System effect factors
It has been known for may years that the ducting adjacent to a fan can have a considerable effect on the air flowrate. This applies to both the fan and ductwork itself.
Reference to Chapter 3, shows that a fan will only achieve its optimum performance when the flow at the inlet is fully developed with a symmetrical air velocity profile. It must also be free from swirl. On the fan discharge a similar situation is present. There is a need for the asymmetric profile at the discharge to diffuse efficiently and again reach a fully developed state.
In the case of fans with an inline casing, e. g. axial and mixed flow fans, there is also the possibility of residual swirl, especially if operating away from the design i. e. best efficiency point. In the case of tube axial fans, the problem can be especially severe with swirl existing up to almost 100 diameters of ducting. The only solution is to incorporate a flow straightener, which destroys the swirl, or guide vanes which can recover the swirl energy.
The system designer should therefore remember that a good arrangement of the ductwork is one that provides the above conditions at the inlet and outlet of the fan. It is his responsibility to make sure that they exist.
Ductwork engineers have been heard suggesting that due allowance should be made for less than perfect connections in fan catalogues. But how bad should they be? The reduction in flowrate for some particularly notorious examples has reached more than 60%. The first attempt in the UK at providing advice was given in the Fan Manufacturers’ Association Fan Application Guide of 1975. It has subsequently been translated into French, German and Italian by Eurovent. This however, was purely subjective — what was good, bad or indifferent.
In the USA, AMCA published the first edition of Publication 201. This attempted to give a number of ductwork examples and quantified the effect as an additional immeasurable pressure loss. It was based on some experimental evidence back up be experience. This basis is not strictly correct as it assumes that the “loss” is proportional to the velocity pressure squared. Whilst reasonably acceptable in the working range of a fan, it is less accurate close to the shut-off (static non delivery) or at the other end of the fan characteristic (free inlet and outlet).
In January 1988 the UK Department of Trade and Industry approved a grant covering 40% of the cost of a project to establish by experimental measurement at NEL (National Engineering Laboratory), the effect of commonly used, fan connected ductwork fittings on fan aerodynamic performance. These would be installed in conjunction with a number of different fan types. The results were subsequently published in abbreviated form by the FMA in 1993 as its Fan and Ductwork Installation Guide.
The ductwork designer is strongly recommended to obtain these publications. They deserve the widest possible readership.
Hopefully there would not then be so many bad examples to amuse the cognoscenti.
For the benefit of those anxious to know more immediately, the following paragraphs are appended. These are based on AMCA 201 which is much easier to use in practice.
Swirl and non-uniform flow can be corrected by straightening or guide vanes. Restricted fan inlets located too close to walls or obstructions, or restrictions caused by fans inside a cabinet, will decrease the usable performance of a fan. The clearance effect is considered a component part of the entire system and the pressure losses through the cabinet must be considered a system effect when determining system characteristics.
Installation type D fans (the Series 28 standard) have been tested with an inlet cone and parallel connection to simulate the effect of a duct. Figure 5.12 shows the variations in inlet flow which will occur. A ducted inlet condition is as (i), the unducted condition as (iv), and the effect of a bell mouth inlet as (vi). Flow into a sharp edged duct as shown in (iii) or into an inlet without a smooth entry as shown in (iv) is similar to flow through a sharp edged orifice in that a vena contracta is formed. The reduction in flow area caused by the vena contracta and the following rapid expansion causes a loss which should be considered a system effect.
V
T
Ii) Uniform flow into fan with smooth contoured inlet
V
Iv) Vena contracta at inlet reduces effective fan inlet area
Figure 5.12 Typical inlet connections for centrifugal fans
Wherever possible fans with open inlet-installation types Aor B should be fitted with bell mouths as (vi) which will enable the performance as installation types C or D to be realised.
If it is not practical to include such a smooth entry, a converging taper will substantially diminish the loss of energy and even a simple flat flange on the end of a duct will reduce the loss to about one half of the loss through an unflanged entry. The slope of transition elements should be limited to an included angle of 30° when converging or 15° when diverging. Where there is additionally a transformation from rectangular to circular; this angle should be referred to the valley.
Non-uniform flow into the inlet is the most common cause of deficient fan performance. An elbow or a 90° duct turn located at the fan inlet will not allow the air to enter uniformly and will result in turbulent and uneven flow distribution at the fan impeller. Air has weight and a moving air stream has momentum and the air stream therefore resists a change in direction within an elbow as illustrated.
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90 round Mction «fco»»—no vanes |
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90 round section «tax*—no vanes |
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I mitred round «action Hbow—no vanes |
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T—m—1 |
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Length |
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Etx>w with inlet transition—no turning vanes |
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Length, at duct |
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B Square etbow with »ile« tranaihon—three long turning vanes |
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V |
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// |
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——— 1 c Square etoow with inlet transition—short turning vanes Figure 5.15 System effects of ducts of given radius /diameter ratios expressed as velocity pressures |
D — ™ vn |
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Inlet swirl may arise from a variety of conditions and the cause is not always obvious. Some common duct connections which cause inlet swirl are illustrated in Figure 5.17, but since the variations are many, no factors are given.
Wherever possible these duct connections should be avoided, but if not, inlet conditions can usually be improved by the use of turning vanes and splitters.
Where space limitations prevent the use of optimum fan inlet connections, more uniform flow can be achieved by the use of turning vanes in the inlet elbow. Many types are available from a single curved sheet metal vane to multi-bladed aerofoils. (See Figure 5.18.)
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-MLK-
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L Distance |
System effect additional fraction of |
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Inlet to wall |
Velocity pressure at inlet |
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0.75 x dia of Inlet |
02 |
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0.5 x dia of Inlet |
04 |
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04 x dia of Inlet |
05 |
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0.3 x dia of Inlet |
0.8 |
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02 x dia of Inlet |
12 |
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Figure 5.20 System effects of fans located in common enclosures |
Mance is reduced if the distance between the fan inlet and the enclosure is too restrictive. It is usual to allow one-half of the inlet diameter between enclosure wall and the fan inlet.
Multiple DIDW fans within a common enclosure should be at least one impeller diameter apart for optimum performance. Figure 5.20 shows fans located in an enclosure and lists the system effects as additional immeasurable velocity pressure.
The way the air stream enters an enclosure relative to the fan also affects performance. Plenum or enclosure inlets of walls which are not symmetrical to the fan inlets will cause uneven flow and swirl. This must be avoided to achieve maximum performance but if not possible, inlet conditions can usually be improved with a splitter sheet to break up the swirl as illustrated in Figure 5.21.
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Figure 5.21 Use of splitter sheet to break up swirl. Above, enclosure inlet not symmetrical with fan inlet: preswirl induced. Below, flow condition improved with a splitter sheet: substantial improvement would be gained by repositioning inlet symmetrically |
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Splitter Sheet |
Figure 5.18 Pre-swirl (left) and contra-swirl (right) corrected by use of turning vanes
The pressure loss through the vanes must be added to the system pressure losses. These are published by the manufacturer, but the catalogued pressure loss will be based upon uniform air flow at entry. If the air flow approaching the elbow is non-uniform because of a disturbance further up the system, the pressure loss will be higher than published and the effectiveness of the vanes will be reduced.
Airflowstraighteners (egg crates) are often used to eliminate or reduce swirl in a duct. An example of an egg crate straightener is shown in Figure 5.19.
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All dimensions ± W%0
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«———————— D——————— • |
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T 45%D 1 |
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Figure 5.19 Example of egg crate air flow straightener
Enclosures (plenum and cabinet effects)
Fans within air handling units, plenums, or next to walls should be located so that airflows unobstructed into the inlets. Perfor-
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A reduction in fan performance can be expected when an obstruction to air flow is located in the plane of the fan inlet. Structural members, columns, butterfly valves, blast gates, and pipes are examples of more common inlet obstructions. Some accessories such as fan bearings, bearing pedestals, inlet vanes, inlet dampers, drive guards, and motors may also cause obstruction. The effects of fan bearings as in Arrangements 3 and 6 are given in Figure 5.22. For these and other examples refer to the manufacturer as they are not part of AMCA 201.
Inlet obstructions such as bearings and their supports reduce the performance of a fan. The loss takes the form of reduction of volume and pressure, the power usually remaining constant. On single inlet fans Arrangement 3 and DIDW fans Arrangement 6, bearings are mounted near the inlet venturi(s). The free passage of air into the inlet(s) is thus affected. Wherever possible Arrangement 1 fans should therefore be selected.
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Figure 5.22 Loss of performance caused by obstruction by inlet bearings and supports |
A measure of this loss is given in Figure 5.22, the degree of obstruction being assessed from the ratio
Minimum free area at plane of bearings Free area at plane of impeller eye
Where the free area is taken to mean the minimum area through which the air has to pass between the bearing and the wall of the venturi. The effect on performance is given as a reduction in volume below that which would be attained by the equivalent open inlet Arrangement 1 or 4 fan having no bearing obstruction, then taken as a percentage reduction down a constant orifice line.
Figure 5.23 gives the compensation necessary in the fan selection process to attain the required performance when using the normal open inlet curves. This adjustment can be either by:
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To compensate for bearings and supports, increase running speed by N% after selection on open inlet curve or Increase duty volume by N% and pressure as the (volume)2 before selecting fan on open inlet curve
Area ratio Figure 5.23 Compensation in fan selection required, using open inlet curves |
Drive guards obstructing the inlet
Arrangement 6 fans may require a belt drive guard in the fan inlet. Depending on design, the guard may be located at the plane of the inlet, or it may be “stepped out”. Depending on the location of the guard, and on the inlet velocity, the fan performance may be significantly affected by this obstruction.
It is desirable that a drive guard in this position has as much opening as possible to allow maximum flow to the fan inlet. However, the guard design must comply with applicable Health & Safety Act requirements.
System effect factors for drive guards situated at the inlet of a fan may be approximated as 0.4 x inlet velocity pressure where 5%ofthefan inlet area is obstructed increasing to 2.Ox inlet velocity pressure where it is 50%.
The velocity profile at the outlet of a fan is not uniform, but is shown in Figure 5.24. The section of straight ducting on the fan outlet should control the diffusion of the velocity profile, making this more uniform before discharging into a plenum chamber or to the atmosphere.
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Figure 5.24 Velocity profile at fan outlet (see also Figure 5.25) |
Alternatively, where there is a ducting system on the fan outlet, the straight ducting is necessary to minimise the effects of bends, etc.
The full effective duct length is dependent on duct velocity and may be obtained from Figure 5.25.
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Duct velocity m/s Figure 5.25 Full effective duct length expressed in equivalent duct diameters |
• Increasing the volume by N% and the pressure as the volume squared before the fan is selected.
The power taken by the fan with inlet bearings will be approximately the same as a fan with open inlet, at the same speed. It will thus be necessary to increase the power for a given duty by N3 % (see Figure 5.23).
If the duct is rectangular with side dimensions a and b, the equivalent duct diameter equals
The effect of outlet bends depends on their orientation relative to the fan and also on the ratio of throat area to outlet area is
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Figure 5.28 Branches located too close to fan |
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Throat area Outlet area |
Outlet Elbow Position |
No outlet duct |
X Effective Duct |
Va Effective Duct |
Vi Effective Duct |
Full Effective Duct |
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0.4 |
1 |
3.0 |
2.5 |
2.0 |
0.8 |
No system effect |
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2 |
5.0 |
4.0 |
2.5 |
1.2 |
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3 |
6.0 |
5.0 |
3.0 |
1.5 |
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4 |
6.0 |
5.0 |
3.0 |
1.5 |
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0.5 |
1 |
2.0 |
1.5 |
1.2 |
0.5 |
No system effect |
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2 |
3.0 |
2.2 |
1.7 |
0.8 |
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3 |
4.0 |
3.0 |
2.2 |
1.0 |
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4 |
4.0 |
3.0 |
2.2 |
1.0 |
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0.63 |
1 |
1.5 |
1.5 |
1.0 |
0.3 |
No system effect |
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2 |
2.0 |
1.5 |
1.2 |
0.5 |
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3 |
3.0 |
2.2 |
1.7 |
0.8 |
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4 |
2.5 |
2.0 |
1.5 |
0.7 |
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0.67 |
1 |
0.7 |
0.5 |
0.3 |
0.2 |
No system effect |
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2 |
1.0 |
0.8 |
0.5 |
0.3 |
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3 |
1.5 |
1.2 |
0.8 |
0.3 |
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4 |
1.2 |
1.0 |
0.7 |
0.3 |
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0.8 |
1 |
0.8 |
0.7 |
0.4 |
0.2 |
No system effect |
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2 |
1.2 |
1.0 |
0.7 |
0.3 |
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3 |
1.5 |
1.5 |
1.0 |
0.3 |
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4 |
1.5 |
1.2 |
0.8 |
0.3 |
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0.88 — 0.89 |
1 |
0.7 |
0.5 |
0.3 |
0.2 |
No system effect |
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2 |
1.0 |
0.8 |
0.5 |
0.3 |
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3 |
1.2 |
1.0 |
0.7 |
0.3 |
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4 |
1.0 |
0.8 |
0.5 |
0.3 |
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1.0 |
1 |
1.0 |
0.8 |
0.5 |
0.3 |
No system effect |
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2 |
0.7 |
0.5 |
0.4 |
0.2 |
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3 |
1.0 |
0.8 |
0.5 |
0.3 |
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4 |
1.0 |
0,8 |
0.5 |
0.3 |
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Table 5.2 System effect factors for outlet elbows for SISW fans |
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Figure 5.26 Outlet duct elbows
Shown in Figure 5.26 and Table 52 gives the system effect factors for SISW fans. (For DIDW fans use the appropriate multiplier from the following: Elbow Position No 2 x 1.25, Elbow Position No 4 x 0.85, Elbow Positions No 1 & No 3 x 1.00.)
The use of an opposed blade damper is recommended when volume control is required at the fan outlet and there are other system components, such as coils or branch takeoffs downstream of the fan. When the fan discharges into a large plenum or to free space a parallel blade damper may be satisfactory.
For a centrifugal fan, best air performance will be achieved by installing the damper with its blades perpendicular to the fan shaft; however, other considerations may require installation of the damper with its blades parallel to the fan shaft. Published
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Throat area outlet area |
SP multiplier |
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0.4 |
7.5 |
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0.5 |
4.8 |
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0.63 |
3.3 |
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0.67 |
2.4 |
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0.8 |
1.9 |
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0.88 |
1.5 |
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0.89 |
1.5 |
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1.0 |
1.2 |
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Table 5.3 Pressure loss multipliers for volume control dampers |
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Pressure losses for control dampers are based upon uniform approach velocity profiles.
When a damper is installed close to the outlet of a fan the approach velocity profile is non-uniform and much higher pressure losses through the damper can result, see Figure 5.27. The multipliers in Table 5.3 should be applied to the damper manufacturer’s catalogued pressure loss when the damper is installed at the outlet of a centrifugal fan.
Where branches are fitted on the fan outlet, a section of straight is especially important, see Figure 5.28. Split or duct branches should not be located close to the fan discharge. Astraight section of duct will allow for air diffusion.
Posted in Fans Ventilation A Practical Guide
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