No one fluid is ideal for all uses. A brief survey of three basic groups -- petroleum-based, synthetic aromatic and specialty fluids -- outlines the general characteristics of each type.

I have in my office information on approximately 70 different thermal fluids, and every time I think that I have them all, I learn about another. The variety of fluids available to the end user is staggering, and each supplier advertises his fluid as “the best.” So, what fluid is best, or at least a good choice for a particular application? A brief survey article can't answer that question specifically, but it can give the reader a general understanding the sources of fluids.

Understanding the vast number of fluids is simplified by arranging the fluids into groups. This article will group fluids into three basic sets -- petroleum-based fluids, synthetic aromatic fluids and specialty fluids. Petroleum fluids can be further divided into mineral oils and technical white oils. The white oils are interesting in that they can be further divided, as will be seen.

Glycol and polyglycol fluids, along with molten salts, will not be discussed in this article, but will be in later articles.

Note: This article will mention typical fluids in each class. Noting a specific fluid should not be taken as a preference or recommendation, but rather an association of familiar brand names to assist the reader's understanding.

Petroleum and Hydrocarbons

Petroleum occurs naturally in various deposits worldwide. As everyone has heard or read, petroleum generally is considered to have come from ancient biological deposits. Because petroleum deposits were randomly laid down worldwide over millions of years, different deposits can vary greatly in their composition, a fact that will be touched upon later.

Before I proceed, a brief review of the chemistry of petroleum is in order to get everyone on common footing.

Hydrocarbons. Imagine hydrocarbon (organic) molecules as being like a set of tinker toys. (Those of you too young to know about tinker toys can look them up on the Internet.) A carbon atom has to have four connections to other atoms; a hydrogen atom can only have one connection; a nitrogen atom has to have three connections, and a sulfur atom has to have four connections. Knowing that, and presented with an endless supply of atoms, you can construct an enormous number of molecules and never repeat yourself.

The simplest hydrocarbon molecule is methane (CH4), which is shown as

Methane is a colorless, odorless gas and is the principal component of natural gas. Odorants are added to natural gas to aid in detecting leaks.

More complex hydrocarbons can be demonstrated by octane (C8H18),

which is more conventionally shown as

or more simply as

Hydrocarbons described as “unsaturated” have one or more double bonds. The most simple depiction of a double bonded carbon is

One last type of chemical to demonstrate is aromatic chemicals, which are unsaturated ring compounds. The chemical of interest is benzene. Years ago benzene's structure was depicted as

but now are shown as the diagram below.

Now, back to heat transfer fluids.

Petroleum-Based Fluids

Mineral Oils. These heat transfer fluids are the ubiquitous “hot oils.” When crude oil is extracted from the earth, it contains a vast mixture of organic compounds, which range from very light hydrocarbons to extremely high molecular weight species. The first operation performed at the refinery is an initial distillation, which yields distillation “cuts” ranging from “lights” (gas and light solvent liquids), fuel (gas oil), a lube cut, and the still bottoms, which include the asphalts. These cuts are further divided and treated as they work their way through the refinery.

Hot oils come from the lube cut. After further distillation and solvent refining, mineral oils are selected for viscosity (which partly defines the heat transfer properties) and stability, and are branded and marketed as heat transfer fluids. Hot oils may or may not have additives mixed in, including antioxidants and rust preventives.

One important thing to consider about mineral oils is that they retain a certain degree of unsaturation (double bonds) and they also contain some sulfur and nitrogen. These attributes make mineral oils slightly more reactive, chemically, than more highly refined petroleum products and allow them to be more susceptible to oxidative degradation (see “Fluid Testing: It Pays to Pay Attention” link at end of article), and makes the fluid less stable at elevated temperatures.

Petroleum base stocks are further classified by their viscosity at 40°C (104°F). It is common to see mineral oil heat transfer fluids with viscosities ranging from as low as 20 centistokes (cst) to as high as 64 cst, so all hot oils are not alike. While many times, hot oils are selected on price alone, it is still a good idea to survey the various offerings to see which product is best for your operation.

Common brands of hot oils are Exxon-Mobil Mobiltherm 603, Shell Thermia C and Sunoco HT-21.

White Oils. White oils are petroleum base stocks that have been chemically treated with hydrogen to make a “cleaner,” more consistent product. This process is referred to as hydrogenation, hydro-finishing or hydro-treating. In general, selected base-stocks are exposed to hydrogen under conditions of elevated temperature and pressure. In most cases, a catalyst is involved to facilitate the reaction. The hydrogenation reaction inserts extra hydrogen atoms at double bond sites to “saturate” the molecule. The hydrogen also reacts with nitrogen and sulfur, removing them from the molecule. The result is a pale yellow to water-white fluid with improved thermal stability and improved resistance to oxidation as compared to mineral oils.

Synthetic Base Stocks. Most readers are familiar with motor oils advertised as being “synthetic.” Heat transfer fluids can be produced by the same processes that yield synthetic lubricants. Synthetic base stocks are similar in appearance to white oils but can offer improved flow at lower temperatures as well as other performance advantages.

Common brand names of white and synthetic oils include Paratherm, Multitherm, Duratherm and Calflo.


Synthetic Aromatic Fluids

The second major class of fluids is the synthetic aromatics. These materials are, for the most part, modified benzene compounds and generally are thermally stable at higher temperatures than are practical for petroleum-based fluids. Some of these fluids also have excellent low temperature properties, as will be seen below. These chemicals are either purposely produced for heat transfer service or other uses, or they are co-produced with other chemicals. While benzene is a listed hazardous chemical and these fluids may carry benzene at part per million concentrations, the derivatives used for heat transfer fluids are generally less toxic and very safe to use. It is recommended, however, that the end user review the material safety data sheets (MSDS) for these products -- and all heat transfer products, for that matter -- before putting them into service.

While there are literally dozens of synthetic aromatic compounds suitable for heat transfer service, this article will concentrate on three common groups: poly phenyls, alkylated benzenes and diethyl benzene.

Poly Phenyls. Poly phenyls consist of two, three or more benzene molecules connected together and include:

Poly phenyls exhibit excellent thermal stability and can be operated as hot as 750°F (399°C). They can freeze or develop high viscosities below 100°F (38°C) and need to be rigorously evaluated to select the best fluid for a particular application.

The mixture of biphenyl and diphenyl oxide (BP/DPO) is sold as Dowtherm A and Therminol VP-1, which can be operated at 750°F in either liquid or vapor phase, but freezes at 54°F (12°C).

Chemically modified terphenyls, blended with similar chemicals, are marketed as Therminol 66 and Dowtherm HT. These products can be operated at 650°F (343°C) and are pumpable down to approximately 30°F (-1°C).

Alkylated Benzene. Alkylated benzene products consist of a benzene molecule with a hydrocarbon chain attached, and are depicted as:

where “n” varies with manufacturer.

Alkylated benzene heat transfer fluids can be operated at 550°F (288°C) and are typically pumpable to 0 to 20°F (-17 to -6°C). Commonly available alkylated benzene heat transfer fluids include Marlotherm N, Therminol 55 and Dowtherm T.

Diethyl Benzene. Diethyl benzene is depicted as:

Diethyl benzene is an interesting chemical because it is stable to reasonably high temperatures (600°F [316°C]) but can also be used to extremely low (-100°F [-73°C]) temperatures. At high temperatures, diethyl benzene can be used as a vapor-phase heat transfer fluid. It is commonly available as Dowtherm J, Therminol LT and Marlotherm X.

Other Synthetic Aromatics. As stated earlier, the types and trade names of fluids listed above represent only a few of the many available synthetic aromatic heat transfer fluids. With the extremely wide range of properties, chemistries and prices available, the user is encouraged to thoroughly research available fluids, or enlist the assistance of experienced resources before making a decision on which fluid to purchase.

Silicone and Fluorocarbon Fluids

Silicone- and fluorocarbon-based fluids are two examples of “specialty fluids” available for highly specific uses. These fluids offer enhanced performance, depending on the specific product, in such areas as superior low temperature performance, broad temperature range, low flammability and low toxicity. They are most often selected when the special demands of a particular process justify the higher cost of these fluids.

Silicone Fluids. Silicone fluids have the same general chemical structure as silicone rubber and silicone lubricants. Silicone heat transfer fluids are generally clear liquids with little, if any, odor. Silicone fluids are noted to have a wide useful temperature range and are often used in systems where the same fluid is used for both heating and cooling (a.k.a. “single-fluid” systems). Silicone fluids have low toxicity and often are found heating and cooling sensitive processes such as pharmaceutical reactions. Silicone fluids can develop significant vapor pressures over time, which should be taken into consideration when a system is designed and specified for these materials.

Dow Chemical's Syltherm fluids are commonly available silicone heat transfer fluids.

Fluorocarbon Fluids. Fluorocarbon fluids are used in semiconductor and pharmaceutical applications where their low temperature, low flammability, and high dielectric properties make them desirable. 3M's Novec products are fluorocarbon heat transfer fluids.

So, which fluid is right for my application?

There is no one fluid that is ideal. Details of specific fluid chemistry, flammability, toxicity and heat transfer properties cannot be addressed in detail in a brief survey article, but nonetheless, need to be researched and considered in the selection of any heat transfer fluid. Evaluation of available product information, including MSDS, and drawing on available experienced resources can lead to a decision that provides value and performance for the process in question.

The vast range of temperature limits, heat transfer properties and price give the end user an opportunity to choose a fluid that offers the best balance of performance vs. price for his particular application.