Did you know there are many options to consider when selecting thermal fluids?

  1. Temperature is Everything – match your bulk temperature but also consider film temperature, heater type
  2. Controlling Oxidation – high temperature air exposure can destroy the wrong fluid quickly
  3. Maximizing Fluid Life – short-term or long-term use – invest wisely
  4. Availability is Critical – thermal fluids are often critical to production – how long can you wait for your thermal fluid to ship?
  5. Reliable Service & Support – thermal fluids are maintenance-intensive, but can your vendor help you if there are problem

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Frequently Asked Questions

Are you new to the subject of heat transfer fluids? Not sure what all the industry jargon means?
We’ve provided answers here to some of the questions we hear most often.

Q: What is a Heat Transfer Fluid?

A: Heat transfer fluids come in many varieties. Each one offers unique properties to allow either indirect heating or cooling of process reactors, molds or extruders, etc., and are also used in portable oil heaters or temperature control units. Heat transfer fluids can also be called thermal oil, thermal fluids, hot oil or heat transfer oil.

Q: What makes a Heat Transfer Fluid better?

A: Some heat transfer fluids function better in closed systems while others offer superior performance in more open systems such as plastic processing. While fluids are made of a variety of base stocks, the use of additive packages such as oxidation, rust and corrosion inhibitors is what makes the difference between a short life and long life fluid. If your system is open to the atmosphere, using a proper fluid such as Duratherm 600 will keep the system running longer and cleaner between oil changes.

Q: How often should the fluids be changed?

A: Oil service life varies with the quality of fluid and the type of equipment it's used in. In extreme cases, this might be a matter of months; in some cases can be more than 10 years. The only way to know for sure if your oil still has usable life is through specific laboratory analysis. Your supplier should provide this service. Duratherm fluids come with free analysis.

Q: Are all fluids compatible?

A: No. Most petroleum-based fluids will be compatible, but some glycol, aromatic chemical or synthetic fluids may not. Always consult your supplier's technical service department for recommendations. If you are changing an incompatible fluid, ask your supplier for a compatible flushing fluid.

Q: Do I need to clean my system and if so how often?

A: Most systems will not require cleaning if they are operated and maintained properly, including regular oil changes. However, if a system is improperly started or shut down, or allowed to overheat, it may require some degree of cleaning. If fluids are not changed regularly and degradation is allowed to set in, systems can require moderate to extensive cleaning to remove carbon and sludge. We have a complete line of system cleaners that will return even severely fouled systems to like-new condition.

Q: The Oil changed colour since I put it in my system should I be concerned?

A: It is normal for fluids to change color after some use, especially in older equipment. If you are sure the system is relatively clean, this is most likely the fluid's normal aging BUT does not necessarily mean it needs to be changed. The fluid should be analyzed to be sure.


The terms and industry jargon we use in the heat transfer industry can be confusing. Below is a list of those terms and their definitions.

  • A-D
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  • E-H
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  • I-L
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  • M-P
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  • Q-T
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  • U-Z
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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.


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.


American Society for Testing and Materials, an agency that standardizes testing petroleum products.


Minimum temperature which a substance must be heated without application of flame or spark to cause substance to ignite.

Average Molecular Weight

Essentially the weight of the molecules that make up a fluid.


A material which promotes some chemical action without itself entering into the reaction.


In a heat transfer system, failure of the material to flow to the suction of the system pump for any reason.

Corrosion Inhibitor

Corrosion inhibitors prevent the oxidation of metal by preventing water from contacting metal system components helping to resist iron-oxide formation.

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.


Density refers to a material’s weight or compactness, which changes with temperature. Higher density fluids often transfer heat more efficiently than less dense fluids. Density is sometimes called spec gravity which refers to the ratio of a fluids weight compared to water.


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.

Distillation Range

Distillation range is a measurement of the temperatures within which a liquid distills(boils). It’s usually expressed as a % of the material that boils between two temperatures. A higher initial boiling point indicates a more thermally stable fluid and lower vapor pressures.

Fire Point

The temperature at which the vapors produced from a fluid will ignite (flash off) if exposed to an ignition source (the fluid will not burn at this point). While important for safety, it’s actually quite common to operate at temperatures above the flash point of the fluid. Watch video

Heat Capacity

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.


Compounds containing only carbon and hydrogen. Petroleum fluids consist chiefly of hydrocarbons.


Having an affinity for water; capable of uniting with or dissolving in water.


Having antagonism for water; not capable of uniting or mixing with water.


A substance that slows or prevents chemical reactions, such as oxidation or corrosion.

Maximum Bulk/Use Temperature

All fluids have a maximum recommended heating temperature. Heating a 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.

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.

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.


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.

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.

Oxidation Stability

The resistance of lubricants to chemically react with oxygen. The absorption and reaction of oxygen may lead to deterioration of lubricants.

Oxidative Degradation

Oxidation occurs when hot fluid (>200°F) reacts with oxygen causing an increase in viscosity and reducing the service-life of the fluid. For every 15°F increase above 200°F the rate of oxidation typically doubles. An increase in acidity(TAN) is an indicator of oxidation.

Pour Point

The lowest temperature at which a liquid will pour under specified conditions.

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.

Seal and Gasket Extender

An additive that conditions seals and gaskets to help keep them from drying out and failing.

Specific Gravity

Specific gravity refers to a material’s weight or compactness, which changes with temperature. The higher a fluid’s specific gravity, the more efficiently it will transfer heat. Specific gravity is sometimes called density which refers to the ratio of a fluid’s weight compared to water.

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.

Suspension Agent

Suspension agents help ensure particulates (for example, carbon, debris, or weld slag and metal shavings in newer systems) are held in suspension and easily filtered or caught in strainers, which could otherwise cause harm to the system if not removed.


Acids form when fluid comes in contact with oxygen (minimal at room temp but increases exponentially above 200°F) TAN levels indicate the extent of which a fluid has been oxidized. New fluids typically have a TAN less than 0.05, most fluids have a condemning TAN limit of 1.0.

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.

Thermal Degradation

Thermal degradation occurs at the heat source when a fluid is overheated past its bulk temperature. ‘Cracking’ the fluid produces light ends which lower flashpoint and viscosity, increase vapor pressure, and can result in coke build-up on the heater.

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 half full when cold and about half full when hot.

Vapor Pressure

Most liquids form vapors when heated. A low vapor pressure helps prevent boiling and pump cavitation. Vapor pressure increases as temperatures rise and also indicates how quickly a liquid evaporates. Fluids with high vapor pressure may need frequent topping up.


A measure of a fluids resistance to flow. Viscosity typically increases as temperatures reduce and decreases as temperatures rise; a higher viscosity essentially means a thicker fluid. Ideally, a heat transfer fluid should have viscosity ranging from 5cSt to 40cSt at 104°F.