ASHRAE REFRIGERATION SI CH 10-2010 INSULATION SYSTEMS FOR REFRIGERANT PIPING《制冷管道系统的绝缘系统》.pdf

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1、10.1CHAPTER 10INSULATION SYSTEMS FOR REFRIGERANT PIPINGDesign Considerations for Below-Ambient Refrigerant Piping 10.1Insulation Properties at Below-Ambient Temperatures 10.1Insulation Systems. 10.2Installation Guidelines 10.7Maintenance of Insulation Systems. 10.9HIS chapter is a guide to specifyin

2、g insulation systems forTrefrigeration piping, fittings, and vessels operated at temper-atures ranging from 2 to 70C. It does not deal with HVAC sys-tems or applications such as chilled-water systems. Refer toChapters 23, 25, 26, and 27 in the 2009 ASHRAE HandbookFun-damentals for information about

3、insulation and vapor barriers forthese systems.The success of an insulation system for cold piping, such asrefrigerant piping, depends on factors such asCorrect refrigeration system designCorrect specification of insulating systemCorrect specification of insulation thicknessCorrect installation of i

4、nsulation and related materials (e.g., vaporretarders)Installation qualityAdequate maintenance of the insulating systemRefrigerant piping includes lines that run at cold temperature,that cycle between hot and cold, and even some that run at temper-atures above ambient. These pipes use various insula

5、tion materialsand systems, and are insulated for the following reasons:Energy conservationEconomics (to minimize annualized costs of ownership and oper-ation)External surface condensation controlPrevention of gas condensation inside the pipeProcess control (i.e., for freeze protection and to limit t

6、emperaturechange of process fluids)Personnel protectionFire protectionSound and vibration controlDesign features for typical refrigeration insulation applicationsrecommended in this chapter may be followed unless they conflictwith applicable building codes. A qualified engineer may be con-sulted to

7、specify both the insulation material and its thickness (seeTables 3 to 12) based on specific design conditions. All fabricatedpipe, valve, and fitting insulation should have dimensions and tol-erances in accordance with ASTM Standards C450 and C585. Allmaterials used for thermal insulation should be

8、 installed in accor-dance with the Midwest Insulation Contractors Associations(MICA) National Commercial and Industrial Insulation Standardsor, for materials not discussed in that standard, the manufacturersrecommendations.DESIGN CONSIDERATIONS FOR BELOW-AMBIENT REFRIGERANT PIPINGBelow-ambient refri

9、gerant lines are insulated primarily to (1) min-imize heat gain to the internal fluids, (2) control surface condensation,and (3) prevent ice accumulations. Other reasons include noisereduction and personnel protection. For most installations, thethickness required to prevent surface condensation con

10、trols thedesign. Given appropriate design conditions and insulation proper-ties, computer programs such as NAIMA 3E Plus may be helpful incalculating the required insulation thickness. Tables 3 to 12 giveinsulation thickness recommendations for several typical designconditions for various insulation

11、 materials. The most economicalinsulation thickness can be determined by considering both initialcosts and long-term energy savings. In practice, this requires thedesigner to determine or assume values for a wide variety of vari-ables that usually are not known with any degree of certainty. Forinsul

12、ation applied to cold pipe, it is more common to specify theinsulation thickness that delivers a heat gain into the insulation sys-tem of 25 W/m2of outer jacket surface. This popular rule of thumbwas used to generate Tables 3 to 12, because the variability ofenergy costs and fluctuations of the myri

13、ad of economic parame-ters needed to do a thorough economic analysis go beyond thescope of this chapter.In many refrigeration systems, operation is continuous; thus, thevapor drive is unidirectional. Water vapor that condenses on the pipesurface or in the insulation remains there (as liquid water or

14、 as ice)unless removed by other means. An insulation system must dealwith this unidirectional vapor drive by providing a continuous andeffective vapor retarder to limit the amount of vapor entering theinsulation.Various insulation and accessory materials are used in systemsfor refrigerant piping. Su

15、ccessful system designs specify the bestsolution for material selection, installation procedures, operations,and maintenance to achieve long-term satisfactory performance,meeting all criteria imposed by the owner, designer, and code offi-cials.INSULATION PROPERTIES AT BELOW-AMBIENT TEMPERATURESInsul

16、ation properties important for the design of below-ambientsystems include thermal conductivity, water vapor permeance,water absorption, coefficient of thermal expansion, and wicking ofwater. See Table 2 for material properties.Thermal conductivity of insulation materials varies with tem-perature, ge

17、nerally decreasing as temperature is reduced. For pipeinsulation, conductivity is determined by ASTM Standard C335.This method is generally run at above-ambient conditions and theresults extrapolated for below-ambient applications. In many cases,conductivity is determined on flat specimens (using AS

18、TM Stan-dard C177 or C518). The designer should be aware of the methodused and its inherent limitations.Water vapor permeance is a measure of the time rate of watervapor transmission through a unit area of material or constructioninduced by a unit vapor pressure difference through two specificsurfac

19、es, under specified temperature and humidity conditions.The lower the permeance, the higher the resistance of the materialor system to passing water vapor. The unit of water vapor perme-ance is the perm, and data are determined by ASTM Standard E96.This preparation of this chapter is assigned to TC

20、10.3, Refrigerant Piping.10.2 2010 ASHRAE HandbookRefrigeration (SI)As with thermal conductivity, permeance can vary with conditions.Data for most insulation materials are determined at room temper-ature using the desiccant method. Water vapor permeance can becritical in design because water vapor c

21、an penetrate materials orsystems that are unaffected by water in the liquid form. Water vapordiffusion is a particular concern to insulation systems subjected toa thermal gradient. Pressure differences between ambient condi-tions and the colder operating conditions of the piping drive watervapor int

22、o the insulation. There it may be retained as water vapor,condense to liquid water, or condense and freeze to form ice, andcan eventually cause physical damage to the insulation system andequipment. Thermal properties of insulation materials are nega-tively affected as the moisture or vapor content

23、of the insulationmaterial increases.The coefficient of thermal expansion is important both forinsulation systems that operate continuously at below-ambient con-ditions and systems that cycle between below-ambient conditionsand elevated temperatures. Thermal contraction of insulation mate-rials may b

24、e substantially different from that of the metal pipe. Alarge difference in contraction between insulation and piping mayopen joints in the insulation, which not only creates a thermal shortcircuit at that point, but may also affect the integrity of the entiresystem. Insulation materials that have l

25、arge coefficients of thermalexpansion and do not have a high enough tensile or compressivestrength to compensate may shrink and subsequently crack. At thehigh-temperature end of the cycle, the reverse is a concern. Highthermal expansion coefficients may cause permanent warping orbuckling in some ins

26、ulation material. In this instance, the possiblestress on an external vapor retarder or weather barrier should beconsidered. The possible negative consequences of expansion orcontraction of insulation can be eliminated by proper system design,including use of appropriately designed and spaced expans

27、ion orcontraction joints.Water absorption is a materials ability to absorb and hold liquidwater. Water absorption is important where systems are exposed towater. This water may come from various external sources such asrain, surface condensation, or washdown water. The property of waterabsorption is

28、 especially important on outdoor systems and when vaporor weather retarder systems fail. Collected water in an insulation sys-tem degrades thermal performance, enhances corrosion potential, andshortens the systems service life.Wicking is the tendency of an insulation material to absorb liq-uid throu

29、gh capillary action. Wicking is measured by partially sub-merging a material and measuring both the mass of liquid that isabsorbed and the volume that the liquid has filled within the insu-lation material.Insulation System Water ResistanceRefrigeration systems are often insulated to conserve energy

30、andprevent surface condensation. An insulation systems resistance towater intrusion is a critical consideration for many refrigerant pip-ing installations. When the vapor retarder system fails, water vapormoves into the insulation material. This may lead to partial or com-plete failure of the insula

31、tion system. The problem becomes moresevere at lower operating temperatures and when operating contin-uously at cold temperatures. The driving forces are greater in thesecases and water vapor condenses and freezes on or within the insu-lation. As more water vapor is absorbed, the insulation material

32、sthermal conductivity increases, which leads to a lower surface tem-perature. This lower surface temperature leads to more condensa-tion, which may cause physical damage to the insulation system andequipment as a result of ice formation. With refrigeration equipmentoperating at 2C or lower, the prob

33、lem may be severe.If a low-permeance vapor retarder is properly installed on theinsulation system and is not damaged in any way, then the insulationmaterials water resistance is not as important. In practice, it is verydifficult to achieve and maintain perfect performance in a vaporretarder. Therefo

34、re, the water resistance of the insulation material isan important design consideration. An insulation materials waterabsorption and water vapor permeability properties are good indi-cators of its resistance to water. Because water intrusion into aninsulation system has numerous detrimental effects,

35、 better long-term performance can be achieved by limiting this intrusion. Forthese reasons, insulation materials with high resistance to moisture(low absorption, low permeability, and low wicking) should be usedfor refrigerant piping operating at temperatures below 2C.INSULATION SYSTEMSThe elements

36、of a below-ambient temperature insulation systemincludePipe preparationInsulation materialInsulation joint sealant/adhesiveVapor retardersWeather barrier/jacketingPipe Preparation for Corrosion ControlBefore any insulation is applied, all equipment and pipe surfacesto be insulated must be dry and cl

37、ean of contaminants and rust. Cor-rosion of any metal under any thermal insulation can occur for avariety of reasons. The outer surface of the pipe should be properlyprepared before installation of the insulation system. The pipe canbe primed to minimize the potential for corrosion. Careful consid-e

38、ration during insulation system design is essential. The prime con-cern is to keep the piping surface dry throughout its service life. Adry, insulated pipe surface will not have a corrosion problem. Wet,insulated pipe surfaces are the problem.Insulated carbon steel surfaces that operate continuously

39、 below5C do not present major corrosion problems. However, equip-ment or piping operating either steadily or cyclically at or abovethese temperatures may have significant corrosion problems ifwater or moisture is present. These problems are aggravated byinadequate insulation thickness, improper insu

40、lation material,improper insulation system design, and improper installation ofinsulation.Common flaws include the following:Incorrect insulation materials, joint sealants/adhesives or vaporretarders used on below-ambient temperature systemsImproper specification of insulation materials by generic t

41、yperather than by specific material properties required for the in-tended serviceImproper or unclear application methodsCarbon Steel. Carbon steel corrodes not because it is insulated,but because it is contacted by aerated water and/or a waterborne cor-rosive chemical. For corrosion to occur, water

42、must be present. Underthe right conditions, corrosion can occur under all types of insulation.Examples of insulation system flaws that create corrosion-promotingconditions includeAnnular space or crevice for water retentionInsulation material that may wick or absorb waterInsulation material that may

43、 contribute contaminants that canincrease the corrosion rateThe corrosion rate of carbon steel depends on the temperature ofthe steel surface and the contaminants in the water. The two primarysources of water are infiltration of liquid water from external sur-faces and condensation of water vapor on

44、 cold surfaces. Infiltration occurs when water from external sources enters aninsulated system through breaks in the vapor retarder or in the insu-lation itself. The breaks may result from inadequate design, incor-rect installation, abuse, or poor maintenance practices. Infiltrationof external water

45、 can be reduced or prevented. Insulation Systems for Refrigerant Piping 10.3Condensation results when the metal temperature or insulationsurface temperature is lower than the dew point. Insulation systemscannot always be made completely vaportight, so condensationmust be recognized in the system des

46、ign.The main contaminants found in insulation are chlorides and sul-fates, introduced during manufacture of the insulation or from exter-nal sources. These contaminants may hydrolyze in water to producefree acids, which are highly corrosive.Table 1 lists a few of many protective coating systems that

47、 can beused for carbon steel. For other systems or for more details, contactthe coating manufacturer.Copper. External stress corrosion cracking (ESCC) is a type oflocalized corrosion of various metals, notably copper. For ESCC tooccur in a refrigeration system, the copper must undergo the com-bined

48、effects of sustained stress and a specific corrosive species.During ESCC, copper degrades so that localized chemical reactionsoccur, often at the grain boundaries in the copper. The localizedcorrosion attack creates a small crack that advances under the in-fluence of the tensile stress. The common f

49、orm of ESCC (inter-granular) in copper results from grain boundary attack. Once theadvancing crack extends through the metal, the pressurized refrig-erant leaks from the line.ESCC occurs in the presence ofOxygen (air).Tensile stress, either residual or applied. In copper, stress can beput in the metal at the time of manufacture (residual) or duringinstallation (applied) of a refrigeration system.A chemical corrosive.Water (or moisture) to allow copper corrosion to occur.The following precautions reduce the risk of ESCC in refrigera-tion systems:Properly seal all seams and join