Trane TRG-TRC013-EN User's Manual
Contents
1. ceeeeceeeeeeeeeeeeeeeeeseeaaeeeeees 44 Inlet VAMOS sn name en mieu 46 Fan Speed Control oo cccccccccceeseeseseseeeseeseeeeeeeeeees 48 Variable Pitch Blade Control 49 period four Application Considerations 52 System Static Pressure Control 53 System ETEC cosmesi asoini ENEN EEES 55 ACOUSTICS scorri manes ia E AaS 56 Effect of Actual Nonstandard Conditions 58 Equipment Certification Standards 59 period five Review 60 OO cos cceccstenaetastaseccistecastandeiatbeasaintiaiecteteeadectestants 64 Answers 66 Glossary aeieea 67 TRG TRC013 EN iii S TRANE TRG TRC013 EN Introduction Air Conditioning Fans centrifugal 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 HVAC applications include centrifugal and axial designs In a centrifugal fan the airflow follows a radial path through the fan wheel In an axial fan the airflow passes straight through the fan parallel to the shaft TRG TRC013 EN TRANE period one Fan Performance Air Conditioning Fans period one Fan Performance Compared to compressors the pressures generated by these air mov2. Equipment Certification Standards The Air Movement and Control Association AMCA establishes testing procedures and rating standards for air moving devices AMCA also certifies performance and labels equipment through programs that involve random testing of a manufacturer s equipment to verify published performance The overall objective of AMCA Standard 210 also known as ASHRAE Standard 51 titled Laboratory Methods of Testing Fans for Rating is to promote consistent testing methods for fans blowers exhausters and some types of air compressors The Air Conditioning amp Refrigeration Institute ARI has similar standards for fans that are a part of equipment such as fan coils central station air handlers unit ventilators and water source heat pumps These standards such as ARI Standard 430 for central station air handlers account for the effects of the air handler casing around the fan and establish rating testing and certification standards for equipment manufacturers 59 S TRANE period five Review Air Conditioning Fans period five Review Let s review the main concepts that were covered in this clinic on air conditioning fans Review Period One design system resistance curve static pressure 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 60 TR
3. Air Conditioning Clinic Air Conditioning Fans One of the Equipment Series TRG TRC013 EN NO POSTAGE NECESSARY IF MAILED IN THE UNITED STATES BUSINESS REPLY MAIL FIRST CLASS MAIL PERMIT NO 11 LA CROSSE WI POSTAGE WILL BE PAID BY ADDRESSEE THE TRANE COMPANY Attn Applications Engineering 3600 Pammel Creek Road La Crosse WI 54601 9985 NO POSTAGE NECESSARY IF MAILED IN THE UNITED STATES BUSINESS REPLY MAIL FIRST CLASS MAIL PERMIT NO 11 LA CROSSE WI POSTAGE WILL BE PAID BY ADDRESSEE THE TRANE COMPANY Attn Applications Engineering 3600 Pammel Creek Road La Crosse WI 54601 9985 abpa Ya WOJ G7 UOIeIO d Comment Card We want to ensure that our educational materials meet your ever changing resource development needs Please take a moment to comment on the effectiveness of this Air Conditioning Clinic Air Conditioning Fans One of the Equipment Series TRG TRC013 EN About me Give the completed card to the presenter or drop it in the mail Thank you Level of detail circle one Too basic Just right Too difficult Rate this clinic from 1 Needs Improvement to 10 Excellent Content 1 Booklet usefulness 1 Slides illustrations 1 Presenter s ability 1 Training environment 1 Other comments 3 N ON NY NY N 3 3 3 3 4 5 6 il 8 9 10 4 5 6 7 8 9 10 4 5 6 7 8 9 10 4 5 6 7 8 9 10 4 5 6 7 8 9 10 Type of business Job funct
4. static pressure and ride up and down its performance curve propeller fan A type of axial fan that is well suited for high volumes of air but which has little or no static pressure generating capability SMACNA Sheet Metal and Air Conditioning Contractors National Association sound trap A device installed in an air duct system to control discharge air noise static efficiency The percentage of input power that is realized as useful work in terms of static energy pressure static pressure Pressure due to the perpendicular outward push of the air against the duct walls surge A condition of unstable fan operation where the air alternately flows backward and forward through the fan wheel generating noise and vibration system effect Turbulence due to fan inlet and outlet restrictions where the air is not allowed to establish a uniform velocity profile influencing fan performance system resistance curve A curve representing the pressure that the system including the supply ductwork duct fittings terminal units supply diffusers return grilles coils filters dampers etc creates over a range of airflows TRG TRC013 EN TRG TRC013 EN S TRANE Glossary total efficiency The percentage of input power that is realized as useful work in terms of total energy pressure total pressure Sum ofthe velocity pressure plus static pressure tubeaxial fan A type of axial fan consisting of a propeller fan mou
5. 3 500 cfm x 2 0 in H O a Static Efficiency 6 362 x2 0 hp 55 3 o _ 1 65 m sx491Pa _ pro Static Efficiency eam 15kW 7 55 22 TRG TRC013 EN period one Fan Performance Constant Volume System design system resistance curve surge region static pressure airflow 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 As the coil and filters become dirty the system resistance increases causing the system resistance curve to shift to the left In response the operating point follows the intersection of the system resistance and fan performance curves to a higher static pressure condition This margin of safety allows a certain increase in system static pressure before the fan s operating point reaches the surge region Of course as the fan is called upon to generate more static pressure to overcome this increased system resistance its airflow is reduced accordingly TRG TRC013 EN 23 TRANE period one Fan Performance Variable Pitch Vaneaxial Fan variable pitch blades The fan performance curves discussed so far are typical of both the centrifugal and fixed pitch vaneaxial fans To complete the discussion the fan performance curves of the variable pitch vaneaxial VPV
6. Using the fan performance curve in Figure 91 answer the following questions What property is plotted on the horizontal axis labeled A What property is plotted on the vertical axis labeled B What property is represented by the curves labeled C aon 9 What property is represented by the curves labeled D Again using Figure 91 what condition is represented by the region to the left of the curve labeled E Questions for Period 2 5 List the three primary types of centrifugal fans TRG TRC013 EN TRG TRC013 EN TRANE 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 1 9 What method of fan capacity control preswirls the air in the direction of fan rotation before it enters the fan wheel 10 How is the capacity of a variable pitch vaneaxial VPVA fan controlled Questions for Period 4 11 List two possible causes of system effect 12 What document establishes laboratory testing methods for air moving devices 65 S TRANE Answers 9 velocity pressure and static pressure a P 0 5in H20 125 Pa b V 2 832 fpm 14 4 m s c Airflow 4 248 cfm 2 m s a airflow b static pressure c fan speed rpm d input power surge forward curved FC backward
7. a higher static pressure The system static pressure controller senses this higher static pressure and sends a signal to the fan speed controller instructing it to slow down the fan motor This causes the performance curve for the fan to shift downward until the balance point C falls TRG TRC013 EN TRG TRC013 EN S TRANE period three Fan Capacity Control along the VAV 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 Variable Pitch Blade Control variable pitch blades Variable Pitch Blade Control Finally the capacity of variable pitch vaneaxial VPVA fans can be modulated by swiveling the fan blades to vary their pitch angle 49 S TRANE 50 period three Fan Capacity Control design system resistance curve TETE 750 ec aE 159 202 3 airflow The performance and control of the direct drive variable pitch vaneaxial VPVA fan is similar to that of a fan equipped with inlet vanes Again as the system resistance curve shifts upward and the fan begins to ride up the current blade pitch curve toward B from its design operating point A it delivers a lower airflow at a higher total pressure The system static pressure controller senses this higher static pressure and
8. inclined BI or backward curved BC and airfoil AF or plug plenum backward inclined BI As the fan airflow increases the nature of the fan s input power curves causes the fan s power requirement to also increase possibly overloading the motor riding the fan curve discharge dampers inlet vanes fan speed control and variable pitch blade control inlet vanes 10 By adjusting swiveling the pitch of the fan blades 11 Here are five possible causes 1 Not enough straight duct at the fan inlet or outlet to ensure a uniform velocity profile 2 A diffuser that connects the fan to the duct system an elbow a branch turning vanes or a damper located too close to the fan outlet 3 An elbow turning vanes air straightener or other obstruction located too close to the fan inlet 4 Preswirling the air prior to it entering the fan wheel 5 Use of an inlet plenum or cabinet 12 AMCA Standard 210 also known as ASHRAE Standard 51 titled 66 Laboratory Methods of Testing Fans for Rating TRG TRC013 EN TRG TRC013 EN e TRANE Glossary 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 AMCA Air Movement and Control Association ARI Air Conditioning amp Refrigeration Institute ASHRAE American Society of Heatin
9. is achieved by selecting a fan that is two or three sizes fan wheel diameter smaller than a fan that would be selected for a constant volume system Then the design operating point will fall farther to the right on the curve permitting a larger range of modulation It should be noted that the addition of vanes to the inlet of a fan introduces a pressure drop that must be overcome by the fan Inlet vane performance curves are established through fan testing and manufacturers typically publish fan performance data with the effect of the inlet vanes included TRG TRC013 EN 47 S TRANE 48 period three Fan Capacity Control Fan Speed Control E 0 variable speed drive Fan Speed Control The third method of capacity control fan speed control modulates fan capacity by varying the speed of the wheel rotation This is commonly accomplished using a variable speed device on the fan motor such as a variable frequency drive a belt speed changer a variable speed mechanical drive or an eddy current clutch Fan Speed Control fan speed curves design system resistance curve static pressure airflow The response of fan speed control to system static pressure variations is similar to that described for inlet vanes Again as the system resistance curve shifts upward and the fan begins to ride up the constant speed performance curve toward B from its design operating point A it delivers a lower airflow at
10. of the performance curves Total efficiencies from 60 to 84 percent are possible with the VPVA fan Again total efficiency is calculated by substituting total pressure for static pressure in the static efficiency equation Like the BI AF and fixed pitch vaneaxial fans the input power lines are essentially parallel to the blade pitch curves Therefore the VPVA fan is a nonoverloading type of fan TRG TRC013 EN 37 S TRANE 38 period two Fan Types Fan Selection a Forward curved FC Lower airflow lower static pressure lower first cost a Backward inclined BI or airfoil AF Higher airflow higher static pressure higher efficiency A Vaneaxial Limited space A Variable pitch vaneaxial VPVA Large systems higher airflow 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 s or less and static pressures of 4 in HO 996 Pa or less The FC fan is also the least costly On the other hand systems requiring in excess of 20 000 cfm 9 4 m s and 3 in H20 747 Pal of static pressure are usually best served by the more efficient backward inclined or airfoil fans With larger fan sizes and larger motors the higher fan efficiencies can result in significant energy savings When space is a prime consideration the vaneaxial fan may be the best solution Straight
11. savings are realized any time the load drops below the design airflow Inlet vanes can however cause significant acoustical tones at part load when applied to BI AF and vaneaxial fans 46 TRG TRCO13 EN S TRANE period three Fan Capacity Control Inlet Vanes surge inlet vane 5 ne position t design system Pa resistance curve Static pressure airflow As the VAV 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 As a result the fan delivers a lower airflow at a higher static pressure The system static pressure controller senses this higher static pressure and sends a signal to the inlet vanes instructing them to begin closing When the inlet vanes are closed the performance curve for the fan shifts downward until the balance point C falls along the VAV system modulation curve and the fan satisfies the system static pressure controller The advantage that inlet vanes provide over discharge dampers is that the fan approaches the surge region at a much lower airflow and static pressure The low end of the fan s modulating range is either the intersection of the modulation curve with the surge line or the leakage rate through the fully closed inlet vanes whichever is larger Typically a satisfactory modulating range
12. static pressure 12 TRG TRC013 EN period one Fan Performance Fan Speed Next the fan laws are used to calculate the performance characteristics of this same fan at other rotational speeds The subscript refers to the tested performance conditions the subscript refers to the calculated performance Airflow Fan Speed Airflow Fan Speed Static Pressure Fan Speed 2 Static Pressure Fan a Input Power Fan Speed 3 Input Power an So The result is a family of curves that represents the specific fan s airflow capacity at various speeds and static pressures 13 14 TRANE period one Fan Performance Input Power 0 5 hp lhp 2 hp 3hp 0 37kKW f 075kW 15kw 2 2 kW NE EE res ere T h Static pressure 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 Fan Surge high pressure low pressure When most fans approach the blocked tight static pressure condition instability is encountered This condition is known as surge Surge occurs when the quantity of air being moved by the fan falls below the amount necessary to sustain the existing static pressure difference between the inlet and outlet sides of the fan When this occurs the pressurized air flows backward through the fan wheel instantaneously reducing the
13. the FC blade imparts a forward motion to the air as it leaves the blade tip This together with the speed of wheel rotation S causes the air to leave at a relatively high velocity V 28 TRG TRC013 EN TRG TRC013 EN e TRANE period two Fan Types 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 Forward Curved Fan 30 static efficiency 50 to 65 static pressure airflow The typical application range of this type of fan is from 30 to 80 percent wide open airflow Selecting an operating point that places the airflow below approximately 30 percent wide open may place the fan in an area of instability Similarly an operating point that places the airflow beyond 80 percent wide open typically produces noise and inefficiency The maximum static efficiency of the FC fan is from approximately 50 to 65 percent and occurs just to the right of the maximum static pressure point on the fan performance curve 29 TRANE 30 period two Fan Types surge line 2hp 3hp 1 5 KW 2 2 kW actual system resistance curve Static pressure airflow 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
14. the operating point A to a higher airflow B At the same time the fan s input power requirement would also rise possibly overloading the motor Consequently the FC fan is referred to as an overloading type of fan As with all fan types the FC fan can exhibit unstable operation or surge However since FC fans are used typically in low speed and low static pressure applications many small FC fans can operate in surge without noticeable noise and vibration Backward Inclined Fan a 3 Als 7 vA Backward Inclined BI Fans The second centrifugal fan design has blades that are slanted away from the direction of wheel rotation These backward inclined or BI fans operate at higher speeds than FC fans TRG TRC013 EN period two Fan Types Backward Inclined Fan 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 EC vs BI Fans forward curved backward inclined Comparing the performance of FC and BI fans for a given wheel speed S the air velocity V off the FC wheel is substantially greater than that off a comparable BI wheel Therefore when a BI fan is selected to handle the same airflow it must be operated at approximately twice the speed of a similarly selected FC fan In spite of this the input power requirement of th
15. 2 686 fpm ft min p 10 0749 Airflow VxA 2 686 x 1 28 3 438 cfm ft min Il P 119 V 1 414 1 414 US 14 06 p 1 204 mys Airflow VxA 14 06 x 0 12 1 69 m s It is determined that at this point the fan operating at 1 100 rpm is delivering 3 438 cfm 1 69 m s against 2 in H2O 491 Pa of static pressure 10 TRG TRC013 EN period one Fan Performance Plotting Fan Performance Points g rer errr re gt o l 2 0 in H30 4 491 Pa Te 3 438 cfm 1 69 mis airflow 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 Plotting Fan Performance Points Static pressure airflow Additional data from the fan tests establish other static pressure and corresponding airflow performance points for a given rotational speed revolutions per minute or rpm TRG TRC013 EN 11 TRANE period one Fan Performance Fan Performance Curve blocked tight static pressure static pressure Pa wide open airflow airflow 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 Notice that the curve extends from blocked tight static pressure with a corresponding zero airflow to wide open airflow with a corresponding zero
16. A fan will be reviewed While this type of fan operates at a constant speed the pitch angle of its blades can be adjusted to match the airflow and pressure requirements of the system 24 TRG TRC013 EN TRG TRC013 EN period one Fan Performance airflow Unlike the previous fan performance curves those of the VPVA 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 To fully evaluate its performance total pressure is used to demonstrate both the static and velocity pressure components The dashed line defines the upper limit of airflow and total pressure above which a surge or turbulent condition is encountered In addition curves are plotted that define fan total efficiency at various airflows total pressures and blade positions Fan total efficiency expresses the percentage of input power that is realized as useful work in terms of total pressure It is calculated by substituting total pressure for static pressure in the static efficiency equation Airflow x Total Pressure Effici Tot ipa We Constant x Input Power 25 26 TRANE period one Fan Performance design system resistance curve Similar to both the centrifugal and fixed pitch vaneaxial fans the intersection of the system resistance curve and the blade pitch curve es
17. Conditions Most fan performance data is published at standard air conditions which are basically sea level elevation and 70 F 21 C If the airflow requirement for a given application is stated at nonstandard conditions a density correction must be made prior to selecting a fan The procedure for selecting a fan at actual elevations and or temperatures is 1 2 3 4 5 Determine the actual air density and calculate the air density ratio density at actual conditions divided by density at standard conditions Divide the design static pressure at actual conditions by this air density ratio Use the actual design airflow and static pressure corrected for standard conditions to select the fan from the performance tables charts and to determine the speed rpm and input power requirement of the fan at standard conditions The fan speed rpm is the same at both standard and actual conditions Multiply the input power requirement by the air density ratio to determine the actual input power required It is important to note that most pressure loss charts for other system components such as ducts filters coils etc are also based on standard air conditions 58 TRG TRC013 EN TRG TRC013 EN S TRANE period four Application Considerations Equipment Certification Standards A Purpose Establish methods for laboratory testing of air moving devices LABORATORY METHOOS OF TESTING FANS FOR RATING
18. G TRC013 EN period five Review Review Period Two cu x EN ak er LE centrifugal Period Two introduced the various fan types including forward curved FC backward inclined BI airfoil AF and vaneaxial Review Period Three A Riding the fan curve a Discharge dampers A Inlet vanes A Fan speed control a Variable pitch blade control Period Three presented various methods for controlling fan capacity including riding the fan curve discharge dampers inlet vanes variable fan speed control and variable pitch blade control TRG TRC013 EN 61 mane period five Review Review Period Four A Application considerations System static pressure control System effect Acoustics Effect of actual nonstandard conditions on fan selection Equipment certification standards 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 62 TRG TRCO13 EN TRG TRC013 EN period five Review S TRANE An American Standard Company For more information refer to the following references Fans and their Application in Air Conditioning Trane literature order number ED FAN Trane Air Conditioning Manual VAV System Optimization Trane Engin
19. M BHP RPW HP RPM BHP APM BHP APM BHP J 174 wm 122 5 175 64 15 74 2 DS 440 Si 624 184 8 Ces00 gt 1969 41 164 566 222 651 28 742 358 C8n XC45 as 498 971 581 104 672 2203 479 215 576 281 664 34 75 42 si aw 901 579 9 65 was 744 8500 248 510 278 601 352 684 424 760 504 837 691 912 676 976 789 WT 84 8350 2009 542 35 629 434 706 5SN 777 593 868 627 S17 722 6 amp 7 WAT 9464 10200 2938 575 439 657 630 730 619 7 704 663 792 98 636 2 1001 ss 10 Sid Ar Outlet Total Static re Ps Flow Velocity HEJ EX 413 TE 5964 us neva APM RPM kW kw APNO IW kW RPM KW APM RW 780A an 423 091 536 131 68 17 7M 20 5 27 Er 7 483 996 451 122 565 165 651 215 742 260 ananas an g 433 1044 501 ai 19 479 160 576 210 664 260 7 319 ost 37 432 969 492 1033 555 4002 1244 510 207 601 262 684 316 76 376 437 441 912 so 976 646 1097 630 4413 1148 542 283 629 324 706 383 777 443 648 513 917 583 966 653 1047 7 22 4814 ua 575 323 657 3295 730 A 73 5 25 863 690 928 670 992 747 1065 823 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 For example assume an application requires 6 800 cfm 3 210 L s at a static pressure of 2 5 in H 0 622 75 Pa Using this sample table this particular fan requires a rotational speed of 821 rpm and 4 25 hp 3 17 kW o
20. aking sleeping or any other activity for which the space was designed The air handling equipment is generally a key noise source that must be addressed in order to ensure a quiet comfortable space Sound from a fan often follows more than one path to the receiver in the space In this example a fan located inside an air handler is installed in a mechanical equipment room next to an occupied space The sound from this fan travels with the supply air to the space breaks out of the supply ductwork over the space radiates from the air handler casing and through the wall and travels from the fan inlet through the return duct system to the space All of these paths must be controlled to achieve the desired sound level in the space TRG TRC013 EN S TRANE period four Application Considerations fan application Acoustical Guidelines a Optimize fan and air handler selection for lowest overall sound a Select fan to operate safely away from surge region a Minimize system effects A Use low pressure drop duct fittings follow SMACNA recommendations a Avoid rectangular sound traps if possible a Use adequate vibration isolation An HVAC 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 Smaller higher speed fans often create more noise then larger lower speed and slightly more ex
21. ard the closed position The pressures are recorded at each throttling device position When the throttling device is fully closed only static pressure is being generated by the fan because there is no airflow This point is called the blocked tight static pressure S TRANE period one Fan Performance Determining Fan Airflow Velocity Pressure P P P Velocity V Constant xa 5 Airflow Velocity x Fan Outlet Area Next the measured velocity pressure is used to calculate the airflow delivered by the fan The manometer measures the velocity pressure P by subtracting static pressure P from total pressure P Next the air velocity V can be calculated by dividing velocity pressure P by the air density p taking the square root of the quotient and multiplying by a constant Finally the fan airflow is determined by multiplying the air velocity V by the outlet area A of the fan For example assume the test readings for a specific throttling device position are as follows Total pressure P 2 45 in H20 62 2 mm H 0 or 610 Pa Static pressure P 2 0 in H20 50 8 mm H 0 or 491 Pal Velocity pressure P P P 2 45 2 0 0 45 in H20 11 4 mm H 0 or 119 Pa Fan outlet area A 1 28 ft 0 12 m Fan speed 1 100 rpm revolutions per minute Air density p at standard air conditions 0 0749 Ib ft 1 204 kg m Proceeding with the calculations P V 1 096 E 1 096 2 45_
22. e BI fan is less often making it a more efficient selection This higher speed requires that BI fans be built with a larger shaft and bearing and it places more importance on proper balance Their rugged construction makes them suitable for moving large volumes of air in higher static pressure applications TRG TRC013 EN 31 S TRANE 32 period two Fan Types Backward Inclined Fan 40 static efficiency 65 to 75 85 static pressure airflow 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 The maximum static efficiency of the BI fan is from approximately 65 to 75 percent and occurs at approximately 50 percent wide open airflow Since the magnitude of surge is related to generated pressure the surge characteristic exhibited by the BI fan is greater than that of the FC fan This is primarily due to BI fans being used in higher static pressure applications surge 75hp l0hp iine Shp 56KW 5KW B 7kw actual system resistance curve airflow Unlike the FC fan the input power lines of a BI fan are for the most part nearly parallel to the fan performance curves If the system resistance were to drop the actual system resistance curve would also drop moving the opera
23. eers Newsletter 1991 volume 20 number 2 Specifying Quality Sound Trane Engineers Newsletter 1996 volume 25 number 3 Acoustics in Air Conditioning Applications Engineering Manual Trane literature order number FND AM 5 ASHRAE Handbook Systems and Equipment AMCA Fan Application Manual publications 200 201 202 and 203 SMACNA HVAC Systems Duct Design The World Wide Web is another helpful resource Sites to visit include the ASHRAE Bookstore at www ashrae org the AMCA Web site at www amca org the SMACNA Web site at www smacna org For information on additional educational materials available from Trane contact your local Trane sales office request a copy of the Educational Materials price sheet Trane order number EM ADV1 or visit our online bookstore at www trane com bookstore 63 TRANE 64 Quiz Questions for Period 1 The total pressure generated by the fan is made up of two components pressure and pressure The total pressure P measured within a duct is 3 5 in H20 872 Pa and the static pressure P is 3 in H20 747 Pa Assume the air is at standard conditions p 0 0749 Ib ft 1 204 kg m a What is the velocity pressure P b Whatis the air velocity V c Ifthe outlet area of the fan is 1 5 ft 0 14 m2 what is the fan airflow ASS z N RS LOUIS A N N z a AN T TRA ner KG NS gt TW he AN ASAA bana A
24. eminating information gathered through laboratory research testing programs and field experience The Trane Air Conditioning Clinic series is one means of knowledge sharing It is intended to acquaint a nontechnical audience with various fundamental aspects of heating ventilating and air conditioning We ve taken special care to make the clinic as uncommercial and straightforward as possible Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text This particular clinic introduces the concept of air conditioning fans 1999 American Standard Inc All rights reserved TRG TRC013 EN Contents Introduction 1 period one Fan Performance 2 Fan Performance Curves ccccccccssseceeeeeeeeneeeees 11 System Resistance Curve cccccccccceeceeeeeeeeeeenanees 17 Fan System INteraction ccccceeceeeeeeceeeeeeeeeeeeeees 19 period two Fan Types 27 Forward Curved FC Fans cccccccssseccceeeeeeeeeeeees 28 Backward Inclined BI Fans ccccccceseeceeaeeeeees 30 Airo AF Fans eeen einai EN raia 33 Vaneaxial Fans cccccccccccsccccesceceeseeeeeeeeeeeaseeeeneeeeas 35 period three Fan Capacity Control 39 Riding the Fan Curve o ccccccccccceeeeeceeeeeeeeeeeeens 40 Discharge Dampers
25. es improve efficiency and reduce turbulence and the resulting sound generated in the downstream ductwork Because a vaneaxial fan is more efficient than a tubeaxial fan it can handle larger volumes of air at higher pressures 35 S TRANE period two Fan Types Vaneaxial Fan 60 static efficiency 70 to 80 static pressure 90 airflow 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 Static efficiencies from 70 to 80 percent can be achieved with vaneaxial fans The airflow and static pressure performance range is similar to that of the BI and AF fans Compared to centrifugal fans vaneaxial fans typically have lower low frequency sound levels and higher high frequency sound levels making them an attractive alternative for sound sensitive applications Variable Pitch Vaneaxial Fan variable pitch blades The variable pitch vaneaxial VPVA fan is similar in construction to the fixed pitch vaneaxial fan The principal difference is that the VPVA fan is a constant speed fan with variable pitch fan blades 36 TRG TRC013 EN period two Fan Types total pressure airflow The VPVA fan is selected so that the operating point is within the most efficient area
26. f power to meet the requirements 16 TRG TRC013 EN TRG TRC013 EN period one Fan Performance System Resistance cooling return air grille coil damper supply diffuser 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 The resistance is the sum of all of the pressure losses experienced as air passes through the ductwork supply air diffusers return air grilles dampers filters coils etc This is the resistance or static pressure loss that the fan must overcome to move a given quantity of air through the system System Resistance 2 0 in H 0 491 Pa Static pressure w 500 cfm 1 65 m s airflow Assume that a system is designed to deliver 3 500 cfm 1 65 m s and that to overcome the system pressure losses the fan must generate 2 0 in H20 491 Pa of static pressure To illustrate how a system resistance curve is developed this point is plotted on the same chart used to develop the fan curve 17 18 TRANE period one Fan Performance System Resistance Curve Static Pressure Airflow Static Pressure Airflow 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 determ
27. g Refrigerating and Air Conditioning Engineers attenuation The process in which sound energy is absorbed or otherwise diminished in intensity axial fan A type of fan where the air passes straight through the fan parallel to the shaft backward curved BC A type of centrifugal fan with blades that are curved away from the direction of wheel rotation backward inclined BI A type of centrifugal fan with flat blades that are slanted away from the direction of wheel rotation BAS Building automation system blocked tight static pressure The point on the fan performance curve where there is no airflow only static pressure is being generated by the fan centrifugal fan A type of fan where the air enters the center of the fan from the side and follows a radial path through the fan wheel constant volume system A type of air conditioning system that varies the temperature of a constant volume of air supplied to meet the changing load conditions of the space DDC Direct digital control a method of unit control using a microprocessor that enables digital communication between the unit controller and a central building automation system discharge damper A device used to control the capacity of a fan by creating a static pressure drop in the system just downstream of the fan dynamometer A device used to measure the power applied to the fan shaft VAV system modulation curve A curve that illustrates the VAV system fan s static pres
28. inal units determining the best sensor location for all load conditions can be difficult often determined by trial and error or by using multiple sensors TRG TRC013 EN 53 mane 54 period four Application Considerations Optimized Static Pressure Control static pressure sensor D VAV terminal units communicating BAS Another method of static pressure control the optimized static pressure control method positions a single static pressure sensor near the fan outlet The static pressure controller dynamically adjusts the static pressure set point based on the position of the modulating dampers or valves in the VAV terminal units The DDC VAV controller in each terminal unit modulates its valve to maintain the airflow required by the zone thermostat and keeps track of the valve position The building automation system BAS continually polls the VAV terminal units looking for the most open VAV damper The controller resets the static pressure set point so that at least one VAV damper the one requiring the highest inlet pressure is nearly wide open The result is that the supply fan generates only enough static pressure to ensure the required airflow through this critical terminal unit Since the pressure sensor is near the fan outlet this method allows the sensor to be factory installed and tested It can also serve as the duct high pressure sensor If the terminal units use DDC controls and the s
29. ined by using the following fan law equation Static Pressure Airflow 2 Static Pressure Ea For example when the same system is delivering 2 000 cfm 0 94 m s the static pressure loss due to the system pressure is 0 65 in H20 159 Pal 2 000 cfm _ Static Pressure 2 0 in H 0 x F500 om 0 65 in H 0 0 94 m s Static Pressure 491 Pa une 159 Pa 1 65 m s TRG TRC013 EN TRG TRC013 EN period one Fan Performance System Resistance Curve system resistance curve 2 0 in H 0 A i 491 Pa i U i 0 65 in H 0 159 Pa CC 9 a 2 000cfm 3 500 cfm 0 94m3 s 1 65 mis airflow 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 Fan System Interaction design system resistance curve Static pressure airflow Fan System Interaction By superimposing the system resistance curve on a fan performance curve the intersection predicts the airflow and static pressure at which the fan and system will balance design operating point A If the installed system resistance is different from that assumed during the design process the fan and system balance point the operating point will not be as intended This means that the fan and system will balance at a
30. ing devices within the ductwork of HVAC systems are relatively small The measurement of these pressures is however essential to the determination of fan performance Measuring Pressure atmospheric Pi pressure One instrument that is available to measure these small pressures is a U tube that contains a quantity of water One end of the tube is open to the atmosphere open leg while the other end is connected to the ductwork closed leg TRG TRC013 EN TRG TRC013 EN period one Fan Performance Positive Duct Pressure atmospheric RE pressure 3 inches 76 2 mm 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 Conversely a negative pressure within the ductwork causes the water column to drop in the open leg and to rise in the closed leg In this illustration a positive pressure in the ductwork forces the water in the closed leg 3 in 76 2 mm lower than the water in the open leg A pressure of 1 psi will support a 27 7 in column of water A pressure of 1 kPa will support a 102 mm column of water Therefore this length of water column is equivalent to a pressure of 0 11 psi 758 Pal However to avoid having to convert units each time a reading is taken inches in or millimeters mm of water H 0 are often used to measure the pressures generated by fans Other common expre
31. ion Optional name phone address S TRANE The Trane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse WI 54601 7599 www trane com An American Standard Company do WO 679 G UOIeJOLad Response Card We offer a variety of HVAC related educational materials and technical references as well as software tools that simplify system design analysis and equipment selection To receive information about any of these items just complete this postage paid card and drop it in the mail Education materials J Air Conditioning Clinic series Software tools Periodicals Other E Engineered Systems Clinic series J Trane Air Conditioning Manual L Trane Systems Manual m E Equipment Selection E System design amp analysis E Engineers Newsletter Thank you for your interest Name Title Business type Phone fax Company Address About me E mail address S TRANE The Trane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse WI 54601 7599 www trane com An American Standard Company Air Conditioning Fans One of the Equipment Series A publication of The Trane Company Worldwide Applied Systems Group S TRANE Preface Air Conditioning Fans A Trane Air Conditioning Clinic The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly diss
32. ning Fans period four Application Considerations Several considerations must be addressed when applying fans in air conditioning systems including m System static pressure control System effect Acoustics Effect of actual nonstandard conditions on fan selection Equipment certification standards While not all inclusive this list of considerations does represent some of the key issues 52 TRG TRC013 EN TRANE period four Application Considerations System Static Pressure Control controller static pressure S sensor VAV terminal units 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 This controller compares the sensed pressure to a set point and modulates the fan capacity to maintain the set point at that sensor location In the most common method for sensing and controlling system static pressure the static pressure sensor is located in the supply duct system typically two thirds of the distance between the supply fan outlet and the critical terminal unit inlet The critical terminal unit is at the end of the supply duct path that represents the largest total pressure drop The sensor is field installed and the controller is set to maintain the pressure corresponding to that location in the duct system at design airflow conditions In larger systems with many term
33. nted in a cylinder vaneaxial fan A type of axial fan with vane type straighteners on the outlet to improve efficiency and reduce turbulence and generated noise variable pitch blade control A method of vaneaxial fan capacity control achieved by adjusting the pitch of the fan blades variable air volume VAV system A type of air conditioning system that varies the volume of constant temperature air supplied to meet the changing load conditions of the space variable pitch vaneaxial VPVA fan A type of vaneaxial fan that adjusts the pitch angle of its blades to match the airflow and pressure requirements of the system variable speed drive A device used to control the capacity of a fan by varying the speed of the motor that rotates the fan wheel VAV terminal unit A sheet metal assembly installed upstream of the space to vary the quantity of air delivered to the conditioned space velocity pressure Pressure due to the axial movement of the air through the duct wide open airflow The point on the fan performance curve where the system offers no resistance to airflow The pressure generated by the fan is velocity pressure only the static pressure is negligible 69 S i RANE Literature Order Number TRG TRC013 EN File Number E AV FND TRG TRC013 1099 EN The Trane Company _9 Worldwide Applied Systems Group Supersedes eee 3600 Pammel Creek Road Stocking Location Inland La Crosse La Crosse WI 54601 7599 www
34. ntrol the capacity of a fan They are discharge dampers inlet vanes fan speed control and variable pitch blade control Discharge Dampers discharge dampers Discharge Dampers The first method to be discussed is the use of discharge dampers Discharge dampers match the airflow and static pressure supplied by the fan with the airflow and static pressure required by the system They accomplish this by adding a static pressure loss to the system just downstream of the fan 44 TRG TRC013 EN S TRANE period three Fan Capacity Control Discharge Dampers discharge surge damper line SP loss design system resistance curve static pressure As the VAV 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 As a result the fan delivers a lower airflow at a higher static pressure The system static pressure controller senses this higher static pressure and sends a signal to the discharge dampers instructing them to begin closing This results in a build up of static pressure at the fan outlet and causes the fan to ride up further on its performance curve until it reaches its new operating point C at a higher static pressure and lower airflow The system balances at D along the desired VAV system modulation curve bringing the sy
35. ore static pressure can be read directly while velocity pressure is derived by subtracting the static pressure from the total pressure An alternate method would be to attach the open end of this manometer to the duct system using it to measure static pressure With one end measuring total pressure and the other end measuring static pressure the difference read on the manometer scale would be equal to the velocity pressure TRG TRC013 EN TRG TRC013 EN period one Fan Performance Fan Performance Test throttling device dynamometer manometer air straightener 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 The throttling device is used to change the air resistance of the duct The fan is operated at a single speed and the power applied to the fan shaft is measured by a device called a dynamometer As discussed on the previous slide a single manometer is used to measure the velocity pressure the difference between the total and static pressures The test is first conducted with the throttling device removed This is called wide open airflow With no resistance to airflow the pressure generated by the fan is velocity pressure only the static pressure is negligible The throttling device is then put in place and progressively moved tow
36. pensive fans Sound is one of the key issues that must be considered during fan selection Additionally the fan should be selected to operate safely away from the surge region Other guidelines for the system include m Minimize system effects since poorly designed ductwork causes turbulent airflow that results in noise m Use low pressure drop duct fittings and follow the best practices published by the Sheet Metal and Air Conditioning Contractors National Association SMACNA for designing and installing duct systems m Avoid rectangular sound traps if possible m On larger fans isolate the fan from the air handler to minimize vibration Lowering the sound level of the source reduces the sound transmitted through all paths In order to treat the paths first analyze them all and determine which are critical Then compare different methods of attenuating the critical path An optimum solution involves addressing the source and the paths during system design Software tools exist to model HVAC system noise in acoustically sensitive projects TRG TRC013 EN 57 manwe period four Application Considerations Effect of Actual Conditions Density 1 Air Density Ratio aca Density standard SP actual 2 SP A Density Ratio SP standard Air Density Ratio 3 Use Airflow etua ANd SP tanaara to Select fan 4 RPMgtandara RPMctuai 5 Power Air Density Ratio x Power ra actual Effect of Actual Nonstandard
37. point that has a higher or lower airflow static pressure and input power 19 S TRANE 20 period one Fan Performance Higher System Resistance actual system lt resistance curve e surge region static pressure airflow 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 The solution to this problem is to either improve the system design or increase the fan speed At the higher speed the system resistance and fan performance curves intersect at C to deliver the design airflow The fan must generate more static pressure to deliver the intended airflow requiring more power than expected Lower System Resistance actual system resistance curve static pressure airflow On the other hand if the system resistance is less than estimated the actual system resistance curve falls to the right of the estimated curve Instead of operating at the design operating point A the actual system resistance curve intersects the fan performance curve at D delivering a higher airflow than intended TRG TRC013 EN S TRANE period one Fan Performance 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 p
38. pressure at the fan outlet This surge of air enables the fan to re establish the proper direction of airflow The resulting fluctuation in airflow and static pressure within the fan and ductwork can result in excessive noise vibration and possibly damage to the fan TRG TRC013 EN TRG TRC013 EN period one Fan Performance Fan Surge Line yj 3 surge EN A gt line static pressure airflow 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 If the fan is operated at a point that falls to the left of this line the fan will surge Percent of Wide Open Airflow M wide open airfl airflow Static pressure 100 airflow Finally to serve as a guide for selection curves are established to indicate the percentage of wide open airflow being delivered by the fan at various operating points This completes the typical fan performance curve It shows the relationship between pressure and airflow and can be used to graphically represent the fan s interaction with the system 15 S TRANE period one Fan Performance Tabular Performance Data Std Ar Out Total Static wal Flow Velocity 05 T0 T5 20 35 35 40 CFM ifini APM BHP APM BHP APM BHP RP
39. rations 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 Additionally the effects of preswirling the air prior to it entering the fan wheel or the use of an inlet plenum or cabinet must also be considered System effect correction factors are published by the Air Movement and Control Association AMCA and fan manufacturers to aid in accounting for these additional losses before the system is installed These factors are velocity dependent and are simply added to the estimated static pressure loss for the rest of the system Finally factory supplied accessories such as silencers flanges screens and guards may also create additional pressure drops that the fan must overcome This information is generally published by the manufacturer and should be accounted for during fan selection Acoustics supply airborne up A breakout radiated Acoustics Proper acoustics are essential for a comfortable environment The sound at any particular location is the sum of sounds emanating from many sources HVAC equipment copiers lights telephones computers and people all contribute to the noise in the space The challenge for the HVAC system designer is to anticipate this and create an environment that allows spe
40. re sensor controller Because of this issue and since many VAV systems are large with high static pressures some form of system static pressure control is generally used A VAV system s static pressure requirement consists of a fixed component and a variable component The system requires a minimum amount of static pressure to properly operate the VAV modulation devices and diffusers This is considered the fixed component The second variable component is the amount of static pressure required to overcome the system pressure losses due to the ducts fittings dampers coils filters etc at various airflows Recall from the discussion of the system resistance curve that these losses vary with changes to the system airflow To ensure adequate static pressure at the VAV terminal units a simple control loop is used First the static pressure is sensed from a location in the system Next a controller compares this static pressure reading to the system s set point Finally the fan capacity is varied to deliver the required airflow at a static pressure that maintains this set point at the location of the system s sensor TRG TRC013 EN S TRANE period three Fan Capacity Control VAV System Modulation Curve design system resistance curve static pressure An exaggerated example is used to illustrate this system operation Assume that the load on the system decreases causing all or some of the VAV terminal units to modula
41. ressure with less power required In these examples it was possible to compensate for the inaccuracies in estimated system resistance through fan speed adjustment However the actual fan operating points fell at conditions other than intended Therefore it may be wise to re evaluate the fan selection Possibly another fan size would perform more efficiently at the revised system conditions Static Efficiency Power Out Static Effici SE atic Efficiency SE ET SE Airflow x Static Pressure Constant x Input Power A term commonly used to express the efficiency of a fan is static efficiency Static efficiency expresses the percentage of input power that is realized as useful work in terms of static pressure The equation used to calculate static efficiency is Power Out _ Airflow x Static Pressure ne oh Alrilow x Static Fressure SAUC ERGENE NA S Power In Constant x Input Power where Airflow is in terms of cfm m s m Static pressure is in units of in H 0 Pa m Constant is 6 362 982 E Input power is in units of hp kW TRG TRC013 EN 21 TRANE period one Fan Performance Static Efficiency sg 3500 cfmx2 0in HO _ o 7 6 362 x 2 0 hp sh GE Homer eee 7 982 x 1 5 kW D Let s assume that the fan from the previous example delivering 3 500 cfm 1 65 m s at 2 0 in H20 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
42. sends a signal to the fan instructing it to change the pitch of the fan blades This causes the performance curve for the fan to shift downward until the balance point C falls along the VAV system modulation curve and the fan satisfies the system static pressure controller The advantage of variable pitch blade control is its broad modulation range from design airflow to virtually zero airflow plus its potential for energy savings TRG TRC013 EN TRANE period three Fan Capacity Control Fan Control Comparison 100 BI fan with 90 discharge 80 dampers 70 AF fan with 60 _ inlet vanes FC fan with 5 50 FC fan with inlet vanes 4 40 discharge x 30 dampers fan speed control variable pitch vaneaxial 10 20 30 40 50 60 70 80 90 100 design airflow 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 Generally the forward curved FC centrifugal fan with inlet vanes the variable pitch vaneaxial VPVA fan and the fan speed control are similar in performance To more accurately compare the various fan types and capacity control methods for a specific application a life cycle cost analysis should be performed TRG TRC013 EN 51 S TRANE period four Application Considerations Air Conditio
43. ssions of this measurement are water gage wg and water column wc S TRANE period one Fan Performance Inclined Manometer atmospheric pressure duct gt pressure reservoir 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 This instrument is called an inclined manometer Knowing the slope of the tube to be 10 1 a pressure applied to the reservoir causes the liquid to travel ten times further up the inclined tube to achieve the liquid level difference between the tube and the reservoir This allows the pressure difference to be read with greater accuracy For example a 0 5 in 12 7 mm H 0 pressure applied to the reservoir end of the device causes the water to travel 5 in 127 mm up the inclined leg Other instruments commonly used to measure pressures related to fans include the electronic manometer and mechanical gages 4 TRG TRC013 EN period one Fan Performance Total Pressure velocity Er 7 pressure static pressure total pressure P static pressure P velocity pressure P The total amount of pressure generated by a fan has two components velocity pressure and static pressure The velocity pressure is d
44. stance curve static pressure modulation range airflow This modulation causes the actual system resistance curve to shift In a VAV system therefore the fan no longer operates at a single point on its performance curve but must operate over a range of such points Riding the Fan Curve static pressure surge actual system increase line resistance curve Q B design system resistance curve static pressure airflow reduction airflow Riding the Fan Curve The simplest form of fan capacity control is called riding the fan curve This method involves no direct form of control but simply allows the fan to react to the change in system static pressure During operation a VAV system experiences changes in resistance as the VAV terminal units modulate closed This increases the system resistance creating a new actual system resistance curve In response the operating point of the constant speed fan simply rides up on its performance curve from the design operating point A to balance the new system condition This new operating point B is at a lower airflow and a higher static pressure 40 TRG TRC013 EN S TRANE period three Fan Capacity Control Forward Curved Centrifugal Fan input power static pressure airflow This method of fan modulation can be used with any type of fan It is most effective however when applied to FC fans The configuration of the inp
45. stem static pressure downstream of the discharge damper down to its set point This method of control is essentially the same as riding the fan curve except that the static pressure drop takes place across the discharge damper instead of across the VAV terminal units While discharge dampers can be used with all types of centrifugal fans they are most effectively used with the FC fan for the same reason mentioned with riding the fan curve Other methods of supply fan capacity control are more energy efficient so discharge dampers are rarely used TRG TRC013 EN 45 S TRANE period three Fan Capacity Control Inlet Vanes Inlet Vanes 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 By changing the air s angle of entry into the fan the modulating inlet vanes lessen the ability of the fan wheel to bite the air This reduces its airflow capacity which in turn reduces its power consumption and its ability to generate static pressure Inlet Vanes inletvane s N Na position 5 input power Static pressure LA P 1 Ca a 1 1 100 airflow Inlet vanes actually alter fan performance creating a new fan performance curve with each vane position Notice that with each increment of vane closing the power requirement becomes less Therefore with inlet vanes fan energy
46. sure requirement over the range of airflows fan performance curve A plot of a specific fan s airflow capacity at a given speed rpm versus the pressure it generates 67 S TRANE 68 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 inlet vanes A device used to control the capacity of a fan by preswirling the air in the direction of fan wheel rotation before it enters the wheel lessening its ability to bite the air and reducing its airflow capacity manometer A device used to measure small pressures within a duct system optimized static pressure control An optimized method of VAV system static pressure control that uses the benefits of DDC control to continuously reset the static pressure set point of the system so that the VAV terminal requiring the highest inlet pressure is nearly wide open plenum fan See plug fan plug fan A type of centrifugal fan it consists of an unhoused fan wheel with airfoil fan blades and an inlet cone that pressurizes the plenum surrounding the fan allowing the air to discharge in multiple directions riding the fan curve A method of fan capacity modulation that involves no direct form of control but simply allows the fan to react to the change in system
47. tablishes the airflow and total pressure at which this fan and this system will balance While the system design conditions are typically stated in terms of static pressure the VPVA fan manufacturer provides conversion factors that enable the system designer to establish the fan system balance point in terms of total pressure TRG TRC013 EN period two Fan Types Air Conditioning Fans period two Fan Types The most common types of fans used in air conditioning applications are the centrifugal and axial designs Centrifugal Fan In a centrifugal fan the airflow enters the center of the fan from the side and follows a radial path through the fan wheel There are three principal types of centrifugal fans each distinguished by the type of fan wheel used forward curved FC backward inclined BI and airfoil AF TRG TRC013 EN 27 S TRANE period two Fan Types Forward Curved Fan 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 These are called forward curved or FC fans FC fans are operated at relatively low speeds and are used to deliver large volumes of air against relatively low static pressures The inherently light construction of the forward curved fan wheel does not permit this wheel to be operated at the speeds needed to generate high static pressures Forward Curved Fan The curved shape of
48. te closed This causes the system resistance curve to shift upwards In response 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 As a result the fan delivers a lower airflow at a higher static pressure The system static pressure controller senses this higher static pressure and sends a signal to the supply fan to reduce its capacity Modulating the fan capacity results in a new fan system balance point C bringing the system static pressure down to the sensor s set point This action results in the fan unloading along a curve the VAV system modulation curve This curve represents the fan modulation needed to balance the static pressure required to offset these variable system losses demand with that produced by the fan supply The equation used to calculate the VAV system modulation curve is Airflow Static Pressure SP Leu 2 x SPy SP SP where E Static Pressure and Airflow are points along the VAV system modulation curve m Static Pressure and Airflow describe the design operating point m Static Pressure is the system static pressure control set point TRG TRC013 EN 43 S TRANE period three Fan Capacity Control Methods of Fan Capacity Control A Discharge dampers A Inlet vanes A Fan speed control A Variable pitch blade control There are four methods used to actively co
49. through airflow permits this fan to be installed in limited space Finally the VPVA fan is generally applied in large built up systems requiring airflows in excess of 50 000 cfm 23 6 m s and greater than 3 in H20 747 Pal of total pressure TRG TRC013 EN TRG TRC013 EN e TRANE period three Fan Capacity Control Air Conditioning Fans 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 This type of system provides a constant volume of variable temperature air to control the environment of a building A variable air volume VAV system however controls the environment by varying the volume of constant temperature air This places additional demands on fan performance and brings up the subject of fan capacity control VAV System thermostat In a VAV system the quantity of air being delivered to each space is controlled by a modulating device a blade damper or an air valve that is contained within a VAV terminal unit box This device is controlled by a thermostat to provide only the quantity of conditioned air needed to balance the space load As the device modulates the overall system resistance changes 39 S TRANE period three Fan Capacity Control VAV System actual system resistance curve design system resi
50. tic efficiencies as high as 86 percent can be achieved with airfoil fans Because surge occurs at a higher airflow the magnitude of the surge characteristics of the airfoil fan is greater than that of the FC and flat bladed BI fans Plug or Plenum Fan plenum inlet cone fan wheel A variation of the airfoil fan is a plug or plenum fan This type of fan consists of an unhoused fan wheel with airfoil fan blades and an inlet cone The fan wheel pressurizes the plenum surrounding the fan allowing the air to discharge in multiple directions This fan can save space by eliminating turns in the ductwork and allowing multiple discharge ducts in different directions TRG TRC013 EN TRG TRC013 EN period two Fan Types Vaneaxial Fan straightening vanes fan wheel or impeller Vaneaxial 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 Tubeaxial and vaneaxial fans are simply propeller fans mounted in a cylinder They are similar except for the vane type straighteners used in the vaneaxial design Since a propeller fan inherently produces a spiral air stream these vanes are installed at the leaving side of the fan to remove much of the swirl from the air and straighten the airflow path These van
51. ting point A to a higher airflow B The fan input power requirement would change only slightly For this reason BI fans are referred to as nonoverloading type fans TRG TRC013 EN period two Fan Types Backward Curved Fan backward inclined backward curved 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 Airfoil Fan Airfoil AF Fans A refinement of the BI fan changes the shape of the blade from a flat plate to that of an airfoil similar to an airplane wing The airfoil blade induces a smooth airflow across the blade surface reducing turbulence and noise within the wheel This results in increased static efficiency and reduced overall sound levels Airfoil AF fans exhibit performance characteristics that are essentially the same as those of the flat bladed BI fan TRG TRC013 EN 33 S TRANE 34 period two Fan Types Airfoil Fan 50 static efficiency 80 to 86 static pressure airflow 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 The reason is that the airfoil fan surges at a greater percentage of wide open airflow placing the surge line farther to the right on the fan curve Sta
52. trane com An American Standard Company Since The Trane Company has a policy of continuous product improvement it reserves the right to change design and specifications without notice
53. ue to the momentum of the air as it moves axially through the duct while the static pressure is due to the perpendicular outward push of the air against the duct walls The total pressure is the sum of the velocity pressure and the static pressure Velocity Pressure vs Static Pressure For example assume a fan is attached to a straight piece of duct that has a damper at the open end To observe air movement a hinged vane is suspended from the top of the duct TRG TRC013 EN S TRANE period one Fan Performance Velocity Pressure vs Static Pressure gt Ley _ _ gt SSS damper fully open 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 The pressure exerted on the vane is due to the velocity of the air moving through the duct not the static pressure exerted on the walls of the duct At this point the outward or static pressure exerted on the duct walls is negligible Nearly all of the usable fan energy is being converted to velocity pressure Velocity Pressure vs Static Pressure partially open Partially closing the damper increases resistance to airflow The fan generates enough pressure to overcome this resistance static pressure loss but this occurs at the expense of velocity pressure Part of the fan s usable energy is now being devoted to generating eno
54. ugh static pressure to overcome the resistance of the damper TRG TRC013 EN TRG TRC013 EN S TRANE period one Fan Performance 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 The reduced velocity pressure on the face of the vane causes it to move to a more neutral position Velocity Pressure vs Static Pressure fully closed Finally when the damper is closed fully airflow stops and no velocity pressure exists in the ductwork All of the usable fan energy is now being converted to static pressure The pressure on the back side of the vane equals the pressure on the face of the vane and it assumes the neutral vertical position Measuring Static Pressure inclined manometer Both velocity and static pressures can be determined using the inclined manometer Static pressure is measured directly by inserting a probe through the duct wall with its open end perpendicular to air movement In this position only the outward or static pressure within the duct is sensed S TRANE period one Fan Performance Measuring Total Pressure inclined manometer 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 Theref
55. ut power curves of the FC fan are such that its power requirement drops as the fan operating point moves upward along the constant speed performance curve Recall that the input power curves of the BI AF and vaneaxial fans closely parallel the fan performance curves Therefore the power reduction of these types of nonoverloading fans would not be as significant when riding the fan curve Additionally when the BI and AF fans surge undesirable noise and vibration result Care must be taken to establish a modulating range that does not take these fans into the surge area The FC fan on the other hand may be operated in the surge area provided the operating point enters the surge area at a relatively low static pressure A satisfactory modulation range may be achieved by selecting a fan that is two to three sizes fan wheel diameter smaller than one that would be selected for a constant volume system This places the design operating point farther to the right on the performance curve Finally riding the fan curve is used most successfully when the system s airflow modulation range is small If the fan is required to modulate over a wide range of airflows the increased static pressure experienced at reduced airflow may overpressurize the VAV terminals resulting in greater than desired space airflow and noise problems TRG TRC013 EN 41 S TRANE 42 period three Fan Capacity Control Fan Control Loop static pressu
56. ystem level communications are already in place this control method provides the highest energy savings at the lowest cost TRG TRC013 EN TRG TRC013 EN S TRANE period four Application Considerations System Effect design system resistance curve system effect x pressure loss Bv static pressure OC desired 1 airflow I I I I airflow 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 This system effect can be attributed to turbulence due to fan inlet and outlet restrictions and to nonuniform air distribution influencing fan performance when the fan is installed in a system If unaccounted for during fan selection the fan system operating point will fall at B instead of A delivering a lower airflow than desired System Effect uniform velocity profile System effects related to the fan inlet or outlet generally occur when the air is not allowed to establish a uniform velocity profile If there is not enough straight duct at its inlet or outlet the fan will not be able to generate the static pressure for which it was rated If a diffuser that connects the fan to the duct system an elbow a branch turning vanes or a damper is located too close to 55 mane 56 period four Application Conside
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