Document N20qDkEz1DxrQLYZxBJYLp9Lp

590 CHAPTER 32 Table 1 .... Outlet Velocities for Optimum Performance of Typical Ventilating Fans 1965 Guide And Data Bd&fe _r 7^-j ' fORWAAO SPIN Rg.' 13 fC REVERSE SPIN, . >,fOJWlAAD SPIN .'; Examples df Spin at Fan Inlet Static PreBoi*. tadm of Wafer x A- ` '.v j*/. ,- ft- 1H ` i ; -2J4-- -*" 3; .' "* .-l! ?-- .-J6 - ;-8^ - 1&> ... CanfriftigcJ Fata Tuboaxiaf end ~ VonoaxJal.roit* - Outlet Velocity fpm Outlet Velodty at wheel dla. fpal.`: 400-1100 650-1450 700-1750 800-2000 ' 950-1500 1350-1900' 1660^-2350 - 1900-2700 1000-2500 ; ' - 2350-3360 1150-2800 ' 2700-3800 1250-3200- ' ' 3000-4300 1400-3500 . 3300-4700.'. 1600--4050 ' !. 2000-4950 '2300-5700 2500-6400 'i' ; .' . ,. . " **''. ' '' FAN INSTALLATION! ;; :!v- ' In designing, heating) yentilating.and aiheonditioning sys tems, the characteristics of the .fans available for. use.tiiere* tnth should not) be ignored. Inlet'and outlet connections,to fans should be deagh^d;without; restrictions and with~roim- fh'tim distUfbihce 6f air 'flow, because, either or both. defecta fldil adversely affect fan performande. If double inlet ,fans or ihultiple fans, in parallel.,are' used, caremiist be taken tirai both inlets hkve the gkme.free 'hfea'and general approach conditions. ,*)~ * The approach to, anddiscitarge from, the fan are very.'im portant to the proper operation of. the system. Since a'fah transfers energy "to the.air by creating.a vortex or whirling flow, any interference with this action.will result:in a change ih jierforinance:'The aif ehould enter'.flie`.fan ihlet axially, Without spin in either direction.' A Spin in .the same direction fis the.fah rotation Will reduce, bdih the'pressure diitput'find power. A reverse spin at .t^e inlet, causes high horsepower,' withpracticaUy no increase in pressure. ` '. Fig.* 13 indiciates^rome.inlet; arrangements which tend' to iniiise spin.u It is will to avoid .tiiirae. conditions inthedesign stages, since it is.pften 'difficiilt to correct ah iulet spin.after ingtft.llft.tinn' . _ , ,, The discharge duct should preferably be a straight sectioii, several duct diameters in length, to allow,full conversion of the fah"energy.' Fig. 14 illustrates both' poor and good pra&r tice-in fan'dischaige'designi* v- .. : .V'v; The ^election of.size of fan usually involves balancing-cost aiul space against.sbund ftr|d efficiency..Unless the pressure involved is so.highi',diat a smaller fan running at greater speed requires.a higher class1 of construction, the smallest fan'is the cheapest in first cost of the fan only, However, as thq.oast of^the driving equipment is also involved in .the total'installation cost, fan. efficiency must be considered for tKat. reason-.as weil'as.ite bearing on the cost.of .operation. In`some cases-whqre,large .fans operate long hours per year, seleqtiijir at absolute maximum' efficiency is indicated. erally, however, the power saved by selecting for optimum efficiency'does not justify,,.the. extra, cost-,, and. a slightly, smaller fan.gives the best-balance of cost and efficiency^ 'fi^eientce'to T^gs.-2 to 4 shows.that'all"types tend t6',have a:.'nuni&\mi"souiid'near maximum.efficiency. When, noise%is a;`donsi'deratidD^therefore,. selection. approaching maximum effita^icy is indicated^ Too' large a fah'may-not only cause, ftri ffiy'estm6nt and an increased powerioon- sdfnptiQii' ahd'sound, but may also give faulty performance, ff it iS of d type' having an unstable pressure characteristic. atldw1 capacity. -) 1 ' Tdble ' 1 'shows' outlet velocities for the range of. optimum ; pei^onnahc^ hf'.typfral,Ventilation. fahi. The dptiirium. per-^ 1fdiroance range. Is dt'or dear peak efficiency'and is alio ih'e fafageof!&wesf nbise. Fans for churches* schools, resittedeeh/ qthet, buildings having a low noise requirement sfi&ula beeeleCted for tew biftlet velocities.' FanS may be selected for higher outlet Vefociths than showfi'm the table if more'noise and lower efficiency are acceptable. '0:" NOT RECOMMENDCO RECOMMOdEO I ' ABRUPT gxfcWsidN' AT PAN OUTVET Fafi bischdrgd D&igft Eon? . tj|n,.,,n,n'r>nH of the ductwork and the size of the, yari- oas devices whose individual resistances determine .the statidr nressure, dictate the fan selection. Often a minor modifica^on of tiie system may permit use of a smaller motor,' and- even a lower class' of fan, with considerable saving of cost.' Invariably, the sound generated is affected, aa fans operating. high pressure produce more noise than at lower pressure^ (see Chapter 14). However, the lowest overall cost may. re-1' suit from selecting the minimum size of system equipment,; pnd ifrtVning adequate acoustical and vibration treatment.'. All ducts should' be connected to fan outlets and' inlets by of unpainted canvas or other flexible material.-' Access should be'provided in the connections for periodic'" removal! of any accumulations tending to qnhalftpge) )lhe,, rotor. When operating against high resistance, or when ? amfrfot noise leyelsare tew, it is preferable to locate the W in a room removed bom occupied areas or ip one acoustically.^, treated to prevent sound transmission. The lighter building constructions, which are common today, make it desirable.; to mount fo? and driving motors on resilient bases designed to prevent transmission of vibrations through floors, to the ' building structure. Conduits, pipes. Mid other rigid members,- should not be attached to fans. Noises due to high velocities, abrupt toms, grilles, and other items pot connected, with" the fan, may be present. Treatment of such problems' as well as the design of sound and vihration absorbents,'are disrnftqpH in Chapter 14. - FAN APPLICATIONS Many fan applications and the corresponding types,of. fan commonly used are listed ip- the following paragraphs.' Reference is also madqto the chapters in which' tfie applica-'; tions are ditoussed. . *. Cmtrakayglem supply, fana are usually, of the centrifugal type, since this application requires a wide range of satisfac tory and quiet operation against high pressures. They can readily be connected to,apparatus oflargp cross-section on the inlet sjde, and to. relatively small ductp on the outlet side. Comparative sizes have been standardized among manp. facturexs,1 and most rating tables, cover, a, .range'of 7QQ-||o 500,000 cfm, and static pressures from to 15 in. of Water. Central-system exhaust fans are predominantly centrito-' gal, but toe space conservation of toe axial is being in-, crearingly utilized. Tubeaxial and vaneaxial fan sizes'are; not yet standardized, but several manufacturers fist cc' pacities from 2000 to 125,000 cfm, and static pressures up* to 3 in. of water. Exhaust fans are found in ail types. Wall fans are pre dominantly of the propeller type, since they operate against little or no resistance. They are listed in capacities from 1000 to 75,000 cfm. They are sometimes incorporated in factory-built penthouses or roof caps, or are provided with matching automatic louvere. Hood-exhaust fans, involving ductwork, are predominantly centrifugal, especially if han dling hot, corrosive, or erosive fumes, where it is best to keep toe bearings and drive remote from the air stream. Otherwise, axial fans are applicable, ,and where Httle or no ductwork is involved, propeller fans are suitable. Spraybooth-exkausi fans are frequently centrifugal, especially if built into self-con- tained booths. Tubeaxial fans lend themselves particularly well to this application, where ease of cleaning and of suspen- son in a section of ductwork are advantageous. For such application, built-in cleanout doors are desirable. Material handling fans are usually straight radial (or modified) blade centrifugal type. Theyare of heavier construction, and have fewer blades and greater -clearances then ventilating fans. Many characteristics are compromised to provide wear re sistance apd.eane <?f maintenance, They, are commonly listed in capacities from 600 to 125,0Q0 cfm, and static pressures 'upL to 18 in. water. . ,, Mine-fan applications?* vary greatly and require fans rang- ing from small portable unite* for local' ventilation, ta large! centrifugal fans, for general or emergency ventilation. Van&- axial fans,are wellBuitedto mine ventilation. For underground location, their compactness saves on cost of excavations/anfr; above ground, their ready reversibility is valuable'in *emer-T gencies) even if reversal causes a reduction in capacity.'' T;Vr' Marinefans are available in both centrifugal and vaneaxial types. The latter are particularly well adapted to combatant'' ships and to noncombatant ships, where compactness andf: light weight are invaluable. - ` n;! `Unitary systems, i.e./ unit heaters, unit ventilators, unit! humidifiers, 'unit air conditioners, unit air coolers,' and unit*1 evaporative condensers are equipped'with centrifugal' br' propeller fans, the latter usually being limited to therrela-': tively small suspended type where no ductwork is involyedP Fans' for unite having considerable internal, or possible ex-' ternal resistance, are mostly of'the forward-curved blade,1 or' the so-called mixed-flow centrifugal type/The'lattor ia-really a' centrifugal type with axial inlets, havmg'a pressure-curvd resembling a'backward-curved-blade centrifugal-fan.-'Both''' of these, types have the high capacities (in relation'-'tiv displacement) requisite for a compact unit. Ratihga 'are frequently given for these units as separate fans, as well-''' as. in conjunction with the various internal resistances. 'In'1 multiple unite on' a' common 6haft, they are. listed up) to 40,000 ?m capacities. 'r' Cooling tower fans are predominantly of the propeller type/-; but axial types are also used for packed; towers, and occa-': sionally a centrifugal fan is used to supply forced draftsf Circulating fans are invariably of propeller or disfe'typeV- and are made in a vast variety of blade shapes and arrange-*: meats. They are designed for pleasing appearance, as well- as utility. .' ' ' ' 'f. General purpose fans &ri centrifugal fans of conventional'- design,-built for service ih' the lower capacity ranges)T^iey' are built with the fan'wheel mounted-on the motbr shaft;' or connected to a! self-contained belt-driven!arrangement:1 They are listed in capacities from 100 to 20,000 cfm, and' static pressures up to'454 in. water. Kitchen fans for domestic use are small propeller fans ' arranged for window or wall mounting, with various useful fixtures. Their capacities range from 300 to 1200 cfm.-: Attic fans are'used during the warm seasons to>'draw' large volumes of outdoor air through a house or other building whenever the indoor temperature exceeds the out door, and thereby utilize the cooling effect of the relatively cool evening or night air. Research by the ASHRAE72 in dicates that a two- to three-minute air change is desirable' in the North while in the South, a one-minute change is recommended to provide the additional cooling effect of air motion. Fans may be centrally located in an attic or other unused space, or in a hallway, and arranged to draw proportion ately from several rooms; or local window units may be installed in a single room. Central units may draw from the living quarters and discharge into the attic, whence the air escapes through windows or grilles, or the air may be drawn through grilles into the attic with the fan discharging directly outdoors. Discharge openings on the lee side are preferred. Attic are usually of propeller type, and should be selected to operate at low velocities to minimize noise. Noise is more of a problem on local unite, but care should be t-ftkpn to prevent transmission, of npise or vibration on.