NACE 6A192-2000 Dehumudification and Temperature Control During Surface Preparation Application and Curing for Coatings Linings of Steel Tanks Vessels and Other Enclosed Areas (Ite.pdf

1、SSPC-TR 3Publication No. SSPC 01-07NACE 6A192 (2000 Revision)Item No. 24083Joint Technical Committee ReportDehumidification and Temperature Control During SurfacePreparation, Application, and Curing for Coatings/Linings of SteelTanks, Vessels, and Other Enclosed SpacesThis NACE International (NACE)/

2、Society for Protective Coatings (SSPC) report represents a consensus of those individualmembers who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect precludeanyone, whether he has adopted the report or not, from manufacturing, marketing, purchasing, or u

3、sing products, processes,or procedures not in conformance with this report. Nothing contained in this NACE/SSPC report is to be construed as grantingany right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or productcovered by Letters Patent, or

4、as indemnifying or protecting anyone against liability for infringement of Letters Patent. Thisreport should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this reportintended to apply in all cases relating to the subject. Unpredictable circumstanc

5、es may negate the usefulness of this report inspecific instances. NACE and SSPC assume no responsibility for the interpretation or use of this report by other parties.Users of this NACE/SSPC report are responsible for reviewing appropriate health, safety, environmental, and regulatorydocuments and f

6、or determining their applicability in relation to this report prior to its use. This NACE/SSPC report may notnecessarily address all potential health and safety problems or environmental hazards associated with the use of materials,equipment, and/or operations detailed or referred to within this rep

7、ort. Users of this NACE/SSPC report are also responsiblefor establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatoryauthorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the us

8、e of thisreport.CAUTIONARY NOTICE : The user is cautioned to obtain the latest edition of this report. NACE/SSPC reports are subject toperiodic review, and may be revised or withdrawn at any time without prior notice. NACE and SSPC require that action betaken to reaffirm, revise, or withdraw this re

9、port no later than ten years from the date of initial publication.Approved December 20002000, NACE International and SSPCNOTICE TO THE READER : The NACE and SSPC releases of this publication contain identical wording in the samesequence. Publication format may differ.SSPC: The Society for Protective

10、 Coatings40 24th Street, 6th FloorPittsburgh, PA 15222-4656+1 (412) 281-2331NACE International1440 South Creek DriveHouston, TX 77084-4906+1 (281) 228-6200Printed by NACE InternationalNACE 6A192/SSPC-TR 32ForewordThe use of dehumidification and temperature control hasbecome more common during coatin

11、g/lining operationsand much has been learned about ways to optimize itsuse to achieve maximum benefits at minimum cost. Thistechnical committee report presents current informationabout why and how dehumidification and temperaturecontrol are being used to achieve higher-quality coating/lining project

12、s. It is intended to be a resource forengineers and coating consultants who write specifi-cations for coating projects involving tanks or enclosedspaces.This report was originally prepared by NACE Task GroupT-6A-60 on The Need for Dehumidification Equipment inthe Application of Linings. This revisio

13、n was prepared byNACE Task Group 003 on Dehumidification. This TaskGroup is administered by NACE Specific TechnologyGroup (STG) 80 on Intersociety Joint Coatings Activities,and is sponsored by STG 03 on Protective Coatings andLinings Immersion/Buried. The Task Group also hasrepresentation from SSPC

14、Group Committee C.2 onSurface Preparation. This report is published by NACEInternational under the auspices of STG 80, and bySSPC.IntroductionThe use of dehumidification and temperature controlduring surface preparation and coating/lining applicationcan be beneficial in a variety of ambient conditio

15、ns.When used properly, dehumidification (DH) provides airdew points well below the surface temperature andreduces the relative humidity (RH) at the surface.Reducing the RH at the surface can retard rust bloom.The health and safety of personnel is also a factor in thedesign of a dehumidification syst

16、em. Dehumidificationequipment that is properly sized for a given spaceprovides air flow for safe working conditions. The lowerexplosive limits ( LELs), toxicity levels, and oxygen levelsare all evaluated at each stage of the project.The volume of coating to be sprayed per hour and thepercentage of s

17、olvent and solids to be added is calculatedusing manufacturers data sheets. The formulas for thesecalculations can be found in NFPA ( 1) 33. 1 Theappropriate air-flow rate of the dehumidified and of theexhausted air through the enclosure and the properinstrumentation to be used for monitoring during

18、 bothstages of the project are also determined.Glossary of TermsTerms used in this report are widely used in severalengineering disciplines. Precise definitions are containedin other references, notably the ASHRAE ( 2) Handbook ofFundamentals . 2 The explanatory definitions containedhere are suffici

19、ent for this report but are not as preciseand detailed as the ASHRAE definitions.Absorbent: A desiccant material that holds water vaporthrough a hydration reaction that is reversible when thematerial is heated. Sodium chloride (table salt) andlithium chloride are examples of absorbent desiccants.Ads

20、orbent: A desiccant material that holds water vaporon its surface without a change in the chemical orphysical structure of the material. Silica gel and thenaturally occurring zeolites used for pet-waste granulesare examples of adsorbent desiccants.Dehumidification: The removal of moisture from the a

21、ir.Desiccant: A material commonly used to absorbmoisture from the air; a solid or liquid material that hasthe ability to collect moisture from the air and laterrelease the water vapor when the material is heated. Adesiccant used for dehumidification has a vapor pressurebelow that of the air to be de

22、humidified is in its active,dehydrated state.Dew Point: The temperature of the air at which themoisture it contains condenses on nearby surfaces orsuspended dust particles. At constant pressure, eachdew point temperature represents a single value of airmoisture content. As a result, air dew point is

23、 often usedto describe air moisture content in absolute terms ratherthan relative humidity, which does not define the absoluteamount of moisture in the air unless the air temperatureis also known.Flash Rusting: (1) Rusting that occurs on metal withinminutes to a few hours after cleaning is complete.

24、 Thespeed with which flash rusting occurs may be indicativeof salt contamination on the surface, high humidity, orboth; (2) Appearance of rust spots on the surface ofnewly applied water-borne film during the drying phase.(1) National Fire Protection Association (NFPA), P.O. Box 9101, Quincy, MA 0226

25、9-9101.(2) American Society of Heating, Refrigeration, and Air-Conditioning Engineers, Inc. (ASHRAE), 1791 Tullie Circle NE, Atlanta, GA 30329-2305. NACE 6A192/SSPC-TR 33Humidity Ratio : The amount of moisture in the air,expressed as the weight of the water vapor compared tothe weight of the air if

26、it were perfectly dry. This results ina small decimal fraction. For example, air at 70 F (21 C)with 50% relative humidity has a humidity ratio of 0.0079.In the SI measurement system, this ratio is expressed asg water vapor/kg dry air. In the U.S. customary system ofmeasurement, the weight of water v

27、apor is converted to awhole number by multiplying the humidity ratio by 7,000(the number of grains of water vapor in 1 lb). Therefore,air at 70 F (21 C) with 50% relative humidity has ahumidity ratio of 55 grains/ lb (7.9 g/kg) of dry air.Process Air : Dry air produced with a dehumidifier.Reactivati

28、on Air : Air used to remove moisture from adesiccant material.Relative Humidity (RH) : The ratio, expressed as apercentage, of the amount of water vapor present in agiven volume of air at a given temperature to the amountrequired to saturate the air at that temperature.Rust Bloom : Discoloration of

29、steel surface indicating thebeginning of rusting.Temperature: A measure of hotness or coldness usuallyrecorded with a thermometer on either the Fahrenheit orCelsius scale; the dry-bulb temperature of the air, whichis the temperature of the air as measured by athermometer with a dry-sensing bulb prop

30、erly shieldedfrom heat radiation sources such as sun or electricheaters.Vapor Pressure : The pressure exerted by watermolecules, either suspended in an air sample or at thesurface of a solid material. A desiccant material attractswater vapor because the vapor pressure at its surface islower than the

31、 vapor pressure exerted by moleculessuspended in the air. In an effort to equalize thispressure differential, water molecules move from the airto the desiccant surface.Wet-Bulb Temperature : The temperature of the airflowing across a thermometer with its sensing bulbsurrounded by a wetted wick. The

32、water evaporatingfrom the wick cools the sensing bulb in proportion to theamount of evaporation. The evaporation effect (thereforea cooling effect) is greater when the air is drier. Bymeasuring the wet- and dry-bulb temperatures andplotting the values on a psychometric chart, the amountof moisture i

33、n the air can be determined.Methods of DehumidificationDehumidification can be accomplished by compression,refrigeration, desiccation (liquid sorption, solid sorption),or a combination of these systems. While compressionand liquid sorption are common methods of dehumidifi-cation, their use is not ge

34、nerally applicable to fieldconditions. Therefore, only the refrigerant-based anddesiccant solid-sorption techniques are discussed indetail in this report.RefrigerationThe cooling of air to below its dew point is an economicalmethod of dehumidification. This method is commonlyused at ambient temperat

35、ures of approximately 85 F(29 C) and high humidity. Ambient air is circulated overa system of refrigeration coils. The surface temperatureof the coils is set at temperatures considerably lower thanthe temperature of the incoming ambient air. As the aircools, it reaches saturation, and condensation f

36、orms.This condensation is collected and removed from thesystem. The air exits the cooling-coil section of thedehumidifier at a reduced temperature, dew point, andabsolute humidity. This refrigeration-based dehumidifi-cation system is illustrated in Figure 1. The cooler air,which has a lower dew poin

37、t, can then be reheated tolower the relative humidity.Refrigeration is often used to pre-cool and dehumidifyinbound air before it reaches a desiccant system in orderto obtain lower dew points after desiccation. The air canbe re-cooled, if necessary, by refrigeration.DesiccantSolid-sorption dehumidif

38、ication systems utilize eithergranular beds or fixed desiccant structures. Thesestructures are contained within machines through whichan air stream is passed. The desiccant used is in anactive, dehydrated state and has a vapor pressure belowthat of the air to be dehumidified. The most commonlyused d

39、esiccants are silica gel and lithium chloride. Air ispassed through beds or layers of the desiccant, whichabsorb moisture from the air stream, producing ahydrated salt. Regeneration of the hydrated salt isaccomplished with heated air, which drives off the waterof hydration, returning the sorbent to

40、its dehydrated state.The previously sorbed moisture is diverted to a separateair stream.The exothermic hydration reaction typically raises thetemperature of the exiting air stream by 10 to 15 F (6 to8 C). Therefore, in hot climates, refrigeration-typedehumidifiers are frequently used in combination

41、withdesiccant equipment to cool the air entering the space. Atypical desiccant dehumidification system is illustrated inFigure 2. Because this type of system absorbs moistureas vapor, it is commonly used at all temperatures andlevels of humidity.NACE 6A192/SSPC-TR 34Compressor raises thepressure and

42、 temperatureof the refrigerant gas.Compressor RefrigerateCondenserRefrigerantExpansion ValveLiquid RefrigerateStorageRefrigerateEvaporatorElectricReheatRefrigerant iscondensed back toa liquid, releasingits heat to the airpassing through thecondenser coil.Air FlowRefrigerate expands inside thecoil, r

43、emoving heat from theair passing through the line.To EnclosureFigure 1: Example of Refrigeration Type Dehumidifier FIGURE 1 : A Refrigeration-Type DehumidifierFIGURE 2: Desiccant WheelSizing EquipmentThe size of dehumidification equipment is typicallydetermined by considering the balance between air

44、extraction from the space and the dehumidificationdesired to accomplish the specified dew point depressionfrom the surface temperature. If the capacity of thedehumidification equipment becomes marginal throughunexpected weather changes, its efficiency can beimproved by reducing the amount of air bei

45、ng extractedfor dust control.The appropriate air-change rate for maintaining aprepared surface during blasting and between shifts whilemaintaining a large differential between dew point andsurface temperature for an extended period of time isdependent on air-space volume, equipment, geographicalloca

46、tion, climate, and season. The number of openingsin the enclosure, the airtightness of the structure, thedistance of equipment from the space, and the amount ofair to be extracted or exhausted by means other than DHequipment also influence the DH capacity. Relativelyairtight enclosures generally req

47、uire less DH volumebecause little or no additional air or moisture isintroduced into the space. Relatively large spaces usuallyrequire fewer air exchanges. Equipment contractorsusually have guides that give volume data for theirequipment.The flow capacity of a dehumidifier for a given number ofair c

48、hanges per hour is calculated using the formulashown in Equation (1):RefrigerantCondenserLiquid RefrigerantStorageRefrigerant expands inside the coil,removing heat from the air passingthrough the line.RefrigerantEvaporatorCoilCoilNACE 6A192/SSPC-TR 35XRACV i 60 )( =Where: V i is the internal volume

49、of the space minus thevolume of any obstructions in ft 3 RAC is the required air changes per hour X is the air-flow capacity in ft 3 /min that correspondswith the specified air change rateOr, for DH equipment with a capacity expressed in m 3 /h,the flow capacity is calculated using the formula shown inEquation (2):XRACV i )( =Where: V i is the internal volume of the space minus thevolume of any obstructions in m 3 RAC is the required air changes per hour X is the air-flow capacity in m 3 /h that correspondswith the specified air change rateExample One: Find the capa