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588 CHAPTB* 32 1965 Guide And Data Book also be used to adjust the tan volume. All of. these methods will give control! From a power consumption consideration, a reduction of fan speed is most.efficient when,accomplished, by direct mechanical method. Inlet vanes save some power,, while dampers save the least. From consideration of first cost, dampers usually are the lowest in cost. In some installations, adjustments of volume are desirahle at various times,during the day, or continuously. In others, an increased supply of air, in Hummer, over that needed in winter, is demanded. The de-. manda in each case'will dictate which type of control is most desirable. Where noise is a factor, a lowering of.f&n speed, if; possible, is preferred as a control means, because of the results ing reduction in sound IcveL ,. - ; s .7 When-speed control is,used, the revised fan.performance, win fjg .Ky moftna of the fan-laws. The;fan perr, formance ischanged, but the system curve remains the siune., System dampers change the system resistance but leave tJio fan performance unchanged. Fig. 5 shows 3 different sys1 tern curves, A, B, and C, such as would be obtained by chang ing the damper setting. 7 Fig. 12 .illustratesithe change, in-.fan performance when inletyane control is used. In addition to the above types of control, tubeaxial aqd vaneaxial fans are'sometimes made with adjustable blades to permit balancing of the fan against the system, or mak ing seasonal adjustment. Rg. 9 Arrangement of Fan Drives Ajt. i. SW, SL, Far belt dmt or direct Wheel orcihung. Two beerin** on brae.' S! Art.8W, SL' Far belt drire or direct eonaectiou. Whed nuVn*ing- Beering* m bracket copported by fen bousin*. Air. & 8W, SL Far belt drive or direct eocaeeliea. One h--oa each rid* and oipported by bousBf. Not in JJ in iti.mrfm wheel end cmdkf. ! Act. S. DW, DL For belt .drive or direct nmnn^inn Ooe bearing an each aid* and tuppottad by'fan hearing.. i Arr. 4. SW, SL* Far direct drive. Wheel overhang oa prune No beeringt on fan' But or - eqairaloBt' far prime mover. r aabaft, Arr. 7, SW, SL Far. belt dnve.ar,direct.connection. Arrangement 1No.'* pttts be** far prime mover. Not recommended hi tiara 27 in. " Arr. 7. DW, DL Far belt drive or direct eaenectioo. Arrangement No! 1 plat h-w far prime move. Act. 8, SW, SL Far belt drive or direct connection. Arrangement No] 1 phts base for prime <do*ct. Arr. 9, SW, SL For belt drive. Arrangement No. 1 for snoot ing prime,movCT.cn ride of been. ..yl > Cfackwiee Clockwise ' Coonter-CSockwira Top Angular Up- Bottom Angular Dowd Bottom Angular Do** Nth: DiracfMO of bhim It determined from fte drive ride for either ringte or double width or ttegte or double intei fbns. (The driving rid* of O tingle Intel fan It conridered to be the ride opporito the fade# regardtett e> fiw actual location of ft* drivej For fan Inverted for ceding nupenrieo, direction of tidulwt end dfidiarg* ft determined when fan ft ratting oa floor. Rg. 10.... Designation of Direction of nr,- ' Rotation and Disdiarge7.............. Fans 589 . MOTIVE POWER ; Heating, ventilating, and air-conditioning fft.no are usually driven by electric motors, although other prims movers may be used. The small sizes of fans, and especially those operat ing in the higher speed range,' are equipped with direct- connected motors. For larger size fans, and those operating at lower speedyV-belt drives'are generally used. " In selecting the sue ;of_; motor, for. operating a fan, it is advisable to select at least the standard size next larger than the fan requirements. Direct-connected motors do not require go great a safety factor as belted units. Justification for liberal power provision exists only in systems .where it is pcrenhlft thatlarger'volumes'of air may'^'required'at intervals and made' available.' by use' of _ bypass dampers! thus greatly redueihg.thie-^rBtem/resistance!'if.suchi'a syai tom irichidcs a fan with forwaril-curved blades, it'would ;b& necessary' that the. motor be sized'for the maximum volume and duty, If such a` system' includes- fans with' backward- curved blades, the volume p^ak would riot make it.necessary to provide' additional motor power.Tn'selecting fans for such 7a system, sound ratings should be given careful considera tion.- / . V /. ! '', "` 'Where a system is constant'.'imd has'no proyision for volume change that would materially reduce the resistance, and when theresistanoe caiculations are reatonablyprecise, there is no necessity .for too liberal a.motor allpwance,(eyen where fans'with forward-eurved .blades'are'used), if the'fan has been properly selected. Fig.'5!shows that the system.'re sistance varies as the 'square' of the. volume^, arid that' the fan. static pressure varies approximately inversely5 as! the volume; thus greatly offsetting the trend toward both in crease in air, delivery and motor' load. Reference `to Fig! 5 indicates'that there is' no justification for' allowing .'.large spare motor capacity. It is generally more economical'to operate motors'well loaded.; ' . V 'J Since the power consumption of fans varies as' the cube of the speed,11very little' starting .torque is' required'of the motor. Refer to Chapter 57 for characteristics of'various types of motors. ... FAN SELECTION / 171. . The following information is required to select the proper type and size.of fan: - _J ; ; 1. Capacity iD cubic feet per minute. .- 2. State pressure or system resistance; / 3. Air density, if other than standard. '*-- J 4. Type of application or service. 5. Arrangement of system. 6. Prevailing sound level or use of space served. 7. Nature of load. S. Type of motive power available. In order to facilitate the choice of apparatus, the various fan manufacturers supply fan tables or curves which usually show the following factors for each sire of fan operating against a wide range of static pressures: (1) volume of air in cubic feet per minute (at a density of 0.075 lb per cubic foot), (2) outlet velocity, (3) revolutions per minute, (4) brake horsepower, (5) tip or peripheral speed, and (6) static pres sure. The most efficient operating point is usually shown by either boldface or italicized figures in the capacity tables. Often the service determines the type of fan. When opera tion occurs with little or no resistance, and particularly with out a duct system, the propeller fan is indicated for con venience and low cost. When resistance is low, the power required is low, and efficiency becomes of secondary im^ portance. When a duct system is involved, the choice is usu ally made between a centrifugal fan and a tubeaxial or vaneaxial Tan. At times, the capacity-pressureispeed relationship (specific speed)* dictates a choice. Usually,.space, efficiency^ sound, cost, and serviceability must sil'be'consideredi*rInf general, centrifugal and axial fans hre. comparable' in effi- ciency ahd sound,' but the latter are lighter and'require con siderably. less space, especially if arranged for straight-! through operation. The comparison' cannot be: made 'on' the cost'of fans only, but the difference in cost of ductwork mounting, and servicing must be included. A varieajtial .is more efficient and quieter than a tubeaxial,' but is more.ex pensive and frequently requires more space! While'requir ing, less space'than the centrifugal, the axial flow fan is inherently les3 accessible for service. When high-temperature air or air containing corrosive elements is being conveyed^ motors and bearings should be located outside "of the mr. stream. This,requirement may determine the' type of.fan' to ;be used. Where the system resistance is`indefinite or! variable, the pressure, horsepower, and noise characteristics of centrifugal fans usually indicate their selection. Under such conditions, a steep and constantly rising pressure curve ,per^ mits less variation in air delivery when the resistance paries! likewise', a flat'sound curve minimizes tho change of moving, into a region of'increased noise. A fan having a high effi^ ciency over a wide range is more desirable than .one .whiph reaches ah .even higher miimum efficiency, but decreases ' more rapidly on either side of a narrow range. A self-limiting horsepower-curve, may,,permit more accurate selectioni of; motor size. v.vi ben