Tips On Starting Your Heat Transfer Fluid Selection Process
New Process Or Retrofit? Use These Tips To Make Your Initial ‘Short List’ Of Fluids
By: Michael R. Damiani
CEO/International Solar Sales Manager
Radco Industries, Inc.
Published in Process Heating magazine, October 2002
Maybe you’ve 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 R&D project, which will require high temperature process heating. In either case, the task of selecting the proper heat transfer fluid from the 90-odd 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 is a selection parameter 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 roughly defined as the temperature where a fluid’s viscosity reaches 2000 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 (especially petroleum-based 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 right 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 even if a fluid is used at a continuous temperature close to or right at its recommended maximum fluid operating limit, there is a point to take into consideration – The thermal degradation rate is not a linear function versus 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 – 20°F 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- 50°F of the fluid’s maximum temperature. The costs associated with increased fluid make-up 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 right in the middle of it’s 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 of thumb, 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 gets 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 he specific type, have a bulk fluid operating range generally from -70°F to 750°F.
Petroleum-based: Also known as “hot oils”. These consist of parafinnic and/or napthenic hydrocarbons. The bulk fluid operating range for these fluids are generally from -10 oF to 600 oF.
It seems that the majority of heat transfer fluid applications fall within the 500°F – 600°F 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 630°F, 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. On the other hand, if your process requirement calls for only 525°F, 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°F to 610°F 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 oF to 610 oF range:
Points For Petroleum-based fluids for 590°F to 610°F applications:
1) High performance, high grade petroleum-based fluids have been proven to be accurately rated to 600°F, demonstrate acceptable thermal stability up to and at 600°F, and have performed well for many years in properly designed systems operating at 600°F.
2) Petroleum-based fluids are 1/3 to ½ the cost of aromatics.
Points Against Petroleum-based fluids for 590°F to 610°F applications:
1) Do not use petroleum-based fluids if bulk fluid temperature exceeds 600°F, or if you think you might have occasional temperature excursions above 600°F.
2) The fluid make-up rate will be on average twice as high as most aromatics at 600°F
Points For Aromatic fluid for 590°F to 610°F applications:
1) Well within the aromatics’ “cushion” range- good heat transfer efficiency and minimal thermal degradation.
2) No degradation concerns should temperature excursions occur.
Some Points Against Aromatics for 590°F to 610°F applications:
1) Cost- two to three times more expensive than petroleum-based fluids.
2) Usually not as personnel-friendly as petroleum-based fluids.
Although there is no “best” answer to which type of fluid to use in the 590°F to 610°F range, you can feel comfortable using a petroleum-based fluid to 600°F, as long as there will not be any possible temperature excursions above that temperature. On the other hand, the more expensive aromatic will be on cruise control at these temperatures, since they are well within their “cushion range”.
Selecting The Fluid
Heat transfer fluid suppliers occasionally see systems using 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 have seen the other side of the coin too- a low cost/low temperature heat transfer fluid (sometimes they are not even heat transfer fluids) put into high temperature applications. There are cases where these fluids have only lasted 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.
CEO/International Solar Sales Manager
Radco Industries, Inc.