1、21.1CHAPTER 21DUCT DESIGNBERNOULLI EQUATION 21.1Head and Pressure. 21.2SYSTEM ANALYSIS . 21.2Pressure Changes in System 21.5FLUID RESISTANCE 21.6Friction Losses. 21.6Dynamic Losses . 21.9Ductwork Sectional Losses 21.12FAN/SYSTEM INTERFACE 21.12MECHANICAL EQUIPMENT ROOMS. 21.14DUCT SYSTEM DESIGN. 21.
2、14Design Considerations . 21.14Duct Design Methods. 21.19Balancing Dampers 21.21HVAC Duct Design Procedures . 21.21Industrial Exhaust System Duct Design. 21.23OMMERCIAL, industrial, and residential air duct systemCdesign must consider (1) space availability, (2) space air diffu-sion, (3) noise level
3、s, (4) air distribution system (duct and equip-ment), (5) air leakage, (6) duct heat gains and losses, (7) balancing,(8) fire and smoke control, (9) initial investment cost, and (10) sys-tem operating cost. For design of residential systems, refer to Man-ual D by ACCA (2009).Deficiencies in duct des
4、ign can result in systems that operateincorrectly or are expensive to own and operate. Poor design or lackof system sealing can produce inadequate airflow rates at the termi-nals, leading to discomfort, loss of productivity, and even adversehealth effects. Lack of sound attenuation may lead to objec
5、tionablenoise levels. Proper duct insulation eliminates excessive heat gain orloss.In this chapter, system design and calculation of a systems fric-tional and dynamic resistance to airflow are considered. Chapter 19of the 2012 ASHRAE HandbookHVAC Systems and Equipmentexamines duct construction and p
6、resents construction standards forresidential, commercial, and industrial HVAC and exhaust systems.BERNOULLI EQUATIONThe Bernoulli equation can be developed by equating the forceson an element of a stream tube in a frictionless fluid flow to the rateof momentum change. On integrating this relationsh
7、ip for steadyflow, the following expression (Osborne 1966) results:= constant, ftlbf/lbm(1)wherev = streamline (local) velocity, fpsgc= dimensional constant, 32.2 lbmft/lbfs2p = absolute pressure, lbf/ft2 = density, lbm/ft3g = acceleration caused by gravity, ft/s2z = elevation, ftAssuming constant f
8、luid density in the system, Equation (1) re-duces to= constant, ftlbf/lbm(2)Although Equation (2) was derived for steady, ideal frictionlessflow along a stream tube, it can be extended to analyze flow throughducts in real systems. In terms of pressure, the relationship for fluidresistance between tw
9、o sections is+ p1+ 1z1= + p2+ 2z2+ pt, 12(3)whereV = average duct velocity, fpspt,12= total pressure loss caused by friction and dynamic losses between sections 1 and 2, lbf/ft2In Equation (3), V (section average velocity) replaces v (streamlinevelocity) because experimentally determined loss coeffi
10、cients allowfor errors in calculatingv2/2gc(velocity pressure) across stream-lines.On the left side of Equation (3), add and subtract pz1; on the rightside, add and subtract pz2, where pz1and pz2are the values of atmo-spheric air at heights z1andz2. Thus,(4)Atmospheric pressure at any elevation ( pz
11、1and pz2) expressed interms of the atmospheric pressure paat the same datum elevation isgiven bypz1= pa az1(5)pz2= pa az2(6)Substituting Equations (5) and (6) into Equation (4) and simpli-fying yields the total pressure change between sections 1 and 2.Assume no temperature change between sections 1
12、and 2 (no heatexchanger within the section); therefore, 1=2. When a heatexchanger is located in the section, the average of the inlet andoutlet temperatures is generally used. Let = 1= 2, and ( p1 pz1)and ( p2 pz2) are gage pressures at elevations z1and z2.pt,12= + g(a )(z2 z1) (7a)pt,12= pt+ pse(7b
13、)pt= pt,1-2+ pse(7c)whereps,1= static pressure, gage at elevation z1, lbf/ft2ps,2= static pressure, gage at elevation z2, lbf/ft2V1= average velocity at section 1, fpsThe preparation of this chapter is assigned to TC 5.2, Duct Design.v22gc-p-gzgc-+v22gc-p-gzgc-+1V122gc-ggc-1V122gc-ggc-1V122gc-p1pz1p
14、z1ggc-1z1+ +2V222gc-p2+= pz2pz2ggc-2z2pt 12,+ggc-ggc-ps 1,V122-+ps 2,V222-+21.2 2013 ASHRAE HandbookFundamentalsV2= average velocity at section 2, fpsa= density of ambient air, lbm/ft3 = density of air or gas in duct, lbm/ft3pse= thermal gravity effect, lbf/ft2 pt= total pressure change between sect
15、ions 1 and 2, lbf/ft2pt,1-2= total pressure loss caused by friction and dynamic losses between sections 1 and 2, lbf/ft2HEAD AND PRESSUREThe terms head and pressure are often used interchangeably;however, head is the height of a fluid column supported by fluidflow, whereas pressure is the normal for
16、ce per unit area. For liquids,it is convenient to measure head in terms of the flowing fluid. Witha gas or air, however, it is customary to measure pressure on a col-umn of liquid.Static PressureThe term pgc/g is static head; p is static pressure.Velocity PressureThe term V2/2g refers to velocity he
17、ad, and V2/2gcrefers tovelocity pressure. Although velocity head is independent of fluiddensity, velocity pressure Equation (8) is not.pv= (V/1097)2(8)wherepv= velocity pressure, in. of waterV = fluid mean velocity, fpm1097 = conversion factor to in. of waterFor air at standard conditions (0.075 lbm
18、/ft3), Equation (8) becomespv= (V/4005)2(9)where 4005 = (10972/0.075)1/2. Velocity is calculated byV = Q/A (10)whereQ = airflow rate, cfmA = cross-sectional area of duct, ft2Total PressureTotal pressure is the sum of static pressure and velocity pressure:pt= ps+ (V/1097)2(11)orpt= ps+ pv(12)wherept=
19、 total pressure, in. of waterps= static pressure, in. of waterPressure MeasurementThe range, precision, and limitations of instruments for measur-ing pressure and velocity are discussed in Chapter 36. The manom-eter is a simple and useful means for measuring partial vacuum andlow pressure. Static, v
20、elocity, and total pressures in a duct systemrelative to atmospheric pressure can be measured with a pitot tubeconnected to a manometer. Pitot tube construction and locations fortraversing round and rectangular ducts are presented in Chapter 36.SYSTEM ANALYSISThe total pressure change caused by fric
21、tion, fittings, equipment,and net thermal gravity effect (stack effect) for each section of aduct system is calculated by the following equation:(13)where= net total pressure change for i sections, in. of water= pressure loss due to friction for i sections, in. of waterpij= total pressure loss due t
22、o j fittings, including fan system effect (FSE), for i sections, in. of waterpik= pressure loss due to k equipment for i sections, in. of water= thermal gravity effect due to r stacks for i sections, in. of waterm = number of fittings within i sectionsn = number of equipment within i sections = numb
23、er of stacks within i sectionsnup= number of duct sections upstream of fan (exhaust/return air subsystems)ndn= number of duct sections downstream of fan (supply air subsystems)From Equation (7), the thermal gravity effect for each nonhori-zontal duct with a density other than that of ambient air is
24、deter-mined by the following equation:pse= 0.192(a )(z2 z1) (14)wherepse= thermal gravity effect, in. of waterz1and z2= elevation from datum in direction of airflow (Figure 1), fta= density of ambient air, lbm/ft3 = density of air or gas within duct, lbm/ft30.192 = conversion factor to in. of waterE
25、xample 1. For Figure 1, calculate the thermal gravity effect for two cases:(a) air cooled to 30F, and (b) air heated to 1000F. Densityof air at 30F is 0.0924 lbm/ft3and at 1000F is 0.0271 lbm/ft3.Density of ambient air is 0.075 lbm/ft3. Stack height is 40 ft.Solution: pse= 9.81(a )z(a) For a(Figure
26、1A), pse= 0.192(0.075 0.0924)40 = 0.13 in. of water(b) For 2500 fpm,Le= (35)For Vo 2500 fpm,Le= (36)whereVo= duct velocity, fpmLe= effective duct length, ftAo= duct area, in2Centrifugal fans should not abruptly discharge to the atmo-sphere. A diffuser design should be selected from Fitting SR7-2 orS
27、R7-3 see ASHRAE (2012).Fan Inlet Conditions. For rated performance, air must enter thefan uniformly over the inlet area in an axial direction without prerota-tion. Nonuniform flow into the inlet is the most common cause ofreduced fan performance. Such inlet conditions are not equivalent toa simple i
28、ncrease in system resistance; therefore, they cannot betreated as a percentage decrease in the flow and pressure from the fan.A poor inlet condition results in an entirely new fan performance. Anelbow at the fan inlet, for example Fitting ED7-2 see ASHRAE(2012), causes turbulence and uneven flow int
29、o the fan impeller.Losses from the fan system effect can be eliminated by including anadequate length of straight duct between the elbow and the fan inlet.The ideal inlet condition allows air to enter axially and uniformlywithout spin. A spin in the same direction as the impeller rotationreduces the
30、 pressure/volume curve by an amount dependent on thevortexs intensity. A counterrotating vortex at the inlet slightly in-creases the pressure/volume curve, but the power is increased sub-stantially.Inlet spin may arise from many different approach conditions, andsometimes the cause is not obvious. I
31、nlet spin can be avoided by pro-viding an adequate length of straight duct between the elbow and thefan inlet. Figure 14 illustrates some common duct connections thatcause inlet spin and includes recommendations for correcting spin.Fans within plenums and cabinets or next to walls should belocated s
32、o that air may flow unobstructed into the inlets. Fan perfor-mance is reduced if the space between the fan inlet and the enclosureis too restrictive. System effect coefficients for fans in an enclosureor adjacent to walls are listed under Fitting ED7-1 see ASHRAE12 fLDh- C+V1097-2Fig. 12 Deficient S
33、ystem Performance with System Effect IgnoredVoAo10 600,-Ao4.3-Duct Design 21.13(2012). How the airstream enters an enclosure in relation to the faninlets also affects fan performance. Plenum or enclosure inlets orwalls that are not symmetrical with the fan inlets cause uneven flowand/or inlet spin.T
34、esting, Adjusting, and Balancing ConsiderationsFan system effects (FSEs) are not only to be used in conjunctionwith the system resistance characteristics in the fan selectionprocess, but are also applied in calculating the results of testing,adjusting, and balancing (TAB) field tests to allow direct
35、 compari-son to design calculations and/or fan performance data. Fan inletswirl and the effect on system performance of poor fan inlet and out-let ductwork connections cannot be measured directly. Poor inletflow patterns affect fan performance within the impeller wheel (cen-trifugal fan) or wheel ro
36、tor impeller (axial fan), and the fan outletsystem effect is flow instability and turbulence within the fan dis-charge ductwork.Static pressures at the fan inlet and outlet may be measureddirectly in some systems. In most cases, static pressure measure-ments for use in determining fan total (or stat
37、ic) pressure are notmade directly at the fan inlet and outlet, but at locations a relativelyshort distance from the fan inlet and downstream from the fan outlet.To calculate fan total pressure for this case from field measure-ments, use Equation (37), where pxyis the summation of calcu-lated total p
38、ressure losses between the fan inlet and outlet sectionsnoted. Plane 3 is used to determine airflow rate. If necessary, useEquation (17) to calculate fan static pressure knowing fan total pres-sure. For locating measurement planes and calculation procedures,consult AMCA Publication 203 (AMCA 2011b).
39、Fig. 13 Establishment of Uniform Velocity Profile in Straight Fan Outlet Duct(Adapted by permission from AMCA Publication 201)Fig. 14 Inlet Duct Connections Causing Inlet Spin and Corrections for Inlet Spin(Adapted by permission from AMCA Publication 201)21.14 2013 ASHRAE HandbookFundamentalsPt= ( p
40、s,5+ pv,5) + p2-5+ FSE2+ ( ps,4+ pv,4) + p4-1+ FSE1+ FSE1,sw(37)wherePt= fan total pressure, in. of waterps= static pressure, in. of waterpv= velocity pressure, in. of waterFSE = fan system effect, in. of water px-y= summarization of total pressure losses between planes x and y, in. of waterSubscrip
41、ts numerical subscripts same as used by AMCA (2011b):1 = fan inlet2 = fan outlet3 = plane of airflow measurement4 = plane of static pressure measurement upstream of fan5 = plane of static pressure measurement downstream of fansw =swirlMECHANICAL EQUIPMENT ROOMSIn the initial phase of building design
42、, the design engineer sel-dom has sufficient information to render the optimum HVAC designfor the project, and its space requirements are often based on per-centage of total area or other rule of thumb. The final design is usu-ally a compromise between what the engineer recommends andwhat the archit
43、ect can accommodate. At other times, the buildingowner, who may prefer a centralized or decentralized system, maydictate final design and space requirements.Total mechanical and electrical space requirements rangebetween 4 and 9% of gross building area, with most buildings in the6 to 9% range. This
44、range includes space for HVAC, electrical,plumbing, and fire protection equipment, as well as vertical shaftspace for mechanical and electrical distribution through the building.Outdoor Air Intake and Exhaust Air Discharge LocationsA key factor in the location of mechanical equipment rooms isthe sou
45、rce of outdoor air. If the air intake or exhaust system is notwell designed, contaminants from nearby outside sources (e.g.,vehicle exhaust) or from the building itself (e.g., laboratory fumehood exhaust) can enter the building with insufficient dilution.Poorly diluted contaminants may cause odors,
46、health impacts, andreduced indoor air quality. Examples are toxic stack exhausts, auto-mobile and truck traffic, kitchen cooking hoods, evaporative coolingtowers, building general exhaust air, trash dumpsters, stagnant waterbodies, snow and leaves, rain and fog, plumbing vents, vandalism,and terrori
47、sm.Chapter 45 of the 2011 ASHRAE HandbookHVAC Applica-tions discusses proper design of exhaust stacks and placement of airintakes to avoid adverse air quality impacts. Chapter 24 of this vol-ume more fully describes wind and airflow patterns around build-ings. Experience provides some general guidel
48、ines on air intakeplacement. As a rule, intakes should never be located on the roof inthe same architectural screen enclosure as contaminated exhaustoutlets. If exhaust is discharged from several locations on the roof,intakes should be located to minimize contamination. Typically, thismeans maximizing separation distance. Where all exhausts of con-cern are emitted from a single, relatively tall stack or tight cluster ofstacks, a possible intake location might be close to the base of thistall stack, if this location is not adversely affected by other exhaustlocations, or is not
