Selecting Heat Transfer Fluids: Synthetic or Hot Oil?
Does fluid chemistry matter?
Table 1. Most commonly used fluids are synthetics or hot oils. Silicone-based fluids are used in specialty applications.
High temperature heat transfer fluids are used in many process applications because their optimum bulk fluid operating temperatures of 300 to 750oF (149 to 399oC) are safer and more efficient than steam, electrical or direct-fire heating methods. Selecting the proper heat transfer fluid -- during the design phase of a new system or to achieve process fluid improvements during a retrofit -- will ensure sufficient and uniform BTU delivery or removal. A properly selected heat transfer fluid also will minimize potential production losses and downtime due to required design changes, mechanical problems or fluid failure.
The best first step in the selection process does not even involve the fluid: Calculate and thoroughly research the energy transfer required by the process as well as the planned and actual service ratings of the heat transfer system's mechanical components. Many companies specialize in heat transfer fluids, and a range of fluids are available. Knowing these key system operating requirements prior to considering any fluid or supplier provides you with a set of criteria to compare various fluids and rapidly eliminate fluids not suited for the application. Before comparing and contrasting various individual fluids, however, much time and effort in the selection process can be saved by comparing and contrasting the various fluid chemistries. Once the fluid chemistry that best meets the performance properties and other criteria required by the application is selected, the resulting list of potential fluids becomes significantly more manageable for more detailed comparisons.
Understanding Fluid Chemistry
High temperature heat transfer fluids can be categorized by chemical structure into three primary groups (table 1):
- "Others," including silicones.
Also referred to as "aromatics," the synthetics group of fluids consists of benzene-based structures and includes the diphenyl oxide/biphenyl fluids, diphenylethanes, dibenzyltoluenes and terphenyls. Depending on the specific product, the overall bulk fluid temperature operating range is -70 to 750oF (-56.7 to 399oC).
Hot oils are petroleum-based, and most consist of paraffinic and/or napthenic hydrocarbons. The overall bulk fluid temperature operating range of petroleum-based fluids is -10 to 600oF (-23.3 to 316oC); however, only the high-grade hydrogenated white oils are recommended for applications requiring bulk fluid temperatures in the 575 to 600oF (302 to 316oC) range.
Silicone-based fluids -- and, to a greater extent, hybrid glycol fluids -- are used primarily in specialized applications requiring process and product compatibility should a heat exchanger leak occur. This group's performance and cost factor disadvantages -- in the comparative temperature ranges of the synthetics and hot oils -- make silicone-based and other specialty fluids unlikely choices for most process applications.
Fluids and System Types. Hot oils and synthetics are used in a multitude of heat processing applications. The type of system used in the process influences the selection of a specific fluid chemistry. Processes utilizing heat transfer fluids may be categorized into three system types:
Nonpressurized liquid-phase systems.
Pressurized liquid-phase systems.
- Pressurized pumped or natural circulation vapor-phase systems.
Nonpressurized liquid-phase systems generally are the simplest to design and operate. Both hot oils and synthetics can be used equally well in this system as long as the heat transfer fluid's operating temperature is below its atmospheric boiling range. Major system components include the heater, heat exchanger, vented expansion tank and circulating pump. With this design, the expansion tank does not require an inert gas blanket to keep positive pressure on the circulating pump. But, to reduce the probability of fluid oxidation, a baffled expansion tank design is preferred to ensure that the fluid is kept below 350oF (177oC) at the fluid/atmosphere interface.
Pressurized liquid-phase systems using both hot oils and synthetics are similar in design to nonpressurized systems, with one exception. With a pressurized system, an inert gas is applied through the expansion tank when the heat transfer fluid's required operating temperature is above its atmospheric boiling range. The pressurized inert gas (usually nitrogen) is used to maintain the heat transfer fluid as a liquid. It also acts as a buffer between the hot fluid surface and the atmosphere in the expansion tank, preventing fluid oxidation. With the exception of the multi-phase fluids like the diphenyl oxide/biphenyl-type, most liquid-phase synthetics and all of the hot oils do not require inert gas pressurization to maintain the liquid phase at their top-end recommended operating temperatures.
Pressurized vapor-phase systems utilize only a handful of synthetic fluids, most notably the diphenyl oxide/biphenyl-type. A simple vapor-phase system can be designed using hydrostatic pressure to gravity return the condensate from the user to the vaporizer, eliminating the need for a condensate pump. More complex systems require a flash tank, condensate return tank and a condensate return pump. The added capital equipment costs and complexity of vapor-phase systems are offset by the increased BTUs delivered per pound of vapor vs. liquid, and increased temperature control (table 2). The precise temperatures that can be achieved and maintained with a vapor-phase system are important in heat-sensitive processes.
Criteria for Selecting the Best Fluid Chemistry
If an existing or proposed design incorporates a vapor-phase system and requires a high temperature fluid, the fluid options are limited: Only a handful of high temperature vapor-phase synthetic fluids are available, and hot oils cannot be used in the vapor phase. By contrast, nonpressurized or pressurized liquid-phase systems allow the greatest variance in potential end-use fluids, both synthetic and petroleum-based. Especially in existing systems, the range of new liquid-phase fluid technology available allows increased system performance and energy savings with a minimum of downtime and cost. Whether researching a potential fluid upgrade in an existing system or specifying the proper fluid type for a new design, five basic criteria should be considered.
1.Think About Thermal Stability.Thermal stability is defined simply as the inherent ability of a heat transfer fluid to withstand molecular cracking from heat stress. Relative thermal stability testing measures a particular fluid's molecular bond strength at a specific temperature vs. another specific heat transfer fluid at the same temperature, under identical testing conditions. Relative is the key word --because the tests are run under ideal laboratory conditions and do not factor in real-world fluid stresses such as mechanical malfunctions, design flaws, oxidation, etc., the data generated is useful only for comparative purposes. Accurate fluid life predictions under actual processing applications should not be implied from thermal stability data.
A fluid's thermal stability is the primary factor determining its maximum bulk fluid operating temperature. This is the maximum temperature at which the fluid manufacturer recommends the fluid can be used and still maintain an acceptable level of thermal stability. Because fluid degradation rates are closely tied to temperature, continuous use above the manufacturer's recommend maximum bulk fluid operating temperature will increase degradation rates exponentially. Potential system problems caused by excessive degradation and the subsequent formation of degradation byproducts include increased coking and fouling, mechanical difficulties and decreased heat transfer efficiency.
Therefore, in selecting a fluid chemistry, the first step in the selection process is to determine the maximum bulk operating temperature required by the process. Most hot oils have a recommended maximum bulk fluid temperature of 550 to 600oF (288 to 316oC) while the aromatics fall between 600 and 750oF (316 to 399oC), depending on the fluid. As the aromatic fluids' molecular structures are significantly more thermally stable than hot oils above 600oF, aromatic-based heat transfer fluids are strongly recommended for applications above this temperature. Process applications requiring bulk fluid temperatures of 300 to 600oF (149 to 316oC) can specify either synthetic or petroleum-based fluids. Within this temperature range, relative thermal stability data is available to compare individual fluids at specific temperatures.
2. Evaluate Heat Transfer Efficiency.
Heat transfer efficiency comparisons between fluids are made using heat transfer coefficients. At a specific temperature, a fluid's overall heat transfer coefficient can be calculated using its density, viscosity, thermal conductivity and specific heat at a determined flow velocity and pipe diameter. The resulting heat transfer coefficients then may be evaluated and compared. At a given temperature, the heat transfer coefficients of the synthetics and hot oils may differ by as much as 30%. Depending on the thermal resistance factors of the system components, a fluid with a substantial heat transfer coefficient advantage may allow the size of the system equipment to be reduced. Likewise, replacing an existing heat transfer fluid with a more efficient heat transfer fluid may significantly increase production output or reduce energy costs.
Most aromatic-based fluids have greater heat transfer efficiency than hot oils from 300 to 500oF (149 to 260oC). Above this temperature range (up to 600oF), petroleum fluids narrow the difference somewhat; additionally, a select number of highly refined paraffinic/napthenic white oils have a slight efficiency advantage over the mid-range aromatics.
Keep in mind that the heat transfer coefficient is calculated using virgin fluid properties. Fluid that has been in service for an extended period of time and has undergone thermal degradation may have a significantly lower coefficient due to fluid viscosity changes and the presence of less efficient fluid degradation byproducts. Therefore, a fluid's thermal stability plays an important role in maintaining its thermal efficiency over time.
3. Understand the Pumpability Point.
Pumpability point -- not freeze point -- is the true low-end temperature at which a heat transfer fluid can operate in heat processing applications. The pumpability point is defined as the temperature at which the fluid's viscosity reaches a point (typically 2,000 cP) where centrifugal pumps can no longer circulate the fluid. Although most high temperature process applications run at bulk temperatures well above hot oil and aromatic fluid's pumpability points, system designs that may encounter cold weather during emergency or maintenance shutdowns, or operate a batch process in a cold climate, should account for the fluid pumpability point. Generally, most hot oils offer adequate startup protection down to the 0 to 25oF (-17.8 to -3.89oC) range. The mid-temperature aromatics (with 650oF maximum bulk temperatures) offer protection from -70 to -20oF (-56.7 to -28.9oC), and the top-end temperature aromatics (with 700 to 750oF maximum bulk temperatures) can handle 40 to 60oF (4.4 to 15.6oC). Processes using a fluid that potentially may have startup problems in cold weather will need to be heat traced.
4. Look at Fluid Serviceability.
Fluid replacement, reprocessing or filtration may be required from time to time due to unexpected temperature excursions, system upsets or contamination. Due to the relatively low cost of petroleum-based fluids, very few suppliers offer reprocessing services for hot oils or a credit program for the off-spec material (to be applied toward the cost of a new charge). By contrast, most synthetics are composed of a limited number of aromatic components and have a narrow boiling range, allowing easy identification of degradation byproducts or contaminants. Reprocessing synthetics using fractional distillation is an economical alternative to disposal and replacement; hence, most synthetic fluid suppliers offer this service at a nominal cost. Some synthetic fluid suppliers also offer credit for off-spec material. These programs eliminate fluid turnaround and reprocessing time: The new fluid can be charged in immediately once the off-spec fluid has been drained. This is especially useful if the system downtime is unscheduled or a short maintenance period has been planned.
Filtration is a cost effective method (vs. reprocessing or fluid replacement) to remove carbon and coke suspended in the heat transfer fluid. Most fluid suppliers recommend slipstream filter loops permanently installed and closely maintained on both hot oil and synthetic systems. Fluid filtering companies with portable units also can set up on-site and remove carbon from the fluid while the unit is still operating.
Almost all heat transfer fluid suppliers offer analytical testing of fluid condition at no-charge. This important service monitors fluid condition over time and provides an early warning should action need to be taken to replace, reprocess or filter the fluid.
5. Weigh Environmental Concerns.
> When selecting a heat transfer fluid chemistry, comparing environmental and personnel guidelines is just as important as fluid performance. In general, heat transfer fluids do not present an appreciable health hazard when used in accordance with acceptable manufacturing practices. Petroleum-based fluids offer simplified handling, reprocessing, shipping and disposal requirements compared to synthetics. For example, most hot oils do not have a reportable spill quantity, and white mineral oils meet the Food and Drug Administration (FDA) and United States Department of Agriculture (USDA) criteria for "incidental food contact." Also, petroleum-based fluids do not form hazardous degradation byproducts, so most spent hot oils can be sent to a local oil/lube recycler for disposal. Finally, the hot oils tend to warrant no special handling precautions, avoid Department of Transportation (DOT) regulations, and require no special storage requirements.
From a personnel standpoint, hot oils are user- friendly. Most have a nondiscernible odor and are nontoxic in case of contact with skin or ingestion.
Due to the aromatic-based chemistry of most synthetics, these fluids can form hazardous degradation byproducts that require special permits, handling and shipping precautions. Some synthetics and their vapors may cause skin and eye irritation after prolonged exposure, or emit pungent odors. In some cases, spills of synthetic fluids require reporting under the Superfund Amendments and Re-authorization Act. Because there is a wide range of chemistries available within the aromatic group, not all fluids have similar properties and environmental/personnel concerns. Regulations and precautions vary from fluid to fluid.
Which Chemistry Is The Best?
Chances are one fluid chemistry is not superior to the other in every criteria required by a new process or retrofit (figure 1). Both major fluid chemistries have advantages -- the aromatics offer superior heat transfer efficiency and stability at elevated temperatures coupled with serviceability and adequate pumpability; the hot oils have cost and environmental/personnel advantages. Identifying the primary criteria required by a new process or the main improvement goal desired in a retrofit will prioritize the criteria by importance. By first selecting the fluid chemistry that best addresses the main concerns, comparisons of individual fluids within the group should solve the little ones.