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 (SI)thickness, which, in this example, is in the double-layer range. View-ing the calculated economic thickness as a minimum thickness pro-
17、vides 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 thi
18、ckness 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,whic
19、h 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. Insulation systemmaintenance costs should also be
20、included, 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 prote
21、ct personnelfrom burns. The potential for burns to human skin is a complexfunction of surface temperature, surface material, and time of contact.Table 1 Minimum Duct Insulation R-Value,aCooling and Heating Only Supply Ducts and Return DuctsClimate ZonedDuct LocationExteriorVentilatedAtticUnvented At
22、tic Above Insulated CeilingUnvented Attic with Roof InsulationaUnconditionedSpacebIndirectly Conditioned SpacecBuriedHeating-Only Ducts1, 2 none none none none none none none3 R-0.62 none none none none none none4 R-0.62 none none none none none none5 R-1.06 R-0.62 none none none none R-0.626 R-1.06
23、 R-1.06 R-0.62 none none none R-0.627 R-1.41 R-1.06 R-1.06 none R-0.62 none R-0.628 R-1.41 R-1.41 R-1.06 none R-1.06 none R-1.06Cooling-Only Ducts1 R-1.06 R-1.06 R-1.41 R-0.62 R-0.62 none R-0.622 R-1.06 R-1.06 R-1.06 R-0.62 R-0.62 none R-0.623 R-1.06 R-1.06 R-1.06 R-0.62 R-0.64 none none4 R-0.62 R-0
24、.62 R-1.06 R-0.34 R-0.34 none none5, 6 R-0.62 R-0.34 R-0.62 R-0.34 R-0.34 none none7, 8 R-0.34 R-0.34 R-0.34 R-0.34 R-0.34 none noneReturn Ducts1 to 8 R-0.62 R-0.62 R-0.62 none none none noneaInsulation R-values, measured in (m2K)/W, are for the insulation as installed and do not include film resist
25、ance. 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 Section6.4.4.2 or Section 5 of 90.1-2010. Insulation resistance me
26、asured on a horizontal plane in accordance with ASTM C518 at a mean temperature of 23.9C 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 United States defined in AS
27、HRAE Standard 90.1-2010.Table 2 Minimum Pipe Insulation Thickness,ammFluid Design Operating Temp. Range, CInsulation Conductivity Nominal Pipe or Tube Size, mmConductivity,W/(mK)Mean RatingTemp., C 177 0.046 to 0.049 121 114 127 127 127 127122 to 177 0.042 to 0.046 93 89 102 114 114 11494 to 121 0.0
28、39 to 0.043 66 64 64 76 76 7661 to 93 0.036 to 0.042 52 38 38 51 51 5141 to 60 0.032 to 0.040 38 25 25 38 38 38Cooling Systems (Chilled Water, Brine, and Refrigerant)d4 to 16 0.032 to 0.040 24 13 13 25 25 2590% rhNew Orleans, LA 26.1 27.8 1253Houston, TX 25.6 27.2 2105Miami, FL 25.6 27.2 633Tampa, F
29、L 25.6 27.2 992Savannah, GA 25.0 26.7 1560Norfolk, VA 24.4 26.1 1279San Antonio, TX 24.4 26.1 932Charlotte, NC 23.3 25.0 1233Honolulu, HI 23.3 25.0 166Columbus, OH 22.8 24.4 531Minneapolis, MN 22.8 24.4 619Seattle, WA 15.6 17.2 1212Insulation for Mechanical Systems 23.5The insulation system also mus
30、t 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 selection. Factors such
31、 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-pressure differenc
32、es 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 higher on one side o
33、f 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 ofwater vapor;
34、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.In many systems
35、, 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 applicable when (1) unidi
36、rectional 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 withlow-dew-poi
37、nt 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 expulsion of accumulat
38、ed 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 2014 ASHRAE HandbookRefrigera-tion, and in ASTM (2001).Freeze PreventionIt is
39、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 content to offset
40、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 0C with no flow, use the followingequation: = (1/3600)Cp(D1/2)2 RT ln(ti ta)/(tf ta) (1)where = time to freezing
41、, h = density of water = 1000 kg/m3Cp= specific heat of water = 4200 J/(kgK)D1= inside diameter of pipe, m (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, Cta= ambient air temperature, Ctf= freezi
42、ng temperature, CAs 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= ln(D3/D2)/(2k)(2)whereD3= outer diameter of insulation, mD2= inner diame
43、ter of insulation, mk = thermal conductivity of insulation material, W/(mK)Table 6 shows estimated time to freezing, calculated using theseequations for the specific case of still water with ti= 6C and ta=28C.When unusual conditions make it impractical to maintain protec-tion with insulation or flow
44、, 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. Then, upon nucleation, de
45、ndriticice 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(335 kJ/kg) through the pipe insulation. Well-insulated pipes maygreatly retard this release of latent heat. Gordon (1996) showed thatwater pipes burst not
46、 because of ice crystal growth in the pipe, butbecause of elevated fluid pressure within a confined pipe sectionoccluded by a growing ice blockage.Table 6 Time to Cool Water to Freezing, hNominal Pipe Size, mmInsulation Thickness, mm13 25 38 50 75 10015 0.1 0.2 0.2 0.3 25 0.3 0.4 0.5 0.6 0.8 40 0.4
47、0.8 1.0 1.3 1.5 50 0.6 1.1 1.4 1.7 2.2 2.580 0.9 1.7 2.3 2.9 3.7 4.5100 1.3 2.4 3.3 4.1 5.5 6.6125 1.6 3.0 4.3 5.4 7.4 9.1150 1.9 3.7 5.3 6.9 9.4 11.7200 5.3 7.6 9.6 13.7 16.9250 6.5 10.2 12.9 17.9 22.3300 8.8 12.5 15.8 22.1 27.7Note: Assumes initial temperature = 5.5C, ambient air temperature = 28C
48、, and insu-lation thermal conductivity = 0.0432 W/(mK). Thermal resistances of pipe and airfilm are neglected. Different assumed values yield different results.Fig. 4 Time to Freeze Nomenclature23.6 2017 ASHRAE HandbookFundamentals (SI)Noise ControlDuct Insulation. Without insulation, the acoustical
49、 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 fromone room to anot
