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Air Conditioning Clinic Air Conditioning Fans One of the Equipment Series TRG-TRC013-EN 85667_Cvr.fm Page 1 Friday, October 8, 1999 10:05 AM.
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Air Conditioning Fans One of the Equipment Series A publication of The T rane Company— W orldwide Applied Systems Group.
Preface © 1999 American Standard Inc. All rights reserved TRG-TRC013-EN ii The T rane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs, and field experience.
TRG-TRC013-EN iii Contents Introduction ........................................................... 1 period one Fan Performance .................................................. 2 Fan Performance Curves ....................................... 11 System Resistance Curve .
iv TRG-TRC013-EN.
TRG-TRC013-EN 1 notes Introduction Efficient distribution of conditioned air needed to heat, cool, and ventilate a building requires the service of a properly selected and applied fan. The types of fans commonly used in HV AC applications include centrifugal and axial designs.
2 TRG-TRC013-EN notes period one Fan Performance Compared to compressors, the pressures generated by these air -moving devices within the ductwork of HV AC systems are relatively small. The measurement of these pressures is, however , essential to the determination of fan performance.
TRG-TRC013-EN 3 period one Fan Performance notes When the pressure within the ductwork is positive, that is, greater than atmospheric, the water column is forced downward in the closed leg and forced upward in the open leg.
4 TRG-TRC013-EN notes period one Fan Performance Since some of the pressures observed in air conditioning systems are very small, the U-tube has been modified to improve the ability to read such small differences in water levels. The modification replaces one leg of the tube with a liquid reservoir and the other leg with an inclined tube.
TRG-TRC013-EN 5 period one Fan Performance notes The total amount of pressure generated by a fan has two components: velocity pressure and static pressure.
6 TRG-TRC013-EN notes period one Fan Performance With the fan operating and the damper fully open, air moves through the duct unimpeded. The impact of the moving air causes the vane to swing in the direction of airflow .
TRG-TRC013-EN 7 period one Fan Performance notes This build-up of static pressure results in reduced air velocity (velocity pressure) and therefore a reduction in the airflow delivered by the fan. Notice that the hinged vane has moved toward a more vertical position.
8 TRG-TRC013-EN notes period one Fan Performance Another probe can be placed in the duct with its open end facing into the air stream. This probe senses total pressure — the combination of velocity pressure plus static pressure.
TRG-TRC013-EN 9 period one Fan Performance notes The characteristics of a fan’ s performance under various duct pressure conditions is tested by an apparatus similar to the one shown here. The fan is connected to a long piece of straight duct with a throttling device at the end.
10 TRG-TRC013-EN notes period one Fan Performance Next, the measured velocity pressure is used to calculate the airflow delivered by the fan. The manometer measures the velocity pressure (P v ) by subtracting static pressure (P s ) from total pressure (P t ).
TRG-TRC013-EN 11 period one Fan Performance notes Fan Performance Curves This point can then be plotted on a chart that has static pressure on the vertical axis and airflow on the horizontal axis.
12 TRG-TRC013-EN notes period one Fan Performance When a series of points is plotted, a curve can be drawn. The resulting curve graphically illustrates the performance of this fan when it is operated at a constant speed.
TRG-TRC013-EN 13 period one Fan Performance notes Next, the fan laws are used to calculate the performance characteristics of this same fan at other rotational speeds. The subscript 1 refers to the tested performance conditions; the subscript 2 refers to the calculated performance conditions.
14 TRG-TRC013-EN notes period one Fan Performance Finally , using the measurements from the dynamometer and the fan laws, curves can be calculated and plotted to represent the fan’ s power consumption at each operating condition. When most fans approach the blocked-tight static-pressure condition, instability is encountered.
TRG-TRC013-EN 15 period one Fan Performance notes A surge line is established during the fan test procedure to indicate the area on a fan performance curve where surge occurs. As long as the fan’ s operating point falls to the right of this line, the fan will operate in a stable manner .
16 TRG-TRC013-EN notes period one Fan Performance Fan manufacturers may present their performance data in graphical and/or tabular form. Similar to using the fan curve, by knowing the desired airflow and pressure-producing capability of the fan, the table can be used to determine the fan’ s speed and input power requirement.
TRG-TRC013-EN 17 period one Fan Performance notes System Resistance Curve Now that a typical fan performance curve has been developed, let’ s see how the fan will perform within a system. With each airflow , an air distribution system imposes a certain resistance to the passage of air .
18 TRG-TRC013-EN notes period one Fan Performance Assuming the system does not change, the static-pressure loss due to the system varies with the square of the airflow . Other points on the system resistance curve are determined by using the following fan law equation: For example, when the same system is delivering 2,000 cfm [0.
TRG-TRC013-EN 19 period one Fan Performance notes By plotting several such points, a curve can be established. This system resistance curve represents the static pressure that the fan must generate, at various airflows, to overcome the resistance — or static-pressure loss — within this particular system.
20 TRG-TRC013-EN notes period one Fan Performance Consider a case where the air resistance through the system is greater than predicted. Instead of the design operating point A , the actual system resistance curve intersects the fan performance curve at B , delivering a lower airflow than intended.
TRG-TRC013-EN 21 period one Fan Performance notes Reducing the fan speed causes the system resistance and fan performance curves to intersect at E . The fan delivers the design airflow at a lower static pressure, with less power required.
22 TRG-TRC013-EN notes period one Fan Performance Let’ s assume that the fan from the previous example, delivering 3,500 cfm [1.65 m 3 /s] at 2.0 in. H 2 O [491 Pa] of static pressure, requires 2.0 hp [1.5 kW] of input power . At these conditions, the fan’ s static efficiency would be: Static Efficiency 3,500 cfm × 2.
TRG-TRC013-EN 23 period one Fan Performance notes In a constant-volume system, where the fan is always delivering the same airflow , the fan is generally selected to balance the airflow and static-pressure requirements at a point on the fan curve that permits a certain margin of safety before surge occurs.
24 TRG-TRC013-EN notes period one Fan Performance The fan performance curves discussed so far are typical of both the centrifugal and fixed-pitch vaneaxial fans. T o complete the discussion, the fan performance curves of the variable-pitch vaneaxial (VPV A) fan will be reviewed.
TRG-TRC013-EN 25 period one Fan Performance notes Unlike the previous fan performance curves, those of the VPV A fan are plotted on the basis of airflow , at various blade pitches, versus total pressure (static pressure plus velocity pressure). This type of fan generates high air velocities and, therefore, high velocity pressures.
26 TRG-TRC013-EN notes period one Fan Performance Similar to both the centrifugal and fixed-pitch vaneaxial fans, the intersection of the system resistance curve and the blade pitch curve establishes the airflow and total pressure at which this fan and this system will balance.
TRG-TRC013-EN 27 notes The most common types of fans used in air conditioning applications are the centrifugal and axial designs. In a centrifugal fan the airflow enters the center of the fan from the side and follows a radial path through the fan wheel.
28 TRG-TRC013-EN notes period two Fan T ypes Forward Curved (FC) Fans The first of these centrifugal fan wheels to be considered has blades that are curved in the direction of wheel rotation.
TRG-TRC013-EN 29 period two Fan T ypes notes The static pressure produced by a fan is a function of the forward motion of the air at the blade tip. The FC fan can perform, within its airflow and static pressure range, at lower rotational speeds than other types of fans.
30 TRG-TRC013-EN notes period two Fan T ypes Notice how the fan input-power lines cross the FC fan performance curves. If the system resistance were to drop, the actual system resistance curve would also drop, moving the operating point ( A ) to a higher airflow ( B ).
TRG-TRC013-EN 31 period two Fan T ypes notes The angle of the backward inclined blade causes the air leaving the wheel to bend back against the direction of rotation. However , the speed of wheel rotation ( S ) causes the air to assume a velocity ( V ) in the direction shown.
32 TRG-TRC013-EN notes period two Fan T ypes The application range of the BI fan is from approximately 40 to 85 percent wide-open airflow . As before, an operating point below 40 percent wide open may place the fan in surge and an operating point above 85 percent wide open typically produces noise and inefficiency .
TRG-TRC013-EN 33 period two Fan T ypes notes A variation of this type of fan, called the backward curved (BC) fan, uses a slight curve in the fan blades, away from the direction of rotation. The performance characteristics of the BC fan are similar to those of the BI fan.
34 TRG-TRC013-EN notes period two Fan T ypes The application range of the airfoil fan is from approximately 50 to 85 percent wide-open airflow . This is a narrower application range than either the FC or BI fan.
TRG-TRC013-EN 35 period two Fan T ypes notes V aneaxial Fans In an axial fan, the airflow passes straight through the fan, parallel to the shaft. There are three common axial fan types: propeller , tubeaxial, and vaneaxial. Propeller fans are well suited for high volumes of air , but have little or no static-pressure generating capability .
36 TRG-TRC013-EN notes period two Fan T ypes The application range of the vaneaxial fan is from approximately 60 to 90 percent wide-open airflow . Similar to the BI and AF fans, the input power lines are essentially parallel to the fan performance curves, and therefore the vaneaxial fan is considered a nonoverloading type of fan.
TRG-TRC013-EN 37 period two Fan T ypes notes The VPV A fan is selected so that the operating point is within the most efficient area of the performance curves.
38 TRG-TRC013-EN notes period two Fan T ypes The selection of the type of fan to be used in a particular application is based on the system size and space availability . The forward curved fan is best applied in small systems requiring 20,000 cfm [9.4 m 3 /s] or less and static pressures of 4 in.
TRG-TRC013-EN 39 notes period three Fan Capacity Control The previous discussions assumed that the fan would perform at a single operating point, located by the intersection of the system resistance and fan performance curves, in a constant-volume system.
40 TRG-TRC013-EN notes period three Fan Capacity Control This modulation causes the actual system resistance curve to shift. In a V A V system, therefore, the fan no longer operates at a single point on its performance curve but must operate over a range of such points.
TRG-TRC013-EN 41 period three Fan Capacity Control notes This method of fan modulation can be used with any type of fan. It is most effective, however , when applied to FC fans.
42 TRG-TRC013-EN notes period three Fan Capacity Control Because of this issue, and since many V A V systems are large with high static pressures, some form of system static-pressure control is generally used. A V A V system’ s static-pressure requirement consists of a fixed component and a variable component.
TRG-TRC013-EN 43 period three Fan Capacity Control notes An exaggerated example is used to illustrate this system operation. Assume that the load on the system decreases, causing all or some of the V A V terminal units to modulate closed. This causes the system resistance curve to shift upwards.
44 TRG-TRC013-EN notes period three Fan Capacity Control There are four methods used to actively control the capacity of a fan. They are discharge dampers, inlet vanes, fan-speed control, and variable-pitch blade control. Discharge Dampers The first method to be discussed is the use of discharge dampers .
TRG-TRC013-EN 45 period three Fan Capacity Control notes As the V A V terminal units modulate shut, the system resistance curve shifts upward. The fan begins to “ride up” its constant-speed performance curve toward B , from the design operating point A , trying to balance with this new system resistance curve.
46 TRG-TRC013-EN notes period three Fan Capacity Control Inlet V anes The next method of capacity control, inlet vanes , modulates a fan’ s capacity by “preswirling” the air in the direction of fan rotation before it enters the fan wheel.
TRG-TRC013-EN 47 period three Fan Capacity Control notes As the V A V terminal units modulate shut, the system resistance curve shifts upward. The fan begins to “ride up” its current vane position curve toward B , from the design operating point A , trying to balance with this new system resistance curve.
48 TRG-TRC013-EN notes period three Fan Capacity Control Fan-Speed Control The third method of capacity control, fan-speed control , modulates fan capacity by varying the speed of the wheel rotation.
TRG-TRC013-EN 49 period three Fan Capacity Control notes along the V A V system modulation curve and the fan satisfies the system static-pressure controller . The low end of the fan’ s modulation range is limited by the surge region. The principal advantages of fan-speed control are its energy saving potential and reduced noise at part load.
50 TRG-TRC013-EN notes period three Fan Capacity Control The performance and control of the direct-drive, variable-pitch vaneaxial (VPV A) fan is similar to that of a fan equipped with inlet vanes.
TRG-TRC013-EN 51 period three Fan Capacity Control notes These curves describe the performance characteristics of various methods of fan capacity control, in terms of the input power required versus the percent of design airflow . Realize that these are generalized curves based on a given set of test conditions.
52 TRG-TRC013-EN notes period four Application Considerations Several considerations must be addressed when applying fans in air conditioning systems, including: ■ System static-pressure control ■.
TRG-TRC013-EN 53 period four Application Considerations notes System Static-Pressure Control Fan capacity control requires a signal from a controller , which monitors static pressure using a sensor located somewhere in the supply duct system.
54 TRG-TRC013-EN notes period four Application Considerations Another method of static-pressure control, the optimized static-pressure control method, positions a single static-pressure sensor near the fan outlet.
TRG-TRC013-EN 55 period four Application Considerations notes System Effect At the end of Period One, we discussed the effect of the air resistance through the system being greater than predicted. This is often caused by failing to allow for the effects of the fan connections to the duct system.
56 TRG-TRC013-EN notes period four Application Considerations the fan outlet, this system effect should be accounted for in the fan selection. If an elbow , turning vanes, air straightener , or other obstruction is located too close to the fan inlet, this system effect should also be accounted for in the fan selection.
TRG-TRC013-EN 57 period four Application Considerations notes An HV AC system can be made quieter by reducing the source (fan) sound level and/or increasing the attenuation of the path. In many cases, fan selection is very important to the final sound level.
58 TRG-TRC013-EN notes period four Application Considerations Effect of Actual (Nonstandard) Conditions Most fan performance data is published at standard air conditions, which are basically sea level elevation and 70 °F [21 °C].
TRG-TRC013-EN 59 period four Application Considerations notes Equipment Certification Standards The Air Movement and Control Association (AMCA) establishes testing procedures and rating standards for air -moving devices.
60 TRG-TRC013-EN notes Let’ s review the main concepts that were covered in this clinic on air conditioning fans. Period One introduced the method of determining and plotting fan performance. It also discussed static pressure versus velocity pressure and the interaction of the fan and the system.
TRG-TRC013-EN 61 period five Review notes Period T wo introduced the various fan types, including forward curved (FC), backward inclined (BI), airfoil (AF), and vaneaxial.
62 TRG-TRC013-EN notes period five Review Period Four covered several considerations in the application of fans in air conditioning systems, including system static-pressure control, system effect, acoustics, the effect of actual (nonstandard) conditions on fan selection, and equipment certification standards.
TRG-TRC013-EN 63 period five Review notes For more information, refer to the following references: ■ Fans and their Application in Air Conditioning (T rane literature order number ED-F AN) ■ T ran.
64 TRG-TRC013-EN Questions for Period 1 1 The total pressure generated by the fan is made up of two components, __________ pressure and __________ pressure. 2 The total pressure (P t ) measured within a duct is 3.5 in. H 2 O [872 Pa] and the static pressure (P s ) is 3 in.
TRG-TRC013-EN 65 Quiz 6 Between the forward curved (FC) and backward inclined (BI) fans, which one can handle higher static-pressure applications? 7 Explain why the forward curved (FC) fan is called an overloading type of fan. Questions for Period 3 8 List three methods of fan capacity control.
66 TRG-TRC013-EN 1 velocity pressure and static pressure 2a P v = 0.5 in. H 2 O [125 Pa] b V = 2,832 fpm [14.4 m/s] c Airflow = 4,248 cfm [2 m 3 /s] 3a airflow b static pressure c fan speed (rpm) d in.
TRG-TRC013-EN 67 adjustable-frequency drive (AFD) See variable-speed drive. airfoil (AF) A type of centrifugal fan that is similar to the backward inclined fan, with the exception that the fan blades are in the shape of an airfoil, like an airplane wing.
68 TRG-TRC013-EN Glossary fan-speed control A method of controlling fan capacity by varying its speed of rotation — commonly accomplished using a variable-speed drive on the fan motor . forward curved (FC) A type of centrifugal fan with blades curved in the direction of wheel rotation.
TRG-TRC013-EN 69 Glossary notes total efficiency The percentage of input power that is realized as useful work in terms of total energy (pressure). total pressure Sum of the velocity pressure plus static pressure. tubeaxial fan A type of axial fan consisting of a propeller fan mounted in a cylinder .
The T rane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse, WI 54601-7599 www .trane.com An American Standard Company Literature Order Number TRG-TRC013-EN File Number E/A V -.
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