ASHRAE FUNDAMENTALS IP CH 23-2013 Insulation For Mechanical Systems.pdf

1、23.1CHAPTER 23INSULATION FOR MECHANICAL SYSTEMSDesign 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, commercial,and industrial

2、 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 design objectives:

3、 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 temperatures to av

4、oidcontact 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 required for heat tracin

5、gsystems 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 buildingsFundamentals of thermal in

6、sulation 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.DESIGN OBJECTIVES AND CONSIDERATIONSEnergy ConservationThermal insulation is commonly used to reduce energy consump-tion of HVAC

7、 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 greater than the optimu

8、m 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, EnergyStandard for Buildings Except Low-Rise Reside

9、ntial Buildings.Interest in green buildings (i.e., 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 v

10、oluntary rating system that sets out sustain-able 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 cert

11、ification areawarded based on the total points earned. 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

12、can be used to (1) select the optimum insulationthickness 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 speci

13、fic period.Life-cycle costing considers the initial 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

14、 increase withthickness. Insulation is often applied 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

15、 application. The slope of the curves is discontinuous andincreases with the number of layers because labor and material costsincrease more rapidly as thickness increases. Figure 1 shows curvesThe preparation of this chapter is assigned to TC 1.8, Mechanical SystemsInsulation. Fig. 1 Determination o

16、f Economic Thickness of Insulation23.2 2013 ASHRAE HandbookFundamentalsof total cost of operation, insulation costs, and lost energy costs.Point A on the total cost curve corresponds to the economic insula-tion thickness, which, in this example, is in the double-layer range.Viewing the calculated ec

17、onomic thickness as a minimum thicknessprovides a hedge against unforeseen fuel price increases and con-serves energy.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. Additiona

18、l insulation re-duces total cost up to a thickness 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 shoul

19、d also consider the time value of money,which can 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. Insula

20、tion systemmaintenance costs should also be included, along with cost savingsassociated with the ability to specify lower capacity equipment,resulting in lower first costs.Chapter 37 of the 2011 ASHRAE HandbookHVAC Applica-tions has more information on economic analysis.Personnel ProtectionIn many a

21、pplications, insulation is provided to protect personnelfrom burns. The potential for burns to human skin is a complexTable 1 Minimum Duct Insulation R-Value,aCooling- and Heating-Only Supply Ducts and Return DuctsClimate ZonedDuct LocationExteriorVentilatedAtticUnvented Attic Above Insulated Ceilin

22、gUnvented Attic with Roof InsulationaUnconditionedSpacebIndirectly Conditioned SpacecBuriedHeating-Only Ducts1, 2 none none none none none none none3 R-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

23、 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-3.53 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-

24、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 and do not include film resistance. The required minimum thicknesses do not consider water vaportransmission and possible surface condensation. Where

25、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 90.1-2010. Insulation resistance measured on a horizontal plane in accordance with ASTM C518 at a mean temperature of 75F at the installed thickness.bInclu

26、des crawlspaces, both ventilated and nonventilated.cIncludes return air plenums with or without exposed roofs above.dClimate zones for the continental United States defined in ASHRAE Standard 90.1-2010.Table 2 Minimum Pipe Insulation ThicknessaFluid Design Operating Temp. Range, FInsulation Conducti

27、vity 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 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

28、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, 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 1233Honolul

29、u, HI 74 77 166Columbus, OH 73 76 531Minneapolis, MN 73 76 619Seattle, WA 60 63 1212Fig. 3 ASHRAE Psychrometric Chart No. 1Insulation for Mechanical Systems 23.5The insulation system should be dry before applying a vaporretarder to prevent trapping water vapor in the insulation system.The insulation

30、 system also must be protected from undue weatherexposure that could introduce moisture into the insulation before thesystem is sealed.Faulty application techniques can impair vapor retarder perfor-mance. The effectiveness of installation and application tech-niques must be considered during selecti

31、on. Factors such as vaporretarder structure, number of joints, mastics and adhesives that areused, as well as inspection procedures affect system performanceand durability.When selecting a vapor retarder, the vapor-pressure differenceacross the insulation system should be considered. Higher vapor-pr

32、essure differences typically require a vapor retarder with a lowerpermeance to control the overall moisture pickup of the insulatedsystem. Service conditions affect the direction and magnitude of thevapor pressure difference: unidirectional flow exists when the watervapor pressure is constantly high

33、er on one side of insulation system,whereas reversible flow exists when vapor pressure may be higheron either side (typically 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

34、 ofwater vapor; the low permeance of some insulation materials canadd to the overall resistance to water vapor transport of the insula-tion system. All vapor retarder joints should be tightly sealed withmanufacturer-recommended sealants.Another fundamental design principle is moisture storage design

35、.In many systems, some condensation can be tolerated, the amountdepending on the water-holding capacity or tolerance of a particularsystem. The moisture storage principle allows accumulation of waterin the insulation system, but at a rate designed to prevent harmfuleffects. This concept is applicabl

36、e when (1) unidirectional vaporflow occurs, but accumulations during severe conditions can be ad-equately expelled during 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

37、 withlow-dew-point air (requires a supply of conditioned air and a meansfor distribution), and (2) using an insulation system supplemented byselected vapor retarders and absorbent materials such that an accu-mulation of condensation is of little importance. Such a design mustensure sufficient expuls

38、ion of accumulated moisture.ASTM Standard C755 discusses various design principles.Chapters 25 to 27 of this volume thoroughly describe the physicsassociated with water vapor transport. Additional information isfound in Chapter 10 of the 2010 ASHRAE HandbookRefrigera-tion, and in ASTM (2001).Freeze

39、PreventionIt is important to recognize that insulation retards heat flow; itdoes not stop it completely. If the surrounding 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 co

40、ntent to offset heat loss. Insulation can pro-long the time required for freezing, or prevent freezing if flow ismaintained 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

41、 freezing, h = density of water = 62.4 lb/ft3Cp= specific heat of water = 1.0 Btu/lb FD1= inside diameter of pipe, ft (see 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,

42、Ftf= freezing temperature, FAs a conservative assumption for insulated pipes, thermal resis-tances of pipe walls and exterior 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, ft

43、D2= inner diameter of insulation, ftk = thermal conductivity of insulation material, Btuin/hft2FTable 6 shows estimated time 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 w

44、ith insulation or flow, a hot trace pipe or electric resistanceheating cable is required along the bottom or top of the water 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. The

45、n, upon nucleation, dendriticice forms in the water and the temperature rises to freezing. Ice canbe formed from water only 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 th

46、atwater pipes burst not because of ice crystal growth in the pipe, butTable 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.

47、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 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 anda

48、ir film are neglected. Different assumed values yield different results.Fig. 4 Time to Freeze Nomenclature23.6 2013 ASHRAE HandbookFundamentalsbecause of elevated fluid pressure within a confined pipe sectionoccluded by a growing ice blockage.Noise ControlDuct Insulation. Without insulation, the aco

49、ustical 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 mechanical equipment noise,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