlet cone is fitted, there will be a noticeable loss of performance. The higher the air velocity, i.e. with large blade angle, the greater the loss. A type A unit is not affected to the same extent, and in most cases the loss of performance is acceptable. FAN SOUND LEVELS Fan Sound Levels were determined from tests to BS848: Part 2: The levels are shown on the specific performance curves in two different ways. The sound power level figures together with the tabulated spectrum corrections are sufficient for all necessary acoustic calculations. It is most important to remember that sound power level is a fixed property of a specific fan at a given duty. Whereas the actual perceived and measured SOUND PRESSURE LEVEL will vary considerably depending on the listener s position relative to the fan and the acoustic environment in which the fan operates. Sound pressure Level (db) as shown by the lower of the two figures on the specific fan performance curves is a measure of the sound directly heard by the person standing near the open inlet or outlet at a distance of three fan diameters. The value shown is the average value under free field conditions. Actual sound pressure levels will vary with the listener s position, as the sound is not radiated uniformly into space. Sound Power Level (dbw) is the total energy radiated by the fan in the audible spectrum. It is an absolute measure and totally independent of the environment in which the fan will finally operate. Hence, sound power level should always be used when comparing different designs of fan for a given application. Unfortunately, it is not possible to measure sound power level directly. It is calculated from measured sound pressure levels under specific standardised test conditions. Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 3 of 15

4 For D-Frame-TEFC Motors, & Motor Shaft E-Frame-Flame Proof Key Size Frame 2 pole 4 Pole 6 Pole 8 Pole Diam. Length WxH Size 2900 rpm 1450 rpm 960 rpm 720 rpm 0.37/ x5 0.75/ / / x S x L x / / L x M x7 5.5/ S x / M x8 11/ / M x L x M x L x / L x S x M x S x12 37/45 30/37 250M x12 45/55 37/45 280S x14 55/75 45/55 280M x14 Most common Motor Power Ratings (kw) Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 4 of 15

5 Quick Selection Guide Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 5 of 15

6 IMPELLERS Two types of high efficiency, tapered aerofoil bladed impellers are available. Both have an outstanding record of proven performance and reliability. Airscrew Aluminium Impellers These combine aluminium alloy blades, clampblocks and side plates with cast iron or steel hubs. The impellers are robust, ensuring long life even under the most difficult operating conditions. Inertia and starting torque is minimised due to the light weight of the impeller. The STANDARD range of axial flow fans utilises three airscrew blade and hub sizes C, D and DX. Smaller diameter aluminium impellers, ie sizes A and B are also available for special applications, eg. High temperature operation. Each blade can be cropped to a number of different lengths to cover the full range of fan diameters from 500 to 2000mm. Each hub is available with a choice of two or more different numbers of blades, refer to ordering procedures, (SELECTION AND APPLICATION GUIDE). Each blade has a fully adjustable pitch through 360. These standard components combine to cover the widest possible range of applications and duties, many of which are shown in the performance curves. Using the QUICK SELECTION GUIDE (previous page) will assist with speedy selections. Fully adjustable pitch ensures that the impeller can be exactly matched to the system and the available motor power. Pitch angles can be changed on site using readily available tools. Provided sufficient access is available, the pitch can be adjusted with the impeller and fan in situ, without dismantling the impeller. Impellers are supplied with Right Handed Blades as standard. All aluminium impellers are dynamically balanced in one plane prior to installation to ensure smooth operation. Hubs are accurately bored and keyed and are retained on the motor shaft by a central set screw. Aluminium Impellers may be used from -40 C to +150 C in the duty and speed ranges indicated by the curves. Temperatures above 150 C are possible but will require de-rating of the blades, depending on the pressure/volume relationship and speed. Fan applications above +40 C require SPECIAL motor winding insulation, and in some cases special bearings, greases and even nonstandard fan cases. Impeller Characteristics Air Radiators axial flow fans are nonoverloading. To simplify selections only the maximum impeller power is shown. Power at a specific duty point may be found by using the computer selection program. Motors however, should always be selected for the maximum impeller power as the fan may run at points other than the design point during system installation and commissioning. Most axial flow impellers are subject to stall at high pressures. The Air Radiators Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 6 of 15

7 selection curves only show the pressure/volume characteristics in the normal operating range. Space restrictions do not allow the inclusion of all available impeller combination characteristic curves. If the QUICK SELECTION GUIDE does not cover your application, please contact Air Radiators for assistance. Air Flow Reversal Reversing the direction of rotation will change the performance approximately as follows; 70% of rated volume 60% of rated pressure 65% of rated power Assembling an impeller with alternative blades turned through 180 gives a Truly Reversible characteristic, giving equal air flow in both directions of operation. CASINGS Two alternative types - Long or Short Casings....plus Steel construction for strength and rigidity Long Case Fully encloses both the motor and the impeller. The long case comes in two different overall lengths, L1 and L2 to cover the full range of motor sizes as shown on the dimensional data table (p4). Two mounting feet form an integral part of the standard case. Two additional mounting feet may be supplied on the top of the fan at extra cost. The additional feet are recommended when fans are to be suspended from the ceiling or are to be vertically mounted. Fans with standard C and D frames, single speed, 3 phase motors up to 5.5 kw (7.5HP) are fitted with an isolating switch. All L-type axial fans with larger or nonstandard motors, ie., two speed, flame proof etc, can be fitted with terminal boxes will be fitted to the top of the case unless otherwise specified by the client. All long cases can be supplied with one or two access doors as an option at extra cost. Unless otherwise specified the first access door will be fitted adjacent to the motor terminal box, which on most metric motors is on the right hand side, when looking at the shaft end of the motor. Short Case Covers the impeller only and is designed primarily for mounting at the intake or discharge end of ducting, or for mounting directly to a radiator, cooling tower or similar application. As they are generally flange mounted, feet are not supplied as standard. Mounting feet are available at extra cost but must be specifically requested, furnishing complete installation details on the order as the fan structure is affected. An inspection door, switch or terminal box are not fitted as the impeller is usually visible and wiring can be run directly to the motor terminal box. If an S-type case is installed in a run of ducting, and access door should be provided in the duct. Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 7 of 15

8 S-type casings come in two different versions, S1 and S2, each with a different size motor plate to cover the full range of motors. Construction All casings are made of steel, and built to our well proven designs, ensuring high strength and rigidity. The cases are suitable for the largest motors in most industrial applications. The motor weights and torque reactions are carried as directly as possible to the mounting points. Flanges are roll-formed on casings up to 1600mm diameter and welded on casings above this size. Standard Finish Casings are hot dip galvanised as standard, however painting options are available. Fans of 1600mm diameter can be hot dip galvanised, but this requires multiple dipping due to bath capability and frequently creates delivery problems and increased costs. Zincilate, or Dimet, which is an inorganic zinc silicate, is suggested as an alternative to hot dip galvanising. MOTORS Air Radiators range of axial flow fans can be used with most foot mounted motors, thus providing: Quickest possible delivery Customer s choice of motor Ready replacement in the event of motor failure. General Specifications Mounting. Fans may be operated in any attitude, subject only to the motor limitations. Typically, this requires that medium to large motors have to be specifically ordered for shaft up or down operation, therefore it is most important to state the attitude that the fan will be operating in, when ordering, refer fan designation table (p 3). Starting Torque. Larger impellers on duties requiring only very low power may need oversized motors to ensure adequate starting torque and bearing life. In certain cases, motor end shields and drain holes must also be considered. Voltage Limitation. Unless otherwise specified, the standard motors supplied will be suitable for the following power supplies; 415 Volt, 3ph,50Hz or 240 Volt, 1ph, 50Hz Voltages or frequencies other than these are subject to negotiation at initial enquiry. Temperature Limitation. Standard motors are supplied with class E or B insulation which are suitable for ambient temperatures up to +40 C. Temperatures in excess of this require special insulation, class F or H, and will incur longer delivery times and additional cost. Most standard motors are suitable for operation down to 20 C. Below this special low temperature lubricants are required. Altitude Effects. Motors operating in altitudes exceeding 1000 metres will require de-rating. Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 8 of 15

9 Wiring. It is the customer s responsibility to ensure that all fans are wired in accordance with local codes and regulations. Rating of Motors. Exhaustive testing has shown that in specific cases motor outputs can be increased above the name plate rating without detriment to the windings. For example: the temperature of the windings is reduced if the velocity of the air over the casing is increased above the minimum required by BS2613. As a result a motor with a smaller frame may be selected. This frequently occurs when a motor is directly driving an axial fan. Note: The name plate on the fan case shows a motor rating which may differ from the values shown on the motor nameplate. The higher set of figures are the ones to use when selecting associated electronic equipment. Motor Lubrication. Grease nipples are not available as an accessory on fractional horsepower motors, ie. B56 frames, or metric motors up to frame 132. Bearings on these motors are sealed for life. Only some motor manufacturers supply greasing facilities on motors above frame size 132, therefore it is important to specify lubrication requirements at the time of enquiry (note: extra cost may be involved if greasing points are required). Based on our experience we recommend that sealed-for-life bearings be used where possible to ensure the longest motor life. Special Motors Special multi-speed, marine motors, and motors for high temperature applications can generally be supplied. Optional Motor Extras Optional motor extras such as, thermistors, tropic proofing, special enclosures, water slingers, non standard insulation, anticondensation heaters, greasing facilities, nonstandard voltages and/or frequencies, porous drain plugs etc, can generally be provided at additional cost. These options are not always available in all motor and frame sizes and should be discussed with your sales engineer during your initial enquiry to avoid delivery delays. OPTIONAL ACCESSORIES Mounting Feet Two mounting feet are supplied as standard on Long case fans only. Two additional mounting feet can be fitted to the top of the L-type case and are recommended for ceiling suspended or vertical mounted applications. Short case fans are designed primarily for flange mounting and do not have feet as standard. Feet may be fitted to S-type cases as an option but require constructional modifications and therefore must be discussed at the time of order. Similar mounting feet can be welded to circular duct attenuators as an extra. This would normally apply to attenuators direct coupled to a fan with both on anti-vibration mounts. Matching Flanges Matching flanges are drilled to match the fan and are available for all sizes to facilitate the installation of flexible connections. Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 9 of 15

10 Inlet Cones Inlet cones must be fitted to fans with a free inlet unless a significant drop from the rated performance is acceptable. For example, an Arrangement B fan (refer fan designation table), with a free inlet could suffer losses of approximately 20% in volume and 50% in pressure. The loss with an Arrangement A fan is not as great but an inlet cone is still desirable. Outlet Cones Outlet cones, providing approximately 30% static regain, are available as a standard accessory. Outlet cones with higher regain can be supplied. FAN CLASSIFICATION BY USE AND INSTALLATION Ducted Fans are used to move air within a duct, and may be defined as, Free Inlet:- direct inlet from free space, ducted outlet. FAN Free Outlet:- ducted inlet, direct outlet to free space. Fully Ducted:-ducted inlet, ducted outlet. FAN FAN Wall or Partition Fans are used for moving air from one free space to another separated from the first by a partition, having an aperture in which the fan is installed. Circulating Fans are used for circulating air within a space, unconnected to any ducting. The following notes do not apply to this class of fan. Performance Testing. Fully Ducted Fan Test Method. Tests are carried out to BS.848: Part 1 : FAN All axial flow fans are tested as Fully ducted types according to Test Method Number 1. Performance ratings are derived from such tests. Standard Performance Testing General. It should be noted that standard test methods are designed to present uniform velocity at the fan inlet, with no obstruction at fan inlet or discharge. Fan pressure is obtained from readings taken in uniform flow at some distance from the fan or in low velocity areas. Volumes are obtained by using a calibrated conical inlet to the test duct or by pilot traverse at some distance from the fan where the flow is more uniform. It is very difficult to obtain these conditions in site testing, with consequent lack of accuracy. Effect on Performance of Installation Variants. Installation requirements may decide the use of a fan in a manner different from that for which the performance ratings are appropriate eg with a free inlet or free outlet in the case of a ducted fan. Some difference in performance is to be expected and the following notes will give guidance. Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 10 of 15

11 Inlet Side 1. Ducted v Free Inlet The standard performance test is carried out with a representative inlet duct. If such a fan is used with free inlet, without a proper inlet cone, the pressure available from the fan can be reduced by as much as 50% and the volume by as much as 20%. The greatest falloff in performance occurs when the fan is installed as an arrangement B with a high blade angle. The fall-off in performance for arrangement A with high blade angle is nearer 9% and 6% respectively. 2. Bends and other fittings The use of such fittings on the inlet side of a fan will cause some reduction in performance unless provision is made to obtain a uniform velocity at the fan inlet. This may be done by using: An adequate settling length between the fitting and the fan of at least four equivalent diameters for an non symmetrical fitting such as a bend, or at least ½ equivalent diameter for a symmetrical fitting such as a diffuser, or by means of turning or distribution vanes combined with a settling length of at least ½ equivalent diameter. Convergent pieces, provided the angle of convergence to the duct axis does not exceed 22.5, will have a negligible effect on the fan performance. Outlet Side 1. Ducted v Free Outlet The standard performance test for the fans is carried out with representative outlet ducting. This can permit some recovery of static pressure in the ducting immediately on the fan outlet, but also could restrict the radial flow from the impeller tips which enables certain types of axial flow fans to have a non-stalling characteristic. Omission of outlet ducting on fans where performance ratings are given with outlet ducting could, therefore, result in a significant difference in the performance obtained. The magnitude of the difference depends on the type of fan and the position on the fan characteristic. 2. Bends, contractions and other fittings A significant reduction in the performance of a fan, and an increase in the pressure loss can occur in a fitting. This is due partly to the distortion of the normal outlet velocity profile from the fan and the lack of recovery of some static pressure, but more seriously to the non-uniform velocity presented to the fitting in place of the uniform velocity on which its calculated loss is based. Unfortunately, there is little information on the magnitude of these effects, but some typical examples are as follows: FAN a. Additional bend loss due to swirling flow from the fan. FAN b. High duct friction losses due to concentrated swirl in the smaller duct seriously reduces pressure. c. Expanders will have negligible effect on fan performance provided the angle of expansion to the duct axis does not exceed 3.5. However, it is recommended that allowance should be Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 11 of 15

12 made for an increase in the nominal total pressure loss in the expander of 20%. INSTALLATION FAULTS Installation requirements agreed to on paper are not always realised in practice. Sometimes this has severe effects on the fan and system performance. Typical instances include: Incorrect flexible connections: Badly fitted flexible connections causing high pressure losses. Badly fitting non-return dampers on stand-by fan installation allowing leakage. Incorrect Fan Rotation Reversed rotation combined with reversed installation is necessary to get the correct direction of airflow through the system. If not the fault is easily identified by the performance and power consumed being greatly reduced. Incorrect System Resistance DUCT FAN CONNECTION Slack connection drawn in to obstruct airflow on fan suction. Misalignment of ducting taken up in flexible connections. Badly fitting non-return dampers. This is classed as an installation fault as the fan has to operate at an incorrect loading. Causes of such a fault include incorrect damper setting, duct leakage, wrongly calculated system losses and duct sizes and configuration not to drawing. The figure shows that it is possible to have a much reduced volume at similar pressure to that of the design duty. Site pressure readings alone are not sufficient to indicate the true picture. Volume measurement together with measurement of pressures on both sides of the fan will show a system pressure loss greatly in excess of the design value, taking into account the low volume associated with it. Poor Maintenance Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 12 of 15

13 Reduction in fan and system performance can occur through poor maintenance, such as build-up of dirt on impeller blades or in filters, slack belt drives or loose dampers. Vibration In fan units, as with other rotating machinery, some level of vibration is inevitable. There are several causes of vibration, the most fundamental one being lack of balance in the rotating masses. Many fan impellers are balanced statically only all aluminium Airscrew impellers are dynamically balanced on two planes to minimise the unbalance couples as well as forces.??? The best commercially achievable degree of balance will still leave a small residual error to produce a rotating out of balance load, which gives the live feel of any rotating machinery. Generally fan impellers are balanced to a tolerance which will give at worst an out-ofbalance rotating load of 5% of the impeller weight, with larger installations restricted to a smaller percentage. This out of balance can be reduced further by a request for units to be installed in unusually sensitive locations. A greater degree of balancing accuracy is also required on light structures. Out of balance produces a vibrating frequency equal to rotational speed with the amplitude substantially at right angles to the axis of rotation, so that vibration from this cause is fairly easily identified. Other causes of vibration can be more difficult to trace, but may be grouped under the headings of resonance, aerodynamic or electrical. A considerable amount of vibration of the whole installation can be used by resonance of one component with the frequency of the small residual unbalance of the fan unit. The resonating part need not be adjacent to the fan; thus a resonating ducting panel some distance away can vibrate the whole installation. Changing the natural frequency of the panel by adding a stiffener would normally stop the resonance. Badly designed installations can cause surging through the fan, eg loose or flexible dampers will produce a fluctuating resistance in the system. Vibration will be of low frequency and erratic. In some instances aerodynamic vibration cannot be avoided and the structure must be adequate to withstand the loading. A typical installation of this type is a simple induced draught cooling tower where variable wind speeds will produce uneven loadings on fan blades. Fluctuating loads will, of course, be transmitted via the drive shaft to the supporting structure. The vibration frequency of electrical origin will be either at motor speed or 1 or 2 times the frequency of the AC supply. This type of vibration is readily identified as it will disappear instantly when the electrical power is switched off. FAN PERFORMANCE TESTING ON SITE Realistic ducted fan performance figures can rarely be obtained from site tests due to inadequate settling sections at the fan inlet and outlet. The ideal would be a duct installation similar to the standard test duct, thus permitting reliable pressure measurements and volumes to be obtained, and ensuring that the fan is presented with uniform flow at the inlet. However, when performance checks are needed and are carried out, the results obtained should be recognised as being of lower accuracy. The following rules should be followed: Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 13 of 15

14 1. Volume measurements in ducting should be made by pitot tube traverse preferably in a section on the inlet side of the fan in a portion of the duct run which is straight and of uniform section. A downstream settling length of at least four times the minimum dimensions of the duct cross section is necessary between a bend or fitting, and the traverse. 2. Static pressure measurements in duct are most reliably made by means of a traverse with a pitot tube. In cases where the duct velocity is low and there is no likelihood of rotational flow or swirl, a reading of static pressure at the duct wall is permissible. Pressures taken immediately after the fan outlet, particularly from an axial flow fan outlet without straighteners, will show wide variation across the duct caused mainly by swirl. Traverse planes should preferably be in a parallel section of the duct and no closer to the fan inlet than ½ diameter, and no closer to the fan outlet than 4 times the minimum dimension of the outlet. An allowance should be made for any system pressure losses between the traverse planes and the fan. It is very difficult to get any reliable pressure reading on the outlet side of an axial flow fan unless it has straighteners or is of the contra-rotating type. It is then more realistic to take a pressure reading at some distance downstream of the fan where a more reliable reading is obtainable, and add to it a nominal value for the pressure loss in the intermediate part of the duct system. 3. To determine the Fan total Pressure, the nominal velocity pressure at each of the pressure traverse sections must be first calculated from the volume and the corresponding duct area. THEN; Total pressure fan inlet = static pressure (inlet traverse) + velocity pressure duct loss Total pressure fan outlet = static pressure (outlet traverse) + velocity pressure + duct loss Fan Total Pressure = total pressure at fan outlet total pressure at fan inlet NB the static pressure and hence the total pressure at the fan inlet is usually negative with respect to the ambient pressure, and the negative sign should then be used in the above equations. 4. To determine the Fan Static Pressure, the nominal velocity pressure at the fan outlet (calculated from the volume and outlet area) is deducted from the Fan Total Pressure. 5. Consideration must be given to the actual Air density (determined from the barometric pressure and air temperature) in the calculation of velocity and velocity pressure, and in relating the fan performance to standard performance ratings. See BS.848 : Part 1 : Noise Levels. The noise levels of Air Radiators Airscrew fans are determined from tests to BS.848 : Part 2: In these tests the sound power level is determined under normal operating conditions with uniform flow into the fan inlet. The sound power level can be used to calculate mean sound pressure levels in a duct or free space. Noise data for Airscrew fans can take the form of sound power level or mean sound pressure level in space under free field conditions, each with its octave band spectrum. It should be noted that the mean sound pressure level under free field conditions can be Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 14 of 15

15 exceeded at particular microphone positions in practical installations because of directivity effects, and reflections from the walls, ceiling, floor and surrounding objects, depending on their vicinity and reflectivity. Noise levels presented in this way are those caused by normal aerodynamic functioning of the fan. They would, however, be increased by abnormal operation (eg operating in the stalled region) and by highly turbulent or non-uniform velocity at the fan inlet (eg. a damper or bend). Mechanical and electrical noises are usually of lower magnitude except outside the fan casing or its associated ducting. FAN ENGINEERING DATA: Standard Conditions: Pressure, 1 Standard atmosphere = kilonewtons/square metre or 14.7 pounds per square inch Standard Air (BS.848:Part1:1963): Barometer pressure 30 in Hg at 68 F with air density of lb/ft 3 Equivalent to kn/m 2 at 20 C with air density of 1.20 kg/m 3 Sound Levels: Sound power level (SWL) = 10 log 10 Watts dbw Sound pressure level (SPL) = 20 log 10 Pa db 2x10-5 Frequency: The preferred values of geometric mid-frequencies for measurements 63Hz, 125Hz, 250Hz etc., are given in BS NOTE: All curves show Total Efficiencies. Fan Efficiency (static or total): = Volume m 3 /s x Fan pressure (static or total) Pa Fan total efficiency Shaft Power (watts): = Volume m 3 /s x Fan total pressure Pa Fan total efficiency Fan Velocity Pressure (Pa) = 0.6 x Volume m 3 /s 2 for Standard Air Outlet area m 2 Fan Total Pressure: Fan Laws: Note: = Fan static pressure + Fan velocity pressure Volume flow varies as speed or rotation Pressure developed varies as (speed of rotation) 2 Power varies as (speed of rotation) 3 Both pressure developed and power absorbed also vary directly with air density. For further information, contact Air Radiators. Ph: Fax: mail@airrads.com.au Issue : 0 Date : 25/06/01 ECN : Approval : Uncontrolled Document - This copy will not be automatically updated. Air Radiators TECHNICAL INFORMATION BULLETIN No th June, 2001 Page 15 of 15