1、21.1CHAPTER 21FANSTypes of Fans 21.1Principles of Operation 21.1Testing and Rating . 21.2Fan Laws 21.4Fan and System Pressure Relationships 21.6Temperature Rise Across Fans . 21.7Duct System Characteristics 21.7System Effects. 21.8Selection . 21.8Parallel Fan Operation 21.9Noise. 21.10Vibration. 21.
2、10Arrangement and Installation 21.11Fan Control 21.11Symbols 21.12FAN is a device that uses a power-driven rotating impeller toAmove air. The impeller does work on the air, imparting to itboth static and kinetic energy, which vary in proportion, dependingon the fan type.TYPES OF FANSFans are general
3、ly classified as centrifugal, axial, mixed, or crossflow according to the direction of airflow through the impeller. Fig-ure 1 shows the general configuration of a centrifugal fan. The com-ponents of an axial-flow fan are shown in Figure 2. Table 1 comparestypical characteristics of some of the most
4、 common fan types.Unhoused centrifugal fan impellers are used as circulators in someindustrial applications (e.g., heat-treating ovens) and are identified asplug fans. In this case, there is no duct connection to the fan becauseit simply circulates the air within the oven. In some HVAC installa-tion
5、s, the unhoused fan impeller is located in a plenum chamber withthe fan inlet connected to an inlet duct from the system. Outlet ductsare connected to the plenum chamber. This fan arrangement is iden-tified as a plenum fan. PRINCIPLES OF OPERATIONAll fans produce pressure by altering the airflows ve
6、locity vec-tor. A fan produces pressure and/or airflow because the rotatingblades of the impeller impart kinetic energy to the air by changingits velocity. Velocity change is in the tangential and radial velocitycomponents for centrifugal fans, and in the axial and tangentialvelocity components for
7、axial-flow fans.Centrifugal fan impellers produce pressure from the (1) centrif-ugal force created by rotating the air column contained between theblades and (2) kinetic energy imparted to the air by its velocity leav-ing the impeller. This velocity is a combination of rotational veloc-ity of the im
8、peller and airspeed relative to the impeller. When theblades are inclined forward, these two velocities are cumulative;when backward, oppositional. Backward-curved blade fans are gen-erally more efficient than forward-curved blade fans.Axial-flow fan impellers produce pressure principally by thechan
9、ge in air velocity as it passes through the impeller blades, withnone being produced by centrifugal force. These fans are dividedinto three types: propeller, tubeaxial, and vaneaxial. Propeller fans,customarily used at or near free air delivery, usually have a small-hub-to-tip-ratio impeller mounted
10、 in an orifice plate or inlet ring.Tubeaxial fans usually have reduced tip clearance and operate athigher tip speeds, giving them a higher total pressure capability thanthe propeller fan. Vaneaxial fans are essentially tubeaxial fans withguide vanes and reduced running blade tip clearance, which giv
11、eimproved pressure, efficiency, and noise characteristics.Table 1 includes typical performance curves for various types offans. These performance curves show the general characteristics ofvarious fans as they are normally used; they do not reflect fan char-acteristics reduced to common denominators
12、such as constant speedor constant propeller diameter, because fans are not selected on thebasis of these constants. The efficiencies and power characteristicsshown are general indications for each type of fan. A specific fan(size, speed) must be selected by evaluating actual characteristics.The prep
13、aration of this chapter is assigned to TC 5.1, Fans.Fig. 1 Centrifugal Fan ComponentsFig. 2 Axial Fan Components21.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)TESTING AND RATINGANSI/ASHRAE Standard 51 (ANSI/AMCA Standard 210)specifies the procedures and test setups to be used in testing fan
14、s andother air-moving devices. The most common type of test usesmultiple nozzle inlet or outlet chambers. Figure 3 illustrates a pitottraverse procedure for developing characteristics of a fan. Fan per-formance is determined from free delivery conditions to shutoffconditions. At shutoff, the fan is
15、completely blocked off; at freedelivery, the outlet resistance is reduced to zero. Between these twoconditions, an auxiliary fan and various airflow restrictions are usedto simulate various operating conditions on the fan. Sufficient pointsare obtained to define the curve between shutoff and free ai
16、r deliv-ery conditions. For each case, the specific point on the curve must bedefined by referring to the airflow rate and corresponding total orTable 1 Types of FansType Impeller Design Housing DesignCentrifugalFansAirfoilBlades of airfoil contour curved away from direction of rotation. Deep blades
17、 allow efficient expansion within blade passages.Air leaves impeller at velocity less than tip speed.For given duty, has highest speed of centrifugal fan designs.Scroll design for efficient conversion of velocity pressure to static pressure.Maximum efficiency requires close clearance and alignment b
18、etween wheel and inlet.Backward-InclinedBackward-CurvedSingle-thickness blades curved or inclined away from direction of rotation.Efficient for same reasons as airfoil fan.Uses same housing configuration as airfoil design.Radial(R)RadialTip(Rt)Higher pressure characteristics than airfoil, backward-c
19、urved, and backward-inclined fans.Curve may have a break to left of peak pressure and fan should not be operated in this area.Power rises continually to free delivery.Scroll similar to and often identical to other centrifugal fan designs.Fit between wheel and inlet not as critical as for airfoil and
20、 backward-inclined fans.Forward-CurvedFlatter pressure curve and lower efficiency than the airfoil, backward-curved, and backward-inclined.Do not rate fan in the pressure curve dip to the left of peak static pressure.Power rises continually toward free delivery.Scroll similar to and often identical
21、to other centrifugal fan designs.Fit between wheel and inlet not as critical as for airfoil and backward-inclined fans.Plenum/PlugPlenum and plug fans typically use airfoil, backward inclined, or backward curved impellers in a single inlet configuration. Relative benefits of each impeller are the sa
22、me as those described for scroll housed fans.Plenum and plug fans are unique in that they operate with no housing. The equivalent of a housing, or plenum chamber (dashed line), depends on the application.The components of the drive system for the plug fan are located outside the airstream.Axial Fans
23、PropellerLow efficiency.Limited to low-pressure applications.Usually low-cost impellers have two or more blades of single thickness attached to relatively small hub.Primary energy transfer by velocity pressure.Simple circular ring, orifice plate, or venturi.Optimum design is close to blade tips and
24、forms smooth airfoil into wheel.TubeaxialSomewhat more efficient and capable of developing more useful static pressure than propeller fan.Usually has 4 to 8 blades with airfoil or single- thickness cross section.Hub is usually less than half the fan tip diameter.Cylindrical tube with close clearance
25、 to blade tips.VaneaxialGood blade design gives medium- to high-pressure capability at good efficiency.Most efficient have airfoil blades.Blades may have fixed, adjustable, or controllable pitch.Hub is usually greater than half fan tip diameter.Cylindrical tube with close clearance to blade tips.Gui
26、de vanes upstream or downstream from impeller increase pressure capability and efficiency.Fans 21.3static pressure. Other test setups described in ANSI/ASHRAE Stan-dard 51 should produce a similar performance curve, except for fansthat produce a significant amount of swirl.Fans designed for use with
27、 duct systems are tested with a lengthof duct between the fan and measuring station. The length of ductevens out the air velocity profile discharged from the fan outlet toprovide stable, uniform airflow conditions at the plane of measure-ment. The pressure loss of the ductwork and flow straightenerb
28、etween the fan outlet and the plane of measurement are added tothe measured pressure at the plane of measurement to determine theactual fan performance. Fans designed for use without ducts, in-cluding almost all propeller fans and power roof ventilators, aretested without ductwork.Not all fan sizes
29、are tested for rating. Test information may beused to calculate performance of larger fans that are geometricallysimilar, but such information should not be extrapolated to smallerTable 1 Types of Fans (Continued )Performance Curves* Performance Characteristics ApplicationsHighest efficiency of all
30、centrifugal fan designs and peak efficiencies occur at 50 to 60% of wide-open volume.Fan has a non-overloading characteristic, which means power reaches maximum near peak efficiency and becomes lower, or self-limiting, toward free delivery.General heating, ventilating, and air-conditioning applicati
31、ons.Usually only applied to large systems, which may be low-, medium-, or high-pressure applications.Applied to large, clean-air industrial operations for significant energy savings.Similar to airfoil fan, except peak efficiency slightly lower.Curved blades are slightly more efficient than straight
32、blades.Same heating, ventilating, and air-conditioning applications as airfoil fan.Used in some industrial applications where environment may corrode or erode airfoil blade.Higher pressure characteristics than airfoil and backward-curved fans.Pressure may drop suddenly at left of peak pressure, but
33、this usually causes no problems.Power rises continually to free delivery, which is an overloading characteristic.Curved blades are slightly more efficient than straight blades.Primarily for materials handling in industrial plants. Also for some high-pressure industrial requirements.Rugged wheel is s
34、imple to repair in the field. Wheel sometimes coated with special material.Not common for HVAC applications.Pressure curve less steep than that of backward-curved fans. Curve dips to left of peak pressure.Highest efficiency occurs at 40 to 50% of wide-open volume.Operate fan to right of peak pressur
35、e.Power rises continually to free delivery which is an overloading characteristic.Primarily for low-pressure HVAC applications, such as residential furnaces, central station units, and packaged air conditioners.Plenum and plug fans are similar to comparable housed airfoil/backward-curved fans but ar
36、e generally less efficient because of inefficient conversion of kinetic energy in discharge air stream.They are more susceptible to performance degradation caused by poor installation.Plenum and plug fans are used in a variety of HVAC applications such as air handlers, especially where direct-drive
37、arrangements are desirable.Other advantages of these fans are discharge configuration flexibility and potential for smaller-footprint units.High flow rate, but very low pressure capabilities.Maximum efficiency reached near free delivery.Discharge pattern circular and airstream swirls.For low-pressur
38、e, high-volume air-moving applications, such as air circulation in a space or ventilation through a wall without ductwork.Used for makeup air applications.High flow rate, medium pressure capabilities.Pressure curve dips to left of peak pressure. Avoid operating fan in this region.Discharge pattern c
39、ircular and airstream rotates or swirls.Low- and medium-pressure ducted HVAC applications where air distribution downstream is not critical.Used in some industrial applications, such as drying ovens, paint spray booths, and fume exhausts.High-pressure characteristics with medium-volume flow capabili
40、ties.Pressure curve dips to left of peak pressure. Avoid operating fan in this region.Guide vanes correct circular motion imparted by impeller and improve pressure characteristics and efficiency of fan.General HVAC systems in low-, medium-, and high- pressure applications where straight-through flow
41、 and compact installation are required.Has good downstream air distribution.Used in industrial applications in place of tubeaxial fans.More compact than centrifugal fans for same duty.21.4 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)fans. Test information can also be used to calculate the per
42、formanceat other speeds by applying fan laws. For performance of one fan tobe determined from the known performance of another, the two fansmust be dynamically similar. Strict dynamic similarity requires thatthe important nondimensional parameters (those that affect aerody-namic characteristics, suc
43、h as Mach number, Reynolds number, sur-face roughness, and gap size) vary in only insignificant ways. Formore specific information, consult the manufacturers applicationmanual, engineering data, or Howden Buffalo (1999).FAN LAWSThe fan laws (see Table 2) relate performance variables for anydynamical
44、ly similar series of fans. The variables are fan size D,rotational speed N, gas density , volume airflow rate Q, pressurePtfor Psf, power W, and mechanical efficiency t. Fan Law 1 showsthe effect of changing size, speed, or density on volume airflow rate,pressure, and power level. Fan Law 2 shows th
45、e effect of changingsize, pressure, or density on volume airflow rate, speed, and power.Fan Law 3 shows the effect of changing size, volume airflow rate,or density on speed, pressure, and power.The fan laws apply only to a series of aerodynamically similar fansat the same point of rating on the perf
46、ormance curve. They can beused to predict the performance of any fan when test data are avail-able for any fan of the same series. Fan laws may also be usedwith a particular fan to determine the effect of speed change. How-ever, caution should be exercised in these cases, because the lawsapply only
47、when all flow conditions are similar. Changing the speedof a given fan changes parameters that may invalidate the fan laws.Unless otherwise identified, fan performance data are based ondry air at standard conditions: 101.325 kPa and 20C (1.20 kgm3).In actual applications, the fan may be required to
48、handle air or gasat some other density. The change in density may be caused by tem-perature, composition of the gas, or altitude. As indicated by the fanlaws, fan performance is affected by gas density. With constant sizeand speed, power and pressure vary in accordance with the ratio ofgas density t
49、o standard air density.Figure 4 illustrates the application of the fan laws for a change infan speed N for a specific-sized fan (i.e., D1= D2). The computed Ptfcurve is derived from the base curve. For example, point E (N1= 650)is computed from point D (N2= 600) as follows:Table 1 Types of Fans (Concluded)Type Impeller Design Housing DesignMixed-FlowMixed-FlowCombination of axial and centrifugal characteristics. Ideally suited in applications in which the air has to flow in or out axially. Higher pressure characteristic than axial fans.The majority of mixed-flow fans are in a tub
