Don't Ignore Thermal Stability
For these high temperature operations, low or no-pressure systems may be preferred to pressurized systems. There are several reasons for this, but the primary one is that nonpressurized systems are less expensive to build. Depending on the requirements for the heat transfer fluid, however, a nonpressurized system may prove more expensive to operate over the long run. Therefore, key engineering and chemical parameters must be considered when selecting the most appropriate synthetic organic fluid for a nonpressurized heat transfer system.
For any heat transfer system requiring a high maximum fluid temperature and liquid-phase operation, a number of fluids could be considered, including diphenyl oxide/diphenyl, di- and tri-aryl ethers, diaryl alkyl, partially hydrogenated terphenyls, terphenyls, dibenzyl toluene, alkylated diphenyls, alkylated napthalenes and dimethyl polysiloxane.
To narrow the selection process, first assume that the designer wishes to eliminate as many handling difficulties as possible. This generally rules out fluids with relatively high freezing points. While any of the remaining fluids could be operated in liquid phase, not all are suitable for a nonpressurized system. This parameter eliminates all but three fluid types: dibenzyl toluene, partially hydrogenated terphenyls and diaryl alkyl. To choose between the three, the potential user should look at the properties that have a direct bearing on the sizing of heat transfer equipment. In addition, the fluid's long-term economic impact on the system's operational life should be considered.
Similar Engineering PropertiesAll three fluids have remarkable similarities in several physical properties -- notably vapor pressure, flammability and viscosity. These properties will influence key design parameters such as pressure drop and film coefficient.
The three fluids combine two properties that enable them to be used in nonpressurized systems over their entire use temperature ranges: relatively high molecular weight and concomitant low vapor pressure. Average molecular weight ranges from 236 for diaryl alkyl to 272 for dibenzyl toluene (figure 1). Additionally, all three fluids have vapor pressures below 1 atm (14.7 psia) at their maximum use temperature (figure 2).
Their flammability properties also are similar, with closed-cup flashpoints above 200oF (93oC), giving each fluid an NFPA rating of Class III B Combustible Liquid (table 1). Given these similarities, electrical, fire safety and containment requirements basically are the same.
A fluid's liquid viscosity impacts heat transfer system design in several areas, including the determination of low temperature limits and the need for heat tracing. Fluid viscosity also is a major factor in pressure drop, which impacts pump sizing, and film coefficients, which determine heat exchanger sizing. The liquid viscosities of each of the three fluids are quite close over their entire use ranges, with the largest differences occurring at the lower temperatures (figure 3).
Another design parameter that must be calculated is film coefficient, which is a function of a fluid's thermal conductivity, density, specific heat and viscosity as well as fluid velocity and a geometric factor. As is the case with pressure drop, the three fluids' physical properties are so close that differences in calculated film coefficients are minimal (figure 5).
If the physical properties examined to this point were all that were necessary, a case might be made that the differences are minor, and that price would be the primary selection criterion. However, there is another critical physical property that must be considered: thermal stability. And here, the differences between diaryl alkyl, dibenzyl toluene and partially hydrogenated terphenyls are pronounced.
A Question of StrengthThermal stability is a measure of the strength of the chemical bonds that hold a particular compound together. Several studies have proved the correlation between molecular structure and thermal stability.1,2,3,4 It is therefore well established that a fluid's maximum recommended use temperature is dependent on fluid structure. Fully aromatic fluids such as biphenyl and diphenyl oxide have the highest use temperatures; alkyl aromatics have lower use temperatures; and aliphatic hydrocarbons have much lower use temperatures. Thus, a fluid's thermal stability is determined by its chemical structure.
Until recently, there was not a standard test method for determining the thermal stability of heat transfer fluids. Each fluid manufacturer had its own test methods and results could vary between manufacturers. Recently, the DIN (Deutsches Institut für Normung, comparable to ASTM in the United States) finalized test method 51528, titled "Testing of Mineral Oils and Related Products. Determination of Thermal Stability of Unused Heat Transfer Fluids." This method describes the procedures to be followed to test fluids for thermal stability. In this test, fluids are degraded for 500 hours at elevated temperatures and then analyzed by gas chromatography (ASTM D-2887) to determine the amount of low boiling and high boiling degradation products produced. The nonvolatile decomposition products are determined by removing the volatile components under vacuum with a ball tube distillation apparatus. The high boilers, low boilers and nonvolatiles are added to determine the total decomposition of the fluid. The data presented in this article was determined in accordance with the DIN 51528 and the analysis was done by ASG Laboratories in Germany.
High boilers and nonvolatiles accumulate in the system over time and cannot be removed by venting. As the high boilers' and nonvolatiles' concentrations goes up, fluid viscosity increases, leading to lower film coefficients and higher film temperatures. These will cause fluid degradation to accelerate, perhaps leading to system fouling. The recommended maximum concentration of high boilers in a heat transfer fluid is 10 percent. Nonvolatiles are very high molecular weight compounds that build up as tar or sludge in a system and can lead to system fouling if allowed to accumulate. Nonvolatiles and high boilers can only be removed by distillation or replacement of the fluid.
The thermal degradation of diphenyl oxide/diphenyl (DPO/DP)-based fluid is a good example. DPO/DP-based fluids are pure, break into easily identifiable degradation products, and do not undergo any significant rearrangement. In fluids with a broad boiling range such as partially hydrogenated terphenyls, some components may break down but will remain within the boiling range of the starting fluid. This is not degradation, but the accumulation of these sorts of compounds has consequences.
When DPO/DP degrades, it forms low and high boilers (figure 6). The former are predominantly single-ring compounds such as benzene and phenol. The high boilers have three or more rings, such as biphenyl phenyl ether and terphenyls. As the temperature increases, the degradation rate increases exponentially.
Evaluating the ResultsBased on the tests, diaryl alkyl had a greater thermal stability than partially hydrogenated terphenyls, which in turn had a greater thermal stability than dibenzyl toluene. The results of the degradation analysis performed by ASG can be seen in table 2. Because all three fluids work for a nonpressurized, liquid-phase system, and their physical properties are similar, what impact would varying thermal stabilities have on an operating system?
When estimating the amount of degradation a given system will undergo, it would be misleading to look at the system's maximum bulk temperature and assume that all of the fluid in the system will degrade at this rate. Industrial heat transfer systems are not isothermal. As any heat transfer fluid flows through the system, it is heated and cooled, and this temperature cycling must be accounted for when estimating degradation. However, for comparison purposes, the test data presented is for fluid held at constant temperature for the duration of the test. Temperatures at or slightly above the fluids' maximum recommended use temperatures are commonly used to accelerate degradation and show trends in the short time span of the test.
When choosing a heat transfer fluid, several choices may satisfy initial criteria. The three major candidate fluids for high temperature, nonpressurized systems have similar properties that would allow them to be used more or less interchangeably. However, another key physical property, thermal stability, differentiates these fluids. Thermal stability is an inherent property of the compounds comprising a particular heat transfer fluid, and use temperature will determine how fast the fluid degrades. Taken together, these two factors will determine the long-term economics of using a particular fluid.