Heat Transfer Fluid

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Glossary of Terms

New to heat transfer fluids and not quite sure what some of the industry lingo means?

Click on a term to skip down to the definition:

Anti-Foam Additive
Causes air bubbles to break on the surface, prevents build up of foam. Can also help prevent oxidation, they cannot prevent air entrainment, but function as an aid to quicker release of entrained air.
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Anti-Oxidant (oxidation inhibitor)
An additive to retard oxidation. Anti-oxidants are critical to any application that is not sealed from the atmosphere. If you don't have a nitrogen blanket on your expansion tank or reservoir it is crucial that your fluid contain an anti-oxidant. Oxidation leads to sludge formation that left unchecked could cause blockages and lead to complete system failure.
Duratherm fluids contain a proprietary dual stage anti-oxidant. You can see the effect of this additive in our Competitive Comparison Report.
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ASTM
American Society for Testing and Materials; an agency that standardizes testing petroleum products.
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Autoignition
Minimum temperature which a substance must be heated without application of flame or spark to cause substance to ignite.
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Average Molecular Weight
Essentially the weight of the molecules that make up a fluid.
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Catalyst
A material which promotes some chemical action without itself entering into the reaction.
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Cavitation
In a heat transfer system, failure of the material to flow to the suction of the system pump for any reason.
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Centipoise and Centistoke
A centipoise is 1/100th of the unit of absolute viscosity (the poise), e.g., the viscosity of water at 20oC is approximately one centipoise. The centipoise is derived from one kinematic unit of viscosity (the centistoke), by multiplying the latter by the density of the liquid, i.e., 1 centipoise = 1 centistoke x the density of the liquid. These units are part of the metric system, commonly used in Europe and becoming adopted in the U.S. and Canada.
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Corrosion Inhibitor
Helps prevent oxidation of metal by displacing water from metal surfaces. It plates to metal with a polarized effect to give an internal "umbrella", helping to resist iron-oxide formation.
Most systems at some point in time will have some water contamination. Whether from leaky heat exchangers or drawn from humid air, moisture venting through the expansion tank or oil reservoir can lead to corrosion inside the tank. Corrosion inhibitors virtually eliminate this risk.
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Defoaming agents
During start-up air can become trapped in a system. Pumping creates air bubbles (foaming) which can lead to pump cavitations, possibly damaging pumps and other system components. Our proprietary additive package contains defoaming agents to help keep air from foaming in the oil.
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Density (Specific Gravity)
Density measures a material's weight or compactness. It changes as temperatures fluctuate. Fluids with a higher density typically transfer heat more efficiently than fluids that are less dense. Density is sometimes expressed as specific gravity, a dimensionless value that has no unit of measure. It refers to the ratio of a fluid's weight in comparison to water. For example, a material with a spec gravity of less than 1 would be considered less dense than water and would therefore float in water.
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Deposits
Oil-insoluble materials that result from oxidation of the oil and contamination from external sources and settle out in system components as sludge and varnish.
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Distillation Range
Distillation range is a measurement of the temperatures within which a liquid distills or boils. It's usually expressed as a percentage of the material that boils between two temperatures. A higher initial boiling point indicates a more thermally stable fluid as well as lower vapor pressures. A narrower boiling range is also more desirable.
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Fire Point
The temperature at which the fluid will sustain a fire if ignited by an outside ignition source. It is quite common for heat transfer systems to be operated at temperatures above the fire point of the fluid as ignition sources should always be far removed from any heat transfer system.
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Flash Point
The temperature at which the vapors produced from a fluid will ignite (flash off) with the presence of an ignition source (the fluid will not burn at this point). The flash point is important from the viewpoint of safety; however it is quite common for heat transfer systems to be operated at temperatures above the flash point of the fluid.
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Heat Capacity (Specific Heat)
This is a measurement of a fluid's capacity to carry heat. It determines indicates how many BTUs it would take to increase 1lb of fluid by 1°F. Heat capacity is used along with other properties to determine efficiency as quantified by the heat transfer coefficient.
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Hydrocarbons
Compounds containing only carbon and hydrogen. Petroleum fluids consist chiefly of hydrocarbons.
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Hydrophilic
Having an affinity for water; capable of uniting with or dissolving in water.
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Hydrophobic
Having antagonism for water; not capable of uniting or mixing with water.
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Inhibitor
A substance that slows or prevents chemical reactions, such as oxidation or corrosion.
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Maximum Bulk/Use Temperature
All fluids have a maximum recommended temperature to which they can be heated. Heating a fluid beyond this point will result in thermal degradation or "cracking." This is characterized by low boilers (or "light ends") coming off, lowering of flashpoint, increased vapor pressure and carbon build up.
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Maximum Film Temperature
Heating elements and pipe walls get much hotter than the maximum bulk/use temperature of a system. A fluid's film temperature is always higher than its max bulk temperature and if exceeded will thermally degrade the fluid.
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Metal Deactivators
Some metals used in the construction of heat transfer systems can actually react with the oil, causing premature breakdown. Metal deactivators ensure compatibility with any system, even those with copper lines, heat exchangers or fittings.
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Oxidation
The process of combining with oxygen. All petroleum products are subject to oxidation to some degree. The reaction increases with rise in temperature. Oxidation produces oil-insoluble oxidized materials, which result in viscosity increase and deposits.
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Oxidative Degradation
Oxidative degradation is the reaction of oxygen (in air) with the fluid by a free radical mechanism to form larger molecules which end up as polymers or solids. These thicken the fluid and increase its viscosity. A more viscous fluid will be more difficult to pump, have poorer heat transfer characteristics as well as an increased chance of coke formation. Oxidation is also accompanied by an increase in the acidity (TAN) of the fluid.
As with all chemical reactions, oxidation occurs more rapidly as the temperature is increased. At room temperature, the reaction rate is hardly measurable. However, it can become a factor in the life of the fluid in certain styles of heat transfer systems. At temperatures encountered in systems in use in the plastics extrusion and die casting industries, as an example, oxidation is the main cause of fluid degradation.
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Oxidation Inhibitor
A chemical additive that minimizes the formation of harmful acids and varnish forming compounds that form when a fluid is subjected to air at elevated temperatures.
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Oxidation Stability
The resistance of lubricants to chemically react with oxygen. The absorption and reaction of oxygen may lead to deterioration of lubricants.
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Pour Point
The lowest temperature at which a liquid will pour under specified conditions.
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Pour-Point Suppressants
Prevent insoluble wax molecules in oil from building a honeycomb (lattice-like structure) at colder temperatures. Particularly useful for paraffinic oils. Gives useful ability to pour at lower temperatures.
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Seal and Gasket Extender
An additive that conditions seals and gaskets to help keep them from drying out and failing.
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Suspension Agent
Some fluids develop carbon and other particulate matter after years of use. Even new systems have things like weld slag and metal shavings that can become trapped in instrument lines or cause problems in other areas. Suspension agents help ensure particulate is held in suspension and easily filtered or caught in strainers.
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TAN ( Total Acid Number )
Acids are formed when a fluid comes in contact with oxygen, TAN levels are a means to show the extent of which a fluid has been oxidized.
New fluids typically have a TAN less than 0.05, most fluids should be changed at and have a condemning limit of a TAN of 1.0.
The rate of oxidation is minimal under 200F however as the temperature climbs the effects of oxidation are exponential. It is an industry accepted standard to assume the rate of oxidation doubles for each 15 degree increase above 200F.
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Thermal conductivity
The rate that heat transfer occurs through conduction. Fluids with low conductivity will transfer heat slower than a fluid with a higher rate of conductivity.
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Thermal Degradation
Thermal degradation or thermal cracking is the breaking of carbon - carbon bonds in the fluid molecules by heat to form smaller fragments which are free radicals. The reaction may either stop at that point, in which case smaller molecules than previously existed are formed, or, the fragments may react with each other to form polymeric molecules larger than previously existed in the fluid. In heat transfer terminology, the two types of degradation products are known as "low boilers" and "high boilers".
If thermal degradation occurs at extreme temperatures greater than 400°C (752°F), the effect is not only to break carbon - carbon bonds but to separate hydrogen atoms from carbon atoms and form coke. In this case, fouling of the heat transfer surfaces is very rapid and the system will soon cease to operate.
The effect of the low boilers is to decrease the flash point and viscosity of the fluid as well as to increase its vapor pressure. The effect of the high boilers is to increase the viscosity of the fluid as long as they remain in solution. However, once their solubility limit is exceeded, they begin to form solids which can foul the heat transfer surfaces.
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Thermal Expansion
Heat transfer fluids change in volume according to temperature. Volume increases when heated and decreases when cooled. This information is helpful when trying to size an expansion tank; typically, an expansion tank should be ⅓ full when cold and about ½ full when hot.
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Vapor Pressure
Most liquids form vapors as they're heated. As temperature increases, so too does a fluid's vapor pressure. Fluids boil when their vapor pressure equals the pressure of the surrounding gas. Knowing your fluid's vapor pressure is important when specifying circulation pumps and piping. A low vapor pressure helps prevent boiling and pump cavitation. It's also an indication of how quickly a liquid evaporates; a fluid with high vapor pressure could require frequent topping up. Vapor also doesn't transfer heat as efficiently as fluid.
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Viscosity
A measure of a fluids resistance to flow. A higher viscosity is essentially a thicker fluid. It is desirable for most high temperature heat transfer fluids to have viscosity in the range of 20cst to 40cst at 104F.
Another aspect of a fluids viscosity is its VI (Viscosity Index) or how a fluids viscosity is affected by temperature. It’s a measure of the rate of change of viscosity with temperature. In lubricants a high VI is desirable to maintain a relatively consistent viscosity throughout the useable temperature range. In heat transfer fluids a lower VI is more desirable in order to allow the fluid to thin out ( reduce viscosity ) with temperature increases. This allows for more efficient thermal transfer properties.
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