ASHRAE FUNDAMENTALS SI CH 21-2013 Duct Design.pdf

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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 21.11FAN/SYSTEM INTERFACE 21.11MECHANICAL EQUIPMENT ROOMS. 21.13DUCT SYSTEM DESIGN. 21.

2、14Design Considerations . 21.14Duct Design Methods. 21.20Balancing Dampers 21.20HVAC Duct Design Procedures . 21.20Industrial Exhaust System Duct Design. 21.22OMMERCIAL, 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:+ gz = constant, Nm/kg (1)wherev = streamline (local) velocity, m/sp = absolute pressure, Pa (N/m2) = density, kg/m3g = acceleration caused by gravity, m/s2z = elevation, mAssuming constant fluid density in the system, Equation (1) r

8、e-duces to+ gz = constant, Nm/kg (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 two sections is+ p1+ g1z1= + p2+ g2z2+ pt,

9、 12(3)whereV = average duct velocity, m/spt,12= total pressure loss caused by friction and dynamic losses between sections 1 and 2, PaIn Equation (3), V (section average velocity) replaces v (streamlinevelocity) because experimentally determined loss coefficients allowfor errors in calculatingv2/2 (

10、velocity pressure) across streamlines.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 ( pz1and pz2) expressed interms of the atmospheri

11、c pressure paat the same datum elevation isgiven bypz1= pa gaz1(5)pz2= pa gaz2(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 and 2 (no heatexchanger within the section)

12、; 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)pt= pt,1-2+ pse(7c)whereps,1= static press

13、ure, gage at elevation z1, Paps,2= static pressure, gage at elevation z2, PaV1= average velocity at section 1, m/sV2= average velocity at section 2, m/sa= density of ambient air, kg/m3 = density of air or gas in duct, kg/m3pse= thermal gravity effect, Pa pt= total pressure change between sections 1

14、and 2, PaThe preparation of this chapter is assigned to TC 5.2, Duct Design.v22-p-+v22-p-+1V122-2V222-1V122- p1pz1pz1g1z1+ +2V222- p2+= pz2pz2g2z2pt 1-2,+ps 1,V122-+ps 2,V222-+21.2 2013 ASHRAE HandbookFundamentals (SI)pt,1-2= total pressure loss caused by friction and dynamic losses between sections

15、 1 and 2, PaHEAD 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 force per unit area. For liquids,it is convenient to measure head in terms of the flowing fluid. Witha gas or a

16、ir, however, it is customary to measure pressure on a col-umn of liquid.Static PressureThe term p/g is static head; p is static pressure.Velocity PressureThe term V2/2g refers to velocity head, and V2/2 refers to veloc-ity pressure. Although velocity head is independent of fluid density,velocity pre

17、ssure Equation (8) is not.pv= V2/2 (8)wherepv= velocity pressure, PaV = fluid mean velocity, m/sFor air at standard conditions (1.204 kg/m3), Equation (8) becomespv= 0.602 V2(9)Velocity is calculated byV = Q/A (10)whereQ = airflow rate, L/sA = cross-sectional area of duct, m2Total PressureTotal pres

18、sure is the sum of static pressure and velocity pressure:pt= ps+ V2/2 (11)orpt= ps+ pv(12)wherept= total pressure, Paps= static pressure, PaPressure MeasurementThe range, precision, and limitations of instruments for measur-ing pressure and velocity are discussed in Chapter 36. The manom-eter is a s

19、imple and useful means for measuring partial vacuum andlow pressure. Static, velocity, 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 p

20、resented in Chapter 36.SYSTEM ANALYSISThe total pressure change caused by friction, 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, Pa= pressure loss due to

21、 friction for i sections, Papij= total pressure loss due to j fittings, including fan system effect (FSE), for i sections, Papik= pressure loss due to k equipment for i sections, Pa= thermal gravity effect due to r stacks for i sections, Pam = number of fittings within i sectionsn = number of equipm

22、ent within i sections = number 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

23、than that of ambient air is deter-mined by the following equation:pse= g(a )(z2 z1) (14)wherepse= thermal gravity effect, Paz1and z2= elevation from datum in direction of airflow (Figure 1), ma= density of ambient air, kg/m3 = density of air or gas within duct, kg/m3g = 9.81 = gravitational accelera

24、tion, m/s2Example 1. For Figure 1, calculate the thermal gravity effect for two cases:(a) air cooled to 34C, and (b) air heated to 540C. Density ofair at 34C is 1.477 kg/m3and at 540C is 434 kg/m3. Densityof ambient air is 1.204 kg/m3. Stack height is 15 m.Solution:pse= 9.81(a )z(a) For a(Figure 1A)

25、, pse= 9.81(1.204 1.477)15 = 40 Pa(b) For 13 m/s,Le= (35)For Vo 13 m/s,Table 4 Duct Fitting CodesFittingFunction Geometry CategorySequential NumberS: Supply D: round (Diameter) 1. Entries 1,2,3.n2. ExitsE: Exhaust/Return R: Rectangular 3. Elbows4. TransitionsC: Common (supply and return)F: Flat oval

26、 5. Junctions6. Obstructions7. Fan and system interactions8. Duct-mounted equipment9. Dampers10. Hoods1000 fLDh- C+V22-Fig. 12 Deficient System Performance with System Effect IgnoredVoAo4500-21.12 2013 ASHRAE HandbookFundamentals (SI)Le= (36)whereVo= duct velocity, m/sLe= effective duct length, mAo=

27、 duct area, mm2Centrifugal fans should not abruptly discharge to the atmo-sphere. A diffuser design should be selected from Fitting SR7-2 orSR7-3 see ASHRAE (2012).Fan Inlet Conditions. For rated performance, air must enter thefan uniformly over the inlet area in an axial direction without pre-rotat

28、ion. Nonuniform flow into the inlet is the most common causeof reduced fan performance. Such inlet conditions are not equivalentto a simple increase in system resistance; therefore, they cannot betreated as a percentage decrease in the flow and pressure from thefan. A poor inlet condition results in

29、 an entirely new fan perfor-mance. An elbow at the fan inlet, for example Fitting ED7-2 seeASHRAE (2012), causes turbulence and uneven flow into the fanimpeller. Losses from the fan system effect can be eliminated byincluding an adequate length of straight duct between the elbow andthe fan inlet.The

30、 ideal inlet condition allows air to enter axially and uniformlywithout spin. A spin in the same direction as the impeller rotationreduces the pressure/volume curve by an amount dependent on thevortexs intensity. A counterrotating vortex at the inlet slightlyincreases the pressure/volume curve, but

31、the power is increased sub-stantially.Inlet spin may arise from many different approach conditions,and sometimes the cause is not obvious. Inlet spin can be avoidedby providing an adequate length of straight duct between the elbowand the fan inlet. Figure 14 illustrates some common duct connec-tions

32、 that cause inlet spin and includes recommendations for cor-recting spin.Fans within plenums and cabinets or next to walls should belocated so 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 ef

33、fect coefficients for fans in an enclosureor adjacent to walls are listed under Fitting ED7-1 see ASHRAE(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 uneve

34、n flowand/or inlet spin.Testing, Adjusting, and Balancing ConsiderationsFan system effects (FSEs) are not only to be used in conjunctionwith the system resistance characteristics in the fan selection pro-cess, but are also applied in calculating the results of testing, adjust-ing, and balancing (TAB

35、) field tests to allow direct comparison todesign calculations and/or fan performance data. Fan inlet swirl andthe effect on system performance of poor fan inlet and outlet duct-work connections cannot be measured directly. Poor inlet flow pat-terns affect fan performance within the impeller wheel (

36、centrifugalfan) or wheel rotor impeller (axial fan), and the fan outlet systemeffect is flow instability and turbulence within the fan dischargeductwork.Static pressures at the fan inlet and outlet may be measureddirectly in some systems. In most cases, static pressure measure-ments for use in deter

37、mining fan total (or static) 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.Fig. 13 Establishment of Uniform Velocity Profile in Straight Fan Outlet Duct(Adapted by permission from AMCA Publ

38、ication 201)Ao350-Duct Design 21.13To calculate fan total pressure for this case from field measure-ments, use Equation (37), where pxyis the summation of calcu-lated total pressure losses between the fan inlet and outlet sectionsnoted. Plane 3 is used to determine airflow rate. If necessary, useEqu

39、ation (17) to calculate fan static pressure knowing fan total pres-sure. For locating measurement planes and calculation procedures,consult AMCA Publication 203 (AMCA 2011b).Pt= ( ps,5+ pv,5) + p2-5+ FSE2+ ( ps,4+ pv,4) + p4-1+ FSE1+ FSE1,sw(37)wherePt= fan total pressure, Paps= static pressure, Pap

40、v= velocity pressure, PaFSE = fan system effect, Pa px-y= summarization of total pressure losses between planes x and y, PaSubscripts 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

41、 = plane of static pressure measurement downstream of fansw =swirlMECHANICAL EQUIPMENT ROOMSIn the initial phase of building design, 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 t

42、otal area or other rule of thumb. The final design is usu-ally a compromise between what the engineer recommends andwhat the architect can accommodate. At other times, the buildingowner, who may prefer a centralized or decentralized system, maydictate final design and space requirements.Total mechan

43、ical and electrical space requirements range be-tween 4 and 9% of gross building area, with most buildings in the 6to 9% range. This range includes space for HVAC, electrical, plumb-ing, and fire protection equipment, as well as vertical shaft space formechanical and electrical distribution through

44、the building.Outdoor Air Intake and Exhaust Air Discharge LocationsA key factor in the location of mechanical equipment rooms isthe source 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 its

45、elf (e.g., laboratory fumehood exhaust) can enter the building with insufficient dilution.Poorly diluted contaminants may cause odors, health impacts, andreduced indoor air quality. Examples are toxic stack exhausts, auto-mobile and truck traffic, kitchen cooking hoods, evaporative coolingtowers, bu

46、ilding general exhaust air, trash dumpsters, stagnant waterbodies, snow and leaves, rain and fog, plumbing vents, vandalism,and terrorism.Chapter 45 of the 2011 ASHRAE HandbookHVAC Applica-tions discusses proper design of exhaust stacks and placement of airintakes to avoid adverse air quality impact

47、s. Chapter 24 of this vol-ume more fully describes wind and airflow patterns around build-ings. Experience provides some general guidelines on air intakeplacement. As a rule, intakes should never be located on the roof inthe same architectural screen enclosure as contaminated exhaustoutlets. If exha

48、ust 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 b

49、e close to the base of thistall stack, if this location is not adversely affected by other exhaustlocations, or is not influenced by tall adjacent structures creatingdownwash.When wind is perpendicular to the upwind wall, air flows up anddown the wall, dividing at about two-thirds up the wall. The down-ward flow creates gro