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Computational Fluid Dynamics Electronically reprinted from November 2019 www.chemengonline.com Computational Fluid Dynamics Heat Transfer Facts at Your Fingertips: Pneumatic Conveying Sustainable Supply Chains Focus on Packaging Water Treatment Production of Nitrile Rubber Security Audits ASTM Standards For Heat Transfer Fluids This practical overview of ASTM standards can serve as a roadmap when selecting and evaluating heat transfer fluids Gerald E. Guffey II Eastman Chemical ability of the fluid, safety in use, effect Eastman Chemical Co. on equipment, efficiency (heat-trans- eat-transfer-fluid systems fer capability) and service life. are often used for pro- Pumpability. Several properties are cess heating to 500°F relevant to the pumpability of a heat Hor higher. To accomplish transfer fluid. Pumpability is briefly heating above 500°F, steam pres- discussed in this standard, but it is sure must be above 600 psig. For discussed more extensively in D8046. many plants, distributing steam Decreased flash point (measured by above 600 psig is impractical due D92 or D93) is indicative of the pres- to high cost and more stringent wa- ence of low-boiling compounds pro- ter-purity requirements. High-tem- duced by thermal degradation of the perature heat-transfer fluids have fluid. Viscosity (measured by D445 or a much lower vapor pressure than D7042) is a measurement of the flu- water at high temperature, so the id’s resistance to flow. The presence design pressure of a heat-transfer- of thermal degradation products can fluid system can be lower. affect the viscosity of the fluid. As fluid Most systems operate with the temperature is decreased, at some heat transfer fluid in the liquid phase. point the fluid viscosity becomes too A few heat transfer fluids have a high to pump. There is some correla- narrow boiling range and can be FIGURE 1. One type of synthetic organic heat tion between hydraulic shock during used in a vapor-phase system, but transfer fluid is shown here pumping with the fluid’s specific grav- the design of the system is much ity (measured by D1298 or D4052) different and the process heating transfer fluids other than inorganic and compressibility properties. Water temperature must match the boiling eutectic salts. This article provides an in a heat-transfer-fluid system will va- range of the fluid. overview of these standards. ASTM porize and expand as system tem- High-temperature heat-transfer standards are listed using their ASTM perature is increased, building pres- fluids are generally classified as inor- designations, for example D5372. sure. It is therefore important to keep ganic eutectic salts, synthetic organ- “D” indicates that the standard is water out of the system as much as ics (Figure 1), aromatic oils, paraffinic administered by one of the D com- possible. Water content can be mea- oils and silicone-based fluids. The mittees. “5372” is the number of the sured using D95. chemistry of each heat-transfer fluid standard. These designations are Safety. Factors related to safety in is different, so each fluid has differ- used throughout this article. use include autoignition temperature ent properties. Considerable back- and flashpoint. Autoignition temper- ground and technical information ASTM D5372 standard guide ature can be measured using E659. about heat transfer fluids is provided ASTM D5372, “Standard Guide for The user should keep in mind that in chapter 21 of the ASTM Fuels and Evaluation of Hydrocarbon Heat a hydrocarbon fluid absorbed on a Lubricants Handbook. A new version Transfer Fluids,” covers the factors porous inert surface (such as insula- of this handbook is expected to be that should be considered to char- tion) can ignite at temperatures more published this month. acterize a heat transfer fluid. These than 50°C lower than the value de- ASTM International (formerly factors are also relevant to selection termined by E659. Flashpoint (mea- known as the American Society for of a new heat-transfer fluid, so it can sured by D92 Cleveland open cup Testing and Materials; West Con- be used as a reference when choos- tester or D93 Pensky-Martens closed shohocken, Pa.; www.astm.org) has ing a new fluid. These factors are cup tester) indicates the temperature developed several standards for heat grouped into five categories: pump- at which flammable vapors given off by a liquid form a mixture with air that ASTM D7665 standard guide of thermal degradation products is can be ignited by contact with a hot ASTM D7665, “Standard Guide for done by gas chromatography (GC), surface, spark, or flame. Evaluation of Biodegradable Heat distillation and weighing. Effect on equipment. Two ASTM Transfer Fluids,” addresses biode- Before D6743 was developed, standards can provide information gradable heat transfer fluids. It cov- each heat-transfer-fluid manufac- about the effect of the heat transfer ers the same factors mentioned in turer had its own method for deter- fluid on equipment. D471 provides D5372 but also includes a few oth- mining thermal stability. There were data about the effect of the fluid on ers. Vapor pressure (measured by differences in the methods, as well rubber or elastomeric seals. G4 is a D2879) is included as a factor that in- as differences in the equipment used guide for conducting corrosion tests in fluences pumpability of the fluid. Bio- to conduct them. These differences field applications. degradation (measured by D5864) is sometimes led to conflicting test re- Efficiency. Several properties — included as a factor to consider for sults. One supplier’s test data might thermal conductivity, specific heat, safety in use. D6384, which covers indicate that fluid A was more ther- density and viscosity — are relevant terminology related to biodegrad- mally stable than fluid B, while an- to the heat transfer capability of the ability and ecotoxicity of lubricants, other supplier’s test data might show fluid. These are the properties used to is listed as a reference document. the opposite. calculate Reynolds number, Prandtl D7044, which is a specification for D6743 provides a standard test number and Nusselt number, which biodegradable fire-resistant hydrau- method for developing data that a determine the heat transfer coeffi- lic fluids, is also listed as a reference. user can evaluate to compare heat cient. D2717 measures thermal con- transfer fluids. Using D6743, ther- ductivity, D1298 or D4052 measures ASTM D6743 standard test method mal stability of two or more different specific gravity, and D445 or D7042 ASTM D6743, “Standard Test heat-transfer-fluid chemistries can measures viscosity. D2766, the test Method for Thermal Stability of Or- be directly compared by heating method listed for specific heat, has ganic Heat Transfer Fluids,” is a them at the same time in the same recently been withdrawn but E1269 test method that measures ther- oven, and then analyzing them and E2716 are other methods that mal stability of a heat transfer fluid. using the same equipment. D6743 could be considered. Thermal stability is the resistance to is very specific about the type of Service life. Heat transfer fluids de- permanent change in fluid proper- equipment that can be used. For grade when exposed to sufficiently ties caused solely by heat. All heat- example, it specifies the type and high temperature. The amount of transfer fluids degrade with expo- length of the GC column, film type degradation increases as tempera- sure to high temperature over time. and thickness, type of detector, ture increases or exposure time Typically, the user wants their heat calibrant range, material and dimen- increases. Thermal stability is mea- transfer fluid to last for many years. sions of the ampoule, temperature sured using D6743. It’s important to select a fluid that is control capability of the oven, and Several tests are used to deter- thermally stable enough to last for accuracy of the balance for weigh- mine the condition of the fluid and many years at the expected system ing. Statistical analysis of test data its remaining service life. These operating temperature. from one laboratory showed very tests include precipitation number The thermal degradation products good repeatability of test results (measured by D91), insoluble con- for most heat transfer fluids include (see the precision and bias state- tent (measured by D893), flash point high- and low-boiling components, ment in the method). (measured by D92 or D93), carbon very low-boiling gases, and residue DIN (Deutsches Institute für Nor- residue (measured by D189, D524 that cannot be evaporated. The mung e.V.; Berlin, Germany; www. or D4530), distillation (D86, D1160 presence of degradation products in din.de) also has a test method for and D2887 are listed but D2887 is the fluid affects its properties. thermal stability, but it is less specific more commonly used), neutralization D6743 quantifies the high-boiling about the test equipment that can number (measured by D664), color components, low-boiling compo- be used. Differences in test equip- (measured by D1500), viscosity, vis- nents, very low-boiling gases, and ment used can cause significant dif- cosity index (measured by D2270) residue that cannot be evaporated ferences in test results. and water content (measured by for a heat transfer fluid that is ex- D95). Typically, a sample of used posed to specified temperature for ASTM D7863 standard guide heat-transfer fluid should be taken a specified time period. The test ASTM D7863, “Standard Guide for from each system at least once per procedure consists of weighing heat Evaluation of Convective Heat Trans- year so that these tests can be con- transfer fluid into a stainless-steel fer Coefficient of Liquids,” provides ducted to evaluate fluid condition. ampoule, purging with nitrogen, information about the calculation of To ensure that the sample is repre- sealing the ampoule, and heating convective heat-transfer coefficient. sentative, it should be taken from it in an oven.
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