1、SSPC-TR 3/NACE 6A192 December, 2000 SSPC-TR 3 NACE 6A192 Joint Technical Committee Report SSPC-TR 3/NACE 6A192 Dehumidification and Temperature Control During Surface Preparation, Application, and Curing for Coatings/Linings of Steel Tanks, Vessels, and Other Enclosed Spaces This SSPC: The Society f
2、or Protective Coatings (SSPC)/NACE International (NACE) report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone, whether he has adopted the report or not, from manufacturing, marketin
3、g, purchasing, or using products, processes, or procedures not in conformance with this report. Nothing contained in this SSPC/NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered
4、by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this report intended to apply in all cases relating to the subject. U
5、npredictable circumstances may negate the usefulness of this report in specific instances. SSPC and NACE assume no responsibility for the interpretation or use of this report by other parties. Users of this SSPC/NACE report are responsible for reviewing appropriate health, safety, environmental, and
6、 regulatory documents and for determining their applicability in relation to this report prior to its use. This SSPC/NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed o
7、r referred to within this report. Users of this SSPC/NACE report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulato
8、ry requirements prior to the use of this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. SSPC/NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. SSPC and NACE require that action be taken to rea
9、ffirm, revise, or withdraw this report no later than ten years from the date of initial publication. Approved December 2000 2000, SSPC and NACE International NOTICE TO THE READER: The NACE and SSPC releases of this publication contain identical wording in the same sequence. Publication format may di
10、ffer. 1SSPC-TR 3/NACE 6A192 December, 2000 Foreword The use of dehumidification and temperature control has become more common during coating/lining operations and much has been learned about ways to optimize its use to achieve maximum benefits at minimum cost. This technical committee report presen
11、ts current information about why and how dehumidification and temperature control are being used to achieve higher-quality coating/ lining projects. It is intended to be a resource for engineers and coating consultants who write specifi-cations for coating projects involving tanks or enclosed spaces
12、. This report was originally prepared by NACE Task Group T-6A-60 on The Need for Dehumidification Equipment in the Application of Linings. This revision was prepared by NACE Task Group 003 on Dehumidification. This Task Group is administered by NACE Specific Technology Group (STG) 80 on Intersociety
13、 Joint Coatings Activities, and is sponsored by STG 03 on Protective Coatings and Linings Immersion/Buried. The Task Group also has representation from SSPC Group Committee C.2 on Surface Preparation. This report is published by SSPC and by NACE International under the auspices of STG 80. Introducti
14、on The use of dehumidification and temperature control during surface preparation and coating/lining application can be beneficial in a variety of ambient conditions. When used properly, dehumidification (DH) provides air dew points well below the surface temperature and reduces the relative humidit
15、y (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 the design of a dehumidification system. Dehumidification equipment that is properly sized for a given space provides air flow for safe working conditions. The lower ex
16、plosive limits (LELs), toxicity levels, and oxygen levels are all evaluated at each stage of the project. The volume of coating to be sprayed per hour and the percentage of solvent and solids to be added is calculated using manufacturers data sheets. The formulas for these calculations can be found
17、in NFPA(1)33.1The appropriate air-flow rate of the dehumidified and of the exhausted air through the enclosure and the proper instrumentation to be used for monitoring during both stages of the project are also determined. Glossary of Terms Terms used in this report are widely used in several engine
18、ering disciplines. Precise definitions are contained in other references, notably the ASHRAE(2)Handbook of Fundamentals.2The explanatory definitions contained here are sufficient for this report but are not as precise and detailed as the ASHRAE definitions. Absorbent: A desiccant material that holds
19、 water vapor through a hydration reaction that is reversible when the material is heated. Sodium chloride (table salt) and lithium chloride are examples of absorbent desiccants. Adsorbent: A desiccant material that holds water vapor on its surface without a change in the chemical or physical structu
20、re of the material. Silica gel and the naturally occurring zeolites used for pet-waste granules are examples of adsorbent desiccants. Dehumidification: The removal of moisture from the air. Desiccant: A material commonly used to absorb moisture from the air; a solid or liquid material that has the a
21、bility to collect moisture from the air and later release the water vapor when the material is heated. A desiccant used for dehumidification has a vapor pressure below that of the air to be dehumidified is in its active, dehydrated state. Dew Point: The temperature of the air at which the moisture i
22、t contains condenses on nearby surfaces or suspended dust particles. At constant pressure, each dew point temperature represents a single value of air moisture content. As a result, air dew point is often used to describe air moisture content in absolute terms rather than relative humidity, which do
23、es not define the absolute amount of moisture in the air unless the air temperature is also known. Flash Rusting: (1) Rusting that occurs on metal within minutes to a few hours after cleaning is complete. The speed with which flash rusting occurs may be indicative of salt contamination on the surfac
24、e, high humidity, or both; (2) Appearance of rust spots on the surface of newly applied water-borne film during the drying phase. (1)National Fire Protection Association (NFPA), P.O. Box 9101, Quincy, MA 02269-9101. (2)American Society of Heating, Refrigeration, and Air-Conditioning Engineers, Inc.
25、(ASHRAE), 1791 Tullie Circle NE, Atlanta, GA 30329-2305. 2SSPC-TR 3/NACE 6A192 December, 2000 Humidity Ratio: The amount of moisture in the air, expressed as the weight of the water vapor compared to the weight of the air if it were perfectly dry. This results in a small decimal fraction. For exampl
26、e, air at 70F (21C) with 50% relative humidity has a humidity ratio of 0.0079. In the SI measurement system, this ratio is expressed as g water vapor/kg dry air. In the U.S. customary system of measurement, the weight of water vapor is converted to a whole number by multiplying the humidity ratio by
27、 7,000 (the number of grains of water vapor in 1 lb). Therefore, air at 70F (21C) with 50% relative humidity has a humidity ratio of 55 grains/lb (7.9 g/kg) of dry air. Process Air: Dry air produced with a dehumidifier. Reactivation Air: Air used to remove moisture from a desiccant material. Relativ
28、e Humidity (RH): The ratio, expressed as a percentage, of the amount of water vapor present in a given volume of air at a given temperature to the amount required to saturate the air at that temperature. Rust Bloom: Discoloration of steel surface indicating the beginning of rusting. Temperature: A m
29、easure of hotness or coldness usually recorded with a thermometer on either the Fahrenheit or Celsius scale; the dry-bulb temperature of the air, which is the temperature of the air as measured by a thermometer with a dry-sensing bulb properly shielded from heat radiation sources such as sun or elec
30、tric heaters. Vapor Pressure: The pressure exerted by water molecules, either suspended in an air sample or at the surface of a solid material. A desiccant material attracts water vapor because the vapor pressure at its surface is lower than the vapor pressure exerted by molecules suspended in the a
31、ir. In an effort to equalize this pressure differential, water molecules move from the air to the desiccant surface. Wet-Bulb Temperature: The temperature of the air flowing across a thermometer with its sensing bulb surrounded by a wetted wick. The water evaporating from the wick cools the sensing
32、bulb in proportion to the amount of evaporation. The evaporation effect (therefore a cooling effect) is greater when the air is drier. By measuring the wet- and dry-bulb temperatures and plotting the values on a psychometric chart, the amount of moisture in the air can be determined. Methods of Dehu
33、midification Dehumidification can be accomplished by compression, refrigeration, desiccation (liquid sorption, solid sorption), or a combination of these systems. While compression and liquid sorption are common methods of dehumidifi-cation, their use is not generally applicable to field conditions.
34、 Therefore, only the refrigerant-based and desiccant solid-sorption techniques are discussed in detail in this report. Refrigeration The cooling of air to below its dew point is an economical method of dehumidification. This method is commonly used at ambient temperatures of approximately 85F (29C)
35、and high humidity. Ambient air is circulated over a system of refrigeration coils. The surface temperature of the coils is set at temperatures considerably lower than the temperature of the incoming ambient air. As the air cools, it reaches saturation, and condensation forms. This condensation is co
36、llected and removed from the system. The air exits the cooling-coil section of the dehumidifier at a reduced temperature, dew point, and absolute humidity. This refrigeration-based dehumidifi-cation system is illustrated in Figure 1. The cooler air, which has a lower dew point, can then be reheated
37、to lower the relative humidity. Refrigeration is often used to pre-cool and dehumidify inbound air before it reaches a desiccant system in order to obtain lower dew points after desiccation. The air can be re-cooled, if necessary, by refrigeration. Desiccant Solid-sorption dehumidification systems u
38、tilize either granular beds or fixed desiccant structures. These structures are contained within machines through which an air stream is passed. The desiccant used is in an active, dehydrated state and has a vapor pressure below that of the air to be dehumidified. The most commonly used desiccants a
39、re silica gel and lithium chloride. Air is passed through beds or layers of the desiccant, which absorb moisture from the air stream, producing a hydrated salt. Regeneration of the hydrated salt is accomplished with heated air, which drives off the water of hydration, returning the sorbent to its de
40、hydrated state. The previously sorbed moisture is diverted to a separate air stream. The exothermic hydration reaction typically raises the temperature of the exiting air stream by 10 to 15F (6 to 8C). Therefore, in hot climates, refrigeration-type dehumidifiers are frequently used in combination wi
41、th desiccant equipment to cool the air entering the space. A typical desiccant dehumidification system is illustrated in Figure 2. Because this type of system absorbs moisture as vapor, it is commonly used at all temperatures and levels of humidity. 3SSPC-TR 3/NACE 6A192 December, 2000 Compressor ra
42、ises thepressure and temperatureof the refrigerant gas.CompressorRefrigerateCondenserRefrigerantExpansion ValveLiquid RefrigerateStorageRefrigerateEvaporatorElectricReheatRefrigerant iscondensed back toa liquid, releasingits heat to the airpassing through thecondenser coil.Air FlowRefrigerate expand
43、s inside thecoil, removing heat from theair passing through the line.To EnclosureFIGURE 1: A Refrigeration-Type Dehumidifier FIGURE 2: Desiccant Wheel Sizing Equipment The size of dehumidification equipment is typically determined by considering the balance between air extraction from the space and
44、the dehumidification desired to accomplish the specified dew point depression from the surface temperature. If the capacity of the dehumidification equipment becomes marginal through unexpected weather changes, its efficiency can be improved by reducing the amount of air being extracted for dust con
45、trol. The appropriate air-change rate for maintaining a prepared surface during blasting and between shifts while maintaining a large differential between dew point and surface temperature for an extended period of time is dependent on air-space volume, equipment, geographical location, climate, and
46、 season. The number of openings in the enclosure, the airtightness of the structure, the distance of equipment from the space, and the amount of air to be extracted or exhausted by means other than DH equipment also influence the DH capacity. Relatively airtight enclosures generally require less DH
47、volume because little or no additional air or moisture is introduced into the space. Relatively large spaces usually require fewer air exchanges. Equipment contractors usually have guides that give volume data for their equipment. The flow capacity of a dehumidifier for a given number of air changes
48、 per hour is calculated using the formula shown in Equation (1): Refrigerant Condenser Liquid Refrigerant StorageRefrigerant expands inside the coil, removing heat from the air passing through the line. Refrigerant Evaporator Coil Coil 4SSPC-TR 3/NACE 6A192 December, 2000 XRACVi60)(= Where: Viis the
49、 internal volume of the space minus the volume of any obstructions in ft3 RAC is the required air changes per hour X is the air-flow capacity in ft3/min that corresponds with the specified air change rate Or, for DH equipment with a capacity expressed in m3/h, the flow capacity is calculated using the formula shown in Equation (2): XRACVi)( = Where: Viis the internal volume of the space minus the volume of any obstructions in m3 RAC is the required air changes per hour X is the air-flow capacity in m3/h that corresponds with the specified air change rate Exa
