ASHRAE FUNDAMENTALS IP CH 23-2017 Insulation For Mechanical Systems.pdf

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1、23.1CHAPTER 23INSULATION FOR MECHANICAL SYSTEMSDesign Objectives and Considerations . 23.1Materials and Systems. 23.9Installation 23.13Design Data 23.18Project Specifications 23.19HIS chapter deals with applications of thermal and acousticalTinsulation for mechanical systems in residential, commerci

2、al,and industrial facilities. Applications include pipes, tanks, vesselsand equipment, and ducts.Thermal insulation is primarily used to limit heat gain or lossfrom surfaces operating at temperatures above or below ambienttemperature. Insulation may be used to satisfy one or more of the fol-lowing d

3、esign objectives: Energy conservation: minimizing unwanted heat loss/gain frombuilding HVAC systems, as well as preserving natural and finan-cial resourcesEconomic thickness: selecting the thickness of insulation thatyields the minimum total life-cycle costPersonnel protection: controlling surface t

4、emperatures to avoidcontact burns (hot or cold)Condensation control: minimizing condensation by keeping sur-face temperature above the dew point of surrounding airProcess control: minimizing temperature change in process fluidswhere close control is neededFreeze protection: minimizing energy require

5、d for heat tracingsystems and/or extending the time to freezing in the event of sys-tem failure or when the system is purposefully idleNoise control: reducing/controlling noise in mechanical systemsFire safety: protecting critical building elements and slowing thespread of fire in buildingsFundament

6、als of thermal insulation are covered in Chapter 25;applications in insulated assemblies are discussed in Chapter 27; anddata on thermal and water vapor transmission data are in Chapter 26.1. DESIGN OBJECTIVES AND CONSIDERATIONSEnergy ConservationThermal insulation is commonly used to reduce energy

7、consump-tion of HVAC systems and equipment. Minimum insulation levelsfor ductwork and piping are often dictated by energy codes, many ofwhich are based on ASHRAE Standards 90.1 and 90.2. In manycases, it may be cost-effective to go beyond the minimum levels dic-tated by energy codes. Thicknesses gre

8、ater than the optimum eco-nomic thickness may be required for other technical reasons such ascondensation control, personnel protection, or noise control.Tables 1 to 3 contain minimum insulation levels for ducts andpipes, excerpted from ANSI/ASHRAE Standard 90.1-2010.Interest in green buildings (i.e

9、., those that are environmentallyresponsible and energy efficient, as well as healthier places to work)is increasing. The LEED(Leadership in Energy and EnvironmentalDesign) Green Building Rating System, created by the U.S. GreenBuilding Council, is a voluntary rating system that sets out sustain-abl

10、e design and performance criteria for buildings. It evaluatesenvironmental performance from a whole-building perspective andawards points based on satisfying performance criteria in several dif-ferent categories. Different levels of green building certification areawarded based on the total points e

11、arned. The role of mechanicalinsulation in reducing energy usage, along with the associated green-house gas emissions, can help to contribute to LEED certificationand should be considered when designing an insulation system.Economic ThicknessEconomics can be used to (1) select the optimum insulation

12、thickness for a specific insulation, or (2) evaluate two or moreinsulation materials for least cost for a given level of thermal per-formance. In either case, economic considerations determine themost cost-effective solution for insulating over a specific period.Life-cycle costing considers the init

13、ial cost of the insulation sys-tem plus the ongoing value of energy savings over the expected ser-vice lifetime. The economic thickness is defined as the thickness thatminimizes the total life-cycle cost.Labor and material costs of installed insulation increase withthickness. Insulation is often app

14、lied in multiple layers (1) becausematerials are not manufactured in single layers of sufficient thicknessand (2) in many cases, to accommodate expansion and contraction ofinsulation and system components. Figure 1 shows installed costs fora multilayer application. The slope of the curves is discont

15、inuous andincreases with the number of layers because labor and material costsincrease more rapidly as thickness increases. Figure 1 shows curvesof total cost of operation, insulation costs, and lost energy costs. PointA on the total cost curve corresponds to the economic insulationThe preparation o

16、f this chapter is assigned to TC 1.8, Mechanical SystemsInsulation.Fig. 1 Determination of Economic Thickness of Insulation23.2 2017 ASHRAE HandbookFundamentals thickness, which, in this example, is in the double-layer range. View-ing the calculated economic thickness as a minimum thickness pro-vide

17、s a hedge against unforeseen fuel price increases and conservesenergy.Initially, as insulation is applied, the total life-cycle cost de-creases because the value of incremental energy savings is greaterthan the incremental cost of insulation. Additional insulation re-duces total cost up to a thickne

18、ss where the change in total cost isequal to zero. At this point, no further reduction can be obtained;beyond it, incremental insulation costs exceed the additional energysavings derived by adding another increment of insulation.Economic analysis should also consider the time value of money,which ca

19、n be based on a desired rate of return for the insulationinvestment. Energy costs are volatile, and a fuel cost inflation factoris sometimes included to account for the possibility that fuel costsmay increase more quickly than general inflation. Insulation systemmaintenance costs should also be incl

20、uded, along with cost savingsassociated with the ability to specify lower capacity equipment,resulting in lower first costs.Chapter 37 of the 2015 ASHRAE HandbookHVAC Applica-tions has more information on economic analysis.Personnel ProtectionIn many applications, insulation is provided to protect p

21、ersonnelfrom burns. The potential for burns to human skin is a complexfunction of surface temperature, surface material, and time of con-tact. ASTM Standard C1055 has a good discussion of these factors.Standard industry practice is to specify a maximum temperature ofTable 1 Minimum Duct Insulation R

22、-Value,aCooling- and Heating-Only Supply Ducts and Return DuctsClimate ZonedDuct LocationExteriorVentilatedAtticUnvented Attic Above Insulated CeilingUnvented Attic with Roof InsulationaUnconditionedSpacebIndirectly Conditioned SpacecBuriedHeating-Only Ducts1, 2 none none none none none none none3 R

23、-3.5 none none none none none none4 R-3.5 none none none none none none5 R-6 R-3.5 none none none none R-3.56 R-6 R-6 R-3.5 none none none R-3.57 R-8 R-6 R-6 none R-3.5 none R-3.58 R-8 R-8 R-6 none R-6 none R-6Cooling-Only Ducts1 R-6 R-6 R-8 R-3.5 R-3.5 none R-3.52 R-6 R-6 R-6 R-3.5 R-3.5 none R-3.5

24、3 R-6 R-6 R-6 R-3.5 R-1.9 none none4 R-3.5 R-3.5 R-6 R-1.9 R-1.9 none none5, 6 R-3.5 R-1.9 R-3.5 R-1.9 R-1.9 none none7, 8 R-1.9 R-1.9 R-1.9 R-1.9 R-1.9 none noneReturn Ducts1 to 8 R-3.5 R-3.5 R-3.5 none none none noneaInsulation R-values, measured in hft2F/Btu, are for the insulation as installed a

25、nd do not include film resistance. The required minimum thicknesses do not consider water vaportransmission and possible surface condensation. Where exterior walls are used as plenum walls, wall insulation must be as required by the most restrictive condition of Section 6.4.4.2or Section 5 of 90.1-2

26、010. Insulation resistance measured on a horizontal plane in accordance with ASTM C518 at a mean temperature of 75F at the installed thickness.bIncludes crawlspaces, both ventilated and nonventilated.cIncludes return air plenums with or without exposed roofs above.dClimate zones for the continental

27、United States defined in ASHRAE Standard 90.1-2010.Table 2 Minimum Pipe Insulation Thickness,ain.Fluid Design Operating Temp. Range, FInsulation Conductivity Nominal Pipe or Tube Size, in.Conductivity,Btuin/hft2FMean RatingTemp., F 350 0.32 to 0.34 250 4.5 5.0 5.0 5.0 5.0251 to 350 0.29 to 0.32 200

28、3.5 4.0 4.5 4.5 4.5201 to 250 0.27 to 0.30 150 2.5 2.5 3.0 3.0 3.0141 to 200 0.25 to 0.29 125 1.5 1.5 2.0 2.0 2.0105 to 140 0.22 to 0.28 100 1.0 1.0 1.5 1.5 1.5Cooling Systems (Chilled Water, Brine, and Refrigerant)d40 to 60 0.22 to 0.28 75 0.5 0.5 1.0 1.0 1.090% rhNew Orleans, LA 79 82 1253Houston,

29、 TX 78 81 2105Miami, FL 78 81 633Tampa, FL 78 81 992Savannah, GA 77 80 1560Norfolk, VA 76 79 1279San Antonio, TX 76 79 932Charlotte, NC 74 77 1233Honolulu, HI 74 77 166Columbus, OH 73 76 531Minneapolis, MN 73 76 619Seattle, WA 60 63 1212Fig. 3 ASHRAE Psychrometric Chart No. 1Insulation for Mechanica

30、l Systems 23.5Faulty application techniques can impair vapor retarder perfor-mance. The effectiveness of installation and application tech-niques must be considered during selection. Factors such as vaporretarder structure, number of joints, mastics and adhesives that areused, as well as inspection

31、procedures affect system performanceand durability.When selecting a vapor retarder, the vapor-pressure differenceacross the insulation system should be considered. Higher vapor-pressure differences typically require a vapor retarder with a lowerpermeance to control the overall moisture pickup of the

32、 insulatedsystem. Service conditions affect the direction and magnitude of thevapor pressure difference: unidirectional flow exists when the watervapor pressure is constantly higher on one side of insulation system,whereas reversible flow exists when vapor pressure may be higheron either side (typic

33、ally caused by diurnal or seasonal changes onone side of the insulation system). Properties of the insulation sys-tem materials should be considered. All materials reduce the flow ofwater vapor; the low permeance of some insulation materials canadd to the overall resistance to water vapor transport

34、of the insula-tion system. All vapor retarder joints should be tightly sealed withmanufacturer-recommended sealants.Another fundamental design principle is moisture storage design.In many systems, some condensation can be tolerated, the amountdepending on the water-holding capacity or tolerance of a

35、 particularsystem. The moisture storage principle allows accumulation of waterin the insulation system, but at a rate designed to prevent harmfuleffects. This concept is applicable when (1) unidirectional vaporflow occurs, but accumulations during severe conditions can be ad-equately expelled during

36、 less severe conditions; or (2) reverse flowregularly occurs on a seasonal or diurnal cycle. Design solutions us-ing this principle include (1) periodically flushing the cold side withlow-dew-point air (requires a supply of conditioned air and a meansfor distribution), and (2) using an insulation sy

37、stem supplemented byselected vapor retarders and absorbent materials such that an accu-mulation of condensation is of little importance. Such a design mustensure sufficient expulsion of accumulated moisture.ASTM Standard C755 discusses various design principles.Chapters 25 to 27 of this volume thoro

38、ughly describe the physicsassociated with water vapor transport. Additional information isfound in Chapter 10 of the 2014 ASHRAE HandbookRefrigera-tion, and in ASTM (2001).Freeze PreventionIt is important to recognize that insulation retards heat flow; itdoes not stop it completely. If the surroundi

39、ng air temperatureremains low enough for an extended period, insulation cannot pre-vent freezing of still water or of water flowing at a rate insufficientfor the available heat content to offset heat loss. Insulation can pro-long the time required for freezing, or prevent freezing if flow ismaintain

40、ed at a sufficient rate. To calculate time (in hours)required for water to cool to 32F with no flow, use the followingequation: = Cp(D1/2)2 RT ln(ti ta)/(tf ta) (1)where = time to freezing, h = density of water = 62.4 lb/ft3Cp= specific heat of water = 1.0 Btu/lb FD1= inside diameter of pipe, ft (se

41、e Figure 4)RT= combined thermal resistance of pipe wall, insulation, and exterior air film (for a unit length of pipe)ti= initial water temperature, Fta= ambient air temperature, Ftf= freezing temperature, FAs a conservative assumption for insulated pipes, thermal resis-tances of pipe walls and exte

42、rior air film are usually neglected.Resistance of the insulation layer for a unit length of pipe is calcu-lated asRT= 12 ln(D3/D2)/(2k)(2)whereD3= outer diameter of insulation, ftD2= inner diameter of insulation, ftk = thermal conductivity of insulation material, Btuin/hft2FTable 6 shows estimated t

43、ime to freezing, calculated using theseequations for the specific case of still water with ti= 42F and ta=18F.When unusual conditions make it impractical to maintain protec-tion with insulation or flow, a hot trace pipe or electric resistanceheating cable is required along the bottom or top of the w

44、ater pipe.The heating system then supplies the heat lost through the insulation.Clean water in pipes usually supercools several degrees belowfreezing before any ice is formed. Then, upon nucleation, dendriticice forms in the water and the temperature rises to freezing. Ice canbe formed from water on

45、ly by the release of the latent heat of fusion(144 Btu/lb) through the pipe insulation. Well-insulated pipes maygreatly retard this release of latent heat. Gordon (1996) showed thatwater pipes burst not because of ice crystal growth in the pipe, butbecause of elevated fluid pressure within a confine

46、d pipe sectionoccluded by a growing ice blockage.Noise ControlDuct Insulation. Without insulation, the acoustical environmentof mechanically conditioned buildings can be greatly compromised,resulting in reduced productivity and a decrease in occupant com-fort. HVAC ducts act as conduits for mechanic

47、al equipment noise,Table 6 Time to Cool Water to Freezing, hNominal Pipe Size, NPSInsulation Thickness, in.0.511.52 3 41/2 0.1 0.2 0.2 0.3 1 0.3 0.4 0.5 0.6 0.8 1 1/2 0.4 0.8 1.0 1.3 1.5 2 0.6 1.1 1.4 1.7 2.2 2.53 0.9 1.7 2.3 2.9 3.7 4.54 1.3 2.4 3.3 4.1 5.5 6.65 1.6 3.0 4.3 5.4 7.4 9.16 1.9 3.7 5.3

48、 6.9 9.4 11.78 5.3 7.6 9.6 13.7 16.910 6.5 10.2 12.9 17.9 22.312 8.8 12.5 15.8 22.1 27.7Note: Assumes initial temperature = 42F, ambient air temperature = 18F, and insu-lation thermal conductivity = 0.30 Btuin/hft2F. Thermal resistances of pipe andair film are neglected. Different assumed values yie

49、ld different results.Fig. 4 Time to Freeze Nomenclature23.6 2017 ASHRAE HandbookFundamentals and also carry office noise between occupied spaces. Additionally,some ducts can create their own noise through duct wall vibrationsor expansion and contraction. Lined sheet metal ducts and fibrousglass rigid ducts can greatly reduce transmission of HVAC noisethrough the duct system. The insulation also reduces cross-talk fromone room to another through the ducts. A good discussion of ductacoustics is provided in Chapter 48 of the 2015 ASHR