Document YGQkjoXk4D9XMN0X5NB4J6310

584 CHAPTER 32 1965 Guide And Data Book points of velocity change, the amount of conversion is sel dom known, and therefore, most fan tables list only the static pressure available to overcome the system resistance. . - According to the Standard Test Code*, the efficiencies may be determined by the formulas: (total) Efficiency = (cfm) X total pressure (inches watcr)^. 6356 X horsepower input Static Efficiency (cfm) X static pressure finches water) 6356 X horsepower, input . .. (a) Q: Varies as cube of wheel diameter. (b) P: Varies as square of wheel diameter. (c) Tip Speed: Varies as wheel diameter. (d) Power: Varies as fifth power of diameter. 4. Variation in Air Density: Constant Volume--Constant System Fixed Fan Size--Constant Fan Speed (a) Q: Constant. (b) P: Varies as density. ;..(c) Power: Varies as density. 5. Variation in Air Density: Constant Pressure--Constant System "] Fixed Fan Sire--Variable Fan Speed i (o) Q: Varies inversely as square rootof density. (b) P: Constant. (c) RPM: Varies inversely as square root of density, r-. :(d)'f Power: .-.-.-"Varies inversely.as square root of density. Static pressure is used more often than total pressure,' and *r,c likewise, static efficiency is used more than mechanical effi ciency. However, where a high outlet velocity can be effec- tively utilised, the static efficiency fails to be a satisfactory measure of performance. When a fan operates against; no resistance, the static efficiency becomes zero and is meaning less^ Under such circumstances, many engineers prefer to use mechanical efficiency. --: The noise characteristics'of a fan are also an important part of the fan performance, and must be evaluated for most fan iwat-ttHatinna Fan noise is reported in terms of sound power level in' night octave bands.. Noise levels of fan installations may be predicted by use of the sound power level values. The methods of calculation are explained in-some detail-in Chapter 14, Sound Control.. . The technique of measuring the sound power, level output ,of afan haa been reasonably well established and is explained -in the Air Moving and Conditioning Association Bulletin 300.* A more detailed method is described in ASHRAE Standard 36-62, entitled Measurement of. Sound Power. Radi- .cded from Heating, Refrigerating, and Air Conditioning Equipment. FAN LAWS4 " 'Q'. Variation'in' Air'Density: Constant Weight of Air--Constant System Fixed Fan Sire--Variable-Fan Speed- - (a) Q: Varies inversely as density. (b) P: .Varies inversely as density. '!; (c)'RPM: ' 'Varies inversely as density.' ;ii,.(d), Power:- Varies inversely as square of density. Examples 1 to 4 illustrate the application of-the preced ing- fan; laws. .Example .!: A. certain fan delivers 12,000 .cfm at a-static pressure of 1 in. 'of water when operating at a speed of 400 rpm and requires an input of 4 hp. If in the same installation' 15,000 cfm are desired, what will-be the speed,-static pressure,' and powerT,. . '< ; ` Speed - 400 X - 500 rpm ' ' ' -t ' .- /500\* -- " Static pressure = 1 X I "I ** l-5 "* -, -/fiOOV : Power - 4 X 7.81 hp ,; . The performances of fans of all types follow certain Jaws which are useful in predicting the effect upon performance of changes in the conditions of operation, the duty required of the installation, or the size of the equipment due to tire space, power, or speed limitations. In the following laws, groups 1 to 6, Q = air volume and P -- static^ velocity, or total pressure. The laws pertaining to fan size apply only to fans which are geometrically similar,- Le., those in which all dimensions are proportional to some linear dimension denoted as size. If the pi number is dsn linearly proportional, it may be used, otherwise, wheel diameter is commonly used as a size criterion. 1. Variation in.Fan Speed: Constant 'Air Density--Constant System 'r (a) Q: - Varies as fan-speed. (b) P.:' Varies as square of fan speed. (c) Power:. - Varies as cube of fan speed. 1 2. Variation in Fan Sire: - 'J Constant Tip Speed--Constant Air Density Constant Fan Proportions--Fixed Point of. Rating . \ (a) Q: , Varies as square of wheel'diameter.- ! (b) P: Remains constant. / . (c) RPM: - Varies-inversely.as wheel diameter.-; -:i (d) Power:* -Varies as square of wheel .diameter. ''Examvtet:-A certain fan delivers 12,000 cfm at TO F and! normal barometric pressure (density 0375 lb per.cubic foot), at a. static pressure of 1* in. of water when, operating at 400. rpm, 'and requires 4' hp. If the air temperature is increased to 200 F (density 00602 Tb) and the speed of'the' fan remains^ this same, what mil' be the static pressure and power? '1 . 0.0602 - . Static preasure 1 X Qjyj$ * 0** m. . 0.0602 . Po'rer = 4x oBs.~3wlp Example.5: If the speed of the. fan of Example t is. in creased so 'as to produce a static pressure of 1 in. of water at. 200F as at -70F, what will be the speed, capacity, static power?!.-i *- Speed => 400 X i /0 Q75 = 446 rpm , ; y 00602 " ' Capacity = 12,000 X j/0-075 V 0.0602 TM 13092cfm (measured at 200 F)' 3. Variation in Fan Sire: At Constant RPM--Constant Air Density `` Constant Fan Proportions--Mixed Point of Rating'. !_. .. " - powe'r = 4 x ,/"II - 4.46 hp ' y 0.0602 ' Fans" Example 4: If the speed of the fan of the* previous ex ample* ^ increased so as to deliver the same weight:of air at 200 F as at 70 F, what will be the speed, capacity, static pressure, and power? Sp' d-' ox--0-075 -s,1rpm T 0.075 ' ' C^acity -- I2fl00 X ** 14045 cfm (measured at 200 F) .: bo7s ... Static pressure-1 X 1^5 m. !' i " /0.075V i ` Powtr - 4 X^^J = 6.20 hp i 585. The fan laws stated may be combined,to give other overall values. One useful combination is the product of Laws 1 and 3, which gives the following relations: \ ~. I -- . Capacity variesas the ratio of size cubed, times the ratio of the rpm._. Pressure varies as the ratio of size" squared, times the ratio of the rpm squared. Horsepower varies as the ratio of the to fifth power, times the. ratio of the rpm cubed. ... Example 6: Assuming that a fan with a 38 in.' diameter blast wheel; wQl deliver 12,000 cfm at'70 F at T in. static pressure, requiring 40 brake hp when operating at 400 rpm,what is the capacity, pressure, and.horsepower of a homologous fan having a 45 in. wheel at the same speed? .. Capacity - (g) X X 12,000 - 23400 c'tm.. Static pressure - (jg)-X (jj)' X 1 - 1.56 in. :r IWpowt, - (s)' x (|jj)'x'4 - 12i hp '. r; ' FAN PERFORMANCE CURVES . Fan performance curves are the. graphical presentation (at a stated speed and air density) of the relation of:total pressure, static pressure, power input,; and nieehani4i pffU <aency, aQd static efficiency, to actual.volume, for the desired range of volumes. Figs. 2, 3 and 4 illustrate performance (sometimes called characteristic) curves of various types of fans. Centrifugal fan* may be roughly divided into three passes: (1) those with the tip of the blades curved forwaid m tiie direction of rotation; (2) those with straight radial Wades, and (3) those with the tip of the blades inclined backward away from direction of rotation. They are also characterized as slow speed, moderate speed,' and high speed fypes, respectively, although the actual speed range of <nMh nay be wide and overlapping. The highest speed type may as high as 200 percent of the speed of the lowest speed type.' to deliver the same volume of air at the differentiating curvature is always the tip of lade, since the inlet edge, if inclined, b always curved !rTui ^ minimize the shock loss at entrance. Straight mmal blades are most frequently found in pressure fans and material-handling fans. - :____ Centrifugal fans produce pressure from two independent from the centrifugal-fdroe created by`.rotating Rg: 2.... Percentage Performance' Curves of a ForwardCurved Blade Centrifugal Fan the enclosed air column, and (2) from the kinetic energy im-1 parted to the air by virtue of its velocity leaving the impeller.This velocity, in turn, is a combination of rotative velocity of the impeller and air speed relative to' the impeller. When' the blades tip forward, these.two velocities are cumulative,' and when'backward, oppositionaL Thus a fan with forward-" curved blades depends less on centrifugal force for its pres-' sure, and more on velocity pressure conversion in the scroll, frith the result that'it may run at relatively low speed. Con-versely, a fan having. backward-curved blades builds up more`of its pressure by centrifugal force (a more efficient fonii of energy transfer) and less by velocity conversion unit; > therefore, must run at a higher speed. likewise, a fan having forward-curved `blades' will produce the greatest capacity1 of any type of the same size when' operating against no re sistance. . "Since the energy imparted to the air depends on the velocities,* and since the velocities are cumulative with a fan' having forward-curved blades,- the theoretical energy per, pound of air rises rapidly with'an increase of air delivery. With the velocities oppositional in the fan having backwardcurved blades, the energy per pound of air may decrease, and in a fan having straight blades it b roughly constant. Thus the shape of -the horsepower curve definitely identifies the blade angle. ....