Heat transfer fluids have thousands of applications involving the removal, the addition and the movement of heat energy from one location to another.
In the fields of physics and engineering all fluid measurements were historically based on the behavior of water. Therefore, this discussion will begin with water as a reference heat transfer fluid against which all other fluids and their characteristics can be compared.
Water As A Heat Transfer Fluid:
While being commonly available and the least expensive of all possible options, water has features and idiosyncrasies that make it ideal for some applications and totally useless in others.
Water has three fundamental disadvantages. First, for low temperature applications, water provides no freezing protection, becoming a solid at 32° F. If that were not enough, this transition to a solid is accompanied by expansion with enormous potential force. Second, water has a limited upper level temperature range governed by its sea level boiling point of 212° F. And finally, the third disadvantage is an inherent lack of corrosion protection which is often further exacerbated by the presence of dissimilar metals in the fluid circuit creating an electrochemical cell promoting galvanic corrosion potentials.
Historically, these issues have been addressed by the addition of a hydrocarbon solvent called ethylene glycol along with various corrosion inhibiting additives. The resulting mixture with water will be recognized as ordinary automotive antifreeze.
The most common mixture ratio of Ethylene Glycol and water is the ubiquitous 50/50 mix in which the freezing point is lowered to –34° F and the boiling point is increased from the 212° F for pure water to 228° F (at atmospheric pressure, of course).
For maximum freezing point protection the ratio is changed to 67% ethylene glycol and the balance (33%) water. The resulting freezing point is suppressed to –84° F. Additionally, it is important to note that the boiling point will be raised somewhat also.
It is often observed that if the addition of ethylene glycol is so beneficial why don’t we just run it at 100%? Since pure ethylene glycol freezes at about 8° F and boils at 330° F, it appears that the freezing level has been improved and the upper end boiling point has been extended by 50%. However, the use of straight ethylene glycol will result in a 25% reduction in heat capacity, or heat carrying, capability compared to water. In an automobile the cooling system (the radiator) would have to be increased in size, therefore, by roughly 25% to provide the same cooling capacity.
Heat Capacity—The First Requirement:
Since the primary purpose of heat transfer fluids is to carry heat, it is reasonable to talk about a given fluid’s ability to absorb and hold a quantity of heat energy.
As energy is absorbed into a fluid, its temperature increases. In the case of water, the definition of the “BTU” (British Thermal Unit) is the amount of energy required to increase the temperature of one pound of water by one degree ˚F.
In the metric system of measurements, this energy is measured in terms of the “Joule” which is defined by the ability to raise one kilogram of liquid by one degree ˚C. These values are called “Specific Heat Capacity.”
As we mentioned earlier, all characteristics of fluids are compared to the performance of pure water. The heat capacities of various fluids are all compared to the heat capacity of water as a percentage or a fractional factor called “Specific Heat Ratio.” Think of it as a correction factor.
The specific heat capacity of water = 1.0 BTU/lb-˚F
The specific heat capacity of pure ethylene glycol = 0.57 BTU/lb-˚F
The ratio of specific heats is 0.57 divided by 1.0 which is 0.57
In the metric system (SI units);
The specific heat capacity of water = 4.19 kJ/kg-˚C
The specific heat capacity of pure ethylene glycol = 2.38 kJ/kg-˚C
The ratio of specific heats is 2.38/4.19 = 0.57
Specific heat capacity values will also vary slightly with temperature. Therefore, specific heat values will typically be defined for a given temperature or temperature range.
We just said that Specific Heat defines the ability of a certain weight of fluid to hold heat energy. It should be readily apparent that for any given volume capacity of a cooling system, there is going to be a fixed weight of fluid circulating within it. Therefore what goes hand in hand with Specific Heat is Specific Gravity—the density of a given fluid compared to the density of water. The higher the Specific Gravity, the more dense is the fluid and therefore the heavier a given volume of fluid is. And the heavier the fluid circulating in a given volume, the more heat energy it can hold.
As with metals and other solids, in addition to their abilities to hold heat energy, fluids have the ability to conduct heat energy. Obviously, this is an important factor since it governs the ability to put energy into a fluid and then get it back out again.
The units for thermal conductivity are defined as a conversion factor that relates the ability to move one BTU of energy through a square foot of material one foot thick for each 1˚F of temperature differential from one side of it to the other. This value is typically defined for a specific temperature or temperature range.
Viscosity—The Resistance To Movement:
Once we have a fluid that can carry the heat volume desired in the temperature ranges of expected operation, and we understand its thermal conductivity—the ability to move energy into and out of it, the next question that arises is just how easy the fluid will be to move around. While certain fluids will have the ability to absorb and hold enormous amounts of heat energy, they are so viscous that the pump horsepower required to move them around would be prohibitive. Additionally, their viscosity might change with temperature making them ideal as long as they are hot but they become solids at ambient temperature (e.g. liquid sodium used in some nuclear reactors).
Additionally, viscosity affects cavitation tendencies within water pumps. In pumps where there is a low pressure area that drops fluid pressure below its vapor pressure there is a tendency to create cavitation with the ensuing erosion effects. In this case a fluid with a higher viscosity will have less of a tendency to cavitate.
Boiling & Freezing Points:
Of critical importance also, is to understand under what temperature circumstances the fluid in question becomes unusable.
This is defined as the temperature at which a fluid boils. Boiling is the transition of a fluid from liquid to gas. The boiling point of any given fluid will vary with the pressure under which it is operating and the degree to which it is diluted with other fluids.
Conversely, the freezing point is defined as the temperature at which a fluid solidifies. Freezing is the transition of a fluid from liquid to solid. The freezing point of any given fluid will vary with the degree to which it is diluted with other fluids.
Let’s return again to water as the base reference. We’ve all seen how water beads up when it is poured onto a smooth surface. This tendency to create beads instead of flowing out uniformly over the surface is due to what is called surface tension. It is the measure of the ability of any given fluid to wet the surface from which it is to conduct heat energy. Surface tension is measured in units of dynes per centimeter and the lower this value, the more “wettability” of a surface a fluid has.
There are products available containing special surfactants (e.g., Red-Line’s “Water-Wetter”) to cut surface tension of water for the purpose of improving the ability of water to transfer heat energy.
It should come as no surprise that one of the chief disadvantages of water as a heat transfer fluid is its proclivity to induce corrosion of metals. Most heat transfer systems are comprised of metals, and dissimilar metals at that, serving to exacerbate the corrosion potential.
It’s The Water:
Water, in its chemically pure state (aka, deionized water), is nearly a perfect electrical insulator. It is however, highly polarized, meaning it will easily dissolve other substances. And when it dissolves substances it can make very strong electrolytic solutions conductive enough to carry significant electrical currents.
Alternatively, tap water is usually laced with both dissolved minerals and chlorine. The chlorine ions can be highly reactive and corrosive toward aluminum and even the stainless steels.
Alternatively, molecules that can’t be dissolved by water are considered to be non-electrolytes and include most of the organic substances (e.g. sugar, alcohol, glycerine, benzene). Note the difference between miscibility and conductivity. Alcohol, for example, is highly miscible in water but still non-conductive.
Most practical heat transfer systems are fabricated from various metals which are selected based on their weight, their conductivity, and their ease of manufacture. Given the inherent corrosiveness of water as an operating fluid, we are faced, therefore, with the need to inhibit the corrosion rate of the materials.
The most common metals found in heat transfer systems are cast iron, steel, aluminum, copper, brass and the constituents of solder (tin, zinc and lead).
Now that we understand the nature of the causes of corrosion, we are ready to consider methods for its inhibition. Corrosion inhibition can be accomplished through three fundamental approaches:
These are chemical compounds intended to prevent the production of ions at the anode (the metal’s surface) by forming an insoluble barrier layer.
Typical examples of anodic inhibitors are the various silicate compounds—sodium silicate, potassium silicate used to protect aluminum. These compounds react with corrosion products and form thick surface films to protect against further corrosion.
These are compounds that capture any ions given up by metal atoms at the anode thus preventing their migration to the cathode—effectively stopping the flow of galvanic current.
Here the surface acquires atoms, ions or molecules from liquid contact. The ions or atoms thus adsorbed are called adsorbates. These ions form strong bonds with the surface through electron sharing. They are insoluble and adhere strongly. Certain organic compounds thus adsorbed into a given metal’s surface become bonded irreversibly.
Acidity & Alkalinity:
The pH scale is a measure of whether a liquid is acidic (low pH, below 7.0) or alkaline (high pH, above 7.0). Knowing that most metals are adversely affected by acid and not alkalinity, one is tempted to simply assure high alkalinity in a heat transfer fluid. But things are never this simple.
It turns out that different metals behave differently regarding corrosion in different and often overlapping ranges of pH. Returning to the ubiquitous water/ethylene glycol mixture, it should be noted that over time, this mixture will become acidic (low pH). To prevent this, a pH buffer such as sodium borate is added to the fluid to maintain the pH within a certain alkaline range.
In addition to corrosion concerns, the selection of a heat transfer fluid must also consider its direct chemical effects on the materials of construction used in the cooling system. Generally, this is not an issue for the metals but rather for the polymers and elastomers—the plastics used for housings, for example, or the hoses used to carry the heat transfer fluids.
Water as a heat transfer fluid couldn’t be safer. But add to it another fluid like ethylene glycol and you have a compound that is toxic to man and animal. This toxicity has led some in recent years to substitute propylene glycol for the more common ethylene glycol. The lowest toxicity level is found with 3M’s Novec HFE-7100 series of engineered fluids. This fluid has many of the properties of water including inertness and lack of toxicity without water’s corrosivity. Its key drawback is its high cost.
Obviously, a fluid with a flammability problem in the presence of hot surfaces is less than ideal. However, for some applications where high temperatures are not an issue, the flammability simply becomes one more characteristic to be traded off against some other benefit or set of ideal characteristics.
Disposal & Environmental Concerns:
More and more attention is being paid to the environmental impacts of various chemicals and the implications of their disposal after use.
Characteristics of The Ideal Heat Transfer Fluid:
As we have seen, water has a series of characteristics; some making it ideal as a heat transfer fluid and others that are distinct disadvantages.
Since the ideal fluid does not exist, the search for a more ideal heat transfer fluid has become a compromise—a trade-off of features and benefits for specific applications. Additionally, there are any number of special additives that have been developed over the years to enhance the properties of a given fluid.
Heat Transfer Fluid Families:
Now that we understand the basic parametric measurements involved in heat transfer fluids, let’s look at the various chemical families of fluid systems in common use:
Ethylene Glycol/Water Formulations:
Most commonly found as antifreeze in your car.
Propylene Glycol/Water Formulations:
A non-toxic alternative to the ethylene glycol in automotive antifreeze.
Organic acid formulations, organic acid technology (“OAT”) (“Dexcool”):
Marketed as a “long-life” or “extended-life” antifreeze by General Motors.
Hydro Fluoroethylene (“HFE”) fluids (3M Novec HFE-7100):
An engineered fluid with many of the characteristics of water (i.e., non-toxic)
Will not corrode metals.
Perfluorinated Polyethers (PFPE’s) (Galden Fluids):
Generally used in high temperature applications for their inherent inertness.
Silicone oil based fluids (Syltherm):
For high temperature applications at lower cost than PFPEs.
Diphenyl oxide/biphenyl fluids (Therminol):
Mineral oil fluids:
High heat capacity
High operating temperatures