In the field, however, inlet and outlet conditions rarely mimic those of laboratory-test setups, as the installation and ducting are influenced by the existing infrastructure and space limitations.
System effect can be avoided by accounting for all factors, including the shape of the transition points between the fan and existing ducts, ductwork configuration close to the fan, and accessories. Even minor improvements to airflow stability can reduce system effect and, in turn, increase fan performance and operating efficiency.
An inlet vane damper is a modulating device that affects fan performance. As a damper is closed, air begins to pre-spin into the fan; the fan wheel no longer can move as much air, and flow, pressure, and brake horsepower all decrease. Even when a damper is fully open, the vanes interfere with normal flow and reduce fan performance.
If the losses are not accounted for, the fan will have to run at a speed higher than the one specified during fan selection. Fans should be tested with accessories that determine the losses and the fan speed required to overcome them. Take, for example, the degree round elbow located at the inlet of a fan shown in Figure 2.
Air entering the fan wheel is not uniform, loading on various parts of the fan wheel instead of at the center as designed for optimal performance. Some air is circulating back into the elbow, creating additional losses.
In this case, the addition of turning vanes in the elbow will help direct air toward the center of the fan wheel. Figure 3 shows the effect of a rectangular inlet box mounted directly to the inlet of a fan. Again, the fan wheel is not being uniformly loaded, resulting in performance loss. With an inlet box, the cavity, or dead area, below the outlet is where air will get hung up, creating additional losses.
Improving the shape of the inlet box or adding straightening vanes can help to redirect the flow of air into the wheel. Figure 4 shows air entering a fan from the side as opposed to straight through the inlet. The air is spinning in the opposite direction of the fan wheel. Consequently, the fan is having to work harder, resulting in greater energy consumption and stress on fan components. Symbols and Subscripts.
Fan Testing. Fan Ratings. Catalog Performance Tables. Air Systems. Post on Nov 94 views. Category: Documents 27 download. Tags: amca international standards amca standards performance of fans international authority system effectfactor duct system designcan duct system configurations duct system design effect. Reproduction or translation of any part of this work beyond that permitted by Sections and of the United States Copyright Act without the permission of the copyright owner is unlawful.
Rick Bursh Illinois Blower, Inc. Sutton G. A reduction in airflow area is caused by the vena contracta and the following rapid expansion causes a loss that should be considered as a System Effect.
If it is not practical to include such a smooth entry, a converging taper d will substantially diminish the a. AMCA R 9. The System Effect Curves shown in Figure 9. The number of blades did not have a significant affect on the inlet elbow SEF. The SEF for a particular elbow is found in Figure 7. This pressure loss should be added to the friction and dynamic losses already determined for the particular elbow.
These improvements help maintain uniform airflow 9. Nonuniform airflow into a fan inlet, Figure 9. AMCA R into the fan inlet and thereby approach the airflow conditions of the laboratory test setup.
Occasionally, where space is limited, the inlet duct will be mounted directly to the fan inlet as shown on Figure 9. The many possible variations in the width and depth of a duct influence the reduction in performance to varying degrees and makes it impossible to establish reliable SEF.
Existing installations can be improved with guide vanes or the conversion to square or mitered elbows with guide vanes, but a better alternative would be a specially designed inlet box similar to that shown in Figure 9. Inlet boxes are added to centrifugal and axial fans instead of elbows in order to provide more predictable inlet conditions and to maintain stable fan performance. The fan manufacturer should include the effect of any inlet box on the fan performance, and when evaluating a proposal it should be established that an appropriate loss has been incorporated in the fan rating.
Should this information not be available from the manufacturer, refer to Section A counter-rotating vortex at the inlet may result in a slight increase in the pressure-volume curve but the power will increase substantially.
There are occasions, with counter-rotating swirl, when the loss of performance is accompanied by a surging airflow. In these cases, the surging may be more objectionable than the performance change. Inlet spin may arise from a great variety of approach conditions and sometimes the cause is not obvious. An example of this condition is illustrated in Figure 9. An ideal inlet condition allows the air to enter uniformly without spin in either direction.
A spin in the same direction as the impeller rotation pre-rotation reduces the pressure- volume curve by an amount dependent upon the intensity of the vortex. The effect is similar to the change in the pressure-volume curve achieved by variable inlet vanes installed in a fan inlet; the vanes induce a controlled spin in the direction of impeller rotation, reducing the airflow, pressure and power see Section AMCA R airflow entering a duct elbow with turning vanes will leave the duct elbow with non-uniform airflow.
Numerous variations of turning vanes are available, from a single curved sheet metal vane to multi-bladed "airfoil" vanes. The pressure drop loss through these devices must be added to the system pressure losses.
The amount of loss for each device is published by the manufacturer, but it should be realized that the cataloged pressure loss will be based upon uniform airflow at the entry to the elbow. If the airflow approaching the elbow is significantly non-uniform because of a disturbance farther upstream in the system, the pressure loss through the elbow will be higher than the published figure.
A non-uniform 9. A single splitter sheet may be used to eliminate swirl in some cases. Straighteners are intended to reduce swirl before or after a fan or a process station. Do not install straighteners where the air profile is known to be non-uniform, the device will carry the non-uniformity further downstream. AMCA R 0. AMCA R half impeller diameter between an enclosure wall and the fan inlet. Adjacent inlets of multiple double width centrifugal fans located in a common enclosure should be at least one impeller diameter apart if optimum performance is to be expected.
Fan performance is reduced if the space between the fan inlet and the enclosure is too restrictive. AMCA R The manner in which the air stream enters an enclosure in relation to the fan inlets also affects fan performance. If this is not possible, inlet conditions can usually be improved with a splitter sheet to break up the inlet vortex as illustrated in Figure 9. Some accessories such as fan bearings, bearing pedestals, inlet vanes, inlet dampers, drive guards and motors may also cause inlet obstruction and are discussed in more detail in Section Obstruction at the fan inlet may be defined in terms of the unobstructed percentage of the inlet area.
Because of the shape of the inlet cones of many fans it is sometimes difficult to establish the area of the fan inlet. Where an inlet collar is provided, the inlet area is calculated from the inside diameter of this collar.
Where no collar is provided, the inlet plane is defined by the points of tangency of the fan housing side with the inlet cone radius. For proper performance of axial fans in parallel installations minimum space of one impeller diameter should be allowed between fans, as shown in Figure 9. Placing fans closer together can result in erratic or uneven airflow into the fans.
The unobstructed percentage of the inlet area is calculated by projecting the profile of the obstruction on the profile of the inlet. The adjusted inlet velocity obtained is then used to enter the System Effect Curve chart and the SEF determined from the curve listed for that unobstructed percentage of the fan inlet area. PreRotational Vortex Induced Figure 9. MIN Figure 9. Effects of Factory Supplied Accessories Unless the manufacturer's catalog clearly states to the contrary, it should be assumed that published fan performance data does not include the effects of any accessories supplied with the fan.
If possible, the necessary information should be obtained directly from the manufacturer. The data presented in this section are offered only as a guide in the absence of specific data from the fan manufacturer.
See Figure AMCA R With a belt driven axial flow fan it is usually necessary that the fan motor be mounted outside the fan housing see Figure 3.
These components may have an effect on the flow of air into the fan inlet and consequently on the fan performance, depending upon the size of the bearings and supports in relation to the fan inlet opening.
The location of the bearing and support, that is, whether it is located in the actual inlet or "spaced out" from the inlet, will also have an effect. In cases where manufacturer's performance ratings do not include the effect of the bearings and supports, it will be necessary to compensate for this inlet restriction. Use the fan manufacturer's allowance for bearings in the fan inlet if possible. If no better data are available, use the procedures described in Section 9. To protect the belts from the airstream, and also to prevent any air leakage through the fan housing, manufacturers in many cases provide a belt tube.
Most manufacturers include the effects of an axial fan belt tube in their rating tables. In cases where the effect is not included, the appropriate SEF is approximated by calculating the percentage of unobstructed area of air passage way and using Figure 9.
Fans located less than 2. National, federal, state and local rules, regulations, and codes should be carefully considered and followed. Arrangement 3 and 7 fans may require a belt drive guard in the area of the fan inlet. Depending on the design, the guard may be located in the plane of the inlet, along the casing side sheet, or it may be "spaced out" due to "spaced out" bearing pedestals.
In any case, 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 located in this position be furnished with as much opening as possible to allow maximum flow of air to the fan inlet. If available, use the fan manufacturer's allowance for drive guards obstructing the fan inlet.
SEF for drive guard obstructions situated at the inlet of a fan may be approximated using Figure 9. Where possible, open construction on guards is recommended to allow free air passage to the fan inlet.
This data should be available from the fan manufacturer. In the absence of fan manufacturer's data, a well-designed inlet box should approximate System Effect Curves "S" or "T" of Figure 7. Either parallel or opposed blades may be used see Figure The parallel blade type is installed with the blades parallel to the fan shaft so that, in a partially closed position, a forced inlet vortex will be generated.
The effect on the fan characteristics will be similar to that of a variable inlet vane control. The opposed blade type is used to control airflow by the addition of pressure loss created by the damper in a partially closed position.
If possible, complete data should be obtained from the fan manufacturer giving the System Effect of the inlet box and damper pressure drop over the range of application. AMCA R When variable inlet vanes are supplied by the fan manufacturer, the performance should include the effects of the variable inlet vane unit.
They are arranged to generate an inlet vortex pre-rotation that rotates in the same direction as the fan impeller. Variable inlet vanes may be of two different basic types: 1 cone type integral with the fan inlet, 2 cylindrical type add-on Figures If data are not available from the fan manufacturer the following System Effect Curves should be applied in making the fan selection. Time The unit of time is the second in both systems Velocity ft-s 0.
Dual Fan Systems - Series and Parallel It is sometimes necessary to install two or more fans in systems that require higher pressures or airflow than would be attainable with a single fan. Two fans may offer a space, cost, or control advantage over a single larger fan, or it may be simply a field modification of an existing system to boost pressure or airflow. The fans may be mounted as close as the outlet of one fan directly attached to the inlet of the next fan, or they may be placed in remote locations with considerable distance between fans.
The fans must handle the same mass airflow, assuming no loss or gains between stages. The velocity pressure corresponds to the air velocity at the outlet of the last fan stage. The static pressure for the combination is the total pressure minus the velocity pressure and is not the sum of the individual fan static pressures.
In practice there is some reduction in airflow due to the increased air density in the later fan stage s. There can also be significant loss of airflow due to non-uniform airflow into the inlet of the next fan. This combination is seldom used in conventional ventilating and air conditioning systems but it is not uncommon in special industrial systems.
It is advisable to request the fan manufacturer to review the proposed system design and make some estimate of its installed performance. These types of systems normally have common inlet and outlet sections, or they may have individual ducts of equal resistance that join together at equal velocities.
In either case, the characteristic curve is the sum of the separate airflows for a given static or total pressure Figure B. The total performance of the multiple fans will be less than the theoretical sum if inlet conditions are restricted or the airflow into the inlets is not straight see Section 9. Also, adding a parallel fan to an existing system without modifying the resistance larger ducts, etc.
Figure B. The closed loop to the left of the peak pressure point is the result of plotting all the possible combinations of volume airflow at each pressure.
If the system curve intersects the combined volume-pressure curve in the area enclosed by the loop, more than one point of operation is possible. This may cause one of the fans to handle more of the air and could cause a motor overload if the fans are individually driven.
This unbalanced airflow condition tends to reverse readily with the result that the fans will intermittently load and unload. This "pulsing" often generates noise and vibration and may cause damage to the fans, ductwork or driving motors. Aileron controls in forward curved fan outlets or dampers near the inlets or outlets may be used to correct unbalanced airflow or to eliminate pulsations or reversing operation See Figure B.
Definitions and Terminology C. The velocity at a plane Vx is the average velocity throughout the entire area of the plane. Barometric pressure pb is the absolute pressure exerted by the atmosphere at a location of measurement per AMCA Static pressure is the portion of the air pressure that exists by virtue of the degree of compression only.
If expressed as gauge pressure, it may be negative or positive per AMCA Static pressure at a specific plane Psx is the arithmetic average of the gauge static pressures as measured at specific points in the traverse of the plane. Velocity pressure is that portion of the air pressure which exists by virtue of the rate of motion only. It is always positive per AMCA Velocity pressure at a specific plane Pvx is the square of the arithmetic average of the square roots of the velocity pressures as measured at specific points in the traverse plane.
Total pressure is the air pressure that exists by virtue of the degree of compression and the rate of motion. It is the algebraic sum of the velocity pressure and the static pressure at a point. Thus if the air is at rest, the total pressure will equal the static pressure per AMCA Total pressure at a specific plane Ptx is the algebraic sum of the static pressure and the velocity pressure at that plane.
A density of 1. The dry-bulb temperature td is the air temperature measured by a dry temperature sensor. Temperatures relating to air density are usually referenced to the fan inlet.
The wet-bulb temperature tw is the temperature measured by a temperature sensor covered by a water-moistened wick and exposed to air in motion. This volume also has good pressure characteristics. Power reaches maximum near peak efficiency and becomes lower, or self-limiting, toward free delivery.
They are not intended to provide complete selection criteria, since other parameters, such as diameter and speed, are not defined. Fans designed for use other than with duct systems are usually rated over a lower range of pressures. The performance of fans intended for use with duct systems is usually published in the form of a "multirating" table. A typical multi-rating table, as illustrated in Figure 5.
Typical fans in this group are propeller fans and power roof ventilators. They are usually available in direct or belt-drive arrangements and performance ratings are published in a modified form of the multirating table. Figure 5. A brief study of this figure will assist in understanding the relationship between curves and the multi-rating tables.
SIZE No. Outlet Vel. Most performance tables do not cover the complete range from no delivery to free delivery but cover only the typical operating range. Comparison of Figure 5.
Air Systems 6. A more complicated system may include a fan, ductwork, air control dampers, cooling coils, heating coils, filters, diffusers, sound attenuation, turning vanes, etc. It should be remembered that fans are generally tested without obstructions in the inlet and outlet and without any optional airstream accessories in place. Catalog ratings will, therefore, usually apply only to the bare fan with unobstructed inlet and outlet.
Fan performance adjustment factors for airstream accessories are normally available from either the fan catalog or the fan manufacturer. Every system has a combined resistance to airflow that is usually different from every other system and is dependent upon the individual components in the system. Fans are usually tested in arrangement 1, or similar see Figure 3. Rating tables will, therefore, also apply only to the tested arrangement.
Allowances for the effect of bearing supports used in other arrangements should be obtained from the manufacturer if not shown in the catalog. The determination of the "pressure loss" or "resistance to airflow," for the individual components can be obtained from the component manufacturers. In a later section, the effects of some system components and fan accessories on fan performance are discussed. The System Effects presented will assist the system designer to determine fan selection. The system curve of a "fixed system" plots as a parabola in accordance with the above relationship.
Typical plots of the resistance to flow versus volume airflow for three different and arbitrary fixed systems, A, B, and C are illustrated in Figure 6. For a fixed system an increase or decrease in airflow results in an increase or decrease in the system resistance along the given system curve only.
Also, as the components in a system change, the system curve changes. If the airflow is changed, the resulting pressure loss, or resistance to airflow, will also change. The relationship between airflow pressure and loss can vary as a function of type of duct components, their interaction and the local velocity magnitude.
In many cases, typical duct systems operate in the turbulent flow regime and the pressure loss can be approximated as a function of velocity or airflow squared. The simplifying relationship used in this publication governing the change in pressure loss as a function of airflow for a fixed system is:. Refer to Figure 6.
In Figure 6. Notice that on a percentage basis, the same relationships also hold for System Curves B and C. These relationships are characteristic of typical fixed systems. Since the system components did not change, System Curve A remains the same.
The point of intersection of the system curve and the fan performance curve determines the actual airflow. System Curve A in Figure 6. The airflow through the system in a given installation may be varied by changing the system resistance.
This is usually accomplished by using fan dampers, duct dampers, mixing boxes, terminal units, etc. The greater airflow moved by the fan against the resulting higher system resistance to airflow is a measure of the increased work done. In the same system, the fan efficiency remains the same at all points on the same system curve. This is due to the fact that airflow, system resistance, and required power are varied by the appropriate ratio of the fan rotational speed.
Figure 6. Similarly, the airflow can be increased by decreasing the resistance to airflow, i. The new operating point. Kp is taken as equal to unity in this and following examples. The resistance of a duct system is dependent upon the density of the air flowing through the system. Note again that the intersection of the actual system curve and the fan curve determine the actual airflow.
However, when system pressure losses have not been accurately estimated as in Figure 6. This condition is generally a result of inaccurate allowances of system resistance. All pressure losses must be considered when calculating system resistance or the actual system will be more restrictive to airflow than intended. If the actual duct system pressure loss is greater than design, an increase in fan speed may be necessary to achieve Point 5, the design airflow.
CAUTION: Before increasing fan rotational speed, check with the fan manufacturer to determine whether the fan rotational speed can be safely increased. Also determine the expected increase in power. Since the power required increases as the cube of the fan rotational speed ratio, it is very easy to exceed the capacity of the existing motor and that of the available electrical service.
Curve C in Figure 6. This condition results in an actual airflow at Point 3, which is at a lower pressure and higher airflow than was expected. The system resistance could also be increased to Point 1 on Curve A, Figure 6. The change in fan operating point should be evaluated carefully, since a change in fan power consumption may occur.
In some cases, safety factors may compensate for resistance losses that were unaccounted for and the actual system will deliver the design airflow, Point 1, Figure 6. If the actual system resistance is lower than the design system resistance, including the safety factors, the fan will run at Point 3 and deliver more airflow.
This result may not be advantageous because the fan may be operating at a less efficient point on the fans performance curve and may require more power than a properly designed system. Under these conditions, it may be desirable to reduce the fan performance to operate at Point 4 on Curve C, Figure 6.
This may be accomplished by reducing. The system designer should also evaluate the fan performance tolerance and system resistance tolerance to determine if the lower or upper limits of the probable airflow in the system are acceptable.
The combination of these tolerances should be evaluated to ensure that the high-side system resistance curve does not fall into the unstable range of performance. Operation in this area of the curve should be avoided and precautions taken to ensure operations outside of the unstable area, especially at the highest expected system resistance.
Any one or a combination of these conditions that alter the aerodynamic characteristics of the air flowing through the fan such that the fans full airflow potential, as tested in the laboratory and cataloged, is not likely to be realized. Other major causes of deficient performance are: The air performance characteristics of the installed system are significantly different from the system designer's intent See Figure 6.
This may be due to a change in the system by others or unexpected behavior of the system during operation. The system design calculations did not include adequate allowances for the effect of accessories and appurtenances See Section The fan selection was made without allowing for the effect of appurtenances on the fan's performance See Section Dirty filters, dirty ducts, dirty coils, etc. The "performance" of the system has been determined by field measurement techniques that have a high degree of uncertainty.
Other "on-site" problems are listed in AMCA Publication Troubleshooting, which includes detailed checklists and recommendations for the correction of problems with the performance of air systems. Use appropriate allowances in the design calculations when space or other factors dictate the use of less than optimum arrangement of the fan outlet and inlet connections See Sections 8 and 9.
Design the connections between the fan and the system to provide, as nearly as possible, uniform airflow conditions at the fan outlet and inlet connections See Sections 8 and 9. Include adequate allowance for the effect of all accessories and appurtenances on the performance of the system and the fan. If possible, obtain from the fan manufacturer data on the effect of installed appurtenances on the fan's performance See Section Use field measurement techniques that can be applied effectively on the particular system.
Be aware of the probable accuracy of measurement and conditions that affect this. It is assumed that the system pressure losses, shown in system curve A, have been accurately determined, and a suitable fan selected for operation at Point 1.
However, no allowance has been made for the effect of the system connections on the fan's performance. To account for this System Effect it will be necessary to add a System Effect Factor SEF to the calculated system pressure losses to determine the actual system curve.
The SEF for any given configuration is velocity dependent and will vary across a range of airflow. This will be discussed in more detail in Section 7. See Figure 7. The actual airflow will be deficient by the difference To achieve design airflow, a SEF equal to the pressure difference between Point 1 and 2 should have been added to the calculated system pressure losses and the fan selected to operate at Point 2.
Note that because the System Effect is velocity related, the difference represented between Points 1 and 2 is greater than the difference between Points 3 and 4. The System Effect includes only the effect of the system configuration on the fan's performance. A System Effect Factor is a value that accounts for the effect of conditions adversely influencing fan performance when installed in the air system. Figure 7.
By entering the chart at the appropriate air velocity on the abscissa , it is possible to read across from any curve to the ordinate to find the SEF for a particular configuration. The SEF is given in Pascals in. The velocity used when entering Figure 7. This will depend on whether the configuration in question is related to the fan inlet or the fan outlet. Most catalog ratings include outlet velocity figures but, for centrifugal fans, it may be necessary to calculate the inlet velocity See Figure 9.
The inlet velocity and outlet velocity of an axial fan can be approximated by using the fan impeller diameter to determine the airflow area. The necessary dimensioned drawings are usually included in the fan catalog.
In Sections 8 and 9, typical inlet and outlet configurations are illustrated and the appropriate System Effect Curve is listed for each configuration. If more than one configuration is included in a system, the SEF for each must be determined separately and the total of these System Effects must be added to the total pressure losses.
The System Effect Curves are plotted for standard air at a density of 1. Determine the configuration being evaluated and use the appropriate loss coefficient, Cp, and application velocity, V. The SEF can then be calculated using the equations shown in Table 7. It also requires that the slope of the transition elements be no greater than 15 for converging elements or greater than 7 for diverging elements.
As previously discussed, fans intended primarily for use with duct systems are usually tested with an outlet duct in place See Figure 3. In most cases it is not practical for the fan manufacturer to supply this duct as part of the fan, but rated performance will not be achieved unless a comparable duct is included in the system design.
The system design engineer. Figure 8. Add 1 duct diameter for each additional 5. Often, fans are installed without an outlet duct, either because of available space or for economic reasons. Centrifugal fans are sometimes installed with a less than optimum outlet duct. If it is not possible to use a. System Effect Curves for centrifugal fans with less than optimum outlet duct length are shown in Figure 8.
If space is not severely constricted, the use of larger ductwork and moving air at a lower velocity may be beneficial. Larger ductwork within reason reduces system pressure requirements. To effectively transition from a smaller duct size to a larger duct size it is necessary to use a connection piece between the duct sections that allows the airstream to expand gradually.
This piece is called a diffuser, or evas. These terms are used interchangeably in the industry. A properly designed evas has a smooth and gradual transition between the duct sizes so that airflow is relatively undisturbed.
An evas operates on a very simple principle: air flowing from the smaller area to the larger area loses. Determine SEF by using Figure 7. This process is called static regain, and is simply defined as the conversion of velocity pressure to static pressure. The fan manufacturer will, in most cases, be able to provide design information for an efficient diffuser.
Any non-uniformity in the velocity profile ahead of the elbow will result in a pressure loss greater than the industry-accepted value. Add 1 duct diameter for each additional fpm. Since the velocity profile at the outlet of a fan is not uniform, an elbow located at or near the fan outlet will develop a pressure loss greater than the industryaccepted value.
Tubeaxial fans with two-piece and four-piece mitered elbows at varying distances from the fan outlet have a negligible SEF see Figure 8. The amount of this loss will depend upon the location and orientation of the elbow relative to the fan outlet. In some cases, the effect of the elbow will be to further distort the outlet velocity profile of the fan. This will increase the losses and may result in such uneven airflow in the duct that branch- takeoffs near the elbow will not deliver their design airflow.
See Section 8. Vaneaxial fans with two and four-piece mitered elbows at varying distances from the fan outlet resulted in System Effect Curves as shown in Figure 8.
Wherever possible, a length of straight duct should be installed at the fan outlet to permit the diffusion and development of a uniform airflow profile before an elbow is inserted in the duct.
If an elbow must be located near the fan outlet then it should be a radius elbow having a minimum radius-to-duct-diameter ratio of 1. The outlet velocity of centrifugal fans is generally higher toward one or adjacent sides of the rectangular duct.
If an elbow must be located near the fan outlet it should have a minimum radius-to-duct-diameter ratio of 1. It also shows the reduction in losses resulting from the use of a straight outlet duct. Determine SEF by using Figures 7.
Note: Fan Inlet and elbow positions must be oriented as shown for the proper application of the table on the facing page. This may result in increased losses in other system components downstream of the elbow.
When partially closed, the parallel bladed damper diverts the airstream to the side of the duct. This results in a non-uniform velocity profile beyond the damper and airflow to branch ducts close to the downstream side may be seriously affected.
The use of an opposed blade damper is recommended when air 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.
For a centrifugal fan, best air performance will usually be achieved by installing an opposed blade damper with its blades perpendicular to the fan shaft; however, other considerations, such as the need for thrust bearings, may require installation of the damper with its blades parallel to the fan shaft.
When a damper is required, it is often furnished as accessory equipment by the fan manufacturer see Figure 8. In many systems, a volume control damper will be located in the ductwork at or near the fan outlet. Published pressure drops for wide-open control dampers are based on uniform approach velocity profiles.
When a damper is installed close to the outlet of a fan the approach velocity profile is nonuniform and much higher pressure losses through the damper can result.
These multipliers should be applied to all types of fan outlet dampers. In Figure 8. Non-uniform airflow conditions will exist and pressure loss and airflow may vary widely from the design intent.
Wherever possible a length of straight duct should be installed between the fan outlet and any split or branch takeoff.
Note: Avoid location of split or duct branch close to fan discharge. Provide a straight section of duct to allow for air diffusion. Inlet System Effect Factors Fan performance can be greatly affected by nonuniform or swirling inlet flow.
Fan rating and catalog performance is typically obtained with unobstructed inlet flow. Any disruption to the inlet airflow will reduce a fans performance. Restricted fan inlets located close to walls, obstructions or restrictions caused by a plenum or cabinet will also decrease the performance of a fan. The fan performance loss due to inlet airflow disruption must be considered as a System Effect.
The slope of transition elements is limited to 15 converging and 7 diverging. An elbow located at, or in close proximity to the fan inlet will not allow the air to enter the impeller uniformly. The result is less than cataloged air performance. Figure 9. The ducted inlet condition is shown as a , and the effect of the bell-mouth inlet as b.
A word of caution is required with the use of inlet elbows in close proximity to fan inlets. Other than the incurred System Effect Factor, instability in fan operation may occur as evidenced by an increase in pressure fluctuations and sound power level. Fan instability, for any reason, may result in serious structural damage to the fan. Axial fan instabilities were experienced in some configurations tested with inlet elbows in close proximity to the fan inlet. Pressure fluctuations approached ten 10 times the magnitude of fluctuations of the same fan with good inlet and outlet conditions.
It is strongly advised that inlet elbows be installed a minimum of three 3 diameters away from any axial or centrifugal fan inlet. Fans that do not have smooth entries c , and are installed without ducts, exhibit airflow characteristics similar to a sharp edged orifice that develops a vena contracta. A reduction in airflow area is caused by the vena contracta and the following rapid expansion causes a loss that should be considered as a System Effect.
If it is not practical to include such a smooth entry, a converging taper d will substantially diminish the. The System Effect Curves shown in Figure 9. The number of blades did not have a significant affect on the inlet elbow SEF. The SEF for a particular elbow is found in Figure 7. This pressure loss should be added to the friction and dynamic losses already determined for the particular elbow. These improvements help maintain uniform airflow. Nonuniform airflow into a fan inlet, Figure 9.
Notes: [1] Instability in fan operation may occur as evidenced by an increase in pressure fluctuations and sound level. Occasionally, where space is limited, the inlet duct will be mounted directly to the fan inlet as shown on Figure 9.
The many possible variations in the width and depth of a duct influence the reduction in performance to varying degrees and makes it impossible to establish reliable SEF. Existing installations can be improved with guide vanes or the conversion to square or mitered elbows with guide vanes, but a better alternative would be a specially designed inlet box similar to that shown in Figure 9.
Inlet boxes are added to centrifugal and axial fans instead of elbows in order to provide more predictable inlet conditions and to maintain stable fan performance. The fan manufacturer should include the effect of any inlet box on the fan performance, and when evaluating a proposal it should be established that an appropriate loss has been incorporated in the fan rating.
Should this information not be available from the manufacturer, refer to Section A counter-rotating vortex at the inlet may result in a slight increase in the pressure-volume curve but the power will increase substantially.
There are occasions, with counter-rotating swirl, when the loss of performance is accompanied by a surging airflow. In these cases, the surging may be more objectionable than the performance change. Inlet spin may arise from a great variety of approach conditions and sometimes the cause is not obvious. An example of this condition is illustrated in Figure 9. An ideal inlet condition allows the air to enter uniformly without spin in either direction.
A spin in the same direction as the impeller rotation pre-rotation reduces the pressure- volume curve by an amount dependent upon the intensity of the vortex. The effect is similar to the change in the pressure-volume curve achieved by variable inlet vanes installed in a fan inlet; the vanes induce a controlled spin in the direction of impeller rotation, reducing the airflow, pressure and power see Section Numerous variations of turning vanes are available, from a single curved sheet metal vane to multi-bladed "airfoil" vanes.
The pressure drop loss through these devices must be added to the system pressure losses. The amount of loss for each device is published by the manufacturer, but it should be realized that the cataloged pressure loss will be based upon uniform airflow at the entry to the elbow.
If the airflow approaching the elbow is significantly non-uniform because of a disturbance farther upstream in the system, the pressure loss through the elbow will be higher than the published figure. A non-uniform. A single splitter sheet may be used to eliminate swirl in some cases. Straighteners are intended to reduce swirl before or after a fan or a process station.
Do not install straighteners where the air profile is known to be non-uniform, the device will carry the non-uniformity further downstream. Adjacent inlets of multiple double width centrifugal fans located in a common enclosure should be at least one impeller diameter apart if optimum performance is to be expected. Fan performance is reduced if the space between the fan inlet and the enclosure is too restrictive.
It is common practice to allow at least. The manner in which the air stream enters an enclosure in relation to the fan inlets also affects fan performance. If this is not possible, inlet conditions can usually be improved with a splitter sheet to break up the inlet vortex as illustrated in Figure 9. Some accessories such as fan bearings, bearing pedestals, inlet vanes, inlet dampers, drive guards and motors may also cause inlet obstruction and are discussed in more detail in Section Obstruction at the fan inlet may be defined in terms of the unobstructed percentage of the inlet area.
Because of the shape of the inlet cones of many fans it is sometimes difficult to establish the area of the fan inlet. Where an inlet collar is provided, the inlet area is calculated from the inside diameter of this collar. Where no collar is provided, the inlet plane is defined by the points of tangency of the fan housing side with the inlet cone radius. For proper performance of axial fans in parallel installations minimum space of one impeller diameter should be allowed between fans, as shown in Figure 9.
Placing fans closer together can result in erratic or uneven airflow into the fans. The unobstructed percentage of the inlet area is calculated by projecting the profile of the obstruction on the profile of the inlet. The adjusted inlet velocity obtained is then used to enter the System Effect Curve chart and the SEF determined from the curve listed for that unobstructed percentage of the fan inlet area.
Building structural members, columns, butterfly valves, blast gates and pipes are examples of more. PreRotational Vortex Induced. Table for Figure 9. Effects of Factory Supplied Accessories Unless the manufacturer's catalog clearly states to the contrary, it should be assumed that published fan performance data does not include the effects of any accessories supplied with the fan.
If possible, the necessary information should be obtained directly from the manufacturer. The data presented in this section are offered only as a guide in the absence of specific data from the fan manufacturer. See Figure Figure Arrangement 3 and 7 fans see Figure 3. With a belt driven axial flow fan it is usually necessary that the fan motor be mounted outside the fan housing see Figure 3.
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