Choosing Heat Transfer Fluids
While most heat transfer applications have unique features that must be considered, having a good understanding of general heat transfer fluid details can ease the selection process.
Industrial users have many high temperature heat transfer fluids from which to choose. Some of these fluids are recommended for open-to-atmosphere systems and some are not. Likewise, some are rated for use as high as 750°F (398°C), while others are only recommended for temperatures up to 450°F (232°C).
All claim to transfer heat efficiently, but what other factors should be considered? It is important to understand the two main culprits of fluid breakdown -- oxidation and thermal degradation -- and fluid types.
Most systems over 250 gal are designed to protect fluid from elements such as oxidation and thermal degradation. Expansion tanks often are blanketed with a buffer or inert gas to help reduce oxidation. Typically, numerous other safety measures such as low-flow or low-fluid-level shutoffs are used that will further help protect the fluid from thermal degradation.
But fluids still break down. The main cause is oxidation. Scientifically speaking, oxidative degradation is the reaction of oxygen (in air) with the fluid by a free radical mechanism. This process forms larger molecules, which end up as polymers or solids. These elements can then thicken the fluid, thereby increasing its viscosity.
The more viscous a fluid becomes, the more difficult it will be to pump. Its heat transfer characteristics also will be compromised, and the acidity (TAN) of the fluid will increase. This also means there will be a greater risk of coke formation.
As with many chemical reactions, oxidation occurs more rapidly as temperatures increase. The reaction rate is hardly measurable at room temperature, but as temperature rises, the risk of oxidative degradation increases exponentially in the absence of special measures such as inert blanketing of expansion tanks.
Put simply, oxidation happens when hot fluid comes in contact with air. Signs of fluid oxidation are seen most often as sludge formation within the system, especially in low flow areas such as reservoirs or expansion tanks.
Thermal degradation, or thermal cracking, is the breaking of carbon-carbon bonds in the fluid molecules by heat. The breakdown forms smaller fragments called free radicals. In some cases, this is as far as the reaction goes. In others, the fragments may react with each other to form larger polymeric molecules. In heat transfer terminology, these outcomes are known as “low boilers” and “high boilers.”
Low boilers decrease the flashpoint and viscosity of the fluid. Its vapor pressure also increases, which can affect overall system efficiency and possibly cause pump cavitation. The reduced flashpoint also can have safety implications.
However, if thermal degradation occurs at temperatures greater than approximately 750°F (400°C), the effect is not only to break the carbon-carbon bonds but to separate hydrogen atoms from carbon atoms, which forms coke. When this happens, high boilers increase the viscosity of the fluid as long as they remain in solution. Once their solubility limit is exceeded, however, they begin to form solids, which can quickly foul the heat transfer surfaces. In this case, fouling of the heat transfer surface is rapid and the system may soon cease to operate.
In essence, thermal degradation is overheating the oil past its boiling point. As the fluid boils, it produces a lighter component, usually in the form of vapors. Continual overheating or cracking can cause reduced viscosity. It also can pose safety concerns with the creation of the lighter components. This, in turn, reduces the overall flashpoint, fire point and auto-ignition temperatures.
In addition to considering oxidation and thermal stability characteristics of a fluid, it is important to know a bit about the four types of fluids. The fluid groups include mineral oils, white/paraffinic oils, synthetics and chemical/synthetic aromatics. Each has its own pros and cons that may make them more suitable for one application over another.
Mineral Oils. Generally available from major refineries, mineral oils usually are low-cost and multi-purpose with very few or no additives for extra protection. These oils are lightly refined, which means they often retain petroleum distillates or aromatic hydrocarbons like naphthalene, xylene or toluene. Mineral oils also can retain sulfur, waxes and other components. All of these contribute to shorter fluid life, particularly at higher temperatures.
White/Paraffinic Oils. In the past 20 years the crude oil refining process has advanced to offer highly refined white and virtually pure paraffinic base oils free of aromatic hydrocarbons. While there are numerous grades or “cuts,” some of these base stocks have been shown to be well suited for heat transfer applications. Some specialty companies also have engineered blends of these base stocks with additives that enhance protection and life expectancy in heat transfer applications.
Synthetics (PAOs and Silicones). Generally some of the more expensive fluids, PAOs provide oxidative and thermal stability (up to about 550°F [287°C]) in heat transfer applications. Silicones are relatively new to the heat transfer market. Though costly, they offer extreme resistance to thermal and oxidative degradation. In applications that involve product finishing, care should be taken because silicones and their vapors can cause problems if they come into contact with surfaces that will be painted or coated.
Chemical/Synthetic Aromatics. Typically made up of modified benzene-based chemical structures, they have wide-ranging temperature characteristics and often can be used up to 750°F (398°C). While chemical aromatics offer good thermal characteristics, they can be costly. They often are not recommended for use in open systems.
The first thing to consider in selecting a heat transfer fluid is the required operating temperature. This should include the maximum operating temperature, as well as any low temperature considerations for processing or pumping ability at ambient temperatures.
Systems operating below 600°F (315°C) have a much wider range of fluids from which to choose. Systems operating between 600 to 630°F (315 to 332°C) have fewer options, and those over 650°F (343°C) are further limited. Petroleum-based heat transfer fluids are often suited for applications requiring fluid temperatures below 600°F (315°C) for reasons of cost, environmental impact and performance. If your system runs between 600 and 630°F (315 and 332°C), fluid options are limited to some high-end petroleum fluids, chemical aromatics and some silicones. Systems needing a fluid to run above 630°F (332°C) are basically limited to chemical aromatic fluids and a few silicone fluids.
Sometimes a high-temperature (650°F [343°C]) fluid is used in applications as low as 400°F (204°C). While a temperature buffer is not a bad idea, overspecifying a fluid can come at the expense of better choices for performance, economic or environmental reasons.
Other factors to consider include life expectancy of the system, process specific limitations (food-grade fluid requirements, for example), and environmental issues, among others.
Life Expectancy of the System. An important consideration is the process or system’s life expectancy. If a system is being commissioned for only a short period of time (perhaps a few years), then the life expectancy of a heat transfer fluid is obviously less critical and cost becomes the biggest factor in your decision.
If you are expecting to maintain and run your system for a number of years, then a fluid’s life expectancy and the costs associated with that should become an overriding factor.
For example, chemical aromatic fluids often have higher vapor pressures, especially as they come closer to the maximum use temperature. If a system is not inertly “sealed” or pressurized, these vapors will escape through the expansion tank. Any vapor loss will then require fresh fluid top-up to maintain sufficient levels.
When used at or close to their limits including maximum use temperature, mineral oils generally tend to thermally degrade. Also, if the system in which they are used is not sealed from the atmosphere, they will oxidize rapidly. This is a result of some of the distillates left in the fluid after refining.
White oils, especially pure paraffinic-based fluids, are effective up to their maximum recommended temperatures. However, if the maximum temperatures are exceeded, these fluids will thermally degrade as well. The resulting light ends will need to be vented from the system, leading to similar issues as with chemical aromatics with respect to fluid makeup. These fluids also have varying degrees of susceptibility to oxidation, which should be considered if your system is not nitrogen sealed.
Food-Grade Rating. Another consideration is whether or not a process requires a food-grade fluid. Of all the heat transfer fluids on the market, there are only a few that are rated as food grade (USDA, USP, H1 etc.). This can make the selection process a little easier, but it is important to understand that there are some restrictions placed on the manufacturing components of a food-grade heat transfer fluid. Oftentimes, these restrictions limit the product’s performance.
A closer examination of a food-grade fluid should be conducted with respect to its resistance to degradation, particularly oxidation. Also worth noting is that food-grade fluids are often specified where, in fact, they are not required. Careful examination of the application should be conducted.
Environmental Issues. When selecting a fluid, it is important to consider its environmental impact as well as any health and safety implications.
The EPA, OSHA or other local agencies governing the use, disposal and leak reporting of fluids should be investigated with regard to any fluid or application.
If your system is open to the atmosphere, of the type that requires plant personnel to regularly expose themselves to the fluid, or could have a probability for leaks, potential environmental issues should be addressed before committing to a heat transfer fluid.
Disposal Factors. Regardless of the heat transfer fluid you select, there will come a day when you need to dispose of either small amounts (if the system leaks or during maintenance), or all of it when the fluid has reached the end of its service life. It is therefore important to consider the costs associated with fluid disposal.
Chemical aromatics generally need to be segregated from other spent fluids and may have to be disposed of as hazardous waste. Silicones also may require segregation but should not be considered hazardous. PAOs as well as petroleum-based fluids can generally be mixed with other waste oils and are, in most areas, the easiest and most economical to discard.
The information provided in this article should help you make and educated decision when selecting a heat transfer fluid. While most applications are unique, this guide is intended to provide background information to make choosing a fluid for a particular application easier.