Maybe you have been given the assignment of designing a new process and you realize that the high temperatures required for production will necessitate the use of a heat transfer fluid. Or, maybe management has decided to convert that old batch distillation column into a production unit and commercialize the new research and development project, which will require high temperature process heating. In either case, the task of selecting the proper heat transfer fluid from the 90-plus fluids available worldwide-- and incorporating the fluid's physical and engineering properties into your initial design-- is as important as it can be daunting.
With so many heat transfer fluids available, how can you initially narrow the fluids down to the best choices for the application? Here are some selection parameters that can easily eliminate many fluid choices and help to quickly make your "short list" of candidates.
Fluid Operating Range
A heat transfer fluid's operating range is the temperature range between the pumpability point and the recommended maximum bulk fluid operating temperature. The pumpability point is defined roughly as the temperature where a fluid's viscosity reaches 2,000 centipoise. At this point, the fluid becomes too viscous for centrifugal pumps to maintain sufficient fluid flow. Although heat transfer fluids technically can be used at temperatures close to their pumpability points, many fluids lose much of their heat transfer ability and efficiency when used close to their pumpability points.A fluid's ability to withstand thermal cracking (thermal degradation) is the primary factor in setting its maximum bulk fluid operating temperature. This temperature is the maximum temperature the fluid manufacturer recommends the fluid can be used and still maintain an acceptable rate of degradation over time.
Typically, a good fit between a heat transfer fluid and an application happens when the required fluid temperature of the process falls in the middle of the operating range of the heat transfer fluid. This "cushion" on either side of the operating temperature allows for good overall heat transfer efficiency and minimal fluid degradation.
One quick rule: There is no reason to consider fluids that have maximum bulk fluid operating temperatures below the bulk heat transfer fluid temperature required by your process. Cross those fluids off your list right away. And, if a fluid is used at a continuous temperature near or at its recommended maximum fluid operating limit, there is another point to consider: The thermal degradation rate is not a linear function vs. temperature. As the bulk fluid temperature reaches, then exceeds, the fluid's maximum recommended temperature, the degradation rate soars asymptotically. Even when used within 15 to 20oF (8 to 11oC) of the recommended maximum temperature, the degradation rate of most heat transfer fluids is significantly higher than when the application requires a temperature within 30 to 50oF (17 to 28oC) of the fluid's maximum temperature. The costs associated with increased fluid makeup rates, the downtime required for heat transfer fluid-related maintenance, and lost heat transfer efficiency due to degradation by-products have to be strongly considered when choosing between a lower cost fluid that will be bumping up against its maximum recommended use temperature and a more expensive fluid that will fit nicely in the middle of its operating range.
Cost vs. Comfort Level
The answer to old car racing adage, "How fast do you want to go?" also holds true with heat transfer fluids: "How much do you want to spend?" Except with heat transfer fluids, the question is "How high do you want to go?" As a general rule, the higher the maximum bulk fluid operating temperature, the more expensive the fluid. This is due to the fact that the chemistries required to achieve acceptable thermal stability and heat transfer efficiency at elevated temperatures get more complex and expensive as the temperature increases. The two primary types of fluids used by the majority of high temperature applications are:
- Aromatics. Also know as "synthetics," these consist of benzene-based chemistries and, depending on the specific type, have a bulk fluid operating range generally from -70 to 750oF (-57 to 399oC).
- Petroleum-Based. Also known as "hot oils," these consist of parafinnic and/or napthenic hydrocarbons. The bulk fluid operating range for these fluids generally are -10 to 600oF (-23 to 316oC).
It seems the majority of heat transfer fluid applications fall within the 500 to 600oF (260 to 316oC) temperature requirement range, which opens the doors to both types of fluids. However, if your process will require a heat transfer fluid to perform at 630oF (332oC), your options are fairly limited in that only the more expensive aromatic-based fluids can be used, so you'll have to dig a little deeper. By contrast, if your process requirement calls for only 525oF (274oC), using an high cost aromatic for an added thermal stability benefit would be overkill. Your best choice here would be a petroleum-based fluid, and you'll be a hero for coming in under budget.
The tough decision is when your application is in that 590 to 610oF (310 to 321oC) range, where higher cost aromatics are in their "cushion" range and you're up against the maximum recommended top operating temperature of the hot oils. Some points to consider when the application falls in this 590 to 610oF range include:
Points For Petroleum-Based Fluids for 590 to 610oF Applications. High performance, high grade petroleum-based fluids have been proven to be accurately rated to 600oF, demonstrate acceptable thermal stability up to and at 600oF, and have performed well for many years in properly designed systems operating at 600oF. Petroleum-based fluids are one-third to one-half the cost of aromatics.
Points Against Petroleum-Based Fluids for 590 to 610oF Applications. Do not use petroleum-based fluids if bulk fluid temperature exceeds 600oF or if you think you might have occasional temperature excursions above 600oF. The fluid makeup rate will be, on average, twice as high as most aromatics at 600oF.
Points For Aromatic Fluids for 590 to 610oF Applications. Applications operating in this range are well within the aromatics' cushion range. Aromatic fluids have good heat transfer efficiency and minimal thermal degradation. There are no degradation concerns should temperature excursions occur.
Points Against Aromatics for 590 to 610oF Applications. They cost two to three times more than petroleum-based fluids and may not be as personnel-friendly as petroleum-based fluids.
Although there is no "best" answer to which type of fluid to use in the 590 to 610oF range, you can feel comfortable using a petroleum-based fluid to 600oF as long as there will not be any possible temperature excursions above that temperature. The aromatic will be on cruise control at these temperatures because they are well within their cushion range.
Selecting the Fluid
Heat transfer fluid suppliers occasionally see systems that use heat transfer fluids intended for applications for significantly higher temperatures. Although these systems will run smoothly (heat transfer fluid-wise) for many years, the same performance could have be achieved from a much more cost-effective fluid.Heat transfer fluid suppliers also have seen the other mismatch -- a low cost, low temperature heat transfer fluid (or sometimes not even a heat transfer fluid!) put into high temperature applications. There are cases where these fluids have lasted only days before significant system trouble occurred. In both cases, it is obvious that the person specifying the fluid did not spend enough time determining the criteria important in making the proper fluid selection. And, although there is no surefire method in selecting the proper fluid for an application, narrowing the field from the many choices is easy with a little thought.
Once the field has been narrowed, the final selection process can begin where each individual fluid can be compared and contrasted and the final selection made. Whether the final choice is a hot oil or a synthetic, making the proper choice should lead to many years of problem-free heat transfer.
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